Susceptibility of Wheat Varieties to Soil-Borne <i>Rhizoctonia</i> Infection

doi:10.4236/ajps.2013.411277

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American Journal of Plant Sciences, 2013, 4, 2240-2258

Published Online November 2013 (http://www.scirp.org/journal/ajps)

http://dx.doi.org/10.4236/ajps.2013.411277

Open Access AJPS

Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia

Infection

Gyula Oros1*, Zoltán Naár2, Donát Magyar3

1Plant Protection Institute, Hungarian Academy of Sciences, Budapest, Hungary; 2Department of Microbiology and Food Technol-

ogy, Eszterházy Károly College, Eger, Hungary; 3National Institute of Environmental Health, Budapest, Hungary.

Email: *gyoros@aim.com

Received September 22nd, 2013; revised October 20th, 2013; accepted October 30th, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Response of 19 wheat varieties cultivated in Hungary varied within large limits to soil borne Rhizoctonia infection. The

most frequent symptom, usually leading to damping off was the root neck necrosis. Four significant factors influencing

the susceptibility of wheat comprised 71% of total variation but none of them was dominant. The inhibition of devel-

opment of survivors in Rhizoctonia infested soil correlated with overall susceptibility of variety concerned. The varie-

ties Emese, Kikelet and Palotás are proved to be less susceptible, but none of the varieties could be certified as tolerant.

No relationships were revealed between pathogenicity of 26 Rhizoctoni a strains studied and their taxonomic position or

origin. The anamorph strains of Athelia, Ceratobasidium, Ceratorhiza and Waitea similar to Thanatephorus ana-

morphs selectively infected the wheat varieties, but the syndromatic pictures were undistinguishable with unarmed eye.

R. solani was proved to be more aggressive against germinating wheat than R. cerealis . Nine significant factors influ-

encing the virulence of Rhizoctonia strains comprised 82% of total variation, and six of them influenced exclusively

Thanatephorus anamorphs.

Keywords: Wheat; Rhizoctonia ; Tolerance; Brown Patch; Soil-Borne; Virulence

1. Introduction

In August 2002, brown patches were observed on turf

grasses in parks at four locations in Budapest. The symp-

toms observed were necrotic lesions on the roots and

stems, as well as brown lesions on the leaves. Two types

of sclerotia, nearly globose, pinkish to orange and ir-

regularly shaped, dark brown were found on roots. On

potato dextrose agar fast growing, colourless colonies with

small reddish/coloured scelrotia uprose of the first type.

This fungus was identified as Rhizo ctonia zeae Voor-

hees (teleomorph Waitea circinata Warcup and P.H.B.

Talbot) [1]. Buff-coloured, fast growing colonies of

Rhizoctonia solani Kühn (teleomorph Thanatephorus cu-

cumeris (A.B. Frank) Donk) arose of the second type.

The study of more than 150 plant species cultivated in

Hungary [2] revealed that R. zeae attacked monocotyle-

donous species more aggressively than dicotyledonous

ones, contrarily to R. solan i. This latter species was for-

merly reported as pathogen of winter wheat [3] and oat

[4] in Hungary.

Traditionally, farmers paid little attention to field dam-

age caused by soilborne Rhizoctonia infection in wheat,

because either seedborne or airborne fungi (rust, mildew,

smut etc.) infecting stem, leaves and spikelets had been

the main constrains of yield. Due to success in breading

and arousal of new synthetic fungicides, these fungi

presently do not cause catastrophic yield losses. However,

in the last two decades increasing number of papers was

published on yield losses (30% to 50%) caused by

Rhizoctonia species [2,5] in main wheat cultivating areas

[6-8]. In Europe and North America winter wheat suf-

fered mainly of R. solani AG-8 strains [7] with the R.

cerealis [6-8], while in Australia AG-1 and AG-8 and in

Turkey five different anastomosis groups of R. solan i

[9,10] were revealed. In South Eastern Hungary damage

by the R. cerealis and R. solani has been observed in

spring wheat [3].

Rhizoctonia species are well known soil borne patho-

gens frequently causing damping off prevalently in moist

and cool conditions that are the main stress factors re-

*Corresponding author.

Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection 2241

quested to induce disposition to increased susceptibility

of potential host plants [11]. These species have no vege-

tative bodies for spreading, but infected seeds and propa-

gating material can distribute infecting propagula. The

disease is more severe in sandy soils, as the fungus can

grow more rapidly [12], and the hyphae tend to colonize

rhizoplane reducing the vitality of plant even without

penetrating into tissues. Infection cushions are formed on

the surface before individual hyphae penetrate with mi-

nor morphological modifications [13]. The ability to pro-

duce discrete appressoria is highly variable and regulated

seemingly by numerous factors. Hyphae may penetrate

without an appressorium through stomata or wounds [14-

16]. All Rhizoctonia species are obligate aerobic fungi,

which are habiting mainly in rhizosphere, however, they

may survive as saprobionts in the upper layer of the soil

forming a mycelial web, thus the undisturbed soil enhance

the risk of the infection of young roots [17]. Rhizoctonia

root rot is more frequent and severe under reduced or

conservation tillage [18,19] as the conventional tillage

disrupts the mycelial web [20]. This underlines the im-

portance of the elaboration of new root health manage-

ment practices, because formerly minor pest problems,

such as Rhizoctonia root rot of wheat became major

problems. In last two decades new research has been

started and several hundred scientific papers reported

data on etiology of root rot and sharp eyespot caused by

Rhizoctonia species, on genetic background of suscepti-

bility of wheat as well as on Rhizoctonia virulence. Ac-

tually, we can conclude that environment dependent

mechanisms regulate the progress of this disease with

complex etiology that is not under gene for gene control

[21], and further research is requested to understand the

multicomponent syndromatic picture.

The disease caused by R. zeae was described for the

first time in Florida, by Voorhees in 1934 [22], as scle-

rotial rot of corn. Since that this fungus distributed in

temperate regions, in Europe firstly reported in 2004 [1].

In a host-range study, the isolate proved to be highly

pathogenic to germlings of several ornamental and culti-

vated plants, including Beta vulgaris ssp. vulgaris , Cal-

listephus sinensis, Dahlia variabilis, Daucus carota, Lu-

pinus polyphyllus, Papaver somniferum, Pennisetum glau-

cum, Phaseolus vulgaris, Sesamum indicum, Solanum

melongena, S. tubero sum, Sorghum bicolor and Triticum

aestivum [1,2,23]. The estimation of risk on wheat pro-

duction caused by this pathogen was the aim of our work.

Actually, both seed and soil borne infections might be

well controlled in early stage of germination with seed

dressing [24], however, we have no effective methods

with reliable cost/benefit ratio for protection of wheat

against Rhizoctonia during the vegetation. Although min-

eral nutrients can manipulate the reaction of plants [25],

such treatments can not combat serious yield loss. Sev-

eral attempts were made to explore antifungal potential

of eubiotic preparations and to desing suppressive soil

[26-32], however, in order to prevent the harm, these de-

velopments have had not satisfactory results. In the case

of soil-borne infections correlative influences among

members of microbial consortia associated to potential

host plant may influence both the invading pathogen and

disposition of potential host to adverse factors of envi-

ronment thus change both the virulence of pathogen and

the susceptibility of host to pathogen at any time de-

pending on the genetic potential of these partners. Due to

complexity of interactions there is difficult to predict the

success of biocontrol measures. Currently the effective

protective method is the appropriate crop rotation, and

the breeding of wheat cultivars for improved tolerance to

factors inducing disposition to increased susceptibility to

the presence of Rhizoctonia in the soil can be considered.

Our objectives of this study were the comparative

evaluation of responses of germinating wheat seeds to

Rhizoctonia strains of various origin and taxonomic posi-

tion as well as to reveal patterns in factors influencing

the wheat/Rhizoctonia interaction.

2. Materials and Methods

Greenhouse experiment was undertaken to compare the

infective potential of R. zeae strain with 25 Rhizoctonia

strains of various taxonomic position. Susceptibility of

two sortiments of Triticum aestivum L., moreover, T.

monococcum L., T. turgidum L. and four small seed

grains were involved into the tests. No seed dressing or

any other manners to depress the microbiota of sper-

mosphere were applied. The potting medium was made

by mixing forest soil with peat before autoclaving (1.15

atm per 20 min), at the ratio of 3:1.

2.1. Test Plants

Seeds of wheat varieties (Table 1) were gifted by Elit-

mag Kft (Martonvásár, Hungary). Except Alkor (Triti-

cum monococcum L.) and Hegyes (T. turgidu m L.) all are

T. aestivum L. cultivars. Small seed grains Eleusine

coroacana Gaertn., Panicum milliaceum L., Phalaris

canadiensis L. and Setaria italica (L.) P. Beauvois were

purchased of the market (HERMES Ltd., Budapest, Hun-

gary).

2.2. Test Fungi

Rhizoctonia strains were originated of different locations

and various hosts:

Rhizoctonia solani strains of CBS collection: B-415

(AG-1, Pinus sylvestris L., Canada, CBS 522.96), B-432

(AG-2, Daucus carota L., Netherlands, CBS 326.84),

B-446 (AG-3, Solanum tuberosum L., Spain, CBS 117248),

B-417 (AG-4, Citrus sp., Argentina, CBS 341.35), B-430

Open Access AJPS

Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

Open Access AJPS

2242

Border limits of the scale of evalutaion: 0 = all germlings killed, 5 = all seedlings survived. (1) Wheat varieties; T. monococcum cv Alkor, T. turgidum cv Hegyes and all others are T. aestivum. (2) Strains 1-13 refer to

anastomosis groups AG-1-AG-11 and AG-E of R. solani of CBS collection, respectively, strains 14-20 are Hungarian isolates of R. solani and R. stahlii (21); all are Thanatephorus anamorphs. Rhizoctonia strains

22-26 are anamorphs of Waitea (22), Cerathorrhiza (23, 24), Ceratobasidium (25) and Athelia (26), respectively. (3) Code; accession numbers of the Mycological Collection of Plant Protection Institute HAS

(

WDCM824

)

. The bold labelled strains had been isolated of wheat.

(

)

tomless = no visual s

toms observed at 8th da

(

)

Kille

= no one survived over 8th da

Table 1. Evaluation of the response of wheat varieties to soil borne Rhizoctonia infection.

Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection 2243

(AG-4, Phaseo lus sp., England, CBS 340.51), B-418

(AG-5, Zeae mays L., Netherlands, CBS 339.84), B-419

(AG-6, Conyza canadensis (L.) Cronquist, CBS 137.82,

USA), B-420 (AG-7, soil, Japan, CBS 214.84), B-421

(AG-8, Triticum aestivum L., Australia, CBS 101782),

B-422 (AG-9, S. tuberosum, USA, CBS 970.96), B-423

(AG-10, T. aestivum, USA, CBS 971.96), B-424 (AG-11,

Lupinus angustifolus L., Australia, CBS 974.96), B-434

(AG-E, Malus sp., Netherlands, CBS 340.84).

R. solani strains isolated in Hungary: B-411 (S. tube-

rosum, cv Desirée) and B-410 (S. tuberosum cv Kis-

várdai rózsa), B-413 (Malu s domestica L.); B-409 (Hi-

biscus rosa-chinensis L., imported of Lybia, Tripoli);

B-245 (Allium cepa L., imported of China, Henan);

B-521 (Impatiens balsamina L.); B-433 (Festuca arun-

dinacea Schreb.).

R. stah lii Burgeff (teleomorph: Thanatephorus sp.): B-

441 (Platanthera chlorantha (Custer) Rchb.), Germany,

CBS 119.92).

R. fraga riae S. Husain & W.E. McKeen (teleomorph:

Ceratorhiza fragariae (S.S. Husain & W.E. McKeen)

R.T. Moore): B-438 (Fragaria × ananassa Duchense,

Canada).

R. ramico la W.A. Weber & D.A. Roberts (teleomorph:

Ceratorhiza ramicola (W.A. Weber & D.A. Roberts) R.T.

Moore): B-427 (Pittospo rum tobira (Thunb.) W. T. Ai-

ton, Florida, USA, CBS 400.51).

R. cerealis E.P. Hoeven (teleomorph: Ceratobasidium

cereale D.I. Murray & Burpee): B-447 (T. aestivum L.,

Germany, CBS 559.77).

R. zeae: B-405 (mixed grass of Festuca and Lolium,

Hungary).

Athelia rolfsii (Curzi) C.C. Tu & Kimbr. (Syn:

Scelotium rolfsii Sacc.): B-442 (S. tuberosum, Italy, CBS

464.48).

The strains were maintained on potato dextrose agar

(Merck, Darmstadt, Germany) amended with 2 g soya

peptone L44 (Oxoid, Basingstoke, UK).

2.3. Test for Pathogenicity

The potting medium was made by mixing forest soil with

peat before autoclaving (1.15 atm per 20 min), at the

ratio of 3:1.

The soil was inoculated with Rhizoctonia by the fol-

lowing manner: the sterile soil prepared as above was

admixed with chickpea seeds previously infected with

the pathogen (10 seeds per 250 g pot), than incubated 96

hours at 26˚C - 28˚C for evolving the mycelial net. The

seeds were put on the surface of infested soil (1 × 1 cm),

than covered with 5 mm layer of sterile soil. Sterile dis-

tilled water was used to moist the surface (15 mL per

pot), and covered with plastic wrap layer to avoid des-

siccation. Subsequently, the pots were evaluated each

day counting the emerged germlings, and observing the

occurrence of disease symptoms (damping off and leaf

spots). The height of seedlings was regularly measured to

nearest millimetre to follow the dinamics of growth. The

control plants were grown up in Rhizoctonia free soil.

The growth inhibition was calculated as a ratio between

control and treated plants.

When the coleptyles of control plants had been fully

developed (8 days after emergence of first germling) the

pathological status of all seedlings was evaluated, their

height and mass of measured to nearest millimetre and

milligramm, respectively. Inhibition rates were calcu-

lated as related to control. The percentages were trans-

formed into probit values, and this transformed data were

analyzed according to Sváb [33]. The state of roots was

assessed as well, and tissue sections were examined un-

der microscope in cases where no visual symptoms were

observed. The method was discussed in detail previously

[23]. The following six fold scale was used to assess the

tolerance of test plants at the 8th days: 0 = all seedlings

were destroyed; 1 = the majority of seedlings was dead,

but at least one survivor was presented either symptom-

less or bearing severe symptoms (the coleoptyle and the

roots damaged, the root neck scoring), 2 = less than half

of seedlings survived, the survivor were either symptom-

less or bearing severe symptoms (the coleoptyle and the

roots damaged, the root neck scoring), 3 = more than half

of seedlings survived, the symtoms of disease syndrome

largely varied, 4 = most of seedlings were similar to con-

trol, but as minimum as one diseased, 5 = none of seed-

lings had any symptoms visible to the naked eye. The

results of observations were compiled into data matrix

((19 wheat varieties + 4 small grains) × 26 Rhizoctonia

strains).

Surviving specimens were grown up to 21 days and

their development and evolution of disease syndrome

were observed.

2.4. Data Analysis

Box plot analysis was applied to demonstrate alterations

both in tolerance of test plants and and virulence of

Rhizoctonia strains as well as variations in growth pa-

rameters of wheat varieties as influenced by the presence

of Rhizoctonia in the soil. The relationships between host

(wheat varieties and small grains) and Rhizoctonia strains

(potential soil borne pathogens) have been analyzed by

multivariate methods: Non-linear Mapping (NLM) [34],

Cluster Analysis (CA), Principal Component Analysis

(PCA), Regression Analysis combined with Canonical

Correlation Analysis (CCA) and Potency Mapping (PM)

technique [35] following a previously described scheme

[36]. PCA was carried out on the correlation matrix [37]

and only the components having an eigenvalue greater

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

2244

than one were included in the evaluation of data to dem-

onstrate potential number of factors influencing on host

parasite system, and the results were not delineated in

details. The protocol of experiments is shown in Figure

Statistical functions of Microsoft Office Excel 2003

(Microsoft, Redmondton, USA) and Statistica5 program

(StatSoft 5.0., Tusla, USA) were used for analysis of data.

The graphical presentation of result of data analysis was

edited uniformly in MS Office Power Point 2003.

3. Results

3.1. Dynamics of Germination

The dynamics of germination of wheat was altered by

Rhizoctonia strains in cultivar dependent manner (Figure

2). In majority of cases first germlings emerged within 2

days after sowing in Rhizoctonia free soil, moreover, the

process was finished rapidly. The seeds of Pannon Pre-

mium sortiment germinated more uniformly, than the

other varieties. In the presence of Rhizoctonia strains

emergence had been delayed with exception of most tol-

erant varieties (Petrence, Emese, Alkor, Hegyes). The

last day means the limit after that no further germlings

outcropped. Several seeds having been destroyed, seem-

ingly, the susceptible individual were killed either before

emergence or suffered damping off within 2 - 3 days

after outcropping. This ratio strongly varied, and it was

not possible to carry out the statistical analysis within

frames of the experimental model applied.

3.2. Syndromatic Picture of Disease

The growth of infected or diseased seedlings of wheat

was conspiciously retarded than the control, however, not

all symptoms of disease syndrome turned up. Even seed-

lings without any visible symptoms suffered damping off

before full development of coleoptyle, the roots of such

individuals were symptomless in many cases, although

brown spots could be frequently observed on roots and

root neck, even in the cases of robust survivors too. In

some cases small black spots (<1 mm) were found on

root necks, but these were not spread later. Leaf spots with

dark brown edge were randomly observed after 6 days.

The development of seedlings which survived the infec-

Antecedents

1 New disease

2 Survay of host range

3 Survay of data bases

Zer o hypothesis

Design of experiments

Collection of test

organism

-Wheat varieties

-Smal l gr ains

-Rhizoctoniastrains

Experimental

Germinating seeds in

Rhizoctoniainfested soil

Sampling

8th day

Observations

Mass Length

Coleoptiles

Box plot

Survivors Obser va tion

until 21 days

Potency

Map Potency

Map

Box plotBox plot

Regression

Analysis

Roots

Microscopic

survay

Dynamics of

germination

Evaluation of

disease syndrome

Data Matrix

(19+4)×26

Susceptibilty

(T olerance) of

wh eat cultivar s

Virulence of

Rhizoctonia

strains

Potency

Map

Potency

Map Nonlinear

Map

Response of

test plants

Regression

Analyisis Potency

Map Cluster

Analysis

F6,7

Cluster

Analysis

Principal

Component

Analysis

Box plot

T2,3

Figure 1. Flow diagram of the experimental protocol. The labels F and T in ellipses mark figures and tables in the body text

where the results of computations were used for demonstration, while V means verbal interpretation of result. The zero hy-

pothesis was: on the base of screening large number of varieties there is possible to select candidates for breeding wheat cul-

tivar tolerant to soil borne Rhizoctonia infection.

Open Access AJPS

Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection 2245

Emergence (days)

1234567

Toborzó

Bodri

Emese

Petrence

Lucilla

Magvas

R23

Kikelet

Karizma

Lona

* Hegyes

** Alkor

Palo tás

Menüett

Suba

Toldi

Ködmön

Kolo

Mazurka

Other varieties Pannon Standard

Pannon Premium

Figure 2. Susceptibility of wheat varieties to soil borne Rhizoctonia infection. The gray and black stri ps mark varieties grow n

up in Rhizoctonia infected (+R) and non-inculated (control) pots, respectively. Wheat varieties are listed on the x-axis,

grouped as: other wheat varieties, Pannon Premium group of varieties, Pannon Standard group of varie ties. F and L are up-

per and lover values of the limits of emergence of first and last seeds, respectively.

tion without root neck symptoms was retarded at various

degree even two weeks later. The incidence of robust

survivors varied greatly within series, the root system of

such individuals was not retarded with exception of most

susceptible varieties (Bodri, Lona, Lucilla, Menüett). The

reaction of germlings to the presence of Rhizoctionia in

the soil was extremely heterogenic, the coefficient of

variation was over 30 percent within pot that made im-

possible the reliable statistical analysis. This was the

reason of use of the six fold scale for assessment of plant

response to Rhizocton ia infection. In several cases half of

seedlings survived while the other half was killed inde-

pendently of host/pathogen pair, most probably due to

environmental effects.

The behaviour of small grains was similar to that of

wheat including symptoms and development of disease

syndrome. The leaf spots on Eleusine appeared rarely,

but this was not in direct contact with virulence of

Rhizoctonia strains. The survivors of small grains were

not analyzed in details.

3.3. Susceptibility of Wheat Varieties

The test plants tolerated the Rhizoctonia in strain de-

pendent manner (Table 1). Among T. aestivum cultivars

Lona, Menüett and Bodri proved to be less tolerant,

while Emese, Palotás and Petrence exhibited low suscep-

tibility. The response of T. monococcum and T. turg idum

was similar to more tolerant T. aestivum cultivars. The

Eleusine was less susceptible than other small grains.

Unfortunately, none of the test plants tolerated the ma-

jority of Rhizoctonia strains at high degree.

The response of wheat cultivars to R. zeae, the new

pathogen in Europe, altered significantly of that of R.

solani strains. This difference manifested clearly, when

variety dependent virulence of R. zeae and associated R.

solani strain (B-433) was compared (R2 = 0.317, n = 19),

although their average pathogenicity (AP) was similar

(2.9 and 3.1, respectively). R. cerealis proved to be mod-

erately aggressive against germinating cereals (AP = 3.6),

and the similarity between activity spectrum of R. solan i

strains and R. cerealis was low (r2 < 0.23).

The susceptibility of wheat varieties to Ceratorhiza

(B-438) isolated of strawberry and Athelia (B-442) was

low, however, the C. ramic ola (B-427) isolated of orchid

heavily injured five T. aesticum cultivars (Toborzó, Bodri,

Lucilla, Toldi and Lona). The orchid symbiont R. stah lii

(B-441) exhibited low pathogenicity. Test plants were

related applying Nonlinear Mapping based on data com-

prised in Table 1 (Figure 3). Plants similarly responding

to soil borne Rhizoctonia infection formed a loose group,

but no clear patterns were revealed.

The taxonomic position did not influence the grouping.

This type of plotting did not gave information on the

structure of relationships among wheat cultivars, so an-

other nonlinear method, the Cluster Analysis was carried

out (Figure 4).

Wheat cultivars clustered into two well separated groups

on the dendrogram. Triticum aestivum cultivars were dis-

tributed between groups, however T. monoco cum and T.

turgidum were in different clusters. The average toler-

ance level of groups was similar (A = 3.1 and B = 2.7),

but alterations were revealed in the spectrum of suscepti-

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

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Toborzó

Bodri

Em ese

Lucilla

Magvas

Palotás

Menüett

Toldi

Suba

Ködmön

Kol o

Mazurka

R23

Ki kelet

Kar izm a

Lona

Hegyes

Al kor

El eusine

Panicum

Phalaris

Setaria

Petrence

Figure 3. Nonlinear map of test plants. The size of balls is proportional to potential tolerance of test plant to Rhizoctonias.

Triticum aestivum cultivars are marked with light grey, those correlated by their response to Rhizoctonia strains (r > 0.5) are

linked with lines. The clusters P and S comprise varieties of Pannon Standard and Pannon Pre mium sortiments. The c urve A

marks possible pattern.

Linkage Distance (1- Pearson’s r)

0.200.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Toldi

M enüett

Ködmön

Suba

**Alkor

Kolo

Magv as

Mazurka

R-23

Palotás

Petrence

Karizma

*Hegyes

Kikelet

Eme se

Lucilla

Bodri

Toborzó

Varieties

1. 2

013

2210

Lona 2. 5

2.1

2.6

3. 2

2.7

2.0

3.5

3.3

3.5

3.7

3. 3

3. 5

2.7

2. 3

2. 4

T(4)

AG1

AG 8

AG10

A(1)

C(2)

C(3 )

W(5)

3.1

1.4 3.3 4.1

2.4 4.7 2.6 3. 7

A2.7

0.6 3.5 4.0

3.3 3.0 2.7 2.8

Mea n (6)

Figure 4. Grouping of wheat varieties based on their responses to soil borne Rhizoctonia infection. The data of Table 1 were

clustered of Pearson’s correlation matrix applying Unweighted Group Average method. Tolerance values to A. rolfsii (1), C.

cerelae (2), C. ramicola (3), Thanatephorus anamorphs (4), W. circinata (5) and anastomosis groups (AG1, AG8 and AG10) of

R. solani are of Table 1 (0 = susceptible, 5 = tolerant). (6) Average values of tolerance calculated for groups A and B.

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection 2247

bility. The difference was particularly spectacular in the

case of AG-1 strain of R. solani isolated in Australia,

which proved to be especially virulent against T. tur-

gidum and cultivars of Pannon Standard sortiment.

3.4. Analysis of Survivors

The inhibition of growth and mass accumulation of sur-

vivors were proportional to the potential susceptibility

and inhibition of emergence (Figure 5). The rates of in-

hibition of growth and mass accumulation well correlated

to each other: (p < 0.001):

Length = 0.8149Mass + 0.8876 (FG = 18, r2 = 0.8066).

Consequently, the response of survivors can be charac-

terized with inhibition of growth. This has importance

when survivors have to be grown up for selection or fur-

ther studies.

3.5. Factors Influencing the Plant Response

Four substantial factors could be revealed by PCA (Table

2) that explained 71% of total variation of data matrix

edited of the Table 1. None of principal components

(PCs) comprised dominant part of variation, and the cul-

tivars clustered into four well defined groups (A = 6, B =

5, C = 3 and D = 4 varieties, respectively). The perform-

ance of majority of cultivars was determined by one

dominant factor except three (Toborzó, Suba and Karizma)

where two factors influenced the response (Table 2). All

this indicates, that tolerance of wheat cultivars to soil

borne Rhizoctonia infection was regulated by different

genetic factors.

3.6. Virulence of Rhizoctonia Strains

Majority of Rhizocton ia strains holded back the germina-

tion (Figure 6) in the case as minimum as one cultivar.

However, this effect was not strictly related to response

of seedling in posterior stages of evolution of wheat/

Rhizoctonia association. For example, the AG-1 strain of

R. solani proved to be later more aggressive than R. ce-

realis, although these two strains altered the germination

by similar manner. Seemingly, the genetic background

regulating the formation of anastomosis between hyphae

is not connected directly to expression of pathogenicity

against wheat, the host spectrum of two AG-4 strains was

different, and these strains inhibited the emergence at

various degree. The AG-8 strain of R. solani that causes

severe yield losses in China and Australia altered the

dynamics of emergence insignificantly, contrary to iso-

lates of imported propagating material of China and

Netherlands (B-245, B-521).

Based on results shown in the Figure 4 two matrices

were edited of the data of Table 1. The Canonical Cor-

relation Analysis resulted three significant canonic func-

tions (R2 = 0.886, 0.847, 0.684 and χ2 = 103.2, 70.5, 42.4,

P a nnon S t a nd a rdPannon PremiumOther varieti es

4.04.55.0 5.5

Susceptibility (1)

To bor zó

Bodri

Emese

Petrence

Lucilla

Magvas

Palotá s

Menüett

Toldi

Suba

Ködmön

Kolo

Mazur ka

R23

Kikelet

Karizma

Lona

* Hegyes

** Alkor

4.0 4.55.0 5.5

Mass accumulation(4)

To bo rzó

Bodri

Emese

Petrence

Lucilla

Magvas

Palotá s

Menüett

To l di

Suba

Ködmön

Kolo

Mazurka

R23

Kikelet

Karizma

Lona

* Hegyes

** Alkor

4.05.0 6.0

Height(3)

To bo rzó

Bodri

Emese

Petrence

Lucilla

Magvas

Palotás

Menüett

To l di

Suba

Ködmön

Kolo

Mazurka

R23

Kikelet

Karizma

Lona

* Hegyes

** Alkor

4.55.0 5.5

Germination(2)

To bo rzó

Bodri

Emese

Petrence

Lucilla

Magvas

Palotás

Menüett

To l di

Suba

Ködmön

Kolo

Mazurka

R23

Kikelet

Karizma

Lona

* Hegyes

** Alkor

Figure 5. Responses of wheat varieties to Rhizoctonia strains. Inhibition rates (%) as related to control are given in probits.

The varieties marked with one and two asterisks are T. monococcum and T. turgidum, while non-marked are T. aestivum cul-

tivars, respectively. 1 = proportion of seedlings bearing symptoms of disease syndrome, 2 = proportion of killed germinating

seeds and seedlings suffered damping off, 3 = growth inhibition, 4 = inhibition of mass accumulation. r1,2 = 0.797, r1,3 = 0.772,

r1,4 = 0.728, r2,3 = 0.941, r2,4 = 0.742, r3,4 = 0.802 < r0,01 = 0.515 (FG = 18).

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

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Table 2. Factors influencing the response of wheat varieties to soil-borne Rhizoctonia infection.

Pricipal Components

No. Varieties Groups PS% PC-1 PC-2 PC-3 PC-4

1 Toborzó A 52 0.513 0.244 0.508 −0.182

2 Bodri C 55 0.416 0.231 0.674 0.080

3 Emese A 27 0.561 0.278 0.107 0.379

4 Petrence D 25 0.459 0.154 0.380 0.683

5 Lucilla - 47 0.435 0.262 0.376 0.407

6 Magvas B 56 0.146 0.575 0.465 0.295

7 Palotás D 27 0.426 0.436 0.312 0.536

8 Menüett B 60 0.234 0.852 0.039 0.230

9 Toldi B 58 0.076 0.683 0.214 0.338

10 Suba C 48 −0.015 0.248 0.656 0.508

11 Ködmön C 53 0.083 0.316 0.765 0.385

12 Kolo B 43 0.241 0.701 0.404 0.146

13 Mazurka D 34 0.306 0.332 0.028 0.744

14 R23 D 34 0.121 0.364 0.245 0.661

15 Kikelet A 32 0.841 0.229 0.017 0.113

16 Karizma A 31 0.626 0.095 0.500 0.251

17 Lona B 76 0.232 0.662 0.224 0.177

18 Hegyes A 25 0.820 0.100 0.146 0.300

19 Alkor A 35 0.558 0.494 0.422 0.204

Eigenvalue 3.73 3.64 3.11 3.01

Proportion (%) of total variation 19.61 19.18 16.37 15.86

The PC loadings influencing the response of cultivars significantly were underlined. Varieties 1-6 and 7-13 are of Pannon Standard and Pannon Premium sorti-

ments, respectively. G = Varieties influenced by the same factor were marked with the same letter. PS = potential susceptibility of variety to Rhizoctonia calcu-

lated in percents of the Table 1.

Emergence (days)

Thanatephorus anamo rphs

1234567

Control

Athelia

R.cereali s

R.ramicola

R.fragariae

Festuca

R.zeae

R.stahli i

Ma lus

Hibiscus

Impatiens

Allium

Desirée

Kisvárdair.

AG‐1

AG‐2

AG‐3

AG‐4

AG‐4P

AG‐6

AG‐5

AG‐7

AG‐8

AG‐9

AG‐10

AG‐11

AG‐E

Figure 6. Influence of Rhizoctonia strains on germination of wheat seeds. The gray strips mark strains that significantly de-

layed the germination. F and L are the limits of emergence of first and last seeds calculated as average for 19 wheat cultivars.

The skew lined area mark the interval (p = 0.05) of germination of seeds in Rhizoctonia free soil.

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

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2249

morphs, indicating that these properties have no decisive

role in expression of pathogenicity. The physiological

characters, which take place in anastomose contact have

also minor importance, for example two R. solani strains

of AG-4 anastomosis group (B-417 and B-430) are in

notably different position. Similarly, the source of strain

had no influence on clustering. For example, the strains

isolated of potato tubers were located in different clusters,

while anamorphs (B-441 and B-427) of Ceratorrhi za and

Thanatephorus of orchid and potato, respectively, were

closely related. The taxonomic position of source (host

plant) also had no importance, the strains of mono- and

dicot plants were placed into the same cluster (see B-417

of Citrus and B-433 of Festuca). Unfortunately, we have

few data on exact geographic origin, but surprisingly, all

isolates of Hungarian origin were separated into the same

cluster. The strains B-413 and B-521 were of imported

propagating material. This might be related to role of

environmental (biogeographic and bioclimatic) factors.

Being typical soil habitants and forming mycelial web in

soil [38,39], the soil biota plays a crucial role in the

microevolution of Rhizocton ia species, while the assem-

blage of soil biota significantly depends on both structure

and composition of mineral matrix and climatic condi-

tions [40].

respectively). Plotting the strains as canonic scores by

first two roots strict linear relationship was revealed, where

only few strains deviated (AG-1, AG-5). However, the

strains on plot of third canonic function (Figure 7) clus-

tered into two well separated groups (p < 0.001). The

members of each group differ by their virulence, and the

strains of the same anastomosis group split within two

clusters, indicating, that the properties responsible for

separation into anastomosis groups have minor impor-

tance in formation of these clusters.

3.7. Relationships among Rhizoctonia Strains

The Rhizo ctonia strains were clustered on the base of

their pathogenicity against wheat cultivars (Table 1) based

on correlation matrix applying Unweighted Pair Group

Average method (Figure 8). No clear patterns can be

revealed on dendrogram. Two highly virulent strains iso-

lated of onion (B-245) and garden jewelweed (B-521)

with poorly virulent AG-E strain separated of others,

most probably due to low diversity of response data. The

properties responsible for taxonomic position of strains

seemingly have minor importance as the anamorphs of

Athelia, Cerato basidium, Ceratorrhiza and Waitea spe-

cies formed mixed clusters with Thanatephorus ana-

B-441

G-3

G-6

G-7

G-10

G-11

B-410

B-413

B-245

B-521

B-433

B-427

G-1

G-2

G-4

G-5

G-8

G-9

G-E B-411

B-405

B-438

B-447

B-442

B-409

Figure 7. Separation of Rhizoctonia strains with Canonical Correlation Analysis. According to groups shown in Figure 3 two

submatrices were edited of the data comprised in Table 1 and were related by means of CCA. Strains B-409 and B-430 (AG-4)

were omitted of calculations. The size of black (A) and gray (B) balls is proportional to potential aggressivity of strains to

wheat, respectively. The fitness of regression was over r = 0.97 (p < 0.001) for both function.

Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

2250

R. solani

AG-E

R. solani

AG-7

A. rolfsii

AG-5

AG-8

R. stahlii

AG-3

AG-4

AG-11

AG-6

AG-2

C. cereal e

C. ramicola

R. solani

W. circinata

R. solani

AG-9

R. solani

AG-4

C. fraga r ia e

AG-1 0

AG-1

Strain (1)

Impatiens

Allium

Malus

Hibiscus

Malus

tala j

Solanum

Zea

Triticum

Orchid

Solanum

Phaseolus

Lupinus

Erigeron

Daucus

Triticum

Orchid

Solanu m

Festuca

Solanu m

Solanum

Festuca

Citrus

Fragaria

Triticum

Pinus

Source (2)

*B-521

*B-245

B-434

B-409

*B-413

*B-420

B-442

B-418

B-421

*B-441

*B-446

B-430

*B-424

B-419

B-432

B-447

*B-427

B-411

B-405

*B-410

B-422

*B-433

B-417

B-438

*B-423

B-415

Code(4)

100

S (5)

100

P (6)

Hungary

China

Netherlands

Lybia

Hungary

Japan

Italy

Netherlands

Australia

Germ

Spain

England

Australia

Japan

Netherlands

Germany

Florida

Hungary

USA

Hungary

Argentina

Canada

Australia

Canada

Origin (3)

Linkage Distance (UW PGA, 1-Pearson’s r)

0.00.20.40.6 0.8 1.0

Figure 8. Grouping of Rhizoctonia strains based on their inhibitory effect on germinating seeds of various wheat varieties.

The data of Table 1 were clustered of Pearson’s correlation matrix applying Unweighted Group Average method. (1) Strains

underlined were isolated of monocots, (2) Source, the monocots are underlined, (3) Location, (4) Code. The codes of members

of the group I on Figure 4 are marked with asterisks. The columns S(5) and P(6) comprise potential tolerance values of varie-

ties of Pannon Standard and Pannon Premium sortiments, respectively (0 = tolerant, 100 = susceptible). H = isolated of

sources collected in Hungary.

3.8. Factors Influencing Virulence of Rhizoctonia

Strains

The Principal Component Analysis resulted in nine nota-

ble components that explain 82% of total variance (Ta-

ble 3). Similarly to factors influencing the response of

wheat cultivars none of them was superior, the major PC

has about twice more weight than the minimally signifi-

cant one. The groups were sharply separated and formed

by strains of various taxonomic positions. Only in two

cases was the performance of strains (B-419 and B-410)

affected by two factors. The quantitative aspect of viru-

lence was not connected per se to groupping, for exam-

ple the most virulent AG-1 strain of R. solani (B-415)

was linked to significantly less virulent AG-10 strain

(B-423). The findings support the concept of multilocal

character of interaction between wheat and attacking

Rhizoctonia.

4. Discussion

We focused on soil borne Rhizoctonia infection that has

the greatest effect on the growth and yield of wheat

among soil borne pathogens [41], and the brown patch

disease became devastating in last two decades. This

might be related to the changes in both pest management

practices and cultivation techniques that resulted the in-

creased frequency of the specialized pathogen genotype

in some geographic areas [42,43]. The metalloorganics of

broad antimicrobial spectrum of activity applied formerly

for seed dressing have been banned, and the monosite

inhibitors either do not inhibit Rhizoctonia like fungi or

they loose activity rapidly due to acquired resistance.

Moreover, the formerly dominant tillage based wheat

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection 2251

Table 3. Factors influencing virulence of Rhizoctonia strains against wheat cultivars.

Rhizoctonia strains Principal Components

No. Code Group PV% PC-1 PC-2 PC-3 PC-4 PC-5 PC-6 PC-7 PC-8 PC-9

1 B-415 A 82 0.03 −0.24 0.07 −0.07 0.15 0.16 *0.81 0.07 0.27

2 B-432 B 13 0.11 0.11 −0.05 −0.14 −0.02 *0.94 0.04 0.10 0.03

3 B-446 C 24 0.09 0.27 0.05 0.07

*0.89 −0.06 0.28 0.14 0.02

4 B-417 D 32 0.47 0.27

*0.64 0.26 0.14 0.15 0.03 0.28 0.22

5 B-430 D 20 0.04 −0.26 *0.90 −0.04 −0.10 −0.04 0.11 0.11 0.02

6 B-418 E 52 0.14 0.03 0.14 0.20 0.14 0.16 0.09

*0.90 0.01

7 B-419 DF 25 −0.07 0.16 *0.76 0.17 0.20 *0.53 0.08 0.07 0.01

8 B-420 F 36 −0.05 0.22 −0.14 *0.85 0.05 −0.04 0.01 0.23 0.16

9 B-421 C 32 0.17 0.20 −0.01 −0.10 *0.51 0.18 −0.40 0.48 0.31

10 B-422 G 52 0.41 *0.56 0.45 0.01 0.21 0.08 0.41 −0.01 0.17

11 B-423 A 19 0.31 −0.44 0.19 0.09 0.25 0.17

*0.57 0.19 −0.35

12 B-424 B 33 0.17 0.11 0.33 0.13 0.27

*0.78 0.20 0.05 0.03

13 B-434 F 28 −0.08 0.17 0.17

*0.61 0.37 0.07 −0.25 −0.31 −0.16

14 B-411 H 58 *0.67 −0.19 0.38 0.02 0.34 0.16 0.17 −0.04 0.14

15 B-410 HG 64 *0.54 0.31 0.36 −0.14 0.07 0.09 *0.51 −0.03 0.34

16 B-413 F 48 0.37 −0.33 0.22

*0.59 0.13 −0.01 0.06 0.02 0.49

17 B-409 I 36 0.07 −0.01 0.06 0.10 0.11 0.05 0.17 0.08

*0.93

18 B-245 G 99 0.00 *0.93 −0.03 0.13 0.05 0.10 −0.14 0.07 −0.04

19 B-521 G 99 0.00 *0.93 −0.03 0.13 0.05 0.10 −0.14 0.07 −0.04

20 B-433 H 42 *0.61 0.11 0.31 −0.21 0.16 −0.04 0.19 *0.51 0.22

21 B-441 H 47 0.24 −0.19 0.02 0.15

*0.68 0.26 0.02 0.03 0.15

22 B-405 C 20 *0.67 0.20 −0.05 0.22 0.30 0.02 0.09 0.10 0.04

23 B-438 H 38 *0.56 −0.25 0.24 0.11 −0.02 0.39 0.44 0.12 0.07

24 B-427 H 27 *0.60 −0.03 0.34 0.48 0.35 0.04 0.10 −0.06 −0.22

25 B-447 H 54 *0.86 −0.05 −0.15 −0.09 −0.11 0.11 −0.10 0.17 0.001

26 B-442 F 41 0.19 0.19 0.36

*0.60 −0.14 −0.04 0.05 *0.50 0.01

Eigenvalues 3.81 3.10 2.96 2.45 2.38 2.22 2.15 1.95 1.80

Proportion of total variance 14.6 11.9 11.4 9.4 9.2 8.5 8.3 7.5 6.9

Strains 1-13 refer to anastomosis groups AG-1-AG-11 and AG-E of R. solani, respectively, of CBS collection, strains 14-20 are Hungarian isolates of R. solani

and R. stahlii (21), all are Thanatephorus anamorphs. Rhizoctonia strains 22 - 26 are anamorphs of Waitea (22), Cerathorrhiza (23, 24), Ceratobasidium (25)

and Athelia (26). PV% = potential virulence of Rhizoctonia strains against wheat cultivars calculated of the Table 1 applying Potency Mapping. The PC load-

ings influencing the virulence of strains significantly, were marked with asterisks. The small grains were omitted of calculations.

cropping system in dryland farming has been changed to

minimum or not-till cropping systems that favours to

survival of mycelial web forming Rh izoctonia [39,44-47].

The efforts to control soil borne infections applying

various eubiotic preparations were not successful yet in

large scale, mainly due to the low reproducibility of the

effect in field conditions which makes the calculation of

cost/benefit ratio unreliable. Actually, seed dressing re-

sults only sufficient control of pathogens threatening the

cultivated plant in early stage of development. Thus the

importance of selection of resistant wheat cultivars in-

creased.

4.1. Performance of Wheat/Rhizoctonia

Pathosystem

Rhizoctonia species are abundant in soils as mutualistic

members of microbial consortium associated with plants.

Their relationship with host plants may change from sym-

biosis (as in various orchids) to destructive parasitism,

and these fungi usually do not cause visible disease symp-

toms. The infection may remain latent for a long period

and can rapidly generalize, when environmental stress

factors overhelm the homeostatic regulation of host plant,

i.e., the disposition of host favours to pathogen either in

phyllosphere of in roots. The amount of thallus varies

within large limits (0.09 - 6 ng per g tissue) in symptom-

less plants [48]. The symptomatic picture is usually vari-

able to a high extent as stunting growth, decrease in mass

accumulation, leaf spots of various size, deformations of

various organs and rooting tissues can be observed on

infected plant alone or in combinations together, the dis-

ease syndrome may evolve rapidly to a fatal consequence

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

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in formerly symptomless host (damping off and wilting).

In our experiments all the above mentioned variations

were observed, and the overall susceptibility of one re-

spective variety influenced only the frequency of various

symptoms of disease syndrome caused by soil borne

Rhizoctonia infection independently on taxonomic posi-

tion or origin of the strains. This indicates that some

traits used for taxonomic classification can not be tightly

associated with the properties determining the character

of wheat/Rhizoctonia interaction. The small black spots

that were occasionally observed on scutellum and meso-

cotyl of surviving individuals may be result of hypersen-

sitive reaction (HR) suggesting the defense mechanism

against Rhizoctonia attack rapidly activates. Further stud-

ies requested to connect this property with wheat re-

sponse to soil borne Rhizoctonia infection as the in-

volvement or manipulation of HR into breeding pro-

grams may open door towards to the control of brown

patch disease [49]. Furthermore, the role of Rhizoctonia

toxins in pathogenesis has to be elucidated as well as

factors regulating development of survivors in presence

of Rhizoctonia should be identified.

The genetic background of tolerance to Rhizoctonias is

not elucidated yet. Discoveries show that both anatomic

and physiological features are involved into manifesta-

tion. Thus, the structure and composition of cell wall

might have importance; the resistance to fungal xylanase

is a tolerance factor [50]. The multivariate analysis of our

experimental results supports the multigenic character of

wheat response, although no dominant factor was revealed.

Unfortunately, the screening of tolerance to Rhizoctonia

in microscale provocative experiments can give data for

only preliminary selection of wheat lines, and the survi-

vors should be evaluated in field conditions as well [51].

Nevertheless, the data obtained are encouraging and on

our opinion the screening of gene banks applying the

method demonstrated here can result germlings useful for

further manipulations (for example cultivars Emese,

Petrence and Toldi). The high variation observed in re-

sponse of wheat to soil borne Rhizoctonia might be

caused by lack of preliminary selection of tested plants.

Members of the genus Rhizoctonia are considered as a

complex mixture of filamentous fungi, having in com-

mon the possession of a non-spored imperfect state, usu-

ally referred to as the anamorphs of five genera: Athelia,

Ceratobasidium, C eratocys t is, Thanathephorus and Waitea.

Here we included data on virulence of strains of the above

five teleomorph genera, all of them attacked germinating

seeds of cereals tested. Our results approve presumtion of

Tomaso-Peterson and Trevathan [52] that the new for

Europe pathogen, R. zeae, is considered as a hazard for

wheat cultivation, with special regard to warm climate

areas.

The multivariate analysis of experimental data re-

vealed nine significant factors influencing the aggressiv-

ity of these strains; moreover, these factors are clearly

not related to traits used for taxonomic purposes. The

hyphal anastomosis interactions has been widely used for

clustering of Rhizoctonia anamorphs within the complex,

since other types of diagnostic features are usually scarce

in these fungi [53]. It was considered, that the anastomo-

sis groups are specialized to defined host plants [54]. Our

experiments do not support this presumption in the case

of wheat. Seemingly, the host range of single strains

might be different, and reversely, strains clustered into

different taxons, may have similar host range. By this

reason, on our opinion, —based on presented results here,

—for primary screening of wheat cultivars as minimum

as six different strains should be used, including A. rolfsii,

C. cerealis and W. circinata.

4.2. Future Prospects

The plant rhizosphere is a dynamic environment in which

many parameters may influence the population structure,

diversity and activity of the microbial community. The

soil C:N ratio has critical role in disease incidence caused

by Rhizoctonia as it was demonstrated by Kuhn et al.

[55]. The roles of mycorrhiza in facilitating the acquisi-

tion and transfer of carbon (C) and nitrogen (N) is well

known. A considerable amount of bidirectional transfer

of C between host plant and its fungal symbiont, and a

fungus-dependent pathway for organic N can be realized

rapidly, thus influencing positively the stress tolerance of

plant [56]. The micro- and mesofauna also can alter the

disease incidence either wounding the roots [57] or de-

creasing the size of inoculum [58]. Induction of suppres-

sive soil by using mixed cropping or applying eubiotic

preparations is contradictory, as the iron deficiency may

harm the wheat although this can be overcame by leaf

nutrition. Nevertheless, we can expect new, usable knowl-

edge of the environmental research on microbial com-

munities and plant microbe interaction studies, which can

help to design and sustain suppressive soil that would be

the most convenient and economic method for comatting

yield losses caused by soil-borne diseases [59].

High number of papers describe antifungal effect of

various plant extracts, but only few of them report con-

vincing comparative data on efficacy, and in minority of

cases these effects have been comparable with marketed

fungicides [60,61]. Most of the active substances identi-

fied are terpenoids, and on our view there is a few prob-

ability to develop potentially effective and environmen-

tally safer alternative fungicide to xenobiotics. However,

these plants might be sources for transgenic modification

to upgrade the sheath blight tolerance of wheat (Table 4).

The breading is the most promising measure to improve

the tolerance to soil-borne pathogen complex, because

the use of any xenobiotic has adverse effects on soil biota

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Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

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2253

Table 4. Potential candidates for tr ansge nic manipulation of wheat for improvement of tolerance to Rhizoctonia.

Plant Factor Results (example) Ref.

Hordeum vulgare rip30 increased tolerance (potato) [71]

Hordeum vulgare chitinase increased tolerance (tobacco) [72]

Pennisetum glaucum lipid transfer protein antifungal [73]

Triticum sp. puroindoline increased tolerance (rice) [74]

Celastrus hypoleucus pristimerin inhibiting the formation of infective body [75]

Celastrus hypoleucus celastrol inhibiting the formation of infective body [75]

Prokaryote 5-enolpyruvoyl-shikimate-3-phosphate synthetase Increased tolerance (wheat/Puccinia) [76]

Oryza sativa thaumatin like protein increased tolerance (rice) [70]

Oryza sativa OsPR-4b gene encoding pathogenesis related proteinenhanced resistance [77]

Solanum tuberosum Potide G proteinase inhibitor [78]

Bacillus subtilis Iturin A Antifungal [79]

Bacillus subtilis flagellin Antifungal [80]

Raphanus sativus defensin increased tolerance (wheat) [81]

Solanum tuberosum Snakin 1 enhanced resistance [82]

Dasypyrum villosum unknown tolerance to AG 8 [83]

Oryza sativa Rice chitinase increased tolerance (Musa/Mycosphaerella) [84]

Oryza sativa Rice chitinase increased tolerance (Eleusine/Magnaporthe) [85]

Tichoderma harzianum glucanase inhibiting the formation of infective body [86]

Tephrosia villosa defensin increased tolerance (tobacco) [87]

Arabidopsis thaliana NADPH oxydase induced resistance [88]

Oryza sativa ACCA synthase induced resistance [89]

as well as can predispose host plant to pathogen [62,63].

In our experiments individual resistant to some Rhizocto-

nia, which attack the majority of its fellows, occurred in

each variety, likely to observations of other authors [64].

Such survivors can be objects of further breeding or ge-

netic engineering. Unfortunately, the mechanistic ap-

proach on concern surrounding the genetically modified

maize with Bacillus thuringiensis toxin or the glyphosate

tolerant crops borne overall social resistance to gene tech-

nology. We should clarify, most of these concerns have

no scientific base. First of all, the usefulness of geneti-

cally modified (GM) crops can be sustained with careful

management [65].

Although, Rhizoctonia species, which cause bare-patch

disease and sharp eyespot in wheat are not among the top

ten pathogens [66], economic improtance of their control

is increasing from severe to catasrophic yield losses re-

ported from main wheat cultivating areas [67-69]. There

is an urgent need in sustainable management strategy to

combat damages induced by soil habiting Rhizoctonia

complex, first of all, new tolerant wheat cultivars. No

major resistance genes to this pathogen have been identi-

fied so far inspite of increasing efforts in studies of

physiology and genetic background of wheat/ Rhizoc tonia

interaction. Nevertheless, candidates for transgenic ma-

nipulation can be selected (Table 4). The possible im-

provement of tolerance demonstrated on rice [70] might

serve as example for wheat.

Some terpenoid phytoanticipins of Pelargo nium grav-

olens [90], Artemisia arborescens [91], Helianthus tube-

rosus [92] exhibited good antifungal effect, but on our

opinion the use of antifungal polypeptides seems to be

more promising for transgenic manipulation. For exam-

ple, a basic oligopeptide of Bacillus subtilis exhibited

excellent and broad spectrum antimicrobial activity in

our experiments [93] and would fit for control of all four

important soil borne pathogens of wheat. The incorpora-

tion of Rhizoctonia specific mycovirus into genome of

cereals also is a promising possibility [94], with special

regard to root border cells. These, detached living cells

form the “front line” in the soil, the special part of

rhizosphere, described as a system first by Hawes [95],

where the plant controls the microenvironment with these

specially progammed cells [96]. The border cells have

indisputably key function in plant defense controlling the

dynamics of adjacent microbial populations in the soil to

foster beneficial associations and inhibit pathogenic in-

vasion [97], thus these cells are plausible objects of ge-

netic engineering to desing wheat plant with optimum

characteristics for root health management.

PCR based molecular methods enable the comparative

analysis of genes involved in plant defense [98]. The task

is complex because the design of PCR based molecular

markers linked to the Rhizoctonia resistance genes of

Susceptibility of Wheat Varieties to Soil-Borne Rhizoctonia Infection

2254

wheat cultivars seems to be complicated due to high

number of factors influencing on the type of plant re-

sponse. Synergic joint action of several minor factor may

result more stabile repel than a single major one as one

gene mutation can not eliminate this type of defense [90].

However, we understand little about how genes interact

because very few possible genetic interactions have been

explored experimentally [99], thus depending on the

insertion event, a particular transgene can have large

effects on the entire phenotype of a plant and that these

effects can sometimes be reversed when plants are

moved from the glasshouse to the field [100]. On our

opinion the more intense research of complex repertoire of

small RNAs (microRNAs [miRNAs] and siRNAs) used

as guides for post-transcriptional and epigenetic regula-

tion would help to design new, either intraspecific or

intrageneric wheat varieties. To promote research in this

field it is necessary to make public the data on cereal

crop genomes and discuss the contribution: what proteins,

and their genome sequence organisation, play in plant

defence. Although these data are extremely valuable tool

for detailed analysis, and the emergence of informatic

market makes difficulties as patent applications back out

of scientific disputation [101], nevertheless, such free

disputation in large scale of scientific community would

significantly accelerate the progress in breeding of wheat

tolerant to soil borne diseases.

5. Conclusions

No relationship was found between taxonomic position

and origin of Rhizoctonia strains, indicating that traits

used for their classification are not closely related to ex-

pression of their pathogenicity against wheat cultivars.

Nine factors were revealed that significantly affect their

virulence in wheat/Rhizoctonia system.

We have got empirical evidence from plant/pathogen

system on the possibility of selection; the wheat phenol-

type resistant to soil-borne Rhizoctonia, that verifies our

approach of using simplified scale for disease assess-

ment.

6. Acknowledgements

This study was supported by The National Office for

Research and Technology, Grant No. K67688.

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