Effect of Various Soil Media on Disease Severity of Fusarium Wilt in Watermelon

Fusarium wilt is a major disease of watermelon (Citrullus lanatus) caused by Fusarium oxysporum f. sp. niveum (Fon). Use of host resistance is the most effective management strategy for the disease, and a major objective for breeding programs. Screening assays rely on the ability to discriminate resistant and susceptible genotypes in segregating populations. However, complex interactions between Fon and the soil environment can influence symptom development and disease severity rating. In the current study, severity of Fusarium wilt (race 1) in sand-peat (1:1 v/v), sand-perlite (1:1), sand-peat-vermiculite (4:1:1), peat-perlite (1:1) and Fafard 3B potting media was compared among five watermelon cultivars: Calhoun Gray (resistant), SunSugar (resistant), Allsweet (moderately resistant), Sugar Baby (susceptible) and Charleston Gray (susceptible). Plant biomass (average dry weight/plant) was lowest in peat-perlite (1.67 g) and sand-peat (2.16 g), and was significantly different (α = 0.05) from that of sand-perlite (3.48 g), sand-peat-vermiculite (4.94 g) and Fafard 3B (6.90 g). Conversely, disease severity [area under disease progress curve (AUDPC)] across cultivars was significantly higher in peat-perlite (AUDPC = 62.96) and sand-peat (AUDPC = 40.87), than in sand-perlite (AUDPC = 11.55), sand-peat-vermiculite (AUDPC = 10.67) and Fafard 3B (AUDPC = 9.29). Consistent discrimination (α = 0.05) of resistant and susceptible cultivars was realized in sand-peat-vermiculite and Fafard 3B, but was not possible in peat-perlite, sand-peat and sand-perlite. Collectively, these findings support suitability of sand-peat-vermiculite and Fafard 3B for routine screening of Fusarium wilt resistance in watermelon.

Management of Fusarium wilt is difficult because of the long-term survival of the pathogen's chlamydospores in the soil and the evolution of new races [3] [6] [8]. Examples of management options for the disease include use of disease-free planting materials, crop rotation, chemical and biological fumigation [3] [6] [9], resistant root-stocks [10], suppressive soils [11] [12] [13] and induced resistance [12] [14]. The ecological risk of the chemical fumigants has led to the phasing out of methyl-bromide (Montreal Protocol and the Clean Air Act, 1998) leaving even fewer options for managing Fusarium wilt. Watermelon growers prefer resistant cultivars as the primary management method for Fusarium wilt [4] [4] reported that high inoculum density elicited higher disease severity than lower inoculum density in watermelon. Other important environmental factors include temperature, light, moisture levels and soil type [18] [19].
The mechanisms and interactions between soil-borne Fusarium species and the soil environment are poorly understood [2] making the choice of soil medium an important factor in disease screening assays. Depending on the plant species, certain soil media can suppress, while others increase severity of soil-borne diseases. For example, compost and other soils have been shown to suppress the development of soil-borne diseases even in presence of pathogens [19] [22]. Cohen et al. [19] reported that peat-based soil media induced susceptibility of melons to Fusarium oxysporum f. sp. melonis. On the contrary, peat-based media has been shown to suppress disease development for some soil-borne pathogens such as Alternaria brassicicola, Leptosphaeria maculans, Rhizoctonia solani and Fusarium oxysporum f. sp. lycopersici [23]. It is therefore imperative to determine a reliable pathogen-media combination for screening assays involving soil-borne diseases. In cucurbit breeding programs, soil media used in disease screening with Fusarium species include sand-peat-vermiculite [16], vermiculite-peat moss [17], commercial potting mix such as Metromix [2], Metromix-sand-vermiculite [24], sand-soil [25], peat-perlite, sand-peat and sand-perlite [19].
Currently, no information is available on the effect of different soil media on

Greenhouse Evaluations
Seeds were sown in 20. The temperature in the greenhouse was maintained at 27˚C ± 3˚C.

Data Collection and Analysis
Plants were evaluated for symptom severity on a scale of 0 to 5, with a score of 0

Results
No significant differences (α = 0.05) in disease severity were observed between the two experiments (data not shown), therefore joint analysis was conducted for the data.

Disease Severity
Disease severity [area under disease progress curve (AUDPC)] was highest in peat-perlite, followed by sand-peat (Table 2). Overall, there was no significantly difference in disease severity in sand-perlite, Fafard 3B and sand-peat-vermiculite ( Table 2). As expected, Sugar Baby and Charleston Gray were the most susceptible across all media, while Allsweet, SunSugar and Calhoun Gray were the most resistant (Table 3). In peat-perlite, disease severity was high across all cultivars, and there was no consistent significant difference between susceptible cultivars and resistant cultivars (Figure 2(a)). However, Calhoun Gray showed significantly greater resistance than all the cultivars in this medium. In sand-peat, the moderately resistant cultivar, Allsweet, showed no significant difference in susceptibility when compared to Sugar Baby and Charleston Gray (Figure 2(b)). However, Calhoun Gray and SunSugar were significantly more resistant than the two susceptible cultivars in this medium. In sand-perlite, all the cultivars exhibited minimal symptoms, and there was no significant difference in disease severity (Figure 2(c)). However, in sand-peat-vermiculite and Fafard 3B, there was a clear delineation in disease severity between the susceptible (Sugar Baby and Charleston Gray) and the resistant cultivars (Allsweet, Calhoun Gray and SunSugar) (Figure 2(d) and Figure 2(e)).

Discussion
Complex interactions between Fon and the soil environment make selection of a reliable soil medium an important factor in Fusarium wilt screening assays [2] [18]. In the current study, severity of Fusarium wilt in five soil media was compared to identify the best media for routine screening assays. Poor plant vigor and high disease severity were observed in peat-perlite and sand-peat media. This was expected because poor plant vigor generally reduces the ability of plants to resist diseases [3] [19] [20] [21]. In the two media, the resistant cultivars (Calhoun Gray, SunSugar and Allsweet) were also highly susceptible. In melon, Cohen et al. [19] reported that seedlings grown in peat-based media developed more severe disease symptoms than those grown in media lacking peat. However, the growth vigor of the seedlings in these media was not reported, thus    difficult to compare. Generally, good plant vigor was observed in sand-perlite; however, disease development in this media was suppressed, even for the susceptible cultivars (Sugar Baby and Charleston Gray). Consequently, it was not possible to discriminate resistant and susceptible cultivars in this medium. Similar observations were reported in melon, whereby resistant and susceptible cultivars inoculated with Fusarium could not be discriminated in a sand-perlite medium [19]. In sand-peat-vermiculite and Fafard 3B, high disease severity was observed in susceptible cultivars, while the resistant cultivars remained asymptomatic. Collectively, this study established that among the media studied, sand-peat-vermiculite and Fafard 3B were ideal for discriminating resistant and susceptible cultivars in Fusarium race 1 screening assays. Although sand-peat-vermiculite is routinely used in Fusarium screening assays [16] [30] [34], Fafard 3B is a suitable commercial-mix alternative [34].

Conflicts of Interest
The authors declare no conflicts of interest.