Cultivar-Specific Induction of Soil Suppressiveness to Fusarium Wilt of Watermelon

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Abstract
In a long-term monoculture of watermelon (Citrullus lanatus (Thunb.) Matsum, and Nakai), cultivars, most of the cultivars wilted severely after 4-5 yr regardless of previously described levels of resistance to Fusarium oxysporum f. sp. niveum. Only the resistance in Smokylee and Crimson Sweet was stable in the monoculture, and only Crimson Sweet continued to have acceptable yields throughout the monoculture. Crimson Sweet, only moderately resistant to Fusarium wilt in greenhouse tests, had a unique resistance that was effective throughout the 7-yr monoculture. When soil was collected from the Crimson Sweet plot and assayed, counts or propagules of F. oxysporum were not significantly lower than in other cultivar plots, but susceptible cultivars did not wilt when planted in this soil. In soil infested with F. o. niveum at 1.5 .times. 103 conidia per gram, there was 70-100% wilt of Florida Giant, Charleston Gray, or Crimson Sweet in fallow soil or Florida Giant monoculture plot soil; however, there was less than 35% wilt in soil from the Crimison Sweet plots. The suppressive factor(s) in Crimson Sweet soil was sensitive to fumigation with methyl bromide and to moist heat at 70 C for 30 min. The unique resistance of Crimson Sweet to Fusarium wilt in monoculture appears to result from the promotion of a biological control factor in the soil.

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