MECHANISMS OF DISEASE SUPPRESSION BY A HAIRY VETCH (VICIA VILLOSA) COVER CROP ON FUSARIUM WILT OF WATERMELON AND THE EFFICACY OF THE BIOCONTROL ACTINOVATE.
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The cover crop Vicia villosa suppresses Fusarium wilt of watermelon but the mechanisms of disease suppression are unknown. Possible mechanisms were examined in field, greenhouse, and in vitro experiments. The effects of cover crop treatments (V. villosa, Trifolium incarnatum, Secale cereale, Brassica juncea) and the biocontrol treatment Actinovate (Streptomyces lydicus WYEC 108) on Fusarium wilt of watermelon and its causal pathogen, Fusarium oxysporum f. sp. niveum (FON) were evaluated. In four of five field experiments there were significant elevations in soil microbial respiration. Arbuscular mycorrhizal colonization of watermelon roots following cover crop amendments of V. villosa and T. incarnatum, were significantly higher compared to bare ground. The elevation in respiration was significantly positively correlated with disease suppression of Fusarium wilt induced by V. villosa and T. incarnatum (both cover crops reduced Fusarium wilt as much as 21%). In greenhouse experiments using infested field soil, Fusarium wilt suppression was observed in pots amended with V. villosa and T. incarnatum. However, there was an increase in Fusarium wilt of watermelon in pots that were amended with V. villosa and T. incarnatum which were also inoculated with FON when compared to plants in nonamended, inoculated pots. These leguminous cover crops may have served as a nutrient source for the pathogen. In addition, in vitro growth experiments showed that media amended with V. villosa leachate significantly stimulated the in vitro growth rates of FON and Trichoderma harzianum compared to nonamended plates. It was hypothesized that V. villosa stimulation of nonpathogenic F. oxysporum spp., which provides cross protection against FON, may have contributed to the wilt suppression. Cover crop leachate amendments did not significantly influence colony forming units of S. lydicus. In both field and greenhouse trials Actinovate applications either had little or no effect on Fusarium wilt of watermelon. However, S. lydicus significantly inhibited in vitro growth of FON. These studies demonstrate that both general and specific disease suppression play a role in V. villosa suppression of Fusarium wilt of watermelon and that T. incarnatum is a viable alternative biocontrol.