In vitro inhibitory activity of xylem exudates from cucurbits towardsFusarium oxysporum microconidia

Experiments were conducted to determine if xylem exudates from three cucurbitaceous plants as well as non-cucurbit species were inhibitory to the growth of microconidia ofFusarium oxysporum f. sp.niveum, causal agent of Fusarium wilt of watermelon. Inhibitory effects of xylem exudates from watermelon(Citrullus lanatus) cultivars differentially susceptible toF.o. f.sp.niveum as well as from watermelon plants previously inoculated with virulent or avirulent races ofF. oxysporum, were compared. Results indicated that xylem fluid contained an inhibitory component that was independent of the resistance status of the cultivar or previous inoculation with virulent or avirulentF.o. f.sp.niveum races. Xylem exudates from cucumber(Cucumis sativis) also inhibited microconidia growth and colony formation while muskmelon(Cucumis melo) and tomato(Lycopersicon esculentum) xylem exudates had little inhibitory activity. Incubation with proteinase K, lysozyme, or boiled xylem fluid suggested that the active component had proteinaceous properties with an oligosaccharide moiety.