Biological control ofSclerotinia minorusing a chitinolytic bacterium and actinomycetes

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Abstract
Isolates of 85 bacteria and 94 streptomycete and 35 nonstreptomycete actinomycetes were obtained from a lettuce‐growing field in Al‐Ain, United Arab Emirates, on colloidal chitin agar, and screened for their ability to produce chitinase. Twenty‐three bacteria and 38 streptomycete and 15 nonstreptomycete actinomycete isolates produced high levels of chitinase and were examinedin vitrofor their ability to suppress the growth ofSclerotinia minor, a pathogen causing basal drop disease of lettuce. The three most suppressive isolates were examined further for their production of β‐1,3‐glucanase and antifungal activity as well as their ability to colonize the roots and rhizosphere of lettucein vitroandin planta. The three isolates,Serratia marcescens, Streptomyces viridodiasticusandMicromonospora carbonacea, significantly reduced the growth ofS. minor in vitro, and produced high levels of chitinase and β‐1,3‐glucanase.Streptomyces viridodiasticusalso produced antifungal metabolite(s) that significantly reduced the growth of the pathogenin vitro. When the pathogen was presented as the sole carbon source, all three isolates caused extensive hyphal plasmolysis and cell wall lysis.Serratia marcescens and St. viridodiasticuswere competent to varying degrees in colonizing the roots of lettuce seedlings after 8 days on agar plates and the rhizosphere within 14 days in pots, with their competency being superior to that ofM. carbonacea. All three isolates, individually or in combination, were antagonistic toS. minorand significantly reduced incidence of disease under controlled glasshouse conditions.