Biological control of Sclerotinia minor using a chitinolytic bacterium andactinomycetes

Isolates of 85 bacteria and 94 streptomycete and 35 nonstreptomycete actino mycetes were obtained from a lettuce-growing field in Al-Ain, United Arab E mirates, on colloidal chitin agar, and screened for their ability to produc e chitinase. Twenty-three bacteria and 38 streptomycete and 15 nonstreptomy cete actinomycete isolates produced high levels of chitinase and were exami ned in vitro for their ability to suppress the growth of Sclerotinia minor, a pathogen causing basal drop disease of lettuce. The three most suppressi ve isolates were examined further for their production of beta-1,3-glucanas e and antifungal activity as well as their ability to colonize the roots an d rhizosphere of lettuce in vitro and in planta. The three isolates, Serrat ia marcescens, Streptomyces viridodiasticus and Micromonospora carbonacea, significantly reduced the growth of S. minor in vitro, and produced high le vels of chitinase and beta-1,3-glucanase. Streptomyces viridodiasticus also produced antifungal metabolite(s) that significantly reduced the growth of the pathogen in vitro. When the pathogen was presented as the sole carbon source, all three isolates caused extensive hyphal plasmolysis and cell wal l lysis. Serratia marcescens and St. viridodiasticus were competent to vary ing degrees in colonizing the roots of lettuce seedlings after 8 days on ag ar plates and the rhizosphere within 14 days in pots, with their competency being superior to that of M. carbonacea. All three isolates, individually or in combination, were antagonistic to S. minor and significantly reduced incidence of disease under controlled glasshouse conditions.