Effect of Microsphaeropsis ochracea on Production of Sclerotia-borne and Airborne Conidia of Botrytis squamosa

Abstract

Onion leaf blight, caused by Botrytis squamosa (Walker), is a destructive disease of onion. Conidia produced on overwintered sclerotia are the main source of initial inoculum, and those produced on lesions are responsible for secondary inoculum build-up. The biological control agent Microsphaeropsis ochracea (Carisse & Bernier) was evaluated for its ability to control sclerotia-borne inoculum, to colonize onion leaves and reduce the production of conidia under field conditions. Colonisation by M. ochracea of onion leaves at different growth stages was monitored and its effect on B. squamosa sporulation on necrotic leaves was evaluated. Onion plots were treated with either Dithane® or with M. ochracea at 7–10-day intervals and according to inoculum production index (IPI). The concentration of airborne conidia and the number of lesions per leaf, on 20 plants per plot, were evaluated throughout the cropping season. The number of conidia produced per sclerotium treated with M. ochracea, was reduced by 75.5%. In the field, M. ochracea colonised only senescent or necrotic leaves and reduced the production of conidia on these leaves by an average of 82% as compared with untreated leaves. Best disease control was obtained by Dithane®, followed by M. ochracea applied at 7–10-day intervals. For the three years of the study, there were no significant differences in airborne concentrations of conidia in plots treated at 7–10-day intervals with Dithane® or M. ochracea. Fall application of M. ochracea could be used as a sanitation practice to reduce initial inoculum or as a part of an IPM program during the season.