Epidemiology of Botrytis cinerea in sweet basil and implications for disease management

Sweet basil (Ocimum basilicum) is a herbaceous annual plant that is highly susceptible to gray mold, caused by Botrytis cinerea. Infections are initia ted on the surface of stem wounds that are caused during harvest; the patho gen then progresses on the stem, killing leaves and secondary buds. When th e infection reaches the main stem, the entire plant dies. A study of the ep idemiology of the disease and of host-pathogen interactions led to the deve lopment of rules for effective disease management. The research was conduct ed in three steps. (i) Natural epidemics in commercial crops were monitored during the 1993 to 1994 and 1994 to 1995 growing seasons. Disease outbreak s were found to coincide with harvests during rainy days, whereas disease i ntensity did not change much when harvests were completed during rainless d ays. (ii) Studies under controlled environmental conditions revealed that c ut ends of stems. were highly susceptible soon after harvest, susceptibilit y diminished gradually, and stem cuts inoculated 48 h after harvest were ra rely diseased. Observations under a scanning electron microscope showed tha t an opaque layer had appeared over the cut surface and, as the entire woun d surface became covered, penetration of the fungus into the tissue was pre vented. Based on these findings, it was hypothesized that avoidance of harv esting during rain events and application of one fungicidal spray, soon aft er harvest, would result in adequate disease suppression. (iii) These hypot heses were tested and corroborated in greenhouses in two experiments, in 19 94 to 1995 and 1995 to 1996. Management of basil crops according to these c oncepts would also lower the risks for contamination of the marketable prod uct with pesticide residues.