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 initiated on the surface of stem wounds that are caused during harvest; the pathogen then progresses on the stem, killing leaves and secondary buds. When the infection reaches the main stem, the entire plant dies. A study of the epidemiology of the disease and of host-pathogen interactions led to the development of rules for effective disease management. The research was conducted in three steps. (i) Natural epidemics in commercial crops were monitored during the 1993 to 1994 and 1994 to 1995 growing seasons. Disease outbreaks were found to coincide with harvests during rainy days, whereas disease intensity did not change much when harvests were completed during rainless days. (ii) Studies under controlled environmental conditions revealed that cut ends of stems were highly susceptible soon after harvest, susceptibility diminished gradually, and stem cuts inoculated 48 h after harvest were rarely diseased. Observations under a scanning electron microscope showed that an opaque layer had appeared over the cut surface and, as the entire wound surface became covered, penetration of the fungus into the tissue was prevented. Based on these findings, it was hypothesized that avoidance of harvesting during rain events and application of one fungicidal spray, soon after harvest, would result in adequate disease suppression. (iii) These hypotheses were tested and corroborated in greenhouses in two experiments, in 1994 to 1995 and 1995 to 1996. Management of basil crops according to these concepts would also lower the risks for contamination of the marketable product with pesticide residues.