USE OF DISEASE PROGRESS CURVES TO STUDY THE EFFECTS OF THE BIOCONTROL AGENT SPORIDESMIUM-SCLEROTIVORUM ON LETTUCE DROP

At the end of the spring 1987 growing season, the mycoparasite Sporidesmium sclerotivorum was applied at 0, 0. 2, 2 or 20 kg ha-1 to lettuce plants infected with Sclerotinia minor. Disease incidence was monitored in the same plots for five subsequent crops (three fall and two spring crops) without additional application of either pathogen or mycoparasite. Logistic growth curves were fitted to the data to describe disease progression over time for each inoculum level within each of the five crops. Within each crop, increasing the quantity of mycoparasite inoculum resulted in positive horizontal displacement of the curve with respect to time. As quantities of inoculum of S. sclerotivorum increased, inflection points of the disease progress curves increased at a decreasing rate. Thus, additional mycoparasite inoculum resulted in ever-smaller increases in inflection point, and after a certain threshold level of mycoparasite inoculum (< 0.2 kg ha-1), increases in inflection point did not result in meaningful increases in harvestable lettuce. Maximum rates of disease increase were not different among the treatments within each crop, but were different between crops. Maximum rates of disease increase averaged 3.4, 3.4, 2.1, 3.6 and 1.5% day-1 for the fall 1987, spring 1988, fall 1988, spring 1989, and fall 1989, respectively. At all inoculum levels, the fall epidemics began later after planting than the spring epidemics.