Epidemiology of pythium damping-off and aphanomyces root rot of peas after seed treatment with bacterial agents for biological control

Field plots were established in 1989 and 1990 to investigate the efficacy of seed treatments with Pseudomonas cepacia strain AMMD, P. fluorescens strain PRA25, Corynebacterium sp. strain 5A, and captan on the epidemiology of root rot of pea caused by Aphanomyces euteiches and preemergence damping-off caused by Pythium spp. Disease incidence was assessed every 2-3 days, and soil-water matric potential and soil temperature were recorded. Strains AMMD and PRA25 significantly (P < 0.05) suppressed initial disease 23-70% and the area under the disease progress curve 15-45% over both years compared to the nontreated control. Strain AMMD also suppressed the final incidence of disease at harvest by 11-19% over both years and PRA25 by 19% in 1989 compared to the nontreated control. Piecewise regression identified two distinct phases of the epidemics during each year, in which changes in the rate of symptom development were associated with periods of change in soil-water matric potential and increasing soil temperature. Disease incidence increased very little during the first phase of the epidemics and increased in rate significantly (P < 0.05) in the second phase of the epidemics during both years. There were no significant differences (P > 0.05) in rates among epidemics in either the first or second phases of the epidemics during both years. Multiple cycles in the average rate of increase of disease incidence were more apparent when expressed in terms of soil water status than chronological time. High negative cross-correlation coefficients at a temporal lag of 2-3 days were obtained for increases in the average rate of change in mortality expressed in terms of soil water status and the average rate of change in soil water status over time. The bacterial strains may be an environmentally sound and effective means of controlling Pythium preemergence damping-off and Aphanomyces root rot in peas.