SPATIAL DYNAMICS OF DISEASE SYMPTOM EXPRESSION DURING PHYTOPHTHORA EPIDEMICS IN BELL PEPPER

Epidemics caused by Phylophthora capsici were monitored in three commercial fields of bell pepper (Capsicum annuum) in order to characterize the spatial progression of symptom expression over time and to provide evidence for possible mechanisms of inoculum dispersal. Spatial dynamics were characterized using two-dimensional distance class analysis. Symptom expression was nonrandom in each field, and quadrats containing dead plants or plants with wilt, crown lesions, or stem lesions were clearly aggregated. Wilting preceded crown symptoms, suggesting that root infection and subsequent colonization to crowns of plants occurred most frequently. Large clusters of dead plants or plants with wilt or crown lesions were observed, and these clusters changed in size over time. Aggregation of pairs of quadrats containing plants with wilt symptoms or dead plants also was greater within than across rows in three of four fields and occurred unidirectionally for long distances down rows, which suggests the importance of movement of surface water along rows in spread of disease. In one field, across-row spread of disease was evident and radial development of symptom expression was observed. Wilted plants occurred in a circular area surrounding a large aggregation of dead plants or plants with crown lesions. Drainage of surface water, possibly containing inoculum, both within and across rows may have been responsible for this radial pattern of disease progression. Incidence of plants with stem lesions was low in all fields. Clusters of plants with stem lesions were initially small and did not increase until late in the season in areas where plants with crown lesions were observed previously. Splash dispersal of inoculum from soil to stems, leaves, or fruits and aerial dispersal were apparently not important mechanisms of dispersal in these fields.