VIEW ARTICLE

Ecology and Epidemiology

Spatial and Temporal Dynamics of Phytophthora Epidemics in Commercial Bell Pepper Fields. Jean B. Ristaino, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; Robert P. Larkin(2), and C. Lee Campbell(3). (2)(3)Postdoctoral research associate, and professor, respectively, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 83:1312-1320. Accepted for publication 16 August 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1312.

Epidemics in bell pepper caused by Phytophthora capsici were monitored in three commercial fields to characterize the spatial pattern of disease and to gain a preliminary indication of dispersal mechanisms that may influence spatial disease progress. Disease incidence increased from 3.8 to 35.8% and from 13.6 to 38.5%, and the final percentage of quadrats with plants with wilt, crown lesions, stem lesions, or dead plants was 15.8, 16.8, 15.5, and 15.3 in field one and 16.5, 23.5, 10.5, and 31 in field two, respectively. In field three, disease incidence increased from 15.9 to 22.6% in the upper portion of the field and from 53.1 to 67.3% in the lower portion of the field. Based on ordinary runs analysis, rows with aggregated patterns of diseased plants within rows ranged from 15 to 35, 30 to 50, 10 to 20, and 77 to 85% over the season in fields one, two, and the upper and lower portions of field three, respectively. Rows with aggregated patterns of disease across rows ranged from 5 to 30, 20 to 70, 31 to 32.5, and 62 to 74% over the season in fields one, two, and the upper and lower portions of field three, respectively. Two-dimensional distance class analysis also indicated nonrandom spatial patterns as well as greater spread of disease within rows than across rows in fields one and two. In field one, minimum core cluster size increased from 4 to 68 distance class units as several small reflected core clusters coalesced over time within and across several rows of the field. In field two, minimum core cluster size increased from 26 to 35 distance class units, whereas a single reflected core cluster increased in size from 5 to 39 distance class units. The core and reflected core clusters in field two did not coalesce over time, and an average intercluster distance of four rows was observed. In field three, a single large core cluster dominated the lower portion of the field and ranged in size from 350 to 366 distance class units. Across-row spread of disease was predominant in the lower portion of field three and coincided with a low drainage area where water flowed across plant rows. In the upper portion of field three, a large core cluster and several reflected core clusters expanded predominantly within rows, and an average intercluster distance of 23 rows was observed at the last disease assessment date. Incidence of quadrats with plants with stem lesions lagged behind the incidence of quadrats with plants with wilt or crown lesions in all fields, whereas plants with leaf or fruit lesions were not observed. Infection of roots by the pathogen and subsequent spread of the disease from roots to the crown of the plant was the predominant mode of infection. Dispersal of inoculum apparently occurred within rows in surface water in these fields, and secondary spread of the pathogen from plant to plant was evident.