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Influence of Rainfall, Drip Irrigation, and Inoculum Density on the Development of Phytophthora Root and Crown Rot Epidemics and Yield in Bell Pepper. J. B. Ristaino, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; Phytopathology 81:922-929. Accepted for publication 25 March 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-922.

Peppers (Capsicum annuum) were grown in three fields in which main plots were irrigated either on a more frequent or less frequent basis. Soil in subplots was left uninfested or was infested 56 wk after transplanting with one of three inoculum densities of Phytophthora capsici. In the Clayton field in 1988, where rainfall was low (16 cm), disease onset occurred 26 days after infestation in plots that were drip irrigated more frequently, whereas disease onset occurred 39 days after infestation in plots that were drip irrigated less frequently. Final disease incidence, the rate of disease increase, and mean area under the disease progress curve (AUDPC) values were significantly greater in infested plots irrigated on the more frequent than the less frequent schedule. More rainfall occurred at the Clinton and Clayton fields in 1988 and 1989, respectively (31 and 22 cm), than at the Clayton field in 1988. Disease in plants in the higher rainfall fields progressed rapidly at all inoculum densities after a single rainfall >2.0 cm, and onset was earlier (57 days after infestation) than at the low rainfall field. Final disease incidence was independent of the density of inoculum applied at all three locations. However, inoculum density and rainfall had significant linear effects on the mean AUDPC values and yield. Mean AUDPC from the three field locations were 24.4, 68.2, and 60.9%-days per day and mean yields were 10.4, 2.3, and 2.1 kg/plot in fields receiving 16, 22, and 31 cm of total rainfall, respectively. Irrigation also increased mean AUDPC values significantly when means were calculated over all locations. Mean AUDPC values were 57.5%-days per day in plots irrigated more frequently and 44.8%-days per day in plots irrigated less frequently. Irrigation increased disease progress to the greatest extent in the low rainfall field in Clayton in 1988. The cumulative centimeters of rainfall between infestation of plots and the final disease assessment was correlated negatively with yield (r = 0.51) and correlated positively with AUDPC (r = 0.47). There was a negative correlation between AUDPC and yield (r = 0.84), and a positive correlation between the time of disease onset and yield (r = 0.73). Plant-to-plant spread of P. capsici resulted in severe disease in uninfested plots in high rainfall fields, whereas less plant-to-plant spread occurred in the low rainfall field. Both rainfall and irrigation had larger effects on the time of onset and final incidence of disease than the range of inoculum densities evaluated. Disease incidence became very high when either a heavy (>2.0 cm) rainfall or more frequent drip irrigations occurred. Final disease incidence was independent of inoculum densities, indicating that Phytophthora root and crown rot is a truly polycyclic disease on pepper in North Carolina.