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Environmental Factors Affecting Rose Downy Mildew and Development of a Forecasting Model for a Nursery Production System

The effect of various environmental parameters on rose downy mildew caused by Peronospora sparsa was determined under controlled conditions and in the field. In growth chambers, optimal temperatures for infection and colonization of rose leaves were 15 to 20°C and 20 to 25°C, respectively. At optimal temperatures, infection required only 2 h of leaf wetness, although disease severity increased significantly with an increasing duration of leaf wetness up to 10 h. Infection of leaves occurred at temperatures as low as 5°C with 8 h of leaf wetness. The latent period of infection varied from 4 to 7 days. Weather and disease incidence data collected from natural field epidemics were used in the development of a predictive model of rose downy mildew. Logistic regression was used to identify those weather variables that explained the largest portion of the variation in disease incidence. The optimum regression model incorporated three weather variables calculated as cumulative totals over the previous 10 days: (i) hours of leaf wetness when temperatures were less than 20°C (positive correlation); (ii) hours between 15 and 20°C (negative correlation); and (iii) hours when temperatures exceeded 30°C (negative correlation). The simplest model, which was also a good fit, included only the 10-day cumulative number of hours of leaf wetness. The critical number of hours of leaf wetness for disease development was an average of 8.4 h per day over 10 days.