Effect of environmental conditions and lesion age on sporulation of Phytophthora ramorum on California bay laurel, rhododendron, and camellia

The objective of our research was to determine the environmental conditions and lesion age favorable for Phytophthora ramorum sporulation under field conditions. For 2 years, new camellia, rhododendron, and California bay laurel (Umbellaria californica (Hook. & Arn.) Nutt.) nursery stock were seasonally inoculated (every 3 months) on foliage. They were covered overhead to prevent rainfall from falling on the plants, but otherwise the plants were completely open to the natural environment. Consistent leaf wetness periods were produced with overhead misting systems and controlling sensors to simulate rainfall, fog, dew, or other conditions that might be supportive of sporulation. For each season, these wetness conditions began when leaf lesions were 3, 6, and 9 weeks old and, at each of these time points, the wetness conditions were maintained for 8 days. Sporulation was evaluated by washing leaf lesions just before the wetness period began (day 0) and at 1, 2, 4, and 8 days during the wetness period. Leaf wetness and temperature were measured near the plants.