The Effect of Leaf Wetness on Phytophthora ramorum Zoospore Infection of Rhododendron ‘Cunningham’s White’ and Viburnum tinus

We performed studies using zoospore inoculum combined from nine isolates of Phytophthora ramorum and determined the effect of leaf wetness on infection of whole plants of Rhododendron ‘Cunningham’s White’ and Viburnum tinus. The mean percentage of infected leaves for both host species increased gradually across a dew chamber moisture period of 1 to 6 h, reaching approximately 80% infection by 6 h. We also evaluated the effect of a postinoculation drying period on infectivity of the two host species with zoospore inoculum. With a 30-min postinoculation drying period, Rhododendron ‘Cunningham’s White’ sustained less than 40% infected leaves, whereas V. tinus had an infection rate of almost 75% infected leaves. Disease percentages for both host species declined sharply with drying periods longer than 30 min. Knowledge of infectivity parameters for P. ramorum will provide a better understanding of epidemic development and lead to improved recommendations for control.