Sporangia Production Over Time by Phytophthora ramorum on Rhododendron ‘Cunningham’s White’ After Placement at Different Relative Humidities

We examined the impact of relative humidity (RH) on Phytophthora ramorum sporangia production on Rhododendron 'Cunningham's White'. When diseased plants were maintained under continuous moisture in a mist tent, sporangia were collected from some plants for 22 weeks. More than 3,000 sporangia/leaf/week were collected over the first 3 weeks but levels declined to <100 sporangia/leaf/week after 7 weeks. We also examined the impact of drying on P. ramorum sporangia production. Diseased, detached leaves were maintained in humidity chambers (100, 96.2, 84.5, 74.9, and 56.2% RH) for up to 9 weeks and removed weekly to assess sporulation. For comparison, diseased leaves were harvested from plants maintained with dry foliage or subjected to 10 h of simulated dew nightly. All leaves supported sporulation following 5 weeks at 100% RH, 3 weeks at 96.2% RH, and 1 week at 84.5% RH. All leaves collected from plants subjected to nightly dew supported sporulation for 3 weeks; however, only 66.7% of leaves collected from plants with dry foliage supported sporulation after 1 week. Knowledge of the effects of RH levels on P. ramorum sporulation capacity will prove useful in terms of disease management recommendations and for development of predictive models and pest risk assessments.