Sporangia production by Phytophthora ramorum on Rhododendron 'Cunningham's White' at different relative humidities

Authors: Tooley, Paul; Browning, Marsha

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2018
Publication Date: 5/10/2018
Citation: Tooley, P.W., Browning, M.E. 2018. Sporangia production by Phytophthora ramorum on Rhododendron 'Cunningham's White' at different relative humidities. Phytopathology. 108:721-729. https://doi.org/10.1094/PHYTO-10-17-0338-R.
DOI: https://doi.org/10.1094/PHYTO-10-17-0338-R

Interpretive Summary: The fungus-like pathogen Phytophthora ramorum causes sudden oak death, a disease that has killed thousands of oaks and infected many other plant species such as bay laurel, big leaf maple, and Douglas fir in California and Oregon. The disease also threatens the nursery industry because it attacks many commercial plants including rhododendrons, azaleas, and camellias that are shipped nationwide, potentially spreading the disease to new regions including the Eastern US which has many oak forests. P. ramorum is disseminated largely by spores called sporangia, which are produced on susceptible infected plants. The factors that result in production of sporangia are important to study, because a better understanding of spore production allows experts to determine through disease models and risk assessments, the severity of disease to be expected under different environmental conditions. It also allows recommendations to be made for reduction of disease severity. We studied the effect of leaf drying on subsequent production of airborne spores by the pathogen and found that the sporangia could be produced over many weeks. We determined how many sporangia were produced per leaf on a common variety of Rhododendron, one type of plant that is attacked by P. ramorum, over 9 weeks time. By week 4, sporangia production had fallen to 50 percent of the production, observed at week 1, and was reduced with additional incubation time at high humidity. We also constructed humidity chambers where lower humidity levels were assessed, and found that for infected leaves held at humidity levels less than 85 percent many fewer spores were produced on infected leaves. These results will prove useful in understanding disease dynamics and disease spread, and can result in recommendations to reduce disease.

Technical Abstract: We examined the impact of relative humidity (RH) and leaf hydration of Rhododendron 'Cunningham's White' on P. ramorum sporangia production. Diseased plants were maintained under continuous moisture in a mist tent for 24 weeks, and sporangia were collected on screens positioned below leaves. Leaves were also harvested from diseased plants maintained in a greenhouse (43 ± 9% RH) with dry foliage and diseased plants maintained in a mist tent (65 ± 13% RH). Under continuous moisture, P. ramorum initially produced >3000 sporangia/leaf, decreasing to ca. 50% by week 4, and falling below 100 sporangia/leaf by week 6 to 7. However, sporangia were collected from some plants for 22 weeks. Diseased, detached leaves were maintained in humid chambers (100, 96, 85, 75, 56% RH) for up to 9 weeks and removed weekly to assess sporulation. All leaves supported sporulation following 5 weeks at 100% RH, 3 weeks at 96% RH and 1 week at 85% RH; production was lower on leaves held at 85, 75, or 56 % RH. Knowledge of the effects of leaf drying on P. ramorum sporulation capacity will prove useful in terms of disease management recommendations and for development of predictive models and pest risk assessments.