Factors Affecting Onset of Sporulation in Phytophthora ramorum

Authors: Tooley, Paul; Browning, Marsha

Submitted to: Sudden Oak Death Science Symposium
Publication Type: Abstract Only
Publication Acceptance Date: 6/19/2009
Publication Date: 4/1/2010
Citation: Tooley, P.W., Browning, M.E. 2010. Factors Affecting Onset of Sporulation in Phytophthora ramorum. Sudden Oak Death Science Symposium. 2010: 373-374.

Interpretive Summary:

Technical Abstract: To elucidate the sporulation potential of the sudden oak death pathogen, Phytophthora ramorum, on rhododendron, we conducted a series of experiments looking at the relationship between moisture period, lesion size, and onset of sporangia production. Inoculations were performed using P. ramorum isolate C5 to generate disease lesions on Rhododendron 'Cunningham's White'. Both detached and attached leaves exhibiting small P. ramorum lesions were positioned on 15 micron-mesh nylon screens in a mist chamber located inside a greenhouse cubicle. Lesions were measured daily, and sporangia were collected from screens daily. Results were similar with detached and attached leaves; sporulation commenced over a range of lesion sizes (1-54 square millimeter) and no correlation was observed between lesion size and first occurrence of sporangia. Sporangia were collected from roughly 50 percent of lesions that had only expanded to 10 square millimeters or less over the incubation period. Sporangia were collected from 19 percent of leaves following 1 day in mist, and from an additional 64 percent following 2 days in mist. Sporangia were collected from 22 percent of infected leaves on 1 year-old plants following 1 day in mist, and an additional 56 percent of leaves following 2 days. With 3 year-old plants, sporangia were collected from 17 percent of leaves following 1 day in mist and from an additional 39 percent of leaves following 2 days in mist. Thus, with age there were fewer leaves that sustained sporangia production by P. ramorum. The effect of temperature (4-30 degrees Celsius) on the onset of sporangia production was also examined with misted detached leaves held in humid chambers. Sporangia were first collected from leaves following a one day incubation at 15 degrees, 2 days at 10 and 20 degrees, and 3 days at 4, 25, and 30 degrees. When infected leaves were first preincubated at 20 degrees for 72 hours prior to incubation, allowing lesions to expand uniformly in all treatments, sporangia were collected from some leaves following a 1 day incubation at all temperatures. We have not observed a critical minimum lesion area to be used in predicting the onset of sporangia production. Future studies will include the sporulation potential of P. ramorum over time on diseased plants subjected to various temperature and moisture treatments.