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Title: Temperature effects on the onset of sporulation by Phytophthora ramorum on rhododendron Cunningham’s White

item Tooley, Paul
item Browning, Marsha

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2015
Publication Date: 4/16/2015
Citation: Tooley, P.W., Browning, M.E. 2015. Temperature effects on the onset of sporulation by Phytophthora ramorum on rhododendron Cunningham’s White. Journal of Phytopathology. DOI: 10.1111/jph.12390.

Interpretive Summary: Phytophthora ramorum is a destructive plant pathogen that causes sudden oak death, a disease of oaks and a wide variety of other plant species. The pathogen moves from the Western U.S., where it is established, to the Eastern U.S. with plants shipped in the nursery industry. Spores called sporangia are produced by the pathogen and can be spread by wind and rain, causing new infections in forests and nurseries. Little is known about requirements for sporangia production and this information would be useful in determining how rapidly the pathogen can spread within forests and nurseries. We studied the ability of P. ramorum to produce spores on plant leaves soon after disease occurs, and at 6 different temperatures. We found that most spores were produced at 15 and 20 degrees Celsius, and could be produced within a single day under high moisture conditions. This is intermediate in terms of the amount of time it takes closely related pathogens to produce their spores. The results are significant because it is the first time that initial sporangia production has been measured in response to different temperature conditions. The information will be useful to regulatory agencies and scientists who develop models of disease prediction.

Technical Abstract: The effect of temperature and moist period on the onset of sporangia production by Phytophthora ramorum on Rhododendron ‘Cunningham’s White’ was examined with misted detached leaves held in humid chambers. Following wound-inoculation with sporangia, leaves were preincubated at 20°C for either 24 or 72 h prior to placement at six different temperatures (4, 10, 15, 20, 25, and 30°C). The overall mean moist period required for first occurrence of sporulation over all six temperatures was 3.24 days with the 24 h preincubation time, compared with 1.49 days for the 72 h preincubation time. Following 24 h preincubation at 20°C and at an incubation temperature of 15°C, sporangia were first collected from leaves following a 24 h incubation. At 10° and 20°C, sporangia were first collected after 48 h, whereas at 4°, 25°, and 30°C sporangia were first collected after 3 days. Following 72 h preincubation at 20°C sporulation generally occurred within one day, even at temperatures such at 4 and 30°C that are suboptimal for sporulation. The highest levels of P. ramorum sporulation were observed at 20°C. P. ramorum formed sporangia on host tissue under moist conditions within the same time frame reported for P. phaseoli, P. palmivora, and P. nicotianae, but substantially more slowly than certain other species such as P. infestans. Quantifying moisture and temperature conditions for initiation of sporangia production provides knowledge which leads to a greater understanding of the epidemic potential of P. ramorum.