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ARS Home » Northeast Area » Frederick, Maryland » Foreign Disease-Weed Science Research » Research » Publications at this Location » Publication #355754

Research Project: Biology, Epidemiology, and Detection of Emerging Plant Pathogenic Oomycetes

Location: Foreign Disease-Weed Science Research

Title: The effect of moisture on Phytophthora ramorum zoospore infection of Rhododendron ‘Cunningham’s White’ and Viburnum tinus

item Tooley, Paul
item Browning, Marsha

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2019
Publication Date: 5/22/2019
Citation: Tooley, P.W., Browning, M.E. 2019. The effect of moisture on Phytophthora ramorum zoospore infection of Rhododendron ‘Cunningham’s White’ and Viburnum tinus. Plant Disease.

Interpretive Summary: Phytophthora is a fungus-like organism that causes major disease on numerous crops in the US and worldwide. One characteristic of Phytophthora species is their ability to produce a type of spore that is able to swim and infect plants under wet conditions. These are called zoospores, and play an important role in the spread of these pathogens. We studied the effects of leaf wetness on infection of two important plant hosts by zoospores of Phytophthora ramorum, which causes sudden oak death in California and also attacks many nursery plant species. After 6 hours in a high humidity environment following inoculation with P. ramorum zoospores, we found that roughly 80% of leaves of both host plant species became infected. Drying plants for 30 min or longer following inoculation substantially reduced the percentage of leaves that became infected. These findings are important because they allow improved recommendations to be made that minimize infection of nursery plants by this damaging pathogen.

Technical Abstract: We performed studies using zoospore inoculum combined from nine isolates of P. 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 over an initial dew chamber moisture period of 1 to 6 h, reaching ca. 80% infection by 6 h. We also evaluated the effect of a post-inoculation drying period on infectivity of the two host species with zoospore inoculum. With a 30 min post-inoculation drying period, Rhododendron 'Cunningham's White' sustained less than 40% infection, while Viburnum tinus had an infection rate of almost 75%. Disease percentages for both host species dropped off sharply at drying periods longer than 30 min. Knowledge of infectivity parameters for P. ramorum will lead to a better understanding of epidemic development and lead to improved recommendations for control.