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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOLOGY, PATHOLOGY, AND EPIDEMIOLOGY OF EMERGING OOMYCETE PATHOGENS

Location: Foreign Disease-Weed Science

Title: Inoculum density relationships for infection of some Eastern forest species by Phytophthora ramorum

Authors
item Tooley, Paul
item Browning, Marsha
item Leighty, Robert -

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 7, 2013
Publication Date: April 4, 2013
Citation: Tooley, P.W., Browning, M.E., Leighty, R.M. 2013. Inoculum density relationships for infection of some Eastern forest species by Phytophthora ramorum. Journal of Phytopathology. 161:595-603.

Interpretive Summary: Forests of the Eastern U.S. represent a vast national resource which is potentially threatened by Phytophthora ramorum, a destructive pathogen that has killed thousands of trees in the Western U.S. The pathogen is a fungus-like organism that produces spores on many native species present in forests, yet accurate estimates of the number of spores needed to cause disease do not exist. We performed experiments using whole plants within specialized growth chamber facilities inside a specialized containment laboratory, and also using leaves separated from the plants and incubated in plastic containers containing high moisture levels. We assessed spore levels needed for disease on several important Eastern forest species using two different statistical approaches. We determined the number of spores needed for disease initiation, and also found that species varied significantly in the number of spores required to produce the disease. This knowledge, combined with knowledge of spore numbers produced by Phytophthora ramorum on various plant species, will allow prediction of disease levels to be expected in Eastern forests should outbreaks of disease occur in the Eastern U.S.

Technical Abstract: The objectives of this work were to establish inoculum density relationships between P. ramorum and selected hosts using detached leaf and whole plant inoculations. Knowledge of levels of initial inoculum needed to generate epidemics is needed for disease prediction and development of pest risk assessments. Sporangia of six P. ramorum isolates were produced by incubating V8-juice agar plugs in 1 percent soil extract for 48 hours, and suspensions were adjusted to 0, 50, 100, 500, 1000, 2000, and 3000 sporangia/ml. Whole plants (2-3 year old) and detached leaves of Quercus prinus (chestnut oak), Q. rubra (Northern red oak), Acer rubrum (red maple), Kalmia latifolia (mountain laurel), and Rhododendron ‘Cunningham’s White' were dip-inoculated and incubated in 20 degrees C dew and moist chambers in darkness and the total number of diseased and healthy leaves recorded. Calibration threshold estimates for obtaining 50 percent infected leaves based on linear analysis ranged from 36 to 750 sporangia/ml for the five hosts. Half-life (LD50) estimates from asymptotic regression analysis ranged from 94 to 319 sporangia/ml. Statistically, significant differences (P = 0.0076) were observed among hosts in rates of infection in response to increased inoculum density. Inoculum threshold estimates based on studies with detached leaves were comparable to those obtained using whole plants. The results provide estimates of initial inoculum levels necessary to cause disease on these five P. ramorum hosts, and will be useful in disease prediction and for development of pest risk assessments.

Last Modified: 12/18/2014
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