Main content area

Inoculum Density Relationships for Infection of Some Eastern US Forest Species by Phytophthora ramorum

Tooley, Paul W., Browning, Marsha, Leighty, Robert M.
Journal of phytopathology 2013 v.161 no.9 pp. 595
Acer rubrum, Kalmia latifolia, Phytophthora ramorum, Quercus montana, Quercus rubra, Rhododendron, foliar diseases, forest trees, fungal diseases of plants, half life, hosts, inoculum density, leaves, lethal dose 50, prediction, regression analysis, relative humidity, risk assessment, sporangia, spores, tree diseases, Eastern United States
Our objectives were to establish inoculum density relationships between P. ramorum and selected hosts using detached leaf and whole-plant inoculations. Young plants 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 dipinoculated with varying numbers of P. ramorum sporangia, and the total number of diseased and healthy leaves recorded following incubation at 20 degrees C and 100 percent relative humidity. Calibration threshold estimates for obtaining 50 percent infected leaves based on linear analysis ranged from 36 to750 sporangia/ml for the five hosts. Half-life (LD50) estimates (the number of spores for which the per cent of diseased leaves reaches 50 percent of its total) from asymptotic regression analysis ranged from 94 to 319 sporangia/ml. Statistically, significant differences (P = 0.0076) were observed among hosts in per cent infection in response to increased inoculums density. Inoculum threshold estimates based on studies with detached leaves were comparable to those obtained using whole plants. The results provide estimates of 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.