Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 2, 2011
Publication Date: July 6, 2011
Citation: Tooley, P.W., Browning, M.E., Leighty, R.M. 2011. Infectivity and sporulation of Phytophthora ramorum on northern red oak and chestnut oak. Journal of Phytopathology. 159:516-521. Interpretive Summary: Sudden oak death is a destructive disease caused by a microbe, currently restricted in distribution to the west coast of the U.S. The causal microbe can attack and cause a serious disease on many plant species including forest and nursery crops. The potential of the pathogenic microbe to reproduce and produce spores on some oak species common to the Eastern U.S. is unknown. This information is needed to assess the impact should the organism travel and become established in the Eastern U.S. We assessed the capacity of the microbe to reproduce on two important oak species and found major differences. One oak species allowed significant reproduction of the microbe while the other allowed much reduced levels of reproduction. The results will help workers assess the potential threat posed to forests of the Eastern U.S. and capacity for spread, should the sudden oak death disease move eastward on infected plants during interstate transit.
Technical Abstract: Branches from northern red and chestnut oak seedlings were dip-inoculated with 5,000 sporangia per milliliter of Phytophthora ramorum and incubated at 100 percent relative humidity in dew chambers for 6 days. Three plants were then used to assess sporangia production, while the other three plants were used to assess chlamydospore production. Sporangia production was evaluated by incubating infected seedlings in a mist chamber and collecting sporangia produced on four misted leaves per plant suspended over 15 micron-diameter nylon mesh screens. Chlamydospore content of leaf disks (6 millimeter diameter) removed from diseased leaves following a one month incubation in a greenhouse was also determined. Chestnut oak exhibited significantly greater disease incidence and severity compared with northern red oak. However, sporulation levels were observed to be much larger in northern red oak. Total sporangia production per plant was not significantly different between the two species but when adjusted by lesion area, significant differences between the two species were observed with northern red oak showing higher levels. Mean chlamydospore production per 6 millimeter-diameter leaf disk also was significantly greater for northern red oak compared with chestnut oak. Knowledge of P. ramorum sporulation capacity in relation to disease incidence and severity on Eastern U.S. oak species will help determine the potential for epidemic development should the pathogen be introduced.