SUSCEPTIBILITY OF SOME EASTERN FOREST SPECIES TO PHYTOPHTHORA RAMORUM

Authors: Tooley, Paul; KYDE, KERRIE - STATE OF MARYLAND

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2006
Publication Date: 4/1/2007
Citation: Tooley, P.W., Kyde, K.L. 2007. Susceptibility of some eastern forest species to phytophthora ramorum. Plant Disease. 91:435-438

Interpretive Summary: Twelve Eastern forest species were screened against the sudden oak death pathogen, Phytophthora ramorum to see if they were susceptible to infection with this emerging pathogen. P. ramorum has killed thousands of oaks on the West Coast in California and Oregon and is threatening the vast oak forests of the Eastern U.S. due to movement of the pathogen on nursery plants which it also infects. We tested young trees via both stem inoculation and leaf inoculation using a virulent strain of the pathogen from California. We found that the pathogen was able to cause stem lesions on all tree species tested when incubated for 72-76 days in a controlled environment greenhouse. The pathogen also was able to produce foliar symptoms on many of the species tested, ranging from less than 1% up to 17% infection. An important species in many Eastern forests, chestnut oak (Quercus prinus) was found to be the most susceptible to both stem and foliar inoculation. The results indicate the potential for P. ramorum to cause infection on Eastern oak species were it to spread to the Eastern U.S.

Technical Abstract: Stem lesions were produced on seedlings of 12 Eastern forest species inoculated with Phytophthora ramorum following 72-76 days incubation in a containment greenhouse cubicle at 20 deg C. Foliar lesions were observed within 7 days when trees were inoculated with 5000 sporangia/ml followed by incubation at 20 deg C in a dew chamber. Chestnut oak (Quercus prinus), an ecologically important species in parts of the Eastern U.S., emerged as the most susceptible host tested in both foliar and stem inoculations. Chestnut oak was followed by tanoak (Lithocarpus densiflorus) and coast live oak (Q. agrifolia) in susceptibility to P. ramorum by foliar inoculation. In response to stem inoculation, chestnut oak and white oak (Q. alba ) were followed by northern red oak (Q. rubra), a very important lumber species, in susceptibility Sugar maple (Acer saccharum) and black walnut (Juglans nigra) were more resistant to stem inoculation than most of the oak species tested. P. ramorum was isolated on selective medium at distances of up to 8 mm from lesion margins. The results indicate that under controlled conditions, P. ramorum is able to infect some important Eastern forest species and cause lesions on stems and foliage. It remains to be determined whether infection of these species by P. ramorum would occur under natural conditions.