Susceptibility of Some Eastern Forest Species to Phytophthora ramorum.

Abstract

We tested some common Eastern forest species for their reaction to stem and leaf inoculation with Phytophthora ramorum, the pathogen that causes sudden oak death. Stem lesions were produced on inoculated seedlings of 12 Eastern forest species following 72 to 76 days of incubation in a containment greenhouse cubicle at 20°C. Chestnut oak (Quercus prinus) and white oak (Q. alba) were followed by northern red oak (Q. rubra) in susceptibility to stem inoculation. 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 stem lesion margins. Foliar lesions were observed within 7 days when seedlings were inoculated with 5,000 sporangia/ml followed by incubation at 20°C in a dew chamber. Chestnut oak was followed by tanoak (Lithocarpus densiflorus) and coast live oak (Q. agrifolia) in susceptibility to P. ramorum by foliar inoculation. Chestnut oak, an ecologically important species in parts of the Eastern United States, emerged as the most susceptible host tested in both stem and foliar inoculations. 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.