Susceptibility of some common Eastern forest understory plant species to Phytophthora ramorum

Authors: Tooley, Paul; Browning, Marsha

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2008
Publication Date: 3/1/2009
Citation: Tooley, P.W., Browning, M.E. 2009. Susceptibility of some common Eastern forest understory plant species to Phytophthora ramorum. Plant Disease. 93:249-256.

Interpretive Summary: Sudden oak death caused by the pathogen Phytophthora ramorum is a devastating disease of oak species in California, and the pathogen also attacks a wide range of other plant species. Recently, the pathogen has been moved several times to the Eastern U.S. on infected nursery stock, but has not become established. If the pathogen becomes established in the Eastern US and infects the vast oak forests present in many states, huge losses could occur both in terms of economic impact and habitat alteration for wildlife. Based on west coast observations, the susceptibility of mature trees is thought to be less than that of certain plant species that make up some of the additional diversity of the forest ecosystem. These are termed understory species, in that they make up the composition of plant species located under the forest canopy. We evaluated the susceptibility to P. ramorum of 25 plant species that comprise a portion of the understory in forests of the Eastern U.S. We found a wide range of susceptibility both in terms of diseased areas produced by the pathogen on plant leaves, and in terms of the pathogen's ability to reproduce (produce spores) on these species. The results provide knowledge that may be used by inspectors monitoring for the presence of the pathogen in Eastern forests, as well as information that can be used in producing maps that outline the risk of establishment of the pathogen in new regions.

Technical Abstract: We evaluated the susceptibility of 25 plant species (21 genera, 14 families), which comprise a portion of the understory in forests of the Eastern US, to infection by Phytophthora ramorum. We also assessed the degree to which P. ramorum is able to form sporangia and chlamydospores on these hosts. Seedlings were spray-inoculated with a mixture of four P. ramorum isolates at a concentration of 4000 sporangia/ml followed by incubation in a dew chamber at 20 degrees C in darkness for 5 days. Percentage infection on individual leaves of each species was assessed visually. Mean percent leaf area infected ranged from 0.7 percent for Smilax rotundifolia to 93.8 percent for Kalmia latifolia. Eight plant species tested developed significantly larger lesion areas than those found on Cunningham’s White rhododendron, the susceptible control. Fourteen species in addition to Cunningham's White rhododendron exhibited infection of over 90 percent of their leaves. Sporangia production varied considerably among plant species as well, ranging from 36 per cm(2) lesion area for Myrica pennsylvannica to 2001 per cm(2) lesion area for Robinia pseudoacacia. Numbers of chlamydospores produced per 6 mm-diameter leaf disk incubated in sporangia suspension ranged from 25 for Ilex verticillata to 493 for Rhus typhina. The results indicate that many common understory species in Eastern US forests are susceptible to P. ramorum and could provide a source of inoculum for forest epidemics should the pathogen be introduced.