Root susceptibility and inoculum production from roots of Eastern United States oak species to Phytophthora ramorum

Authors: Widmer, Timothy; Shishkoff, Nina; Dodge, Stephen

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/8/2012
Publication Date: 11/1/2012
Citation: Widmer, T.L., Shishkoff, N., Dodge, S.C. 2012. Root susceptibility and inoculum production from roots of Eastern United States oak species to Phytophthora ramorum. Plant Disease. 96:1675-1682.

Interpretive Summary: The pathogen that causes Sudden Oak Death is a threat to forests in the eastern United States. Oak species are an important part of these forests covering approximately 43 percent of eastern timberland. It is known that the pathogen that causes Sudden Oak Death causes disease on the leaves of many oak species. It is not known if this pathogen can cause disease on oak roots. It was found that the pathogen could infect the roots of the six oak seedlings tested at very low pathogen numbers and at very little exposure time. However, the root weights were not impacted by infection. In addition, it was found that the roots were capable of providing an environment favorable to multiplying the pathogen, which could then be released from the roots into the environment. This research is important in that it shows that the roots of oak trees common in the eastern U.S. are susceptible to this pathogen and will give a clearer picture of what might happen if this pathogen invades the eastern U.S. This information will be important to personnel in forestry and regulatory agencies (Forest Service and APHIS) who are impacted by this pathogen and can make strategic plans to help understand the spread and impact of this pathogen.

Technical Abstract: Little is known about root susceptibility of eastern U.S. tree species to Phytophthora ramorum. In this study, we examined root susceptibility and inoculum production from roots. Sprouted acorns of Q. rubra, Q. palustrus, Q. coccinia, Q. alba, Q. michauxii and Q. prinus were exposed to motile zoospores (3000/ml) of P. ramorum for 1, 6, or 24 h and transplanted to potting soil. After 4 weeks, the roots were weighed, surface sterilized, plated on Phytophthora selective medium (PARPH+V8). Developing P. ramorum was identified visually. Results showed that the primary roots of all oak species tested were susceptible to P.ramorum zoospores, and that infection could occur when exposed for only 1 h to the inoculum. Root weights were not negatively impacted by exposure to P. ramorum after 4 weeks, regardless of the oak species (P = 0.952). To test inoculum threshold levels, roots of sprouted acorns from each oak species were exposed to zoospore suspensions of 1, 10, 100, or 1000 zoospores per ml. After 24 h exposure, they were planted in pots and placed in the greenhouse. After 4 weeks, the roots were removed, surface sterilized and plated on PARPH+V8 medium. A root was recorded as positive if P. ramorum was observed on the medium. Infection of oak radicles occurred at a concentration as low as 1 zoospore per ml. Differences were observed among the species tested. To test inoculum production, the roots of oak seedlings were inoculated with sporangia, washed after 24 hr and transplanted into 2 x 2 inch pots containing Turface®. Periodically, 20-25 ml samples of runoff were collected from each pot and plated on PARPH; the resulting colonies were counted. Counts from oaks were compared to a positive control, Viburnum tinus, using regression analysis. Root segments were plated to calculate percent colonization. After 16 days, inoculum production from oak seedlings was variable and lower than V. tinus. Colonization of roots was lower than that in V. tinus. After 35-days, results were similar. This study shows that sprouted oak acorns are very susceptible to P. ramorum and may be important epidemiologically.