Poster Abstract
  Monitoring

Evaluation of a Novel Diagnostic Procedure to Detect the Presence of Phytophthora ramorum by Sampling Ooze from Infected Cankers

S. A. Tjosvold¹, D. L. Chambers² J. Tse³, J. M. Davidson⁴, M. Garbelotto³, and S.T. Koike⁵

Phytophthora ramorum, the causal agent of the disease commonly known as Sudden Oak Death is a prevalent pathogen in California with its effects evident in 12 counties and found on 14 different oak, tanoak and non-oak hosts. On oak and tanoak, cankers are formed along the lower trunk and often ooze seeps from them. The traditional field diagnostic technique to confirm the presence of the pathogen in these species involves shaving away the outer bark near the perimeter of the canker, collecting pieces of inner bark at the perimeter of the canker, and placing these pieces on selective PARP media. This sampling technique is often very destructive, leaving large wounds on the lower trunk of the tree. Also, this technique is not very dependable; at best, it may require multiple sampling to get an affirmative result on known diseased trees.

Since diseased trees commonly have cankers where ooze seeps to the surface of the bark- and P. ramorum sporangia have been observed in this ooze- it is possible that sporangia or other fungal components could be detected with the right detection technique. This study evaluates the reliability of using PCR (polymerase chain reaction) and traditional PARP selective media to confirm the pathogen’s presence in canker ooze.

For 9 months, ooze samples were taken weekly from naturally infected, seeping cankers of 10 coast live oaks (Quercus agrifolia) and 4 tanoaks (Lithocarpus densiflora) in several locations in north Santa Cruz County. Ooze was tested for the presence of P. ramorum with PCR or by plating on PARP selective media. In addition, ooze was periodically examined for fungal particles under a light microscope. At each sampling, cankers were sampled using traditional techniques, where inner bark is placed on selective media.

The three detection techniques, 1) inner bark plated on PARP, 2) ooze tested with PCR, and 3) ooze plated on PARP resulted in 26.3, 17.6, and 1.8 % with positive findings of P. ramorum respectively. Traditional sampling of inner bark was less effective on tanoak than on live oak. No fungal structures of Phytophthora were observed in ooze samples. Ooze was not always present for sampling on some trees during some dry months, particularly following exceptionally warm weather.

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¹University of California Cooperative Extension, 1432 Freedom Blvd, Watsonville, CA 95076; (831) 763-8040; satjosvold@ucdavis.edu
²University of California Cooperative Extension, 1432 Freedom Blvd, Watsonville, CA 95076
³Department of ESPM-ES, University of California, Berkeley, CA 94720
⁴Department of Plant Pathology, University of California, Davis, CA 95616
⁵University of California Cooperative Extension, Salinas, CA 93901