Phytophthora Leaf Spot and Foliar Blight of Pieris japonica Caused by Phytophthora citricola in California.

During 2004, containerized nursery stock of lily-of-the-valley-bush (Pieris japonica 'Flamingo', family Ericaceae) in Santa Cruz County was affected by a foliar disease. Symptoms consisted of large leaf spots, many developing at the leaf tips that ranged in size from 1 to greater than 4 cm in diameter. Spots were dark brown to almost black, generally oval to round, visible from both sides of the leaf, and did not exhibit signs of any pathogen. Lesions typically expanded and affected the entire leaf, leaf petiole, and stems, resulting in blight-like symptoms. Severely affected leaves abscised from the plant. In advanced stages of the disease, the foliage of the plant was killed. These symptoms resembled those caused by the sudden oak death (SOD) pathogen, Phytophthora ramorum (3). A Phytophthora sp. was isolated consistently from symptomatic leaf tissue. However, the species was identified as P. citricola based on morphological traits that included the following: production of semipapillate, noncaducous sporangia that were irregular in shape and occasionally had more than one apex; presence of oospores with paragynous antheridia in single culture; and radiate to slightly petaloid colony morphology (1). P. ramorum and other fungi were not recovered. Pathogenicity of four representative isolates was confirmed by gently abrading the adaxial surfaces of attached leaves with a sterile wire brush, placing a colonized agar plug (5 mm in diameter) on the surface, misting the leaf with sterile water, and then covering the plug with a plastic cap that was secured with a wire clip. Control leaves were treated in the same manner but received sterile agar plugs. Plants were maintained in a greenhouse at 23 to 25°C. After 2 days, all leaves inoculated with the isolates exhibited dark brown lesions and by day 6, lesions measured 3 cm in diameter. P. citricola was reisolated from symptomatic lesions. Sterile plug control leaves developed no symptoms. The test was repeated and the results were similar. To our knowledge, this is the first report of P. citricola causing a foliar disease of Pieris japonica in California. P. citricola has been reported as a pathogen on Pieris spp. in Ohio (2). Our finding is important because P. ramorum causes very similar symptoms on this same host (3). The occurrence of these two foliar Phytophthora spp. on this ornamental plant may complicate P. ramorum field detection during inspections and laboratory confirmations as established by quarantine regulations. References: (1) D. C. Erwin and O. K. Ribeiro. Morphology and Identification of Phytophthora Species. Pages 96-144 in: Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (2) W. W. P. Gerlach et al. Phytopathology 64:1368, 1974. (3) P. W. Tooley et al. Plant Dis. 88:993, 2004.