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First Report of Phytophthora heveae and Pythium spp. on Tropical Tree Seedlings in Panama

Damping-off was observed on experimentally planted seedlings of Ana-cardium excelsum (wild cashew), a timber tree, and Tetragastris panamensis, a canopy tree, within lowland tropical rain forest in Panama. Disease impact was greatest during the wet season (May through December). During the 1995 wet season, 40.7% (572/1,404) of T. panamensis seedlings died due to damping-off disease. Sixty-eight percent (703/1,034) of A. excelsum seed lings died due to damping-off during the 1996 wet season. Symptoms included leaf, cotyledon, and stem necrosis. Phytophthora heveae sporangia were observed on both host species, and oospores were found within stems of T. panamensis. Plating of diseased A. excelsum seedlings on potato dextrose agar with rifampicin (25 mg/ml) and pimaricin (10 mg/ml) produced cultures of Phytophthora heveae and Pythium from 27.4% (110/402) and 44.5% (179/402) of seedlings, respectively. Pythium isolates included P. vexans, P. splendens, and P. chamaehyphon species types, but P. vexans species types accounted for 70% of the Pythium isolates. Disease symptoms on experimental seedlings also were evident on naturally occurring seedlings. Mycelial plugs from six A. excelsum isolates of Phytophthora heveae were used to separately inoculate stems of three A. excelsum seedlings each. Of 18 seedlings inoculated, 88.8% developed characteristic symptoms and died in an average of 8.7 ± 1.0 (standard error [SE]) days. Nine Pythium isolates were used to separately inoculate stems of one to three A. excelsum seedlings each; three of these isolates were known to be P. vexans species types. All of the 20 seedlings inoculated with a Pythium isolate developed characteristic symptoms and died in an average of 6.1 ± 0.3 (SE) days. Both Phytophthora heveae and Pythium isolates were reisolated readily from diseased seedlings. Cotyledons and stems of seven to eight T. panamensis seedlings per isolate were inoculated with two Phytophthora heveae isolates originating from T. panamensis. Necrotic lesions on cotyledons consistent with field symptoms developed on 33.3% of 15 seedlings, but disease did not spread within the stem. Measurements of key morphological structures and cardinal temperatures of four Phytophthora heveae isolates from A. excelsum were consistent with published species descriptions (1), except (i) sporangia with two apices were present, although infrequent; (ii) chlamydospores were produced; and (iii) antheridia were narrower and often shorter than published measurements (7 to 12 m long; 2 to 6 m wide). Internal transcribed spacer (ITS) sequences from Phytophthora heveae isolates cultured from A. excelsum and T. panamensis were matched to reference sequences of Phytophthora heveae with only 3-bp differences (2). ITS sequences for isolates of Pythium vexans, P. splendens, and P. chamaehyphon species types clustered within clades of reference strains of these species (C. A. Lévesque, personal communication). Phytophthora heveae and Pythium spp. have been reported from the tropics. However, this is the first report of these pathogens on seedlings of A. excelsum and T. panamensis.

Reference: (1) D. C. Erwin and O. K. Ribeiro. 1996. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, pp. 100--107, 336--337. (2) D. E. L. Cook et al. A molecular phylogeny of Phytophthora and related Oomy-cetes. Fungal Genet. & Biol. In press.