From 1999 to 2001, a Massachusetts nursery received a number of shipments of Pothos, Epipremnum aureum (Lindl. & André) Bunting, with significant crown, petiole, and leaf rot. The plants were imported from Costa Rica. Sporangia were observed on diseased tissues, and five presumptive isolates of Phytophthora were recovered from infected petioles and stems for species identification. The five isolates were morphologically indistinguishable from each other. Sporangia were produced in water and on V8 juice agar under fluorescent light at 22°C. Mating type was determined by pairing isolates with A1 and A2 mating types of Phytophthora capsici Leonian. Sporangial measurements were taken from water cultures. Determination of caducity, and measurements of pedicels and oospores were taken from V8 agar cultures. Measurements represent an average of 50 observations a single isolate. In water culture, sporangia were borne in umbellate clusters. Sporangium length/breadth was 48.29 and 22.33 μm respectively; length/breadth ratio 2.16. On solid media, sporangia were upright and caducous. The bases of the sporangia were mostly tapered. Pedicel lengths were 22 to 49 μm (average 35 μm). Oogonia had amphigynous antheridia and developed only in the presence of an opposite mating type, and oospores measured 25.74 μm diameter. All five isolates were the A1 mating type. Chlamydospores were absent in V8 and corn meal agar (CMA) cultures. Metalaxyl sensitivity was determined at 0, 0.1, 0.5, and 5 ppm in CMA with five replications. The isolate was completely sensitive to 5 ppm metalaxyl, but grew as well as the controls at 0.1 ppm metalaxyl. Growth response to temperature was determined on V8 agar at 15, 20, 25, 30, and 35°C in five replications. After 4 days, colony diameters at 20, 25, and 30°C were not significantly different (P = 0.01) and colonies filled the 100-mm petri dishes. At 15 and 35°C, average colony diameter was 65.7 and 71.4 mm, respectively. Based on the above characteristics, the isolates were identified as P. capsici. Koch's postulates were carried out on pepper, Capsicum annuum ‘Italia’, squash, Cucurbita pepo ‘Patty Pan’ seedlings, and rooted cuttings of pothos. Pepper and squash seedlings and rooted pothos were transplanted in 4-in. (10 cm) pots containing a soilless growing medium (Metro Mix 360, W.R. Grace, Columbia, MD). Phytophthora cultures were grown on V8 juice agar for 4 days. An agar culture was added to 200 ml of sterile distilled water and briefly blended. Ten milliliters of the resulting mycelial slurry were pipetted in the soil one cm from the crown on two sides of the plant. Controls received no mycelial slurry. Petiole, leaf, and crown rot of pothos developed within 2 weeks following inoculation. Squash and pepper plants did not become diseased. In a second pathogenicity test, a 1-cm-diameter plug of mycelial growth from a V8 agar culture was placed between the stem and petiole of the lowest leaf of pothos cuttings directly after transplanting. Inoculated plants died within 3 days. The development of umbellate clusters of sporangia, sporangial shape, length/breadth ratio, and lack of pathogenicity to pepper suggest that the P. capsici isolated from pothos belong to the CAPB (tropical) subgroup of Mchau and Coffey (2).
References: (1) S. S. A. Al-Hedaithy and P. H. Tsao. Mycologia 71:392, 1979. (2) G. R. Mchau. and M. D. Coffey. Mycol. Res. 99:89, 1995.
