Chlamydospores of Phytophthora cactorum: their production, structure, and infectivity

Chlamydospores of Phytophthora cactorum are described and compared with oogonia and sporangia. Chlamydospores (average diameter 39.7 μm) were generally larger than either oogonia or sporangia, were usually produced terminally on long supporting hyphae and had single, two-layered walls. They were readily formed in vitro in V8 juice broth supplemented with 2 g L−1 CaCO3 after incubation for 20 days at 4 °C. No chlamydospores were produced at 8, 12, 16, 20, or 32 °C, but chlamydospores sometimes developed at 24 and 28 °C. Chlamydospores also formed on mycelial mats buried in pasteurized potting mix after incubation for 20 days at 4 °C. Germination of chlamydospores after freezing at −23 °C for 24 h was 60–80%. Chlamydospores prepared in this way were infectious to American ginseng (Panax quinquefolium) seedlings. All 11 isolates of P. cactorum examined produced chlamydospores.