Detecting Multiple Species of Phytophthora in Container Mixes from Ornamental Crop Nurseries

A baiting bioassay was developed to detect species of Phytophthora, i.e., those typically associated with ornamental crops, in container mixes that are used routinely in producing container-grown landscape plants. Both fresh and air-dried subsamples of container mixes were baited to improve detection of species that persist as dormant propagules. Leaf disks of Camellia japonica detected Phytophthora spp. most frequently and consistently, but intact needles of shore juniper also were effective baits and less likely to become contaminated. Phytophthora spp. were detected at baiting durations of 24, 48, and 72 h; both detection and contamination were greatest at 72 h. To minimize problems from contamination and maximize detection, camellia leaf disks and shore juniper needles were used simultaneously; half of the baits were removed at 24 h and the other half were removed at 72 h. Baiting at temperatures of 15, 20, and 25°C did not have a dramatic effect on detection; however, Phytophthora spp. occasionally were detected more frequently at 20 and 25°C than at 15°C. Both camellia leaf disks and shore juniper needles were colonized readily by zoospores of P. cinnamomi, P. nicotianae (= P. parasitica), P. cryptogea, and P. citricola but were not colonized as readily by zoospores of P. cactorum. Disks from leaves of C. sasanqua and six cultivars of C. japonica were effective as baits; however, some differences among camellia types occurred. P. cinnamomi, P. nicotianae, P. citricola, P. citrophthora, P. cryptogea, and P. cactorum have been detected in naturally infested container mixes using this baiting bioassay.