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Botrytis porri in Onion Seed Crops and Onion Seed

Nine fields direct-seeded with onion (Allium cepa L.) were surveyed in central Washington in the spring and summer of 2001 for Botrytis species associated with onion seed crops produced in this semiarid region. Forty plants were sampled from each field in a ‘W’ pattern in April, and 20 plants were similarly sampled from each field in June and July. Each plant was placed in a separate plastic bag, stored at 4 ± 2°C for 3 to 5 weeks, sliced lengthwise using a knife sterilized with 70% ethyl alcohol, incubated in a moist chamber for 5 days, and examined under a dissecting microscope. Fungal growth resembling Botrytis spp. was transferred to acidified potato dextrose agar (PDA) for species identification based on colony morphology, rate of growth, and spore and sclerotium characteristics (3). Cultures were incubated on a laboratory bench at 24 ± 4°C with 8 to 16 h of daylight. A species resembling B. porri (3) was detected in 3 fields in April at an incidence ranging from 3 to 28%, and in 2 of the same 3 fields in each of June and July at incidences ranging from 5 to 10%. Infected plants were asymptomatic at the time of sampling. The isolates formed brown, cerebriform sclerotia and sporulated sparsely. Subsamples of seed harvested from each field were assayed for Botrytis spp. To detect internal infection, 400 seeds from each of the nine fields were soaked in 0.525% NaOCl for 60 s, triple-rinsed in sterile deionized water, air dried, placed on a selective agar medium (2) with 20 seed per 9-cm-diameter petri plate, and incubated at 24°C (12 h day/night) for 14 days. Seeds were examined 5, 10, and 14 days after plating, and fungi resembling Botrytis spp. were transferred to acidified PDA for species determination. Isolates resembling B. porri were detected in 0.75% of seed from two of the three fields in which this species was isolated from plant samples. The internal transcribed spacer 1 region of ribosomal DNA of four isolates of the putative B. porri (two from plant samples and two from seed) were sequenced, and all four sequences matched that of B. porri registered in GenBank (Accession No. Z99666) most closely. Botrytis porri is a pathogen of garlic (A. sativum L.), leek (A. porrum L.), and wild garlic (A. vineale L.), but can infect onion and shallot (A. ascalonicum L.) when inoculated on these hosts (1). To our knowledge, this is the first report of natural infection of onion by B. porri, and the first report of seedborne B. porri on onion.

References: (1) W. R. Jarvis. Pathology. Page 62 in: Botryotinia and Botrytis Species: Taxonomy, Physiology, and Pathogenicity. Canada Department of Agriculture, Monograph No. 15, 1977. (2) G. Kritzman and D. Netzer. Phytoparasitica 6:3, 1978. (3) A. H. Presly. Plant Pathol. 34:422, 1985.