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First Report of Root Rot Caused by Pythium aphanidermatum on Artichoke

In the deserts of Southern California, globe artichokes (Cynara scolymus L.) are grown as annuals. Greenhouse-grown seedlings (5 to 6 weeks old) are commonly transplanted into minimum-tilled, drip-irrigated beds in late summer (August to September) and harvested in winter and spring (December to April). By mid-October of 1997 and 1999, up to 30% of the plants in some commercial fields were either stunted or dead. There was no further progression of the disease over the remainder of the production season. Primary and secondary roots of symptomatic plants submitted for diagnosis in October of 1997 and 1999 exhibited extensive root rot. The organism consistently isolated from rotted roots that were plated onto water agar grew optimally at 37°C, produced inflated sporangia, intercalary antheridia, and oospores characteristic of Pythium aphanidermatum Edson (Fitzp.). Soil temperatures at the 10 cm depth during late summer range from 25 to 34°C. These high soil temperatures are known to be favorable to the pathogenic activity of P. aphanidermatum. To confirm Koch's postulates, 6-week-old artichoke seedlings were transplanted into potting soil that was artificially infested with the pathogen (20 oospores per g of soil). Oospores were obtained from 2-week-old V8 agar cultures of the fungus. Inoculated and noninoculated control plants were incubated at 28°C and the experiment was repeated once. All inoculated plants died within 2 weeks and P. aphanidermatum was recovered only from the rotted roots of inoculated plants. This is the first report of P. aphanidermatum causing root rot on artichoke.