Jerusalem artichoke or sunchoke (Helianthus tuberosus) is a specialty vegetable that is grown commercially in California. The fleshy tubers are harvested and used as a fresh salad ingredient or cooked vegetable. During 2003, field plantings of Jerusalem artichoke in coastal California (Santa Cruz County) showed symptoms of an unfamiliar disease. Initial symptoms consisted of wilting of new shoots and leaves followed by browning and collapse of all foliage. Crown and lower stem tissues turned tan to brown. In advanced stages of the disease, crown and stem tissues were colonized internally and externally by white, cottony mycelium. Tan, spherical sclerotia that measured approximately 1 mm in diameter formed on the surfaces of the affected crowns and stems. Mycelia and sclerotia also grew on the soil adjacent to infected plants. Isolations from symptomatic crowns, mycelia, and sclerotia produced colonies that were identified as Sclerotium rolfsii. Pathogenicity was tested using two methods that included sclerotial inocula collected from five isolates grown on potato dextrose agar plates. With the first method, sclerotia of each isolate were applied to sets of tubers (10 tubers per isolate) prior to planting tubers into a soilless, peat moss-based medium in pots. With the second method, 3-week-old potted plants were inoculated by placing sclerotia of each isolate adjacent to stem tissue that was 3 cm below the surface of the soilless medium. Noninoculated controls were included for both methods. All plants were incubated in a greenhouse at 21 to 24°C. For the first method, by the third week after planting, 10 to 40% of plants did not emerge because the tubers were rotted and decayed. For the plants that did emerge, wilting of foliage and browning of crown and stem tissue occurred approximately 6 weeks after planting and by 10 weeks, all plants were diseased. S. rolfsii was reisolated from all necrotic tuber, crown, and stem tissues. For the second method, disease symptoms and signs of the pathogen occurred 5 weeks after inoculation and by week 10, 75% of test plants were symptomatic. S. rolfsii was again reisolated from all necrotic tuber, crown, and stem tissues. Symptoms were not observed on any of the noninoculated plants. To my knowledge, this is the first report of southern blight of Jerusalem artichoke in California. This disease has been reported on Jerusalem artichoke in several southern U.S. states (1,2). The two inoculation methods demonstrated that the pathogen could infect propagation organs (tubers) and also emergent stems of this host.
References: (1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989. (2) S. M. McCarter and S. J. Kays. Plant Dis. 68:299, 1984.
