Hibiscus trionum L. (Venice mallow) is an annual weed widely distributed in the United States. In September of 2008, Venice mallow plants with bleached stems and necrotic tissues were observed in a commercial field of cabbage (Brassica oleracea L. cv. Moreton) in Geneva, NY. White, cottony mycelium and dark sclerotia were readily found on the stems and in the stem pith. Cabbage plants in direct contact with diseased Venice mallow also displayed signs and symptoms of infection by Sclerotinia sclerotiorum (Lib.) de Bary. Sclerotia from within diseased Venice mallow stems were placed in 9-cm-diameter petri plates on potato dextrose agar amended with 0.1 g/liter each of chloramphenicol and streptomycin (ABPDA) and incubated at room temperature. In addition, diseased stem tissue was surface disinfested for 3 min in 0.525% sodium hypochlorite solution, rinsed for 3 min in sterile distilled water, and placed on ABPDA. After 5 days, hyphae from the colony margin were excised and transferred to potato dextrose agar (PDA) plates. Fungal cultures consisting of white mycelia and medium-sized (~4 mm), black, irregular sclerotia were consistently recovered and identified as S. sclerotiorum based on morphological characteristics (1). Pathogenicity of two isolates (one from a sclerotium and one from stem tissue) was determined by inoculating seven 43-day-old Venice mallow plants growing in greenhouse pots (65 mm in diameter). Mycelia plugs (7 mm in diameter) were excised from 2-day-old PDA cultures of each isolate and placed on the stems at the soil line. Seven control plants were inoculated with noncolonized PDA plugs. All plants were enclosed in plastic bags for 72 h and placed under shade in the greenhouse with temperatures from 20 to 38°C (average 27°C). Symptoms similar to those observed in the affected fields were evident within 2 days after inoculation, while control plants remained symptomless. S. sclerotiorum was successfully recovered from infected plant tissue, fulfilling Koch's postulates. The experiment was repeated with similar results. To our knowledge, this is the first report of Sclerotinia stem rot of Hibiscus trionum caused by S. sclerotiorum (2,3).
References: (1) L. Buchwaldt. Sclerotinia White Mold. Page 43 in: Compendium of Brassica Diseases, 1st ed. S. R. Rimmer et al., eds. The American Phytopathological Society, St. Paul, MN, 2007. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, MN, 1989. (3) C. Wehlburg et al. Index of Plant Diseases in Florida. Fla Dep. Agric. Consum. Serv. Bull. 11, 1975.