In June 2013, a commercial organic planting of Swiss chard (Beta vulgaris subsp. cicla) in Monterey County, CA, showed symptoms of a soilborne disease. Early symptoms consisted of delayed and stunted growth, with wilting of foliage during the warmer times of the day. Initially, a light brown discoloration developed on stems at the soil line. As disease progressed, a dark brown necrosis extended up the main stem and down along the upper portion of the taproot. In advanced cases, the plants collapsed and died. Extensive white cottony mycelium and numerous brown, spherical sclerotia, approximately 1 mm in diameter, developed externally on the lower stem, crown, and adhering adjacent soil. For this particular planting, approximately 10% of the 0.4 ha was lost. Sequentially planted sets of chard placed in other parts of the farm were unaffected. Isolations from necrotic plant tissues, sclerotia, and white mycelium all resulted in recovery of the same white fungus that in culture produced identical sclerotia but no other reproductive structures. Based on white mycelium, sclerotia morphology, and the presence of clamp connections at hyphal septa, the fungus was identified as Sclerotium rolfsii (1). Pathogenicity was tested by growing isolates on potato dextrose agar, drying the resulting sclerotia for 48 h, then burying 5 to 8 sclerotia adjacent to the crowns of healthy Swiss chard plants grown in pots. Three isolates were tested using 24 plants per isolate. Six control plants were inoculated with sterilized sand. All plants were incubated in a greenhouse at 22 to 25°C. After 8 days, inoculated plants began to wilt. By 14 days after inoculation, 100% of the inoculated plants showed symptoms identical to those observed in the field. One half of the plants were used for re-isolations, from which S. rolfsii was recovered from all necrotic crown and stem tissues. The other half of the plants were maintained in the greenhouse; these plants later supported the development of sclerotia. Sand-inoculated control plants did not develop any disease symptoms. The experiment was repeated and the results were the same. To our knowledge, this is the first report of southern blight of Swiss chard in California. Southern blight has not previously been found in this cooler, western part of the county adjacent to the Pacific Ocean; southern blight has been documented on other crops such as pepper, tomato, and chives (3) in the warmer eastern and southern parts of Monterey County. S. rolfsii has been reported on Swiss chard in Louisiana, South Carolina, and Cuba (2).
References: (1) K. H. Domsch et al. Compendium of Soil Fungi, 2nd edition. IHW-Verlag, Eching, Germany, 2007. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab. Online publication, ARS, USDA. Retrieved July 26, 2013. (3) S. T. Koike et al. Plant Dis. 78:208, 1994.
