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Ecology and Epidemiology

Epidemiological Factors of Sugar Beet Powdery Mildew. E. G. Ruppel, Research Plant Pathologist, Agricultural Research Service, U. S. Department of Agriculture, Colorado State University, Fort Collins, CO 80523; B. J. Tomasovic, Research Assistant, Beet Sugar Development Foundation, Crops Research Laboratory, Colorado State University, Fort Collins, CO 80523. Phytopathology 67:619-621. Accepted for publication 26 October 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-619.

Susceptibility to powdery mildew (caused by Erysiphe polygoni) in sugar beet inoculated 2, 4, 8, 12, and 16 wk after planting increased with plant age. Of 33 plant species representing 19 genera in nine families, only Beta atriplicifolia, B. lomatogona, B. macrocarpa, B. macrorhiza, B. maritima, B. patula, B. trigyna, and B. vulgaris (red beet, sugar beet, Swiss chard) were highly susceptible to the fungus from sugar beet; B. patellaris was highly resistant. Atypical infection spots occurred on senescent leaves on one Chenopodium capitatum and two Rumex crispus plants, but the hyphae did not spread, sporulation was minimal or nonexistent, and the fungus ultimately disappeared. Powdery mildew conidia (Erysiphe spp.) from Amarathus retroflexus, R. crispus, and Solanum sarachoides growing in sugar beet fields did not infect sugar beet in the greenhouse. Vegetative mycelia or conidia of the fungus remained infectious in sugar beet leaf debris buried outdoors in soil for 60, but not 90, days. Infected leaf debris stored at room temperature, in a refrigerator at 3-4 C, or in a protected area outdoors over winter was noninfectious when tested after 60 days of storage. Erysiphe polygoni-infested seed yielded healthy seedlings, and inoculum prepared from infested seed was noninfectious to sugar beet. The brief life of the fungal vegetative stage, the absence of the perfect stage, specificity for the genus Beta, and the yearly sequential spread of the disease support the theory that overwintering of the fungus mainly occurs in the southwest and that conidia are carried northward and eastward by prevailing winds.