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Disease Thresholds and Increases in Fall Sucrose Yield Related to Powdery Mildew of Sugar Beet in California. F. J. Hills, Extension Agronomist, University of California, Davis. G. F. Worker, Jr., Specialist, University of California, Imperial Valley Field Station, El Centro. Plant Dis. 67:654-656. Accepted for publication 23 November 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-654.

Near El Centro, in California’s Imperial Valley, powdery mildew appeared 18 wk after seedling emergence. Little or no additional crop loss of sugar occurred when the topical applications of sulfur for disease control were not started until 40% of the mature leaf area was diseased. At Davis, where disease control was initiated at the first sign of disease, retreatment after 30% of the mature leaf area became diseased was tolerated without crop loss. Also at Davis, after 1 October, there was little difference in increase in sucrose yield for plants where disease had not been controlled compared with plants where disease was well controlled all season. These experiments support the importance of controlling this disease of sugar beet during periods of rapid root growth but indicate that disease control is less critical before and after this period.

Keyword(s): Erysiphe polygoni.