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Some Biotic and Climatic Factors Influencing Sporadic Occurrence of Sunflower Downy Mildew. D. E. Zimmer, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Fargo, North Dakota 58102; Phytopathology 65:751-754. Accepted for publication 27 January 1975. DOI: 10.1094/Phyto-65-751.

The sporadic occurrence of systemic downy mildew of sunflower, Helianthus annuus, incited by Plasmopara halstedii, was closely associated with seedling age and rain. Seedlings become increasingly resistant with age, and the rapidity of symptom development was a function of seedling age at inoculation. The period of maximum susceptibility to systemic infection was as short as 5 days in a 22-25 C greenhouse. In the field, where mean air temperatures during seed germination and emergence were as low as 13.2 C, plants remained susceptible to systemic infection for at least 15 days. With enough rain during this period to provide free-soil water for only a few hours, systemic infection was likely on infested land. Any managerial practice that supports rapid seedling emergence, and reduces the chance of free-soil water during the period of susceptibility, should reduce losses from systemic downy mildew. When seed harvested from plants with systemic symptoms was planted, systemically infected plants were not produced. However, more of these plants had root and basal-stem infection than did plants from seed harvested from healthy plants.

Additional keywords: epidemiology, Helianthus annuus, Plasmopara halstedii.