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Factors Affecting Sclerotium Populations of, and Apothecium Production by, Sclerotinia sclerotiorum. H. F. Schwartz, Former Graduate Research Assistant, Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583, Present address of senior author is CIAT, Apartado Aéreo: 67-13, Cali, Colombia, South America; J. R. Steadman, Associate Professor, Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583. Phytopathology 68:383-388. Accepted for publication 4 August 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-383.

Sclerotium populations of Sclerotinia sclerotiorum were variable during this 3-yr study; however, sclerotia did not accumulate in increasing numbers in fields planted to susceptible dry edible bean (Phaseolus vulgaris) cultivars despite annual white mold epidemics. Populations ranged between one and three sclerotia/kg air-dried soil in bean fields. A 3-yr crop rotation did not reduce sclerotium populations significantly. A low sclerotium population of 0.2/kg soil produced sufficient inoculum (ascospores) to infect 46% of the plant canopy during 1975. Sclerotia were redistributed within a field by irrigation water. During August, numerous sclerotia germinated to form 11-14 and 7-11 apothecia/m2 in bean and sugar beet fields, respectively. An average of two apothecia were produced by each germinated sclerotium in both crops. An apothecium continued to produce ascospores in the field for about 7 days. Apothecium production was less beneath the open bush canopy of dark red kidney Charlevoix and the upright semi-vine canopy of small white Aurora when compared to production beneath the dense compact bush canopy of Great Northern (G.N.) Code P #92 and the dense viny canopies of G.N. UI #59 and G.N. Tara. Over 90% of the apothecia were located either adjacent to the plant or on the side of the irrigation furrow, regardless of plant growth habit. An irrigation application every 5 days increased apothecium production, especially beneath Tara, when compared to a 10-day irrigation frequency. Each apothecium produced about 2.3 × 106 ascospores under laboratory conditions.

Additional keywords: Whetzelinia sclerotiorum, inoculum production, epidemiology.