Factors Affecting Sclerotium Populations of, and Apothecium Production by, Sclerotinia sclerotiorum

SCHWARTZ, H. F. , and J. R. STEADMAN. 1978. Factors affecting sclerotium populations of, and apothecium production by, Sclerotinia sclerotiorum. Phytopathology 68: 383-388. Sclerotium populations of Sclerotinia scierotiorum were germinated sclerotium in both crops. An apothecium variable during this 3-yr study; however, sclerotia did not continued to produce ascospores in the field for about 7 days. accumulate in increasing numbers in fields planted to Apothecium production was less beneath the open bush susceptible dry edible bean (Phaseolus vulgaris) cultivars canopy of dark red kidney Charlevoix and the upright semidespite annual white mold epidemics. Populations ranged vine canopy of small white Aurora when compared to between one and three sclerotia/kg air-dried soil in bean production beneath the dense compact bush canopy of Great fields. A 3-yr crop rotation did not reduce sclerotium Northern (G.N.) Code P #92 and the dense viny canopies of populations significantly. A low sclerotium population of G.N. U I #59 and G.N. Tara. Over 90% of the apothecia were 0.2/ kg soil produced sufficient inoculum (ascospores) to located either adjacent to the plant or on the side of the infect 46% of the plant canopy during 1975. Sclerotia were irrigation furrow, regardless of plant growth habit. An redistributed within a field by irrigation water. During irrigation application every 5 days increased apothecium August, numerous sclerotia germinated to form 11-14 and 7production, especially beneath Tara, when compared to a 10II apothecia/m 2 in bean and sugar beet fields, respectively, day irrigation frequency. Each apothecium produced about An average of two apothecia were produced by each 2.3 X 106 ascospores under laboratory conditions. Additional key words: Whetzelinia sclerotiorum, inoculum production, epidemiology. Sclerotia are the primary survival structures of the tions and disease severity are lacking, especially for white mold fungus, Sclerotinia sclerotiorum (Lib.) de western Nebraska. Bary = Whetzelinia sclerotiorum (Lib.) Korf and Apothecium production can be influenced by depth of Dumont (13). In western Nebraska 78% of the sclerotia sclerotium burial in the soil (14, 25), presence of a cover buried at various depths survive for at least 3 yr under crop (25), chemicals (5, 10, 12, 17, 18), or periodic soil fallow conditions, and 72% of these sclerotia germinate cultivation (18, 23). However, there are no reports on the and form stipes on water agar (4). In the field, sclerotia influence of frequency of furrow-irrigation and bean produce secondary sclerotia which can prolong S. plant-growth habit upon apothecium production. sclerotiorum survival (1, 4, 25). Sclerotium populations contributing to local disease Sclerotia formed on or within host tissue may be epidemics and cultural factors affecting apothecium dislodged onto the soil surface by the wind or during production in western Nebraska are reported in this harvesting and threshing. Sclerotia are distributed within paper. A quantitative estimate of ascospore production the vertical soil profile by land preparation (4) and also is reported. between fields by irrigation runoff water (3, 22). The fungus also can be distributed throughout a beanproducing area in sclerotiaor mycelia-infested seed lots Sclerotium populations. --Soil samples were collected (2, 11,21). randomly along a zigzag path which traversed each Suzui and Kobayashi (24) reported that 3.2 selected field from end-to-end and side-to-side. Each soil sclerotia/m 2 caused 60-95% infection in a kidney bean sample was collected from a trench 10-cm wide by 20-cm field. Additional data concerned with sclerotium populalong by 7.5to 15-cm deep. In each field, 7 to 22 soil 00032-949X/78/000 064$03.00/0 samples ranging from 1.34 to 3.67 kg dry weight were Copyright © 1978 The American Phytopathological Society, 3340 collected. Pilot Knob Road, St. Paul, MN 55121. All rights reserved. Soil samples were dried at 25-30 C for 2 to 5 days,