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Source of Primary Inoculum and Effects of Temperature and Moisture on Infection of Beans by Whetzelinia sclerotiorum. G. S. Abawi, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva, N.Y. 14456; R. G. Grogan, Department of Plant Pathology, New York State Agricultural Experiment Station, University of California, Davis, California 95616. Phytopathology 65:300-309. Accepted for publication 2 October 1974. DOI: 10.1094/Phyto-65-300.

White mold epidemics of beans in New York are initiated by ascospores produced by sclerotia of Whetzelinia sclerotiorum. Sclerotia are abundant in nonplowed bean fields in the spring; but after plowing, sclerotia are rare near the soil surface. Ascosporic inoculum late in the season originates mainly from sclerotia under duff in noncultivated wooded areas and fruit orchards adjacent to bean fields. In 1973, apothecia were produced abundantly from 23 April to 15 June but not during dry weather in June, July, and August. Potted bean plants exposed in commercial fields for 4 days when apothecia were abundant, became infected after incubation in a mist chamber. Also, there was a good correlation between presence of ascospores on bean tissue in the field, and subsequent development of white mold. An exogenous energy source such as bean blossoms, steamed celery stems, turnip extract, or sucrose solution was required for ascospores to infect healthy bean plants in the prebloom stage; however, ascospores readily infected injured plants at any stage of development, or noninjured plants in the blossom stage. Infection of beans by mycelium from sclerotia was not observed in the field. Presumably, this is because in the absence of an exogenous energy source sclerotia are incapable of infecting bean plants, even under ideal conditions in a mist chamber. Moisture is a limiting factor in the development of white mold on beans. Apothecia were produced only in saturated or near-saturated soils, and infection of beans occurred only if free moisture was maintained for a relatively long period at the interface of bean tissue and inoculum. Moisture was also essential for lesion expansion. Ascospore germination was not drastically affected by temperature, but 25 C was optimum for germ tube growth. Mycelial growth, sclerotial production, and lesion initiation and development were optimum at 20-25 C. The temperature range for apothecial production was 10 to 25 C, with an optimum of about 10 C. Temperature does not appear to be a limiting factor in the development of white mold under New York conditions.

Additional keywords: epidemiology, Sclerotinia sclerotiorum, ascospores, Phaseolus vulgaris.