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Influence of Moisture, Temperature, Leaf Maturity, and Host Genotype on Infection of Elms by Stegophora ulmea. G. H. McGranahan, Graduate research assistant, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706, Current address of senior author: USDA-ARS, Department of Pomology, University of California, Davis 95616; E. B. Smalley, professor, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. Phytopathology 74:1296-1300. Accepted for publication 30 May 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1296.

The influence of environmental and host factors on the initiation and development of black spot disease on both resistant and susceptible elm clones was investigated. Optimum disease development occurred when plants were exposed to 100% relative humidity at 16 C for 24 hr following inoculation with 106 spores per milliliter of conidial suspension. Low temperatures (12- 16 C) resulted in increased disease incidence in moderately susceptible clones but did not have a significant effect on a highly susceptible clone of Ulmus laevis. Temperature did not influence the type (sporulating vs nonsporulating) of lesion present. Young, growing leaves were most susceptible; no lesions were found on leaves below the third internode from the growing tip. Repeatability estimates were high (0.88- 0.91), which suggests a high heritability for clonal response to S. ulmea and a good potential for breeding and selection for resistance.