A Leaf Disk Assay for Detecting Resistance to Early Blight Caused by Alternaria solani in Juvenile Potato Plants. Melinda J. Bussey, Former Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. W. R. Stevenson, Professor, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. Plant Dis. 75:385-390. Accepted for publication 16 October 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0385.

Resistance to early blight, caused by Alternaria solani, was examined using leaf disks from plants of 10 cultivars of potato (Solanum tuberosum) grown in a growth chamber or greenhouse. Disks (14 mm diameter) were cut from five leaves of equivalent maturity per cultivar, 21–80 days postemergence. The leaf disks were spray-inoculated with a conidial suspension of A. solani, floated on solutions containing either 1-naphthaleneacetic acid (NAA), indoleacetic acid (IAA), 2,4-dichlorophenoxyacetic acid (2,4-D), 6-benzylaminopurine (BAP), abscisic acid (ABA), or H₂O, and incubated at 22 C in the dark for 3–5 days. Estimates of susceptibility, which correlated highly with observations of early blight in Wisconsin field trials, were obtained when leaf disks were floated on 10.7 µM NAA (r = 0.86) or 4.0 µM 2,4-D (r = 0.71).