Gray Leaf Spot Disease of Maize: Rating Methodology and Inbred Line Evaluation. M. A. Saghai Maroof, Associate Professor, Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, 24061. S. W. Van Scoyoc, Y. G. Yu, and E. L. Stromberg. Research Associate, Graduate Student, Department of Crop and Soil Environmental Sciences, and Associate Professor, Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg, 24061. Plant Dis. 77:583-587. Accepted for publication 28 January 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0583.

Development of gray leaf spot (GLS) of maize (Zea mays), caused by Cercospora zeae-maydis, was evaluated in seven inbreds in 1990 and 19 inbreds in 1991 in field plots with naturally infested corn debris. In 1990, nine ratings over 35 days were made beginning 9 August. In 1991, 10 ratings over 43 days were made beginning 17 July. Individual plants were assessed by two methods: 1) a disease index (DI) score was assigned on a 1–5 scale (1 = no lesions, 5 = all leaves dead); and 2) disease severity (DS) was expressed as diseased leaf area divided by the total area multiplied by 100, assessed on the ear –1 leaf and all leaves above it. The mean of ratings over time and the area under the disease progress curve (AUDPC) rankings for seven inbreds were similar both years. Rankings for resistance among inbreds based on all 1991 individual ratings, as well as various combinations of ratings for both DI and DS methods, were highly correlated (P ≤ 0.001). Such close associations between the two rating methods indicate that the DI method could replace the DS procedure, which was perceived as the more precise rating method. Of the seven inbreds tested both years, each rating procedure identified T222, NC290, and Pa875 as the most resistant; NC250 as resistant; H93 as intermediate; NC286 as susceptible; and B87 as the most susceptible to GLS. Furthermore, the use of fewer and more widely spaced rating times, especially toward the end of the disease epidemics, could distinguish inbred GLS response nearly as well as the use of many evenly spaced ratings.

Keyword(s): percentage leaf area affected.