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Tolerance and Resistance to Plant Disease: An Epidemiological Study. Kathleen Politowski, Graduate Research Assistant, Department of Botany and Plant Pathology, Iowa State University, Ames, IA 50011, Present address of senior author: Pfizer Genetics, Inc., P.O. Box 166, Olivia, MN 56277; J. Artie Browning, professor of Plant Pathology, Department of Botany and Plant Pathology, Iowa State University, Ames, IA 50011. Phytopathology 68:1177-1185. Accepted for publication 28 February 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1177.

Oat crown rust was used as a model for an epidemiological evaluation of concepts of tolerance and resistance to plant disease. The concepts included: “true tolerance,” in which a cultivar has a susceptible infection type and supports the same amount of the pathogen as another cultivar but has significantly better yield and quality, or the same yield and quality as another cultivar but supports significantly more of the pathogen; “discriminatory resistance or susceptibility,” in which the host rejects or favors certain components of the pathogen population; and “dilatory resistance,” in which the host reduces the rate of pathogen development. We compared final cumulative spore counts of Puccinia coronata from large plots with host yield and kernel-weight ratios (rusted: healthy) from hill plots. Some cultivars had true tolerance relative to others; dilatory resistance, however, was responsible for the lower spore counts which result in higher yields compared to a susceptible check. For example, cultivar Otter was tolerant relative to Cherokee; both had nearly the same yield ratios (Otter, 0.58; Cherokee, 0.56) and kernel-weight ratios (Otter, 0.70; Cherokee, 0.67), but Otter produced more spores (177/100 liters of air) than Cherokee (90). Both had dilatory resistance compared with the susceptible check, which had the highest spore count (490) but low yield (0.32) and kernel-weight (0.55) ratios. In the greenhouse, we determined the numbers of pustules, spore yield, and pustule area on selected cultivars. Cultivars with lower spore counts in the field tended to have fewer pustules per leaf and less pustule area per leaf in the greenhouse. This may allow greenhouse selection of lines with dilatory resistance. More precise methods of pathogen assessment, such as spore collection, should be used with host yield and quality data to determine if a cultivar has tolerance or resistance relative to another; dilatory resistance may not be apparent by visual assessment.

Additional keywords: Avena sativa, Puccinia coronata.