Tolerance and Resistance to Plant Disease: An Epidemiological Study

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Abstract
Oat [Avena sativa] crown rust was used as a model for an epidemiological evaluation of cencepts 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. Final cumulative spore counts of Puccinia coronata from large plots were compared 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 l of air) than ''Cherokee'' (90). Both had dilatory resistance compared with the susceptible check, which had highest spore count (490) but low yield (0.32) and kernel-weight (0.55) ratios. In the greenhouse, the numbers of pustules, spore yield and pustule area on selected cultivars were determined. 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.

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