Probits for Analyzing Latent Period Data in Studies of Slow Rusting Resistance

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Abstract
Probit analysis was applied to latent period data of wheat [Triticum aestivum] leaf rust [Puccinia recondita]. Analysis of data from monocyclic infection experiments in the greenhouse revealed a linear relationship between probit percent of uredinia erupted and days after infection. Coefficients of determination from linear regression analysis were greater than 0.91 for 32 of 33 sets of data analyzed. T50, the day by which 50% of the uredinia have erupted, was calculated from the linear regression coefficient and y-intercept. T50, which is analogous to the LD50 of toxicology, characterizes the latent period of the disease and can be used to compare host genotypes for degree of slow rusting resistance. Latent period curves (percent uredinia erupted vs. days after infection) generated from probit analysis statistics were quite similar to the original curves. The advantage of using T50 over calculating a weighted latent period or directly estimating the day on which 50% of uredinia erupt, is that plants need not be inspected each day for percent uredinia erupted in order to calculate T50. Alternate day inspections are sufficient and greatly reduce the time and effort associated with studying slow rusting or using it in a breeding program.