Wheat stem rust (WSR), caused by Puccinia graminis subsp. graminis Pers., is a highly destructive disease of wheat and several other small grains. The discovery of P. graminis subsp. graminis race Ug99, which overcomes previously effective resistance in wheat, raises concerns for global wheat production and food security. There is currently no mathematical model that describes the duration of the WSR latent period based on temperature or heat units. A study using P. graminis subsp. graminis race GFCDC in ‘Stephens’ wheat was conducted at a range of temperatures (from 4.7 to 33.4°C), to determine their effect on latent period duration. There were significant differences in latent period duration among temperatures, and temperatures above 30°C generally were not conducive for pustule development. A mathematical model to predict latent period duration based on temperature was formulated; the model can be applied to data consisting of varying temperature readings measured at any desired time increment. The model was validated in outdoor experiments under natural conditions on Stephens and ‘McNair’ wheat. In field and outdoor experiments, the latent period durations predicted with the model were within 16 h of the observed latent period, and most fell into the 99% confidence interval of the observations. For experiments conducted on field-grown plants, no significant differences were found between predicted and observed latent period duration. Factorial experiments conducted under growth chamber conditions, using four wheat cultivars (Stephens, McNair 701, ‘Scout 66’, and ‘Kingbird’) and four P. graminis subsp. graminis races (GCCNC, GCCSC, QFCSC, and GFCDC) at three temperatures (5, 15 and 30°C) showed significant differences among cultivars at each temperature. A quantitative model for latent period duration could help in estimating epidemic development, and also in improving our understanding of WSR epidemiology.
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