Quantitative Relationships Between Climatic Variables and Stripe Rust Epidemics on Winter Wheat

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Abstract
Climatic variation at Pullman, Washington, USA, since 1958 has contributed to an increase in the frequency of epidemics and severity of stripe rust (caused by P. striiformis) on winter wheat. For 1963-1979, rust intensities on ''Omar'' (CI 13072) (very susceptible) and on ''Gaines'' (CI 13448) (susceptible in the seedling stage at all temperatures but resistant at later growth stages at high temperatures) were positively correlated with Jan. temperatures and negatively correlated with April and June temperatures. Spring temperatures were more highly correlated with disease development in ''Gaines than in ''Omar''. Frequency of precipitation in June was correlated with stripe rust intensity in both ''Gaines'' and ''Omar''. To mathematically relate stripe rust intensity to cumulative temperatures, negative degree days (NDD) and positive degree days (PDD) were calculated by using a 7.degree. C base. Disease intensity was negatively correlated with NDD and PDD; the best correlations were with NDD that accumulated from 1 Dec.-31 Jan. and PDD that accumulated from 1 April-30 June. Slopes of linear regression equations for disease intensity on ''Gaines'' vs. NDD in Dec. and Jan. and vs. PDD in April-June differed significantly from zero at P < 0.001. Regression equations for disease intensity on ''Omar'' as a function of NDD or PDD had significantly different Y-intercepts but slopes that were similar to the comparable equations for ''Gaines''. These relationships help explain why stripe rust was not severe in 1941-1958 and may be useful for predicting stripe rust intensity in the Pacific Northwest.

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