VIEW ARTICLE

Ecology and Epidemiology

Interannual Variations in Wheat Rust Development in China and the United States in Relation to the El Niņo/Southern Oscillation. H. Scherm, Department of Plant Pathology, Iowa State University, Ames 50011; X. B. Yang, Department of Plant Pathology, Iowa State University, Ames 50011. Phytopathology 85:970-976. Accepted for publication 2 June 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-970.

The El Niņo/Southern Oscillation (ENSO) is one of the most important and best-characterized mechanisms of global climatic variation. Because regional temperature and precipitation patterns are influenced by the ENSO and plant diseases are responsive to these factors, historical disease records may contain an ENSO-related signal. We used cross-spectral analysis to establish coherence and phase relationships between the Southern Oscillation Index (SOI), which is a measure of the ENSO, and long-term (>40 years) data on wheat stripe rust in five regions of northern China and wheat stem rust in four climatic divisions of the midwestern United States. Monthly SOI values were averaged from March to June and October to March for analysis of the rust data from China and the United States, respectively, based on the times of the year at which weather patterns in these regions are influenced by the ENSO. The coherence relationships showed consistent and significant (0.01 ≤ P ≤ 0.10) cooscillations between the rust and SOI series at temporal scales characteristic of the ENSO. The five stripe rust series were coherent with the SOI series at periodicities of 2.0 to 3.0 and 8.0 to 10.0 years, and three of the four stem rust series were coherent with the SOI series at a periodicity of 6.8 to 8.2 years. The phase relationships showed that, in most cases, the rust and SOI series cooscillated out of phase, suggesting that the associations between them are indirect. In a separate analysis of a shorter (18 years) stripe rust series from the Pacific Northwest of the United States, disease severity was significantly lower during El Niņo years (warm phases of the ENSO) than during non-El Niņo years (P ≤ 0.0222) or during La Niņa years (cold phases of the ENSO) (P ≤ 0.0253). Although no cause-and-effect relationships could be deduced, this analysis identified methods and directions for future research into relationships between climate and disease at extended temporal scales.