Modelling the potential distribution of Karnal bunt of wheat in South Africa

Abstract

The objective of this paper was to use established meteorological modelling methods to determine wheat growing regions in South Africa that would be favourable to infection by Tilletia indica, and to determine whether irrigation schedules influence climatic suitability for the pathogen. Under natural climatic conditions, only locations planted to rainfed spring-type wheat in the western and southern production regions of the Western Cape experienced climatic conditions suited to T. indica development. Humid Thermal Indexes (HTI) were usually within the suitable range of 2.2 to 3.3 (HTI range 2.09–3.20, mean 2.58) and most locations experienced at least one Suitable Rain Event (SRE) (range 0.78–2.44) during the susceptible period. In contrast, central and eastern wheat growing regions in South Africa were likely to be too hot and/or dry under natural climatic conditions (HTI range 0.61–1.67, mean 1.06) and there was usually less than one SRE (range 0–0.63) for early and mid-planted wheat. HTI usually increased to levels within the suitable range for T. indica development (HTI range 1.56–2.57, mean 2.14) in early-planted irrigated wheat. For mid- and late-planted irrigated wheat, HTI also increased, however, levels mostly remained below the minimum critical level of 2.2 (range 1.13–2.29, mean 1.59). These results indicate that in irrigated wheat, the later the crop is sown the less likely suitable climatic conditions for pathogen development will prevail. This may not apply to farms where crops are irrigated every 24 h, as minimum critical relative humidity (RH) is likely to be maintained and the probability of T. indica spores surviving would potentially be increased. Farmers may need to consider altering irrigation schedules so that suitable climatic conditions for development and spread of T. indica are limited.