Consecutive wheat sequences: effects of contrasting growing seasons on concentrations ofGaeumannomyces graminisvar.triticiDNA in soil and take-all disease across different cropping sequences

SUMMARY The extent and severity of wheat take-all (caused byGaeumannomyces graminisvar.tritici(Ggt)) can vary considerably between growing seasons. The current study aimed to identify climatic factors associated with differing concentrations ofGgtDNA in soil and take-all disease at different stages of a sequence of wheat crops. Pre-sowing soilGgtDNA concentrations and subsequent take-all disease in consecutive wheat crop sequences were compared across six seasons in 90 commercial cropping fields in Canterbury and Southland, New Zealand, between 2003 and 2009. Disease progress was assessed in additional fields in 2004/05 and 2005/06. While a general pattern in inoculum and disease fluctuations was evident, there were exceptions among wheat crop sequences that commenced in different years, especially for first wheat crops. In three consecutive growing seasons, there was very low inoculum increase in the first wheat crop, while increases in first wheat crops during the following three seasons was much greater. Low spring–summer rainfall was associated with low build-up of inoculum in first wheat crops. The inoculum derived from the first wheat then determined the amount of primary inoculum for the subsequent second wheat, thereby influencing the severity of take-all in that crop. Differing combinations of weather conditions during one wheat crop in a sequence and the conditions experienced by the next crop provided explanations of the severity of take-all at grain fill and the resulting post-harvest soilGgtDNA concentrations in second wheat crops. Examples of contrasting combinations were: (a) a moderate take-all epidemic and high post-harvest inoculum that followed high rainfall during grain fill, despite low pre-sowing soilGgtDNA concentrations; (b) severe take-all and moderate to high inoculum build-up following high pre-sowing soilGgtDNA concentrations and non-limiting rainfall; and (c) low spring and early summer rainfall slowing epidemic development in second wheat crops, even where there were high pre-sowing soilGgtDNA concentrations. The importance of the environmental conditions experienced during a particular growing season was also illustrated by differences between growing seasons in take-all progress in fields in the same take-all risk categories based on pre-sowing soilGgtDNA concentrations.