Influences of crop sequence, rainfall and irrigation, on relationships between Gaeumannomyces graminis var. tritici and take-all in New Zealand wheat fields

Abstract

In a study, over three growing seasons, of Gaeumannomyces graminis var. tritici (Ggt) and take-all disease in commercial wheat fields (that were in their first, second, third or fourth year of consecutive wheat) in New Zealand, Ggt concentrations in soil (the amount of Ggt DNA measured using quantitative PCR) and take-all incidence and take-all index (TAI) were least at the start of a wheat sequence and stabilized in third and fourth consecutive wheat crops. Median Ggt concentrations in soil increased 10-fold during a first wheat crop, reaching 78 pg Ggt DNA/g soil after harvest. Growing season rainfall and position in a continuous wheat crop sequence were most closely associated with TAI and post-harvest soil Ggt concentrations, but other factors were also associated with inoculum and disease. Inoculum concentrations, disease incidence and TAI were greater where the frequency of crops susceptible to Ggt (wheat, barley or triticale) was greatest in the crop rotations and where crops were irrigated. Irrigation in particular was associated with high post-harvest soil Ggt concentrations in the driest of the three growing seasons assessed, when environmental conditions were least favourable for disease development. There was evidence of take-all decline following two wheat crops where environmental conditions had enabled severe epidemics to occur. Our study showed that frequency of cereal crops in the rotation, length of breaks between host crops, irrigation and growing season rainfall interact over the course of a cropping sequence to influence TAI when wheat is sown.