Infection of stem bases and grains of winter wheat by Fusarium culmorum and F. graminearum and effects of tillage method and maize-stalk residues

The effects of tillage (ploughing vs minimum tillage) and application of chopped maize stalks on winter wheat cv. Hereward by Fusarium culmorum and F. graminearum were investigated in two 2-year experiments in eastern England. Supplementary inoculum of each fungus (five isolates) was applied to the ground to the first wheat crop in each experiment. Infection of shoot bases (spring), stem bases (summer) and harvested grain were determined by isolating the fungi on agar media and by quantitative PCR. Stem bases were infected more frequently by F. culmorum (up to 35% where inoculum was applied) than F. graminearum (up to 22% where inoculum applied; otherwise scarce). Despite mist-irrigation to encourage ear infection, the incidence of pathogens in grain was usually low, always less than 4% for F. culmorum, but up to 30% for F. graminearum where inoculum was applied to the ground. Lack of rainfall probably limited the dispersal of conidia from the ground to the ears. Ascospores, produced by F. graminearum (Gibberella zeae) but not F. culmorum, are less dependent on rainfall for dispersal. Infection of grains by F. graminearum was usually correlated significantly with stem infection and sometimes, negatively, with grain yield. Both conidia and ascospores were produced on the previous crop's debris, apparently in small amounts, throughout much of the year. Little horizontal (plot to plot) dispersal was evident. Non-inversion cultivation and maize-stalk amendments tended to decrease stem-base disease, presumably through the effects of microbial antagonism or competition. However, the incidence of F. graminearum was increased in stem bases where maize stalks were present and in grain by both the presence of maize stalks and minimum tillage. The results support experimental data and UK survey data that non-inversion tillage increases the risk of ear infection, especially by F. graminearum, and hence of mycotoxin contamination of grain. Maize presents an additional risk where weather conditions do not limit the proliferation of the pathogens. This is consistent with evidence from surveys that the region in which the wheat crop is grown is the main risk factor for ear infection and mycotoxin accumulation in grain.