Wet soil conditions
Ergot requires moisture on soil surface to germinate and then wet, cool, cloudy conditions in the summer to extend the flowering and increase the amount of time spores can infect the florets.
Low soil copper levels increase pollen sterility which leads to high susceptibility to ergot infection.
High Nitrogen application rates
High rates of applied nitrogen combined with normal nitrogen mineralization can induce a copper deficiency even on soil with adequate soil test copper.
Herbicide stresses are known to induce copper deficiency, delay maturity and cause pollen sterility, thereby allowing for a longer infection window.
Riesselman, Montana State University