In the spring, parts of the fungus within the sclerotia begin to grow and after breaking the sclerotia coating, form one to several mushroom-like growths called stroma. Once infected, a grain flower is replaced by a sticky fungal mass called 'honey-dew' which contains millions of new fungal spores. These new spores are spread to other healthy kernels on the same grain head or to other heads by insects, wind or rain. As infected plants mature, the fungus develops into the hard sclerotia once again. The fungus is favored by cool, wet weather for sclerotia germination, dry conditions for initial spore spread, and cool weather, which prolongs the grain flowering period to allow honey-dew spread.
Florets are most susceptible to infection just before anthesis.
Yields are reduced by formation of the sclerotia. More importantly, ergot is poisonous and can cause serious losses if fed to livestock and poultry. Animals fed large quantities of ergot over an extended period of time develop a dry gangrene resulting in the loss of hooves, ears, tails, combs and wattles. Spontaneous abortion and loss of milk can occur in cows or sows fed even small amounts of ergot. Brood sows, bred and milking cows are most susceptible and should not be fed ergoty grain.