However, the delay often reduces the final severity of the disease. The rust fungus moves back to volunteer wheat around harvest time. Leaf rust epidemic severity increases exponentially over time. The infection process requires moisture, which can be provided by rain or dew. Heavy rain is unfavorable for rust because it tends to wash the spores off the leaves.
Infection can occur in as little as four hours during favorable weather.
Dispersal of spores to upper leaves and between fields is favored by dry, windy conditions. The earlier leaves are lost, the more severe the yield loss. Losses may vary depending on the variety's ability to fill from the stem, glumes, and awns. The following table provides a rough estimate of percent yield loss due to leaf rust at various flag leaf severities and different growth stages. Resistant varieties possess one or more special leaf rust resistance genes called Lr genes.