Impact
Powdery mildew is caused by the fungus Erysiphe graminis. which affects the foliage and heads of small grains. Each type of small grain is infected by a separate subspecies.
The disease damages plants by competing for plant nutrients, destroying leaf surfaces, reducing plant photosynthesis, and increasing plant respiration and transpiration rates. Serious economic loss is rare from powdery mildew due to typically low humidity in the San Luis Valley despite the higher humidity under sprinkler irrigation.
Light infections of powdery mildew occur throughout the San Luis Valley. Serious problems are rare.

Description and Symptoms

White, cottony patches of the fungus initially form on the upper surfaces of lower leaves. The patches can spread to all aerial portions of the plant.

Opposite sides of an infected leaf may become pale green to yellow.

The patches turn dull gray or brown with "age and develop dark specks.

Dense plant stands, heavy nitrogen fertilization, lush growth, high humidity, and cool temperatures favor disease development.

Overwintering occurs on crop residue. In the spring, inoculation occurs-by wind-borne spores.

Manage crop to limit excessive vegetative growth (i.e., proper fertilizer levels) to limit disease development.

Integrated Management

Use crop rotation to reduce disease inoculum from crop residue.

Plant resistant varieties when available. Resistance depends on the strain of the fungus.

Apply systemic foliar fungicides only for severe infestations. Refer to the current Colorado Pesticide Guide for Field Crops.

This material is based on work supported by the San Luis Valley Water Quality Demonstration Project Best Management Practices Advisory Committee, Colorado State University Cooperative Extension, Farm Service Agency, Natural Resources Conservation Service, and the U.S. Department of Agriculture under authorization as 1991 U.S. Department of Agriculture Water Quality Demonstration Project.