Powdery Mildew

Washington State University Bulletin

SP0004 -- 1993

Diseases of Washington Crops.

Otis C. Maloy and Debra Ann Inglis

 

Small Grains Home * Wheat Diseases * Barley Diseases

Cause

Hosts

Symptoms and Signs

Disease Cycle

Control

1997 PNW Pesticide

Recommendations

Another Picture

Other interesting Powdery Mildew sites

Powdery mildew is common on wheat in Washington, especially in early spring. As yet there is no evidence to suggest it causes significant damage. We have included discussion here only because of the common occurrence and conspicuousness of the disease and because of its frequent concern to farmers and others.

Cause
Powdery mildew of wheat is caused by the ascomycete, Erysiphe graminis f. sp. tritici, a fungus that produces characteristic barrel-shaped conidia in chains. The sexual stage forms round, brown cleistothecia that bear simple, flexuous appendages. There are 15 to 20 asci per cleistothecium .

Hosts
Wheat is the only host of E graminis f. sp. tritici. Although similar powdery mildews occur on other small grains and many grasses, these mildews are other strains of the group species, E. graminis. The barley cultivar, 'Advance,' is very susceptible to powdery mildew.

Symptoms and Signs
Powdery mildews are among the easier plant diseases to diagnose, since the fungus forms patches of a white to gray, powdery superficial coating consisting of mycelium and conidia on green above-ground parts of the plant, that is, leaf blades and sheaths, stems and heads. As the season progresses, many small, dark cleistothecia form in the mycelium (Photo 18). Chlorotic patches develop on infected leaves, but often the tissue directly under the fungus remains green. This pattern of symptom development is called the "green island" effect. Lower leaves are usually the most severely infected because of the high humidity around them.

Disease Cycle
The fungus over-winters as cleistothecia, or in mild climates as mycelium and conidia. Wind- or rain-borne ascospores or conidia are the primary inoculum. The fungus requires high humidity but not free water for spore germination and infection. The fungus penetrates only into the epidermal cells, where it forms specialized food absorptive structures called haustoria. Sporulation on the plant surface ensues and resultant conidia are dispersed by wind to induce secondary disease cycles. Heavy, lush growth favors the disease by promoting high humidity through the plant canopy.

Control
Powdery mildew is not considered economically important in Washington, and control is not required. However, appropriate fungicides can control the disease. Use of resistant cultivars is the best defense, unless they succumb to new races of the pathogen.

References
Kingsland, G.C. 1982. Triadimefon for control of powdery mildew of wheat. Plant Disease 66: 139-141.

Purdy, L.H 1967. Wheat powdery mildew, a 1966 epiphytotic in eastern Washington. Plant Dis. Rep. 51:94-95.

Other Interesting Powery Mildew sites:
University of Saskatchewan, Winter Wheat Production Manual -- description and pictures.

Alberta Agriculture, Food and Rural Affairs -- description and pictures.