Wheat Powdery Mildew

Powdery mildew on wheat is recognized by small, effuse patches (colonies) of cottony mycelia (masses of fungal threads of hyphae that make up the body of the fungus). These occur on the upper and lower surfaces of the leaves. As these patches sporulate and age, they become a dull tan color. Chlorotic (yellow) patches may later surround the mildew colonies. As the wheat and the mildew colonies mature, the sexual stage of the fungus or cleistothecia are produced. The mildew fungus survives the summer in the absence of wheat in infested wheat debris in the cleistothecia. When the new crop develops as seedlings and fall rains occur, the cleistothecia within the infested wheat debris rupture to release spores. This process is favored by moderate fall temperatures and lush wheat growth. The mildew fungus, survives the winter on the infected wheat seedlings. In the spring, with the return of moderate temperatures, the typical cottony mildew colonies develop and sporulate (asexual reproduction) to infect and colonize the newly developing wheat leaves. This stage of the disease cycle is favored by moderate (59 degree to 72 degree F) temperatures and high relative humidity. The canopy within a lush stand of wheat is an ideal environment for powdery mildew to develop.

As the wheat crop matures and the temperatures rise, the mildew fungus produces the oversummering cleistothecia. Typically, the cottony colonies turn tan and are dotted with the cleistothecia. When the crop is harvested, the cleistothecia remain attached to the infested straw.