Stem Rusts of Wheat, Barley, and Oats

Brick-red urediniospores are formed in elongated pustules that erupt through the surface of host tissues. The pustules mainly form on stems but can also occur on leaves and leaf sheaths. The pustules are distinguished from those of the leaf rust fungi by their brick-red color and conspicuously tattered edges. As the plants mature, the pustules turn dark and shiny as teliospores are formed.

Stem rusts of wheat and barley are not significant diseases in California because the commonly grown cultivars are resistant. Some cultivars of oats, however, can be severely affected by the strain of rust that is specific to oats. The rust fungi grow only on living host plants and are specialized to a narrow range of hosts. Infected plants begin to show disease symptoms 7 to 15 days after infection, when urediniospores break through the plant tissue. Crop infections can come from volunteer small grains.  Urediniospores can be dispersed over great distances by air currents and can spread disease between distant fields of the respective small grain crops (wheat, barley, and oats). Windblown spores from pustules of initial infections start secondary infection cycles in 7- to 10-day intervals. Disease occurs when leaf surfaces are wet. It is favored by daytime temperatures of 77° to 86°F (25° to 30°C) and nighttime temperatures of 59° to 68°F (15° to 20°C).

Urediniospores infect the plant through stomata, and a film of moisture is required for infection. The fungus then grows between host cells just under the plant epidermis. Tiny structures, called haustoria, penetrate host cells to obtain nutrients. Fungal tissue proliferates beneath the epidermis. As masses of spores are formed, the epidermis bursts and characteristic rust pustules appear. Infections increase water loss and decrease the amount of photosynthate available for grain filling. This reduces the number and weight of kernels.

Cultural Control

Select cultivars of barley and wheat with higher resistance ratings. Some cultivars of oats, specifically Montezuma and Swan, can be severely affected and should not be grown in areas subject to significant disease pressure.