Stem Rusts of Wheat, Barley, and Oats

Brick red spores 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 races (strains) of the form of fungus that is specialized to oats. The rust fungi grow only on living host plants and are specialized to narrow host ranges (wheat stem rust does not affect barley; barley [rye] stem rust does not affect wheat). Sources of primary inoculum (urediospores) for crops include volunteer cereal plants and, because urediospores can be dispersed over great distances by air currents, distant fields of the respective cereal crops (wheat, barley, and oat). Spores from pustules from initial infections are windblown to initiate secondary cycles (7- to 10-day intervals) when temperatures are above 60°F (16°C) and moisture is not limiting. Urediospores infect the plant through stomata; a film of moisture is required for infection. The fungi then grow between host cells just under the plant epidermis. Tiny structures, called haustoria, penetrate host cells to obtain nutrients. Fungal tissue proliferates beneath the epidermis and 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, resulting in reductions in the number and weight of kernels.

Control is achieved through the use of resistant cultivars. Some cultivars of oat, specifically Montezuma and Swan, can be severely affected by races (strains) of the fungus that are specialized to oat and should not be grown in areas subject to significant disease pressure.