Leaf Rusts of Wheat and Barley, Crown Rust of Oats

Symptoms on the foliage are similar for wheat, barley, and oats, although the species of Puccinia are different for each host. Pustules on barley are small, round, and yellowish brown. Pustules on wheat are reddish orange and scattered or clustered on upper leaf surfaces. Pustules on oat are oblong and orange colored. The shape of the spore and its ornamentation are the reasons that oat leaf rust is termed crown rust. The lack of ragged edges on pustules of leaf rusts distinguishes them from stem rusts. As the plants mature, the pustules turn dark and shiny as teliospores are formed. These spores do not play a role in disease development or survival in California.

Leaf rusts are late season diseases that cause losses in years of lower than normal late spring temperatures and high humidity conditions. The leaf rust fungi grow only on living host plants and are specialized to narrow host ranges (wheat leaf rust does not affect barley; barley leaf 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 of initial infections are windblown to initiate secondary cycles (7- to 10-day intervals) when temperatures are 60° to 72°F (16° to 22°C) and moisture is not limiting. The spores 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. Fungus 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 (see BARLEY, OAT, or WHEAT CULTIVAR tables). A statewide monitoring program exists for early detection of susceptible genotypes.

In the event that new races of the fungus render current sources of resistance obsolete, fungicides such as propiconazole (Tilt) can be applied at 4 oz per acre to control disease outbreaks. Applications should be made between tillering and heading to protect the flag leaf.