Leaf Rusts of Wheat and Barley, Crown Rust of Oats

Symptoms on the foliage of wheat, barley, and oats are similar, although the species of Puccinia is different for each host. Pustules (raised areas or blisters) on barley are small, round, and yellowish brown. Pustules on wheat are reddish orange and scattered or clustered on upper leaf surfaces. Pustules on oats are oblong and orange. Oat leaf rust is called crown rust because of the shape of the spore and its ornamentation. 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 form. These teliospores do not affect disease development or survival in California.

Leaf rusts are late-season diseases that cause losses in years with high humidity and lower-than-normal temperatures in late spring. The leaf rust fungi grow only on living host plants and are specialized to a narrow host range (e.g., wheat leaf rust does not affect barley, and barley leaf rust does not affect wheat). Sources of primary inoculum (urediniospores) for crops include volunteer cereal plants and fields of the same small grain crop (wheat, barley, or oats) because urediniospores can be dispersed over great distances by air currents. After initial infection, spores from newly formed pustules are windblown to initiate secondary infection cycles (7- to 10-day intervals between sporulation and infection) when temperatures are 60° to 72°F (16° to 22°C) and there is sufficient humidity, dew, or water in the crop canopy. 

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 root-like 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.

Cultural Control

Plant resistant cultivars (see wheat and barley cultivar information; see oat cultivar information) to manage these diseases.

Chemical Control

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