Crown rust on oat

Cloud of crown rust spores from infected oat

COMMON NAME:
Oat crown rust

SCIENTIFIC NAME:
Puccinia coronata Corda var. avenae W.P. Fraser Ledingham

DISTRIBUTION:
Oat crown rust occurs worldwide wherever cultivated or wild oat species occur except in very arid areas. Crown rust is most important where dews are frequent and temperatures are mild (15-25 C) during the oat growing season.

YIELD LOSS:
Crown rust is the most widespread and damaging disease of oat. There have been severe epidemics in virtually every oat-growing region of the world. Moderate to severe epidemics can reduce grain yield by 10 to 40%. Individual oat fields may suffer total crop failures. Weather conditions most favorable for oat growth also favors crown rust, so greatest yield losses commonly occur in years when oat yields should be highest. Damage to leaves, particularly the flag leaf, reduces photosynthesis and interferes with transport of photosynthesized sugars from leaves to the developing grain. This causes shriveled grain with reduced feed value. In forage oat crops, crown rust can severely damage leaves, limit growth, and reduce forage quality. Badly rusted plants have stunted root systems and poor drought tolerance.

SYMPTOMS:
Uredinia form on both the upper and lower surfaces of infected leaves. In severe epidemics, leaf sheathes also become heavily infected. Uredinia are round to oval pustules, up to 5 mm long, and contain masses of orange-yellow spores exposed by rupture of the leaf epidermis. After a week or two, the margins of uredinia may become black with formation of the dark colored teliospores. Often telia form as secondary, black rings abound the orange-yellow uredinia. The leaf epidermis remains intact over the telia until after the leaves have died.

ENVIRONMENTAL CONDITIONS:
Crown rust develops best during mild to warm (20-25 C) sunny days and mild nights (15-20 C) with adequate moisture for dew formation. Wind at mid-day is important in moving spores around to infect new oat plants. Under ideal conditions, rust spores can infect oat plants even after being carried in the wind for hundreds of miles.

INOCULUM SOURCE AND INFECTION:
In sub-tropical and warm-temperate regions, oat grows during the winter. Initial infections on fall sown oat generally come from uredinia on volunteer oat plants that survived the hot summer in moist protected habitats near streams or irrigation canals. The alternate host, Rhamnus spp., is an important source of inoculum for oat in temperate areas of Europe and North America. Teliospores on infected straw from the previous summer germinate in the spring producing basidiospores that infect young leaves of Rhamnus. These infections produce aecia, which release aeciospores that infect oat plants. Rhamnus cathartica is an important source of inoculum for oat in the north central states of the U.S. In countries of the Middle East, native Rhamnus species become infected in December or January from teliospores that have survived the hot dry summer and germinate after the fall rains stimulate new leaf growth on Rhamnus plants.

Urediniospores and aeciospores germinate in free water on leaf surfaces. Germination and infection of leaves through stomates occur readily at temperatures from 10 to 25 C. Infection is inhibited at temperatures above 30 C.

SURVIVAL:
Winter oats are not hardy in most parts of the U.S. north of Tennessee. Therefore, crown rust does not survive in the uredinial stage through the winter except in the South. As crown rust epidemics develop during early summer in the South, urediniospores may be blown north to infect spring sown oat crops. For northern states, however, aeciospores from buckthorn bushes (Rhamnus cathartica) are usually a more important source of inoculum. Teliospores survive the winter in temperate regions or the hot dry summers in regions with Mediterranean climates. Mild, wet weather stimulates dormant teliospores to germinate and produce basidiospores that infect newly formed Rhamnus leaves.

METHOD OF DISSEMINATION:
Urediniospores and aeciospores are wind borne and can survive long distance dissemination. Teliospores remain with the straw. Germinating teliospores produce short-lived basidiospores that are released when the humidity is very high at night or during rain showers. Basidiospores must infect quickly after their release. They rarely survive long enough in the air to infect Rhamnus plants more than several hundred meters from their source.

HOST RANGE:
There are several varieties of crown rust that infect a wide variety of grasses. Individual varieties of the crown rust fungus are limited to specific sets of grass species. Varieties that infect cultivated oat (Avena sativa) also infect wild species of Avena as well as some other related wild grasses. Among cultivated cereal crops, rye (Secale cereale) and barley (Hordeum vulgare) may be slightly infected by crown rust, but the forms that infect rye or barley will not infect oat. Ryegrass (Lolium spp.) and fescue (Festuca spp.) are common lawn and pasture grasses that may be severely infected by crown rust. Recently a new form of crown rust was found attacking smooth brome grass (Bromus inermis), an important pasture grass, in North and South Dakota, Minnesota, and Wisconsin.

Oat crown rust resistance genes 

SELECTED REFERENCES:

Simons, M. D., 1970. Crown Rust of Oats and Grasses. APS Press, St. Paul, MN. 47 pp.

Simons, M. D., 1985. Crown rust. Pages 132-172 in: The Cereal Rusts Vol II: Diseases, distribution, epidemiology and control. A. P. Roelfs and W. R. Bushnell eds., Academic Press, Orlando, FL.

J. Chong, J., Leonard, K. J. and J. J. Salmeron 2000. A North American System of Nomenclature for Puccinia coronta f. sp. avenae. Plant Disease 84:580-585.

Contact at Cereal Disease Laboratory: Dr. Marty Carson