Common Disease Name: Stem rust or Black rust.

Pathogen: Puccinia graminis f. sp. tritici.

Symptoms:

• Oval lesions of uredinia (mass of spores) generally appear on leaves, sheaths, stem, glumes, spikes and awns.
• Stems and leaf sheaths are the main tissues affected.
• On stems, the uredinia are reddish-brown and can be seen to rupture the host epidermis. On the leaves; uredinia generally penetrate through.
• Sometimes black lesions are also observed these may indicate the presence of telia (lesions with resting spores).
• Uredinia fuse to cover large areas of the host tissue during heavy infection, releasing a powdery mass of spores.
• From the seedling stage to the five- or six-leaf stage, stem rust infections are most obvious on the leaves.

Confirmation:

• In the field the main time for inspection is when the crop begins to head until near maturity. However, in some highlands where wheat crops or volunteer plants can be found throughout the year early inspections are necessary. Stem rust lesions are easily detected by sweeping aside the canopy and looking into the crop with strong sunlight coming from behind. The red-brown uredinia are clearly recognizable against the normally blue-green colour of healthy stem tissue.
• Infected areas are detected by gently feeling the plant tissue between the thumb and forefinger they are rough textured in comparison to uninfected areas.
• Uredinia of stem rust are larger than those of stripe and leaf rust.
• If there is a doubt scraped lesion(s) can be observed under a microscope. Place spores in a droplet of water on a glass microscope slide under low magnification (100 x). The urediniospores of P. graminis appear as oblong, spiny spores, 20 x 30 µm in diameter.

Why and where it occurs:

• Presence of mycelium or uredinia on volunteer wheat or barley plants, other species related to wheat,, wild grasses or berberry plants sustains survival of the inoculum.
• The pathogen (through urediniospores) is effectively dispersed over great geographical distances by air currents and climatic conditions. Rain or dew is necessary for effective deposition and germination of uredinospores on susceptible host plants.
• Favorable conditions for infection are hot days (25-30°C) and mild nights (15-20°C) with adequate moisture for night time dews.
• P. graminis occurs worldwide wherever wheat is grown.

Causal agent or factors:

• P. graminis is a macrocyclic, heteroecious rust (i.e. spends different stages of its life cycle on different, hosts).
• Genetic studies with stripe rusts have shown that race-specific resistance genes in the host plant (Yr genes) follow the “Gene-for-Gene” concept, i.e. corresponding virulence genes in the fungus is selected soon after their deployment allowing the fungus to overcome the gene conferring the host plant’s resistance.
• Pathogenic races in rust fungi are determined by which combinations of virulence genes are present in the fungus.
• It is reported that the barberry plant is an important alternate host, and it is here that sexual recombination of the pathogen occurs, resulting in new races. However, in areas where barberry plant is absent, stem rust fungus can still produce new races through mutations and rarely through asexual recombination.

Host range:

• Major hosts include Triticum aestivum (bread wheat), Triticum turgidum (durum wheat), and Hordeum vulgare (barley).
• Minor hosts include wild grasses.
• The alternate host Berberis vulgaris (European barberry) was historically an important source of inoculum in North America and Europe, but are now rare since the implementation of barberry eradication laws. However, some regions where barberry is common, particularly in East Europe and West Asia may facilitate continued cyclic rust infection and evolution of new combinations of virulences.

Life cycle: to be added

Mechanism of damage:

• Spores germinate when they come into contact with free water (dew or rain). The germinated fungus infects plant tissues through the stomata. Favorable conditions for spore germination and penetration include low light intensity and field temperatures of 15-30 °C. The latent period varies from 10 to 15 days.
• The infection damages the conducting tissue which decreases the amount of nutrients transported to the grain, causing it to shrivel. During severe infection loss of spikes occurs due to straw breakage.

When damage is important:

• Yield loss due to fungal damage is greatest when the disease becomes severe before the grain is completely formed. In areas favorable for disease development, susceptible cultivars should not be grown.

Economic importance:
Stem rust has been largely under control worldwide since the deployment of resistant varieties. Widespread epidemics are infrequent, but disease within a region or in individual fields can frequently be severe, resulting in the destruction of between 50 to 70% of the crop. More recently, a virulent race Ug99 has evolved in East-Africa that threatens current sources of resistance.

Management principles:

• Quarantine of the pathogen is impossible because of the airborne nature of rust fungal spore dispersal.
• Removal of susceptible barberry bushes eliminates local sources of the fungus and importantly removes the sexual stage of the pathogen’s life cycle, hence reducing the formation of new races with new combinations of virulence genes.
• The use of resistant varieties is the most effective, least expensive and environmentally friendly means of controlling the disease. Multigenic resistance (combination of specific or minor resistance genes) may be a more long-term form of controlling the disease.
• Because new rust races can and do arise, global monitoring of new rust races is not only important for breeding of rust resistance but also in predicting epidemics.
• Where possible, a reduced exposure time of wheat to the pathogen by planting early or through the use of early-maturing cultivars can reduce damage severity in some areas where inoculum is transported from other areas.
• Effective fungicides against rusts are available; however they have not been widely used mainly because cost-benefit analysis has shown it to be non-profitable to the farmer.

References:

See the Global Rust Initiative website: http://www.globalrust.org/

USDA Cereal Disease Laboratory: http://www.ars.usda.gov

Prescott, J.M., P.A. Burnett, E.E. Saari, J. Ransom, J. Bowman, W. de Milliano, R.P. Singh and G. Bekele. 1986. Wheat diseases and pests. A guide for field identification. Mexico, D.F.: CIMMYT.

Singh, R.P., J. Huerta-Espino and A.P. Roelfs. 2002. The wheat rusts. In B.C. Curtis, S. Rajaram and H. Gómez Macpherson (eds.), Bread improvement and production. FAO Plant Production and Protection Series. Rome: Food and Agriculture Organisation of the United Nations.

Wiese, M.V. 1987. Rusts. In M.V. Wiese (ed.), Compendium of wheat diseases. St. Paul, MN: The American Phytopathological Society (APS Press). Pp. 37-42.

Zillinsky, F. 1983. Common diseases of small grain cereals. Mexico, D.F.: CIMMYT.

Contributors: H. K. Buhariwalla, R. Singh and P. Kosina