Common Disease Name: Stripe rust or yellow rust.

Pathogen: P. striiformis West. f. sp. tritici Eriks. & E. Henn.

Symptoms:

• The fungus forms clusters of tiny yellow- to orange-colored pustules, called uredinia. On young plants the pustules appear as blotches covering large areas of affected leaves. In older plants pustules appear in linear rows on the leaf forming narrow stripes along the length of the wheat leaf, which eventually merge and whole leaf can be covered with uredinia in susceptible cultivars. Resistant cultivars can show from small necrotic stripes to blotches carrying uredinia that have little or moderate sporulation.
• Affects leaves, spikes and occasionally leaf sheaths.
• During infection of the wheat spikes pustules appear on the inner surface of the glumes and lemmas.
• Individual pustules are oval and about 0.5-1 mm in diameter.
• Stunting of plants is common with severe early infections.
• Narrow black stripes form on the leaves later on in the season during reproductive development and are called telia.

Confirmation:

• The distinguishing characteristics of stripe rust are colour and arrangement of the uredinia. Stripe rust spores are yellow to orange in colour whereas stem and leaf rust spores are brick-red and orange-brown, respectively. Additionally, uredinia of stripe rust are arranged in stripes whereas stem rust and leaf rust form random patterns.
• Microscopic observations of urediniospores appear spherical, 20-30 µm in diameter with thick, spiny walls containing 6-12 scattered germ pores.
• Teliospores are two-celled, ellipsoid, orange to brown, and 12-20 x 36-68 µm.

Why and where it occurs:

• Disease usually is more important when wheat grown in cooler climates (2 to 20°C). Stripe rust is mainly an important disease of wheat either grown in highlands, mountainous regions or during the winter or early spring in cool lowland locations.Urediniospores of the pathogen germinate, penetrate, infect and produce more wind-dispersed spores on susceptible wheat plants that are capable of causing epidemics when conditions are favourable, i.e. cooler temperatures with dew or free moisture on the plants.
• Under ideal conditions the pathogen life cycle may take as little as 7 days to complete.
• During late summer, telia may form which give rise to teliospores.
• The pathogen survives the period between wheat harvest and sowing on volunteers (self-sown) of wheat, barley or wild grasses which provide a ‘green bridge’ for stripe rust to survive across seasons.
• The fungus is inhibited by high temperatures. Urediniospores of stripe rust are susceptible to ultraviolet light hence viable spores are generally not thought to travel on air-currents over large distances.

Causal agent or factors:

• P. striiformis is an obligate parasite i.e. it requires living green plant material in order to survive.
• 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 stripe rust fungi are determined by the combination of virulence genes present together.. It is thought that virulence probably exists for most known yellow rust resistance (Yr) genes. However virulence for certain combinations of Yr genes may be absent on a regional basis.
• Presence of adult-plant resistance to stripe rust based on minor genes is very common in wheat. Such resistance is additive and combining 4-5 minor genes together results in plants that are highly resistant. Such resistance is believed to be “durable” or long-lasting and is highly recommended for use in wheat improvement.

Host range:
Major hosts include: Triticum aestivum (bread wheat), Hordeum vulgare (barley), Secale cereale (rye), and various wild grasses.

Life cycle:

Mechanism of damage:

• In wheat the urediniospores germinate in high humidity with temperatures of less than 15°C, and the germ tubes enter the leaves or other parts of the plant via the stomata forming sub-stomatal vesicles and haustoria that penetrate and colonize the mesophyll cells.
• Once inside the leaf, the mycelium spreads along the leaf. In mature leaves it spreads longitudinally between the veins of the leaf.
• Stripe rust may debilitate and kill young plants. In older plants, damage is caused by disruption of the epidermis by the fungus; thus reducing the water retention capacity of the leaves. Additionally, stripe rust affects yield by reducing the sugar supply to the developing seed, resulting in smaller poor quality grain.

When damage is important:

Stripe rust reduces grain quality and quantity. The percentage losses are dependent on; susceptibility of the cultivar, earliness of the initial infection, rate of disease development, and duration of disease. Losses can be minimized by the widespread cultivation of resistant varieties. In areas favorable for disease development, susceptible cultivars should not be grown.

Economic importance:
In most wheat-producing areas, yield losses caused by stripe rust range from 10% to 70% if resistant varieties are not grown.

Management principles:

• Prior removal of volunteer wheat plants which support disease growth between wheat cropping seasons will reduce the amount of rust inoculum in the new season crop.
• The use of resistant varieties is the most effective, least expensive and environmentally friendly means of controlling the disease. Multigenic resistance (combination of major or minor resistance genes) may be a more long-term form of controlling the disease. New stripe rust races can and do arise, thus monitoring of new races is not only important for breeding of stripe rust resistance but also in predicting epidemics.
• Avoid planting of very susceptible and susceptible varieties and select more resistant varieties. Widespread cultivation of resistant varieties minimizes the level of stripe rust inoculum in the environment and reduces the occurrence of new races.
• If susceptible varieties are grown then seeds should be treated with fungicides which suppress early infections by stripe rusts.
• Crop monitoring during the growing season is important as early application of foliar fungicide is effective in the control of stripe rust. The number of applications and timing of fungicide treatments required is dependent on the progress of the disease and the level of resistance within the variety.

References:

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

Chen, X.M. 2005. Epidemiology and control of stripe rust [Puccinia striiformis f. sp. Tritici] on wheat. Canadian Journal of Plant Pathology 27:314–37.

McMullen, M. and J. Rasmussen. 2002. Wheat leaf rust. PP589. North Dakota State University Extension Service. Available online at http://www.ag.ndsu.edu/pubs/plantsci/smgrains/pp589w.htm

Murray, G., C. Wellings, S. Simpfendorfer and C. Cole. 2005. Stripe rust: Understanding the disease in wheat. Fact sheet NSW (Australia) Department of Primary Industries. Available online at http://www.agri.nsw.gov.au.

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. 41.

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

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