Disease Name: Leaf rust or brown rust.

Pathogen: Puccinia triticina and Puccinia tritici-duri

Symptoms:

• The fungus forms small reddish-orange pustules (uredinia), which rupture the upper surface of the leaf blade as the spores mature.
• Affects primarily the leaves and leaf sheaths.
• As the leaves age, the pustules begin to produce dark black spores. These glossy, black pustules (telia) are mostly seen on the lower leaf surface and leaf sheaths covered by epidermal tissue. Teliospores do not infect wheat. Telia may not develop when plants become infected near maturity.
• Individual pustules are circular to oval with a diameter of 1.5 mm.
• On resistant wheat cultivars pustules may appear as small, necrotic spots which does not develop spores or have tiny pustules with little sporulation. On moderately resistant wheat cultivars, the uredinium is surrounded by a halo of pale green or yellow host plant tissue, limiting the extent of infection.

Confirmation:

• When leaf rust severity is high, walking through a field of infected wheat may result in orange dust (spores) depositing on hands and clothes.
• Problems may arise in distinguishing between leaf rust and stem rust: the distinguishing characteristics are the colour, size and area of infection by uredinia.
• Leaf rust pustules are smaller than those of stem rust. Leaf rust spores are orange-brown whereas stem rust spores are brick-red in colour. Additionally, stem rust pustules are surrounded by torn epidermal tissue whereas, leaf rust pustules are not. Leaf rust pustules are usually concentrated on the upper surface of the leaf but stem rust pustules grows on both sides of the leaf.
• Leaf rust pustules occur randomly across the leaf, whereas stripe rust produces yellow stripes of pustules.
• Stem rust usually only occurs late in the season because it requires warm temperatures.
• Microscopic observations of urediniospores appear subgloboid, 15-30 µm in diameter, with a spiny wall surrounding three to eight germ pores.

Why and where it occurs:

• Occurs worldwide wherever wheat is grown, excellent conditions for disease development are cool nights, high atmospheric moisture combined with warm days (15 to 25°C).
• Urediniospores of the pathogen germinate and infect susceptible or moderately susceptible wheat plants and produce more spores under ideal conditions. During ideal conditions the pathogen life cycle may take 7-10 days to complete. A minimum of 7-9 cycles would be required to cause an epidemic.
• The leaf rust pathogen survives the period between wheat harvest and sowing by infecting volunteers (self-sown) of wheat or wild grasses which provide a ‘green bridge’ for survival of the pathogen across seasons.
• Leaf rust pathogen which has survived on volunteers, initiate disease from the lower leaves whereas the disease initiated from upper leaves are due to wind dispersed spores.

Causal agent or factors:

• P. triticina or P. tritici-duri are obligate parasites i.e. they require 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 of leaf rust fungi are determined by the combination of virulence genes.. It is thought that virulence probably exists for most known race-specific leaf rust resistance (Lr) genes.
• Presence of adult-plant resistance to leaf rust based on minor genes is common in wheat. Such resistance is additive and combining 3-5 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), Triticum turgidum (durum wheat), triticale, and wild grasses.
• Alternative hosts are not important in the leaf rust life cycle in some global regions. This slows the development of new races through sexual recombination of the pathogen. However, in certain global regions meadow rue (Thalictrum spp.) or Anchusa italica are the alternate host for wheat leaf rust.

Life cycle:

Mechanism of damage:

• Under ideal conditions, in wheat the urediniospores germinate and enter the leaves via the stomata and can cover most of the leaf surface.
• The effects of leaf rust are that it reduces: the number of tillers, the amount of grain per spike, grain weight, and grain protein content because the infected leaves have a reduced area for photosynthesis.
• The severity of the disease in wheat is dependent on the amount of leaf area covered by leaf rust pathogen which is scored as a percentage. Yield losses are related to percent severity, especially of the top (flag) leaf which is a major contributor of nutrients during grain filling.

When damage is important:

• The percentage losses are dependent on: susceptibility of the wheat cultivar, weather, the rate of disease development and the crop stage when disease initiated.
• Severity is greatest when the initial infection occurs before or during flowering, especially when the flag leaf becomes severely infected.
• Leaf rust is more damaging on late maturing susceptible varieties or when wheat is planted late.
• Often losses are greatest in years most favorable for wheat growth, hence, high yields and higher losses often occur together.

Economic importance:
The disease causes more damage worldwide than other wheat rusts. On average losses range from 5 to 20%, however in severe cases can reach 50%.

Management principles:

• The use of resistant varieties is the most effective, least expensive and environmentally friendly means of controlling disease. Multigenic resistance (combinations of major or minor resistance genes) is be the most long-term form of controlling the disease.
• New leaf rust races do arise, thus monitoring of new races is not only important for breeding of leaf rust resistance but also in predicting epidemics.
• Prior removal of volunteer plants which support disease growth between wheat cropping seasons will reduce the amount of rust inoculum. However, disease can still occur from wind dispersed spores from distant sources.
• Where possible damage severity can be reduce by minimizing exposure time of wheat to the pathogen by planting early or through the use of early-maturing cultivars.
• Fungicide application is only recommended if; the wheat variety is susceptible to leaf rust, the yield potential of the crop is high or the disease started early and the flag leaf is in danger of becoming infected.

References:

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

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

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