Puccinia graminis (stem rust of cereals)

Uredinial StageThe uredinia may occur on leaves, stems, leaf sheaths, spikes, glumes, awns and occasionally on grains of their grassy hosts; stems and leaf sheaths are the main tissues affected. On stems, the uredinia are elongated and reddish-brown; loose epidermal tissue is conspicuous at the margins of the uredinia, giving a roughened feel to the stem surface. The uredinia coalesce to cover large areas of the host tissue in heavy infection. Since the urediniospores are dehiscent, they are released as powdery masses from the uredinia.Telial StageThe telial stage occurs in the same tissue as the uredinial stage, but becomes shiny-black. The teliospores are sessile, and the telial tissue is, therefore, firmer than the uredinial tissue; no spores are released.Pycnial StageThe pycnial stage occurs on the young leaves of the alternate host, mainly Berberis vulgaris. Pycnial infections initially appear as light, chlorotic areas on the adaxial leaf surface, then become light orange-brown lesions, consisting of individual small cone-shaped eruptions (the pycnia), often occurring in clusters.Aecial StageThe aecia develop on the abaxial surfaces of the leaves of the alternate host. When mature, they appear as bright-orange, closely-packed, raised clusters of individual aecia. The aecia are cylindrical in shape and flare out at their apices, appearing as a grouping of rings within the aecial cluster.

Historically, stem rust has caused major devastation to wheat crops in most of the wheat-growing areas of the world (Roelfs et al., 1992). In ancient Rome, and probably a wide area around the city, damage to crops, mainly caused by stem rust, was so severe that a number of ancient authors referred to the problem. Rites and processions were organized to appease the numen (spirit), Robigo (Zadoks, 1985).

Globally, stem rust was the most important disease of wheat until the late 1950s, when the use of resistant cultivars became more widespread (Saari and Prescott, 1985). Epidemics of stem rust can be spectacular, reducing an apparently healthy crop to a tangle of blackened stems and shrivelled grain within a few weeks. Widespread epidemics occur relatively infrequently, but disease within a region or in individual fields is frequently severe, often completely destroying the crop. Widespread epidemics have been documented for Australia (Luig, 1985), Europe (Zadoks and Bouwman, 1985) and North America (Roelfs, 1985a).

Epidemics also occur regularly in Africa, China and Asia (Saari and Prescott, 1985). Accurate assessment of losses is difficult and, as a result, losses are often poorly documented. Losses in North Dakota, during the severe epidemics of 1935 and 1954, were estimated at US $356 million and US $260 million, respectively, based on wheat prices in late 1995 (Roelfs, 1978).

The annual value of stem rust resistance for eastern Saskatchewan and Manitoba, Canada, was estimated at $217 million (in 1977 Canadian dollars), based on the annual acreage yield loss (25%) expected if susceptible cultivars were grown in this area (Green and Campbell, 1979). The value relates to about $307 million at late 1995 US $ prices for wheat.

Wheat losses to stem rust in Chile, monitored over a 30-year period (1960-90), averaged about 0.25% (Hacke, 1992). Losses in southern Europe, mainly in Portugal, Spain, France, Italy, southern Germany, Romania and Bulgaria can average 10%, but losses as high as 60-80% have been reported (Santiago, 1961). Stem rust on wheat is, at present, largely under control worldwide. Even with the widespread use of resistant cultivars, P. graminis remains ubiquitous and heavy localized losses are possible.

The major loss due to stem rust currently is the costs incurred to find, incorporate and evaluate resistance in new cultivars. The need is constantly demonstrated by the appearance of new pathogen virulences. In recent years Enkoy rusted in Ethiopia, Sr24 in South Africa (LeRoux, 1985), Sr31 in Uganda (Pretorius, 2000) and the Rpg-1 barley cultivars in North America (Roelfs et al., 1997).

The economic impact of stem rust has been reduced mainly through the breeding of cultivars with resistance to wheat stem rust. However, resistance in cultivars must continue to be improved to keep up with pathogen evolution (McIntosh and Brown, 1997). In most years, 10-20% of the cost of cultivar development is related to stem rust resistance. A novel stem race in Africa now is of great concern. Records of yield losses caused by cereal rust diseases in the USA have been maintained by the USDA since the early twentieth century (Roelfs, 1978) (see the USDA Cereal Disease website for information from recent years). Losses in dollars are difficult to estimate as rust reduces the quantity of grain, which in turn increases the price. Stem rust also reduces the quality of grain, resulting in a lower price. The USDA data only estimates the reduction of quantity.