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Infections occur mostly on leaves, often on petioles, and less frequently on stems. On susceptible species/cultivars, infections result in small yellowish-brown or greyish-brown spots or lesions (TAN-type) which are delimited by vascular bundles. Several pustules of urediniospores are formed on both adaxial and abaxial surfaces of the lesion, but more frequently on the abaxial surface. The lesions coalesce, become dark brown and are covered by buff or pale-brown spore masses as sporulation progresses. Later in the season, the lesions become dark reddish-brown and crust-like; these are subepidemal telial clusters. When resistant species/cultivars are infected, minute, reddish-brown spots (RB-type) appear, on which only a few uredinial pustules are formed. Sporulation on RB-type lesions is much less than on TAN-type lesions.
Successful rust disease management can be achieved by selecting durable resistant/tolerant cultivars with desirable agronomic properties, employing necessary good husbandry, and applying appropriate fungicides at the correct stages of soyabean growth and disease development. No single measure can provide successful disease management. In each of the soyabean-growing areas, a specific management programme must be developed according to the economic factors, the type of soyabeans to be grown (grain vs. vegetable), time when soyabeans are to be grown, climatic conditions, soil types, and the number and frequency of prevalent rust races.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Estimated yield losses due to the rust infection were 15-40% in southern Japan and up to 70-80% in individual fields and 20-30% in the total crop in Taiwan (Bromfield, 1976). In field trials in Taiwan, yield losses were 18-57% (Chen, 1989).
In a plot trial in Thailand, yield losses were 100% in the most susceptible cultivars and 0-38% in the most tolerant cultivars (Sangawongse, 1973). In Thailand, seed-yield losses in the wet season were 100% for the most susceptible cultivars, while the losses were reduced to 10-15% in the dry season (Sangawpmgse et al., 1977).
In a field trial in Korea, yield losses were 68.7% in a susceptible cultivar and 22.3% in a tolerant cultivar (Shin and Tschanz, 1986).
In a field trial conducted in Austria, seed-yield losses were 60-70% in the most severely infected plots without chemical control (Ogle et al., 1979).
Kuchler et al. (1984) analysed the potential economic consequences if a virulent race of the soyabean rust fungus were to become established in the USA using an econometric-simulation model under two alternative environmental and grower response assumptions. Total losses to consumers and other sectors of the USA economy are forecast to exceed $7.2 billion/year even with a conservative estimate of potential damage, while profits to some soyabean farmers and producers of other feed grains would rise.