Pathogenic Effects of Stem Rust

Stem rust damages wheat in several ways, and a severe infection a few weeks before harvest can destroy an entire crop.Loss of nutrients. Haustoria absorb sugars and amino acids from host cells. Sucrose that would be transported in the phloem to the developing grain is diverted to the fungus, so grains are small and shrivelled and yield falls.Water loss. Each pustule bursts through the epidermis and cuticle. The broken surface loses water by evaporation, so transpiration increases and the plant can wilt and dry out, especially in hot weather.Reduced photosynthesis. Pustules and dead tissue cover the green stems and leaves, reducing the area available to absorb light. Less photosynthesis means even less sucrose for the grain.Weakened stems. Damage to stem tissues weakens the stems, which may break or fall over (lodging), making the grain hard to harvest.The overall result can be yield losses of 50 to 100% in a badly infected field. Because wheat is a staple food for billions of people, stem rust epidemics threaten food security.Worked example: a field normally yields 8.0 tonnes of wheat per hectare. After a stem rust epidemic it yields 2.4 tonnes per hectare.percentage loss = (8.0 − 2.4) ÷ 8.0 × 100 = 70%