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Causal organism - Puccinia striiformis
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Puccinia striiformis requires living green plant material in order to survive. The fungus survives the winter as dormant mycelium or active sporulating lesions on volunteers or early autumn-sown crops. Set-aside provides an excellent source of yellow rust overwintering inoculum . Yellow rust within plant tissue can survive very low temperatures so once infected the fungus will usually survive the coldest of UK winters. Only if the wheat is killed by frost will the yellow rust fungus within the plant tissues die. In the spring, particularly in cool moist weather, the fungus starts to grow and produces active sporulating lesions. Temperatures of 10-15°C and a relative humidity of 100% are optimal for spore germination, penetration and production of new, wind-dispersed spores. The fungus is inhibited by temperatures over 20°C. The complete cycle from infection to the production of new spores can take as little as 7 days during ideal conditions. The disease cycle may therefore be repeated many times in one season. During late summer, the dark teliospores may be produced. These can germinate to produce yet another spore type, the basidiospore , but no alternate host has been found and the teliospores seem to have no function in the disease cycle in the UK.
Importance
The disease is very sporadic in occurrence in the UK. Severe epidemics are usually associated with very susceptible varieties, mild winters and cool moist summers. The disease occurs most frequently in coastal areas which may have cool summer weather accompanied by regular mists. Yield losses of 40-50% have often been recorded in susceptible varieties grown in high-risk areas. However, in the UK yield losses are, on average, about 2%.
