The crown rust fungus affects only oats: it does not attack other cereals. It can infect a number of grasses, particularly ryegrass but the forms on grass do not cross infect to oats.
The first symptoms of crown rust are very similar to brown rust of wheat and barley. Orange brown pustules appear scattered over the leaf surface. Leaf sheaths, and later the oat panicle, can also become infected. The disease is favoured by high temperatures (20-25oC) so epidemics
usually occur in June-July. Late in the season black pustules appear within the existing crown
The orange spores (uredospores) produced on leaves are air-borne and spread the disease long
distances to other plants and adjacent crops. Later in the season black pustules containing teliospores are produced. These remain dormant on crop debris until the spring when they germinate to produce basidiospores. The basidiospores of this fungus infect the alternate hosts, the buckthorn (Rhamnus catharticus) and alder buckthorn (Frangula alnus) on which are produced a third spore type, the aeciospores, which can then infect oats once again.
Uredospores produced in the oat crop also infect volunteer oat plants and then emerging winter
The disease is favoured by warm and humid weather and mild winters. Severe attacks have been
more common in recent years. Such epidemics can reduce yield by 10-20%.