Yellow rust belongs to the stand fungi (Basidiomycota) and is an obligate parasite. This means that it requires surviving host plants throughout the year in order to survive. It has an incomplete development cycle because it does not produce all spore forms. When an infestation occurs, so-called running hyphae are formed inside the leaf along the leaf veins for rapid dispersal and numerous spore deposits are formed, which appear as rust spore pustules arranged in stripes. This is why yellow rust has also been given the name stripe rust.
At first, bright yellow rust pustules can be seen scattered irregularly on the leaves, which develop into numerous strip-shaped pustules between the leaf veins as the disease progresses. The glumes, awns and culms are also affected, and rarely the leaf sheaths. In very heavily infested and highly susceptible varieties, the rust pustules arranged in stripes merge together and cover larger areas on the leaves. This eventually leads to emergency ripening of the plants. In the early stages, yellow rust occurs in a nest-like manner in a stand, from where it then spreads. Before the plants mature, brown-black spore deposits are formed on the undersides of the leaves. These spore deposits remain covered by the epidermis for a very long time. Some varieties also react to infestation with atypical chlorotic brightening.
Propagation and transmission
Yellow rust survives the winter on volunteer and winter cereals by means of spores (uredospores) and/or as mycelium. In frost the spores die, in severe winters also the fungal mycelium.
A special feature of yellow rust is that the uredospores are embedded in thin mucilage substances in their spore stores. In spring, therefore, wind and rain increasingly transport spore packages rather than individual spores. If a spore gets onto a host plant, liquid water must be present for it to germinate. Infections can occur as early as 0 °C. The fungal spores germinate in a film of water in darkness and penetrate their host plants through stomata. Even relatively few spores are sufficient to cause severe infections. The risk of an epidemic is greatest at 10 to 15 °C, high humidity and high light intensity. However, a yellow rust epidemic can also continue to develop during very warm to hot weather periods. However, for this to happen, dew must form at night and temperatures must fall below 15 °C.
The best conditions for the risk of a yellow rust epidemic are: Failure cereals become infected in the fall, early sowing of winter cereals, mild winters or a protective snow cover, damp-cool weather in the spring, cultivation of susceptible varieties, and a high nitrogen supply to the soil.
Prevention and control
- Preventive careful removal of volunteer cereals.
- Avoid cultivation of winter crops too early
- As far as possible, spatial separation of fall and spring sowings of the same cereal variety
- Cultivation of less susceptible varieties
- Possibly also use of plant protection products (see list of plant protection products approved in Austria)
Yellow rust epidemics occur when a new breed appears in the already complex breed mixture, either through mutation or sexual recombination. In Austria, this was the case from 1998 to 2001 and has also been observed since 2013. For this reason, varietal resistance also does not remain stable in some cases. Therefore, regular monitoring of the stocks is recommended even for varieties described as more resistant. When new races appear, they are included in the race mixture for the tests of specific variety resistance in the following year.
Last updated: 22.11.2021