Stem rust
Stem rust is caused by the fungus Puccinia graminis var. avenae. The disease appears as elongated reddish-brown pustules mainly on stems (Figure 5) but also on leaves and heads. The powdery spore masses in the pustules can dislodge readily.
Figure 5 Stem rust (Puccinia graminis var. avenae) and closeup
Rusts grow and reproduce only on living plants and must continually infect new hosts. They survive over summer by infecting volunteer or wild oats and infect crops in the next season.
Seasons are at greater risk of a rust epidemic if:
- Rust was present in the previous season;
- Summer and autumn rains allow wild or volunteer oats to grow over summer, harbouring and building up the rust; and
- Spring conditions are suitably wet.
Each factor depends on locality, so it is possible to assess rust risk in your locality.
Oat stem rust will not attack wheat and wheat stem rust does not attack oats. Under favourable conditions, stem rust is very damaging and can destroy a crop (Figure 6) Epidemics are more frequent in the rust-prone, very high rainfall districts in northern and southern agricultural areas.
Figure 6 The red discolouration throughout this crop indicates a severe stem rust infection. Early infection and a susceptible variety, combined with seasonal conditions which favoured the disease, led to crop destruction
Control
Avoid highly susceptible varieties in rust-prone areas. In seasons with increased risk of rust, resistant oat varieties are the main practical control measure (see Oats: choosing a variety). Foliar fungicide registrations exist for control of this disease, refer to Registered foliar fungicides for cereals in WA. Grazing, spraying or tilling may help to control volunteer and wild oats over summer but the effect could be limited because of the abundance of wild oats.