Risk factors
While variety resistance will influence individual crop risk, the overall risk of serious rust outbreaks is influenced by several factors which can be considered each season. These include;
- the amount of stem rust present in the previous season – more stem rust in a given year means there is more chance of carry over into the next season. The PestFax map can show you where stem rust finds were reported last season.
- summer and autumn rains – summer rains permit the development of volunteer hosts (green bridge) and autumn rains permit the early build-up of rust on these volunteers. It primarily infects wheat and triticale but can infect barley and some grasses, including barley grasses and common wheat grass. For more information see the seasonal climate information page.
- spring rains and humid warm conditions – if all the other factors occur and early rust breaks out in crops, then a stem rust epidemic is more likely if the spring is suitably wet. Stem rust prefers temperatures of 15-30°C and humid conditions.
- keep up to date with stem rust finds across the wheatbelt by subscribing to PestFax.
- wind can spread spores large distances so any disease outbreaks in a season (even distant ones) pose a risk.
Yield losses
Yield loss will depend on the disease resistance of the variety and on how early the disease starts in the crop (Table 1). Quality reductions such as increased screenings and lower hectolitre weights can add to high yield loss impacts.
| Resistance rating | Definition | Potential yield loss (%) |
|---|---|---|
| Very susceptible (VS) | Early high disease build-up; can promote epidemic development | 50-90 |
| Susceptible (S) | High disease build-up | 25-50 |
| Moderately susceptible (MS) | Develops disease less quickly and so reduces loss risk | 10-35 |
| Moderately resistant to moderately susceptible (MRMS) | Some partial resistance; losses depend on disease pressure | 5-25 |
| Moderately resistant (MR) | High partial resistance; generally few losses | 5-15 |
| Resistant (R) | Highly effective resistance; no or slight losses | 0-5 |
| Highly resistant (HR) | Complete resistance | 0 |
Fungicide control may be required for varieties rated VS-MS. Varieties with a MRMS disease rating may benefit from fungicide control if early disease occurs or high spore loads are present (for example, next to another severely rusted paddock). High crop yield potentials also increase the rate of return from fungicides. Disease ratings for current wheat varieties are available from the Wheat variety guide for Western Australia.