A bit about rust…
- Rusts are caused by fungi. Rust spores are spread readily by the wind over large areas in a short time.
- There are three rust diseases of wheat in Australia – stripe, stem and leaf. Cultivars resistant to one may be susceptible to another.
- Rust becomes a problem in areas where susceptible varieties are grown. These varieties also enable inoculum levels to build up on volunteer plants during summer and autumn and give the rust an early opportunity to re-establish in commercial crops. Rust epidemics are more common following wet summers and in wet growing seasons.
- Rusts can mutate (alter or change form) to overcome resistance genes it is essential to use a current disease guide to check the resistance rating of your cultivars.
- The three rusts can be distinguished on the basis of spore colour (see below). These are the urediospores which move readily on the wind and are responsible for disease transmission between plants, within fields and across large regions. In high disease pressure and as crops begin to turn, darker coloured spores become evident. These are teliospores which, under Australian conditions, have no role in disease spread.
- All rusts can cause significant loss to wheat yields, given appropriate environmental conditions and susceptible varieties.
- The earlier in a season a rust epidemic starts, the greater the potential yield loss.
- Crops must be monitored to detect rust early. Timing is critical for the effective control of rust diseases with fungicides. Varieties known to be Susceptible (S),
- Moderately Susceptible (MS) and Moderately Resistant (MR)-MS will need to be monitored regularly from emergence if fungicide protection has not been applied at sowing. In these cases, fungicide application should be considered at the first appearance of symptoms. Monitoring should continue as protection periods following
foliar fungicides will vary according to chemical product and growth rate of the crop.
- Varieties S and MS will need further protection where disease begins early.
- A note of caution when applying fungicides. The withholding period should be carefully observed when applying chemical protection to crops intended for grazing. Similar precautions need to be taken when considering fungicide applications to crops in the flowering to grain fill stages late in the season.