- 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.
- Stripe rust forms long yellow/orange stripes of small pustules on the leaf. The pustules, which run parallel to the long axis of the leaf, consist of masses of spores. The stripes can turn black as the disease progresses
and teliospores develop. The teliospores are nonfunctional in Australia.
- The infection of leaves stresses the plant and reduces the ability to fill grain.
- Stripe rust requires temperatures of less than 18°C (optimum 6-12°C) with a minimum of three hours of leafwetness (for example, dew) for new infections to occur.
- Of the three wheat rusts, stripe rust is best suited to cooler temperatures that occur during late autumn and early spring.
- Stripe rust is potentially less damaging than stem rust, but suitable conditions for stripe rust infection occur regularly.
- Once an infection is established the fungus can survive short periods of temperatures as high as 40°C.
- Stem rust is characterised by reddish-brown, powdery, and longer pustules.
- The pustules have a ‘torn margin’ that can occur on both sides of the leaves, on the stems and the glumes. They tend to run parallel to the long axis of the leaf or stem.
- Stem rust spores are darker in colour than leaf rust spores.
- Stem rust prefers warm conditions and will often not be evident in a crop until late spring and early summer.
- Outbreaks are more likely following above average growing season rainfall.
- Stem rust is a significant problem in seasons following wet summers, particularly, if this is followed by a mild winter and a warm wet spring.
- While fungicides may work very well on leaf and stripe rust, they are less effective on stem rust especially in thick canopies where fungicides may not reach all stems.
- The leaf rust fungus forms small circular, orange-brown pustules with spores that will rub off on your finger. These are usually on the top of the leaf, and rarely on the under-side.
- The most notable signs of an infection are the reddish-orange spore masses of the fungus breaking through the leaf surface.
- Leaf rust is a significant problem for susceptible cultivars during seasons following wet summers, particularly if this is followed by a mild winter and a warm wet spring.