Dr Grant Hollaway in a paddock of self-sown barley heavily infected with stem rust and leaf rust, which provides a ‘green bridge’ for these rust pathogens to survive between cropping cycles.
PHOTO: Victorian DEDJTR
Many growers would be familiar with the term ‘green bridge’ and its significance in helping pathogens such as rust survive between cropping cycles. The principle of green-bridge control applies to all crops that are infected by rust, not just cereals.
Rust pathogens can only grow on living plant tissue and will only survive on dead plants for a matter of weeks.
The same is true for mildew pathogens. The lack of living cereal plants during the non-cropping summer period ordinarily causes their populations to crash.
Following a hot and dry summer, rust pathogens need to build up again to reach epidemic levels. Where wet conditions occur during summer, volunteer (self-sown) cereals may become established, forming a green bridge. Destroying the green bridge well in advance of the following cropping cycle – at least four weeks prior to sowing – is an important management strategy in reducing the risk of rust epidemics.
In some regions it is not just self-sown cereals that can form a green bridge. In east Africa, wheat can be planted on any day of the year. As a result, the region comprises a tapestry of overlapping crops at all growth stages, providing an ideal environment for rust to flourish. It is not surprising that stem and stripe rust continue to challenge wheat production there.
In Australia, extending the growing period for cereal crops beyond the traditional winter/spring period can assist the survival of rust pathogens. The introduction of early-sown dual-purpose winter wheats during the early 1990s caused some initial concerns in this regard, but fortunately the winter wheat breeders worked closely with the Australian Cereal Rust Control Program and most of the cultivars released thereafter had good rust resistance.
Ensuring resistance in these wheats and other longer-season cereals is a vital component of integrated rust control. Avoiding sowing cereal crops during summer is just as important.
While ‘green-bridging’ of rust pathogens is clearly undesirable, it is important to note that it does not always translate into a rust epidemic in the following cropping cycle. For example, extremely wet conditions in South Australia during the summer of 2010-11 led to large areas of volunteer cereals and a rapid increase in rust inoculum.
A stem rust epidemic in 2011 did not eventuate, mainly due to careful management by growers (destruction of volunteers, selection of resistant cultivars, seed-applied fungicides) followed by a dry autumn.
Rust-resistant cultivars = rust-resistant green bridge
Ensuring low inoculum levels at the beginning of each season by destroying volunteer cereals and avoiding overlapping crops are important management strategies in minimising the probability of a rust epidemic in the following cropping phase.
Growers should also consider the use of fungicide applied to seed or fertiliser in districts known to have rust surviving on volunteers. Variety selection to reduce areas sown to wheats considered vulnerable in the early establishment phase is also valuable.
While there are obvious benefits from growing rust-resistant cultivars in terms of rust control, these cultivars will also give growers the added bonus of a rust-resistant green bridge.
Professor Robert Park,
02 9351 8806,
Dr Will Cuddy,
02 9351 8871,
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