Grains Research and Development

Date: 01.09.2003

Managing disease - resistance versus fungicides

Have you seen these rusts?: Left:Stem rust pustules are quite large, oval to elongated in shape, and are full of reddish-brown powdery spores. They can be found on stems, leaves (both front and back), leaf sheaths or heads. Middle: Leaf rust pustules are smaller than stem rust and rarely appear on the back of the leaf. They are circular to oval in shape and produce a mass of light-brown spores. Right: Stripe rust pustules are yellow to orange in colour, full of powdery spores and in adult plants form a stripe along the leaf veins. It is usually first detected as yellow patches in paddocks.

FOLIAR FUNGICIDE applications on cereal crops have become far more common in Australia. This has been the result of falling costs of fungicides with the appearance of many generic products, and the clear benefits of controlling diseases in susceptible varieties. On the face of it, this may seem to be a sensible economic decision. After all, serious disease outbreaks do not occur each year and, in many situations, varieties that don't have adequate resistance have yield or other advantages, which make them more attractive than more resistant varieties.

This reasoning has a serious flaw in it however, which, when multiplied across many farms around the country, brings significantly greater risks to cereal growing and threatens the sustainability of profitable wheat production in a relatively low-input, low-yielding environment.

It is unlikely that all growers will apply adequate fungicide at the first sign of rust and keep the infection sufficiently under control so that the disease will not spread to neighbouring properties. Equally it is unrealistic to expect that all growers will sufficiently monitor or destroy volunteers growing between seasons to ensure that those volunteers will not harbour rust infections which may be responsible for starting new epidemics.

Control of rust diseases is, of necessity, a community exercise. A disease, which recognises no property boundaries and, initially unseen, can spread so very rapidly, requires a community or industry strategy to manage it. This means that all growers need to participate in minimising the level of rust both within crops during the season and in volunteers between seasons.

Arguably the most sustainable way of doing this is to avoid growing rust-susceptible varieties. These varieties also become rust-resistant volunteers and thus provide a double advantage in controlling rust diseases.

In recognition of the great importance of this strategy, wheat-breeding programs around the country have agreed to abide by a set of Minimum Disease Resistance Standards (MDRS) for the release of new varieties. These standards were drawn up by wheat breeders and pathologists working together with the Australian Cereal Rust Control Program and supported by the GRDC and the Grains Council of Australia. These standards need to be supported by growers, and varieties that are susceptible must be discarded as soon as possible.

One of the characteristics of rusts is their ability to mutate into new strains, which can infect previously-resistant varieties. Many years of work developing new varieties, from finding new genes to introducing them into new varieties, can be lost. The supply of new genes is not limitless, so unless GM technologies can provide a permanent solution at some time in the future, we will need to work with a limited resource. This resource is best maintained if we can reduce the rate at which new mutant strains develop. And how is this achieved? By ensuring that the number of rust spores that can mutate are kept to an absolute minimum. This is done by growing resistant varieties. It will be much easier and more effective if everyone plays a part.

Photographs, information and control options on these and other cereal diseases are detailed in Cereal Leaf and Stem Diseases, edited by Dr Hugh Wallwork and published by the GRDC. This book and Cereal Root and Crown Diseases are available through Ground Cover Direct, free call 1800 110 044; fax 1800 009 988; email ground-cover-direct@canprint.com.au or go to www.grdc.com.au.