Diseases may severely affect yield and quality in wheat.
In some cases, these diseases are controlled through the use of simple cultural practices and good farm hygiene. One of the major practices used in the control of diseases is crop rotation.
To minimise the effect of diseases:
- use partially resistant varieties
- use disease-free seed
- use fungicidal seed treatments to kill fungi carried on the seed coat or in the seed
- keep farm free from weeds which may carry over some diseases
- rotate crops.
Rusts
In Queensland, there are three rust diseases of wheat:
- stripe rust
- stem rust
- leaf rust.
They are caused by three closely related fungi all belonging to the genus Puccinia.
The ‘rusts’ are so named because the powdery mass of spores which erupt through the plant’s epidermis have the appearance of rusty metal. These spores can be spread over considerable distances by wind but may also be spread on clothing and equipment.
Wheat rusts have a number of features in common. They can only infect a limited number of specific host plants (mostly volunteer wheat, triticale and barley) and can only survive on green growing plant tissue. Plants facilitating the survival of rust fungi through the summer are known as the ‘green bridge’.
Rust diseases of wheat can be eliminated or significantly reduced by removing this green bridge. This should be done well before the new crop is sown, allowing time for any herbicide to work and for the fungus to stop producing spores.
Rust diseases occur throughout the wheat-growing northern regions, frequently causing economic damage. In Queensland, in recent times, stripe rust has been the more important of these diseases.
Wherever possible, resistant wheat varieties MR (moderately resistant = 6) and above should be sown.
Rust fungi continuously change, producing new pathotypes. These pathotypes are detected when disease is found on a previously resistant variety. Even if a resistant variety has been sown, the crop should be monitored for foliar diseases on a regular basis.
This should start no later than growth stage 32, the second node stage on the main stem and continue to at least growth stage 39, the flag leaf. This is because the flag leaf and the two leaves below it are the main factories contributing to yield and quality. It is very important that these leaves are protected from diseases.
There are a number of fungicides registered in Queensland for the control of the rusts. Early intervention reduces damage to the important ‘factory’ leaves.
Yellow spot
Yellow spot is caused by the fungus Pyrenophora tritici-repentis. It survives in wheat and occasionally triticale stubble.
In rare cases, the fungus may survive in barley stubble. Wet spores (ascospores) develop in fungal fruiting bodies on wheat stubble, spread during wet conditions and can infect growing wheat plants.
As the crop develops, masses of a second type of spore (conidia) are produced on old lesions and dead tissues. Conidia result in rapid development of the epidemic within a crop and spread of the disease to other crops and areas.
Again, wet conditions are necessary for spore production and infection. Strong winds are needed to spread the disease any great distance.
Severe yellow spot may result in short, spindly plants with reduced tillering and root development. Where conditions are favourable, plants may be fully defoliated soon after flowering.
Grain yield can be substantially reduced and losses of more than 50% may occur in extreme situations. Pink grain with reduced value is also a frequent result of severe yellow spot epidemics. Where wheat follows wheat and some stubble is left on the soil surface, losses may be around 10-15%, and up to 30% in wet seasons.
Yellow spot is likely to develop in wet years in fields where wheat residues remain on the soil surface. The impact of the disease can be reduced by:
- planting partially resistant varieties
- rotation with resistant crops such as barley, oats or chickpea
- incorporation of stubble into the soil
- grazing or burning the stubble late in the fallow period.
Incorporation or burning stubble is not recommended unless infestation levels are very high. Correct identification of the yellow spot fungus in infected stubble should be carried out before the stubble is removed. Varieties partially resistant to yellow spot offer the only long-term solution and should be considered for planting where yellow spot could be a problem.
Fungicides used against yellow spot in Australia include:
- Propiconazole
- Tebuconazole
- Azoxystrobin + Cyproconazole
- Propiconazole + Cyproconazole.
Timing for applying the chosen fungicide is crucial. The most effective time of application is at 90% flag leaf emergence with disease levels of less than 10% on the flag leaf.
The higher rate of application has been shown to provide longer protection under periods of high disease pressure.
Fungicide effectiveness is greater on susceptible varieties and is reduced with increasing levels of resistance.
Information on fungicide effectiveness has been gathered from irrigated field trials and does not confirm the economic viability of such applications during the extreme pressure of large-scale epidemics.
