Management of Diseases in Wheat - Rot and Smuts

Common root rot

A soil-borne fungal disease which attacks wheat, barley and triticale. It survives from one season to the next through fungal spores, which remain in the cultivated layer of the soil. The disease increases in severity with continuous wheat or wheat-barley sequences.

Barley increases the soil population of fungal spores rapidly. Infection is favoured by high soil moisture for six to eight weeks after planting.

Common root rot symptoms:

• a dark-brown to black discolouration of the stem just below the soil surface
• black streaks on the base of stems
• slight root rotting.

Common root rot can cause yield losses of between 10-15% in susceptible varieties.

The disease may be controlled by planting partially resistant varieties or by crop rotation. Where the disease is severe, rotation to non-susceptible crops for at least two years is recommended. Summer crops such as sorghum, sunflower, or white French millet can be used for this purpose.

Crown rot

This is a fungal disease that affects wheat, barley and triticale. It survives from one season to the next in the stubble remains of infected plants. The disease is more common on heavy clay soils.

Infection is favoured by high soil moisture in the two months after planting. Drought stress during elongation and flowering will lead to the production of ´deadheads´ or ´whiteheads´ in the crop. These heads contain pinched seed or no seed at all.

If the leaf bases are removed from the crowns of diseased plants, a honey-brown to dark-brown discolouration will be seen. In moist weather, a pink-purple fungal growth forms inside the lower leaf sheaths and on the lower nodes.

The disease may be controlled through planting partially resistant varieties or crop rotation. If the disease is severe, rotation to a non-susceptible crop for at least two and preferably three years is recommended. A winter crop such as chickpea, oats or any summer crop may be used as a disease-free rotation crop.

Smut

Bunt or stinking smut

This disease affects mature wheat ears in which a mass of black-fungal spores replaces the interior of the grain and forms a bunt ball. Infected plants are shorter and have darker green ears and gaping glumes than healthy plants. Bunt is usually only noticed at harvest when bunt balls and fragments are seen in the grain. Grain deliveries with traces of bunt balls are not accepted by AWB Limited.

If a bunt ball is crushed, a putrid fish-like odour is released. Spores released during harvest contaminate sound grain.

The spores germinate with the seed when planted and infect the young seedling. The fungus then grows inside the developing wheat plant, finally replacing each normal grain with a mass of spores.

Bunt has not been recorded in commercial wheat crops in Queensland for more than 30 years. This is probably because of the widespread use of fungicidal seed dressings.

Bunt control recommendations:

• Seed which is sown to provide the following season´s wheat seed should be treated with a fungicidal seed dressing.
• Seed obtained from plants grown from untreated seed should be treated with a fungicidal seed dressing before planting.
• All seed entering Queensland should be treated with a fungicidal seed dressing which will control bunt.
• Grain from a crop with bunt should not be used for seed.
• On farms where a crop has been affected by bunt, all wheat seed should be treated with fungicidal seed dressing for at least six years.

These recommendations could be adopted in one of two ways:

1. Treat all wheat seed with a fungicidal seed dressing every second year.
2. Treat a small quantity of seed of each variety with a fungicidal seed dressing every year and use the grain from this as planting seed in the following year.

Loose smut

Loose smut is a fungal disease which becomes evident at head emergence. A loose, powdery mass of fungal spores is formed in the head; these spores are readily blown away leaving a bare, ragged stalk.

If the spores settle on healthy flowers, they may germinate and infect the embryo of the developing seed. When this seed is planted, the smut grows inside the plant until flowering when the disease appears. Because loose smut is carried inside the seed, systemic seed dressings are needed to control it. These are more expensive than the others and should be used only when a high incidence of loose smut is expected.