More Maize Diseases - Blight - Rust - Smuts and Wallaby Ear

Blights

Turcica leaf blight or turcicum leaf blight

Turcica leaf blight can occur in all growing regions. It is favoured by warm weather when leaves are wet from dew or rainfall for extended periods. Under conducive weather conditions, it can cause significant yield losses in susceptible hybrids.

Long, spindle-shaped, greyish-green, water-soaked spots (up to 150 mm long and usually less than 20 mm wide) develop on leaves and later turn light purplish-brown or grey. After flowering, the disease can develop rapidly, resulting in blighting of infected leaves.

The fungus survives on volunteers and maize residues, and its spores are spread long distances by wind.

Use less susceptible hybrids and do not practise maize monoculture. Eliminate volunteer maize. Early plant can reduce the risk of this disease. Control with fungicides is not economical.

Maydis leaf blight

Infection and disease development is favoured by warm, showery, humid weather. Although it can occur in all maize growing areas, maydis leaf blight is more common in the tropics.

Symptoms can vary depending on the hybrid. Spots range from tan, elongated spots up to 40 mm long and 6 mm wide with parallel sides to tan, spindle-shaped or elliptical spots up to 25 mm long and 12 mm wide. Spots often have dark red-brown margins and/or a narrow yellow halo. This fungus can be seedborne and cause seedling rot.

The fungus survives on diseased residues, volunteer maize and grasses.

Plant resistant hybrids to combat this disease.

Rusts

Common rust

This rust is found in most maize growing areas but is more common in temperate and subtropical regions. Moderate temperatures (16-25 o C), humid weather and leaf wetness durations of at least six hours from rain or dew favour infection and disease development.

The disease is recognised by the abundant oval-elongate, red-brown pustules up to 2 mm long, which erupt through both leaf surfaces in scattered groups. This distinguishes common rust from polysora rust, which has little development of rust pustules on the lower leaf surface. The pustules contain numerous powdery spores that can be spread long distances by wind.

Common rust survives between seasons only on living maize plants.

The only practical control measure is to plant resistant hybrids. Sweetcorn hybrids tend to be most susceptible.

Polysora rust, or tropical rust, or southern corn rust

Polysora rust is favoured by warm wet weather and is, therefore, a disease of tropical regions. Similar to common rust, it needs a long period of leaf wetness for infection. Severely affected leaves can die, which results in lighter than normal ears.

Small red-brown or orange pustules develop evenly over the upper leaf surfaces, and larger elongated ones may also develop on the midribs, ear husks and tassels. Polysora rust survives between seasons only on living maize plants.

Smuts

Boil smut or common smut

Boil smut occurs in most production areas but is often sporadic and minor.

All above-ground parts of the plant can be infected, but particularly actively growing tissues on cobs, tassels and stems. Blisters or galls develop, initially with a thin white membrane and later containing black powdery spores. Mature galls can grow as large as 20 cm in diameter.

Spores can be spread by wind, seed, clothes or farm machinery and can survive in the soil for many years. The spores germinate under the right conditions and produce another type of spore that is transported to the plant in the air.

Most hybrids have a reasonable level of resistance to boil smut. Howver, practise good crop hygiene and ensure that seed is treated with a registered fungicide.

Head smut

This fungus causes the replacement of part or all of the cob, and often the tassel, with black masses of powdery spores. At first, these masses are covered by a white membrane, which later bursts. Leafy structures may replace the reproductive tissues. Badly affected plants may be stunted and have profuse tillering.

Infection initially occurs when spores in the soil germinate and infect seedlings. Then the fungus grows through the plant, ultimately invading the developing cobs and tassels. Soil with low moisture levels and temperatures of 21-28°C favour infection.

Plant resistant hybrids and avoid early planting of susceptible varieties.

Wallaby Ear

Affected plants are often stunted, and their leaves are dark green or green-blue, have thickened veins and are held at an acute (upright) angle. The disorder is more common in subtropical coastal areas, where it can severely impact on yield. It occurs when a toxin is injected into the plant by the leafhopper (Cicadulina bimaculata) while feeding. Use tolerant hybrids, particularly if planting late in coastal districts, and control leafhoppers with registered insecticides when plants are young.

Virus

Johnson grass mosaic virus

Johnson grass mosaic virus is a disease of temperate and subtropical areas, and can be a serious problem in southern Queensland.

Many maize hybrids have good resistance, though sweet corn varieties, especially supersweet types, are highly susceptible if planted late in the season.

Symptoms are either a ring-spot pattern or a mosaic of light and dark green patches on the leaves. Plants of highly susceptible hybrids can have yellow leaves, may be small and suffer considerable yield loss.

The virus survives in Johnson grass, and old or ratoon forage and grain sorghum crops between seasons. It is spread by aphids and transmitted after a short feeding time.

Planting resistant hybrids is the main control measure for this disease, though sweet corn is highly susceptible. Control Johnson grass on your farm.