On this page:

• Stalk and cob rots
• Blights
• Rusts
• Smuts
• Wallaby ear
• Virus

Stalk and cob rots

Rots in maize can be caused by a wide range of fungal pathogens.

Following are some of the more common ones.

Fusarium stalk rot (Fusarium verticillioides [Gibberella fujikuroi] and other Fusarium species)

Although fusarium stalk rot is common in many of the corn-growing areas of Australia, it tends to be more severe in warm, dry regions. On the Atherton Tablelands it often occurs in the first maize crop after a lengthy pasture phase. Fusarium species and other fungi can also cause seedling blight but this is uncommon due to improved seed production and handling, and planting techniques.

Infected seedlings are usually stunted, and have pale green or purple leaves and poor roots. Symptoms of fusarium stalk rot in mature plants are difficult to distinguish from those of other stalk rots, but the internal tissues of affected stalks are usually reddish-brown and rotted. The discolouration may also be seen on the surface of the stalks near nodes. Stalks are weak and lodge easily.

Good agricultural practices, including crop rotation, correct plant densities and the minimisation of stress, assist in reducing fusarium stalk rot. Use resistant hybrids to manage this disease.

Fusarium cob rot or ear rot (Fusarium verticillioides and other Fusarium species)

Fusarium cob rot is favoured by warm, dry weather at or after flowering and can occur in all maize-growing areas of Australia. Fusarium verticillioides, the main pathogen, produces the mycotoxin fumonosin, which is toxic to livestock, particularly horses.

Individual or groups of infected kernels are scattered at random on the cobs but, in severe cases, the entire cob can be affected. Whitish pink-lavender fungal growth occurs on and between the kernels, often at the tip of the cob as the result of insect damage. Individual kernels may also exhibit a ´starburst' symptom where white streaks radiate from the point of attachment.

The Fusarium species over winter in infected residues. During the growing season, airborne spores infect the silks at flowering. They are also known to be systemic in maize plants and grow rapidly when plants are stressed.

The practices outlined above for fusarium stalk rot, as well as the management of insect pests and proper storage of kernels, reduce the risks of mycotoxin contamination. Use resistant hybrids where available.

Gibberella stalk rot (Gibberella zeae; Fusarium graminearum)

This fungal disease is favoured by moist conditions. It is more common on the Atherton Tablelands, and in coastal and inland northern New South Wales (NSW), where it can cause serious yield loss. The disease is much less serious in other regions, including southern Queensland. It is common where maize monoculture is practised.

The surfaces of infected stalks are often reddish-brown, particularly around the nodes, and the tissues internally are red-pink. Stalks are weak and break easily, resulting in lodging and plant death. Later in the season, small, round, bluish-black fruiting bodies may be found around the nodes of dead stalks.

Similar to the Fusarium species that cause fusarium stalk rot, F. graminearum survives in infected residues and can also systemically infect plants. The fungus causes head blight of winter cereals and has been a problem in some years on the Liverpool Plains of NSW in maize-wheat rotations.

Use crop rotation (avoiding maize-winter cereal rotations), good agronomic practices to minimise stress, and/or resistant maize hybrids to combat this disease.

Gibberella cob rot, ear rot or pink ear rot (Gibberella zeae; Fusarium graminearum)

Gibberella cob rot is favoured by cool, moist conditions at flowering. Therefore, like the stalk rot phase, it is more common in wetter, cooler growing regions. Several mycotoxins are produced by F. graminearum, including zearalenone and the trichothecene group, which are harmful to a wide range of livestock, especially pigs. Maize monoculture, maize-winter cereal rotations and plant stress during grain fill are factors in the disease.

The most common symptom is a reddish-pink or whitish-pink fungal growth that appears at the tip of the cob and grows down. Husks tend to bind to the kernels and there may be black fruiting bodies on external husk leaves. Infection occurs from windborne spores, which grow down the silks at flowering.

The use of good agronomic practices, resistant hybrids, prompt harvesting and proper storage minimise the risk.

Diplodia cob rot or ear rot (several Diplodia species)

Most of the species that cause diplodia cob rot are also capable of causing seedling death, stalk rot, and/or leaf spot, though cob rot is the most significant disease. Wet weather favours infection of cobs and leaves.

If infection occurs after flowering, the husks covering the cobs are bleached. Cobs are usually shrunken, lighter than normal and covered in a white-grey fungal growth. Black fruiting bodies (smaller than the fruiting bodies of Gibberella spp.) develop in the husks and cobs towards the end of the growing season.

The Diplodia species survive in infected residues and spores produced in fruiting bodies are splashed on the silk, which remain susceptible to infection as they are drying.

The only known management options to hasten residue breakdown are crop rotation and agronomic practices.

Image: Gibberella ear rot disease

Blights

Turcica leaf blight or turcicum leaf blight (Exserohilum turcicum)

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 (Bipolaris maydis)

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.

Image: Maydis leaf blight

Rusts

Common rust (Puccinia sorghi)

This rust is found in most maize growing areas but is more common in temperate and subtropical regions. Moderate temperatures (16-25^oC), 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.

Image: Common rust

Polysora rust, or tropical rust, or southern corn rust (Puccinia polysora)

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 (Ustilago zeae formerly known as Ustilago maydis)

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 (Sporisorium reilianum)

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.

Image: Head smut

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.

Image: Wallaby ear disease

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.

Image: Dwarf Mosaic virus