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
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 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
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 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
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.