By Joshua Kato
This is a disease caused by the pathogenic plant fungus Ustilago maydis. It can cause significant economic damage in dry, hot maize growing areas as well as in mild hill zones and under temperate conditions.
The infection is systemic: the fungus penetrates the seedlings and grows inside the plant without showing symptoms, until the tasseling and silking stages.
The most conspicuous symptoms are:
Abnormal development of the tassels, which become malformed and overgrown.
Black masses of spores develop inside individual male florels. Masses of black spores in place of the normal ear, leaving the vascular bundles exposed and shredded. The smutted ear develops no grains.
Avoiding mechanical damage to plants will reduce the primary means of infection by using fungi. Removing galls before they rapture will limit the spread, but is not practical in large-scale maize cultivation.
A well-balanced fertilizer regime will reduce disease severity, and high levels of nitrogen fertilization increase disease severity although the application of phosphorous reduces disease incidence.
Maize Streak Virus
The virus is transmitted by Cicadulina spp. Early disease symptoms begin within a week after infection and consist of very small, round, scattered spots in the youngest leaves.
The number of spots increases with plant growth; they enlarge parallel to the leaf veins. Soon spots become more profuse at leaf bases and are particularly conspicuous in the youngest leaves.
Fully elongated leaves develop chlorosis with broken yellow streaks along the veins, contrasting with the dark green colour foliage.
Severe infection causes stunting and plants can die prematurely without developing cobs. Many cereal crops and grasses serve as reservoirs of the virus and vectors.
Control: It is managed by growing resistant maize varieties.
Grey Leaf Spot (GLS)
The disease is caused by Cercospora zeae-maydis, Csorghi var maydis. It may occur in subtropical and temperate, humid areas.
Lesions begin as small, regular, elongated brown-grey necrotic spots growing parallel to the veins.
Occasionally, lesions reach 3.0 x 0.3 cm. Minimum tillage practices have been associated with an increased incidence of GLS.
Development is favoured by extended periods of leaf wetness and cloudy conditions and can result in severe leaf senescence following flowering and in poor plant grain fill.
Management of GLS
Use of resistant cultivars. Fungicides are important for GLS control
Because moisture on the leaf surfaces is important throughout the disease cycle, efforts should be made to avoid practices that extend dew periods.
Therefore, irrigation should not be scheduled during the afternoon or early evening, especially after outbreaks have already occurred.
Turcicum Leaf Blight (TLB)
The disease is caused by Exserohilum turcicum. An early symptom is the easily recognized, slightly oval, water-soaked, small spots produced on the leaves.
These grow into elongated, spindle-shaped necrotic lesions. They may appear first on lower leaves and increase in number as the plant develops and can lead to the complete burning of the foliage.