These are the major pests and diseases that affect maize crops in New Zealand
Can be a major threat to seedling maize plants.
Larvae enter the maize shoot soon after emergence then bore towards the base of the plant and sever the growing point.
ASW feeds on pasture tillers, therefore maize crops sown into paddocks previously in pasture or with pasture weeds with a short cultivation period, are most at risk.
Visible damage includes emergence failure, wilting and collapse.
Tends to stay near the soil surface, eating roots of immature plants and also brace roots and can bore into stems of mature plants or ears lying on the surface.
They can survive through winter to affect crops in the new season. On average each adult can kill one October planted maize plant.
Early planted crops are generally most susceptible.
Ground inspection should ideally occur before planting.
Caterpillars are usually found at or shortly before silking. CAW caterpillars graze on leaf laminae of maize and leave only the midribs in severe cases.
CAW will consume the bottom half of plants before moving to the upper parts.
Silks and seeds may also be consumed.
Cutworm damage is determined by the number and growth stage of the caterpillars in relation to the crop growth stage.
The first stages of a cutworm's life is spent on the soil surface feeding on plant leaves. As the cutworm grows it begins to sever plants and consume them, this is referred to as the “cutting stage”.
The most devastating effect occurs where “cutting stage” coincides with crop emergence. Damage though is more frequently caused by caterpillars tunnelling up the maize stem after the 4-leaf stage.
Head smut can cause serious yield losses. The cobs and tassels are invaded by spores, taking over the zone of the kernels and stamens. It is more prevalent in low-lying valleys or on land prone to flooding and in sandy or silty soils. Affected cobs are bulbous, soft and with no visible silks. The fungus gets into the plant through the mesocotyl and roots before the 8-leaf stage and spreads systemically, going up to the developing cob and tassel. Cold temperatures will slow the rate of germination and establishment of the pathogen, which survives in heat. Disease development is helped by any factor that slows early growth, for example, early dry conditions or compacted soil.
Protection is in the form of hybrid resistance and fungicidal seed treatment. Seed crops are monitored carefully to ensure no carry-over of infection.
Leaf blight is an endemic leaf disease, with visible symptoms in the form of large spindle-shaped lesions, running in the direction of the veins and causing premature drying of the leaves. In damp weather, dark brown fructifications develop, causing the disease to spread, with airborne spores infecting the upper leaves. The impact on the crop occurs through the loss of leaf area for photosynthesis and can happen rapidly – 5-12 days from initial infection to visible symptoms.
NLB is often restricted to certain specific regions of New Zealand due to the occurrence of the required climatic conditions. Risk is increased with the following factors: planting in a known hotspot, planting in a paddock subject to a previous outbreak or in close proximity to one, and close to a grain dumping site or a grain storage facility.
Control is helped by hybrid selection and shredding/mulching of previous crop residues.
Rhizoctonia/Fusarium - Root rot diseases are caused by a number of different pathogenic fungi affecting the roots. Rhizoctonia affects main and brace roots, causing severe necrosis with a blackening of the affected areas. Fusarium fungi causes rotting with a characteristic browning of the roots. Visible effects in the crop are often seen during early grain fill with leaves turning to green-grey, then to pink before drying out. The plant is weak, easy to pull out and therefore prone to lodging.
Higher risk situations are: sandy soils, irrigated land and in single-crop conditions where there are soil structure problems.
Maize, like many other grasses, can suffer from rust infection – and maize rust (Puccinia sorghi) has its own characteristic symptoms. The disease is not found to be hybrid specific and occurs in areas of higher temperature/humidity. The orange/brown spots are found on any leaves, stalks and husks that are exposed to the light and the spots darken and appear scab-like, when the next cycle of spores are produced. Damage to the crop occurs through this loss of leaf area for photosynthesis.
Rust has a very short biological cycle – spores are formed approximately seven days after infection, so several cycles can infect a single crop.
Most common: brown shield bug and green shield bug. Commonly called “stink bugs”.
They use their hypodermic-like mouth parts to suck sap from plants.
They have a powerful unpleasant odour when squashed.
Can be found on plant foliage usually that is exposed to sunlight.
Slugs may damage emerging maize crops. Cavities are gouged into the seed, sometimes affecting the growing point, and early leaves often appear shredded as a result of slug feeding. Crops sown on heavy, moist soils which have a cloddy structure and direct drilled crops are most at risk.