Pests & Diseases

These are the major pests and diseases that affect maize crops in New Zealand


Argentine Stem Weevil

Argentine Stem Weevil


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.


Black Beetle

Black Beetle Adult


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.


Cosmopolitan Armyworm CAW

Cosmopolitan armyworm


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.





Eyespot appears as a large number of small circular spots, usually less than 4 mm in diameter and often just confined to the leaves. Eyespot is cream to grey coloured in the centre, with a purply -brown edge and surrounded by a translucent halo when seen against the light. Initial symptoms can appear at the seedling stage and spread to the lower leaves; but the disease does not become fully established until after flowering, when it attacks the leaves above the cob. Leaf loss is the result, with severe infection causing kernels to shrivel. Lower summer temperatures and leaf surface moisture help the disease to manifest itself, with several hours of leaf moisture required for the disease to spread.

The fungus survives easily on crop residues, so it is important to bury the trash if the disease is present in a mono-crop situation.


Fusarium Head Blight

Fusarium Head Blight


Caused by a number of Fusarium species, with the disease occurring on crop residues and spreading to the growing plant. Symptoms appear after flowering and infects the cob via the silks, forming a mycelium sheath starting at the apex of the ear and progressing downwards. The decomposing kernels and the mycelium produce a characteristic pinky-brown colouring. Often an attack can occur opportunistically, following corn earworm damage or similar.

There is potentially some varietal resistance, but management of previous crop residue is the key to control.

Greasy Cutworm

Greasy cutworm


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

Corn smut


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.


Northern Leaf Blight


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.


Root Necrosis

Root Necrosis


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.


Shield Bugs

Shield Bug


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.

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