Armyworm or grasshoppers. If the problem is severe consider insecticide application. In the case of armyworm, a parasitic wasp (Apanteles ruficrus) often keeps the population under control, but if damage is severe and unstung caterpillar numbers are above 3-4 caterpillars per plant, then it may be worthwhile spraying. Refer to Pests for more information on this insect pest.

Check for tracks and droppings, usually accompanied with pushed over/leaning plants.

Strong winds in exposed places will often result in leaf shredding which can range from <5% to >50% loss of leaf area. In severe cases leaves are usually snapped off either at the base or somewhere up the midvein, with accompanying leaf blade loss.

Even light hail is usually accompanied with strong winds which can combine to cause significant damage to leaves.

Some hybrids have strong leaves which are more resistant to leaf shredding than others.

Various pathogenic microorganisms including common rust, northern leaf blight and eyespot cause spots or disease lesions on leaves, and in severe cases these lesions can coalesce resulting in total leaf senescence/death.

Mites can give the appearance of tiny yellow-brown spots on both the upper and underside of leaves. Close inspection may reveal the presence of mites which will be accompanied by the presence of a fine web-like film on the underside of leaves. More common in sheltered areas. This is unlikely to be of economic concern unless there is widespread infestation.

Spray drift from neighbouring crops.

Lower leaves will “fire-off” in cases of N or K deficiency. This can be induced by dry topsoil where these nutrients are often in their highest concentrations.

Lower leaves will yellow and start to fire off in cases of N or K deficiency. This can be induced by dry topsoil where these nutrients are often in their highest concentrations. See Leaf Abnormalities for specific details.

If pollination or kernel set is unsuccessful entire plants may slowly turn purple during this growth phase.

Mites or aphids. Leaves infected with mites display thousands of fine spots with a fine web and tiny light-coloured mites on the underside of leaves.

Dark blotches caused by saprophytic fungi colonising pollen trapped between the stalk and leaf sheath. Eyespot can also affect leaf sheaths.

Aphids.

Purpling due to anthocyanin deposition.

Each hybrid has characteristically straight or zigzagged internodes. Expression of the zigzag trait can be influenced by environmental conditions with some hybrids sometimes expressing both straight and zigzagged internodes to varying degrees within the same field.

At this growth stage plants will more usually lodge from the roots but if they are well anchored into firm soil, stalks can snap; but only in severe or turbulent wind events in exposed areas, on ridgetops or downwind of some form of shelter or obstacle. Snapping can occur above or below the ear but occurs more typically at or just above the ear.

Uncommon at this stage but can become an issue in crops that have experienced or are experiencing significant stress which has allowed disease to take hold and weaken the stalks. Often the pathogenic organism(s) will come up from the roots. Check the stalks and roots of affected plants for signs of rot by cutting open some stalks and digging up some plants.

Check for tracks and droppings of stock or wild animals. Determine if stock is still present in the crop. Remove the stock if warranted. Fence off entry points to avoid this occurring again.

In-season field operations can result in stalk injury. Check for signs of mechanical injury on stalks of affected and unaffected plants.

High populations can reduce stalk diameter, stem strength and can increase crop and ear height, which interact to increase the risk of stalk snapping/lodging. Scout and check these characteristics on plants in the outside and inside rows, and in overlap areas around headlands to help understand the problem.

N-K imbalance (excessive N, insufficient K) can predispose plants to stalk lodging by increasing plant and ear height, and reducing stem diameter and strength.

Commonly occurs in soft, saturated peat soils or heavy or poorly drained soils.

Commonly occurs in fields that have been side dressed with N using knives or discs during an earlier growth stage. Check lodged plants for signs of root cutting.

Usually caused by compaction and/or ponding either before or after planting, but pests (insects or nematodes) or diseases (fusarium etc.) should not be ruled out. Subsoil acidity or compaction may also be involved.

Late applied phenoxy herbicides such as Dicamba and 2-4D can cause damage to brace roots making plants more susceptible to root lodging. Check for signs of malformed brace roots if these herbicides have been applied, focussing on overlap areas around headlands etc.

High populations can increase crop and ear height and reduce root strength, which interact to increase the risk of root lodging. Scout and check these characteristics on plants in the outside and inside rows, and in overlap areas around headlands to help understand the problem.

N-K imbalance (excessive N, insufficient K) can predispose plants to root lodging by increasing plant and ear height.

Usually caused by pathogenic fungi killing stressed plants from the roots. May start to occur two or more weeks before R6 (black layer), particularly under high plant populations and/or stressed conditions.

Splotches, marks or discolouration from within the stalk may indicate developing stalk rot. Cut a few affected plants stalks open to investigate. Refer to Stalk Diseases for stalk rot identification.

Some hybrids naturally produce red to purple coloured stalks, but stalks for any given hybrid within a field or localised area should be similar in colour. Significant variation in stalk colour may be indicative of an underlying problem and/or differences in local field conditions.

Check ear development of affected plants. Poor kernel set usually results in anthocyanin accumulation in leaves and stalks, giving them a marked red/purple colouration.

Currently not present in NZ but something all growers should be aware of in case of a future biosecurity incursion. This pest is also responsible for mid-grain fill stalk lodging and chewing holes in the ear peduncle/shank causing premature ear drop.

Drought-like conditions can cause plants to flower early while the crop is relatively short; in extreme cases flowering can occur when crops are below waist height.

Drought-like conditions can cause a delay in silk emergence relative to tassel emergence. If this phenomenon occurs in an otherwise healthy looking crop, it is almost certainly indicative of a recent critical soil-moisture deficit developing. Irrigate immediately if possible.

Drought-like conditions can result in poor anthesis or pollen shed. Usually occurs in conjunction with delayed silking. If these phenomena occur in an otherwise healthy looking crop, it is almost certainly indicative of a recent or critical soil-moisture deficit developing.

A variety of caterpillars can be found feeding on maize silks. Refer to Pests for more information on these insects.

The cause of this phenomenon is a topic of some debate but anecdotal evidence suggests that large diurnal (night/day) temperature differences can impede normal silk development in some hybrids, typically those with long and naturally tight husks.

This is perfectly normal in any decent growing season. Unless plant populations are very low, most secondary cobs will abort soon after pollination and/or kernel set on these lower order cobs will be severely limited.

Usually occurs on tillers on the outside rows of the crop. Perfectly normal and nothing to be concerned about.

Occurs rarely, certainly less frequently than naked/tassel cobs, but again, nothing to be concerned about.

Commonly called parasitic ears, which die off as grain fill progresses.

Often resulting from a combination of delayed silking and poor anthesis/pollen shed as described under Delayed silking and Poor anthesis and/or pollen shed .

Cloudy or short days during or soon after flowering period (e.g. late planting date) and/or high plant population density can result in poor pollination at the cob tips.

Head smut.

Corn earworm or cosmopolitan armyworm. See Pests for more details on these insects.

A variety of birds and rodents are fond of feeding on cob tips. Common symptoms are chewing of the husks and developing grain. Rodents tend to concentrate on outside rows near shelter, whereas damage from birds tends to be more random and widespread, often accompanied by droppings on affected cobs.

Corn earworm or green vegetable bug (GVB) which pierce husks causing damage/discolouration to underlying kernels. Refer to Pests for more information on these insect pests.

Particularly eyespot. See Diseases for more details.

Harmless fungi feed on pollen trapped between husk leaves causing pale to dark drown splotches on the first few layers of husk leaves.

Often resulting from a combination of delayed silking and poor anthesis/pollen shed.

Low light levels during pollination can result in poor pollination at the cob tips which may not be apparent until the R2 stage.

Chaffy, shrivelled kernels at the cob tip can be indicative of developing or previous (post R1) drought conditions and/or excessive plant population density.

In the absence of drought conditions chaffy, shrivelled kernels at the cob tip can be indicative of low solar radiation levels (e.g. cloudy days) post R1 and/or excessive plant population density.

The occurrence of a near-frost event during or soon after pollination can result in the abortion of random kernels in some sensitive hybrids. Even moderate symptoms do not seem to affect yield.

The proboscis of the green vegetable bug (GVB) is capable of piercing the husks of some hybrids creating off-coloured (splotchy brownish-grey) tips on random kernels on affected cobs. Refer to Pests for more information on this insect pest.

Certain diseases including Fusarium (white/salmon discolouration) or Cladosporium (black discolouration) can be isolated to specific kernels ranging from several to many on a cob.

Currently not present in NZ but something all growers should be aware of in case of a future biosecurity incursion. This pest is also responsible for mid-grain fill stalk lodging and holes chewed in the ear peduncle/shank causing premature ear drop.

Ears falling off plants during silage operations. See Plants dead or dying prematurely for more information.