Maize Pests and Diseases

Which are the most important pests and diseases in maize?

Maize is a high-yielding crop, but in order to reach its potential, the farmer should protect the health of the plants during the entire growing season. Except for the weeds that strike early in the plant’s life, there are various pests and pathogens that can jeopardize the growth and production of maize at different times during its life cycle.

The farmer needs to be aware of them and take preventative measures or suppressive actions to avoid or limit any damage. It is highly advised for the farmer to

Visit the field often, observe the plants carefully and take samples-measures to define the “enemy” and its spread.
Be aware of the favorable factors for the spread of maize pests and the economic threshold for the crop to decide if, when, and how you need to take action.
Taking action before the disease or pest has infected the whole field can reduce the amount of chemicals used and increase the efficacy of crop protection.

The most common and important pests in maize

Some pests have a preference or are more abundant during specific growth stages of the maize plants, while others threaten the health of maize during the whole growing season.

Corn earworm

The Corn earworm (Helicoverpa armigera or Helicoverpa zea) is an important and common insect enemy of maize and can cause extensive damage to sweet corn. The larva can cause damage (due to feeding) to leaves, tassels, and especially the silks and ears, where the insect prefers to be fed. Such damage can reduce pollination and grain-set, while damage in the cob may help the later infection from mycotoxins. The insect overwinters in the soil as pupae. Other host plants are cotton, tomato, and some legumes.

The control measures should take place at egg hatch and young caterpillars (up to 5 cm), but the hatched larva monitoring should continue later in the season. Except for chemical pesticides, a farmer can use pheromone traps, Bacillus thuringiensis, Entrusts SC or nucleopolyhedrovirus (NPV) and of course benefit from insect’s natural enemies (1).

Maize stem (or stalk) borer

The Maize stem borer (Busseola fusca, Chilo partellus, Chilo orichalcociliellus, Sesamia calamistis) is still one of the most important pests that infect maize. It can cause significant damages (up to 50-75% yield loss) in plantations of maize in developing countries (affects 30 million hectares(2)) but also in Europe. Millet, Sorghum and sugarcane are other host plants of the insect. The caterpillars (larva) are yellow-brown with brown heads and feed on young plants, causing “dead hearts” and damaging the leaves. As the larva matures, it enters the stems. The feeding damage results in weak, hollow stems and underdeveloped plants (due to disturbance of nutrient and water flow in the plant) (3).

The farmer should start monitoring the plants 3 weeks after planting (2 times per week) until the flowering stage. The control measures should take place before the larva enters the stems. For direct control, the farmer can use chemical solutions (4) and biological control, like natural enemies, parasites, B. thurigiensis and biopesticides (3). There are many resistant varieties available: TELA hybrids (5), KDH4SBR, KDH5, KEMBU 214, EMB 0702, KATEH 2007-3, MTPEH 0703 (6).

Cutworms (Black cutworm, Variegated cutworm)

The Cutworms (Agrotis ipsilon, Peridroma saucia) are major enemies for maize seedlings. The caterpillar overwinters in the soil and starts its activity in spring. The larvae feed on the stems and leaves and can cut down the young plants from their base. Other host plants of cutworms are pea, alfalfa, and many vegetables (like potato, tomato, crucifers, lettuce, etc.).

There are properly treated seeds (dressed seeds) in the market for controlling the pest. The spread of bait on the ground is another effective control solution. Since the insect can migrate to the maize crop from neighboring areas with pasture and weeds or from the residues of previous (host) crops remaining on the field, the farmer should take action to minimize these risks. Cutworms have many natural enemies (predators, parasitoids, and diseases) that are farmers’ allies and can facilitate the reduction of the insect population. For that reason, it is advised not to apply severe spraying with chemical compounds that can harm these natural enemies.

Corn Aphids and Maize thrips

Both insects can decrease the productivity of maize plants and cause remarkable damage, especially in water-stressed plants and under favorable environmental conditions. In severe infections, the plants become chlorotic (yellowish patches in the leaves). The population is usually larger at the end of Spring and the start of Summer. Usually, neither enemy is actively controlled since spraying is not cost-effective. However, aphids have many natural enemies that can “keep it under control” (e.g., ladybird larvae, Wasp parasitoids, etc.) (1). In areas where aphids are a common problem, farmers can choose to plant resistant maize hybrids (varieties that have resistance in Ostrinia can work). The population should remain under control, especially in areas where the virus causing the Maize Lethal Necrosis Disease has been reported (see more information below).

White grubs, black field earwigs, and migratory locusts

Each one of them can cause significant problems and losses in maize (during the plant’s growth stages), with bigger problems reported in Africa and China. In contrast to other cereal crops, there is a limited number of certified insecticides in the market for maize.

The most common and important diseases in maize

Monoculture, reduced or no-tillage, overuse of chemical protectants, and climate change have contributed to the increase in infection severity of numerous maize diseases, putting the crop’s final yield at great risk. Farmers should visit their fields often and be able to recognize the most important maize diseases already from an early stage to take action. Usually, an integrated approach of best management practices has the best results. It includes foliar spraying, seed treatments, crop residue management, crop rotation, and a balanced nutrient and water supply for the plants. The diseases can be caused by phytopathogenic funguses, bacteria or viruses.

The most important Fungal Maize Diseases

Gray Leaf Spot

The Gray Leaf Spot (pathogen: Cercospora zeae-maydis) is considered the major threat to maize crops in most parts of the world, causing even up to 100% grain yield losses. The fungus survives in crop residues, which is why the problem becomes more severe when the farmers plant corn in the same field every year and apply no-tillage techniques. The infection starts from the lower leaves as lesions, and it gradually spreads upwards. In the first stages, the lesions are small in size, enveloped by yellow halos that expand, creating larger grey spots, oblong (up to 5cm long, 0.3cm wide) parallel in the leaf veins. Warm, humid, cloudy weather with a lot of rainfalls helps in the spread of the infection. Progressively the infected leaves dry and die (become necrotic).

To avoid yield losses, a farmer can select to cultivate a hybrid resistant to gray leaf spot. This is highly recommended in areas with a known history of infection from the fungus. In combination with crop rotation, this measure could be very useful in disease management when the farmer wants to apply no-tillage. Alternatively, there are some appropriate fungicides available in the market. Still, the spraying should be performed as early as possible, especially when favorable conditions for the growth of the pathogen are expected and the cultivated hybrid is susceptible.

Northern (Turcicum) and Southern Corn Leaf Blight

The Exserohilum turcicum (Helminthosporium turcicum) is responsible for the Northern Corn Leaf Blight, and the Bipolaris maydis (Helminthosporium maydis) for the Southern Corn Leaf Blight. They are distinct diseases caused by different funguses that have in common the symptoms of elongated grayish-green to tan lesions appearing on the leaves and the significant yield losses that these pathogens are causing in maize fields nowadays. In the case of Northern Leaf Blight, the lesion expands gradually in the whole leaf and is not restricted in leaf veins. Humid, rainy, windy, and warm weather is favorable for the growth and spread of the disease in the cornfield.

The symptoms of this disease could be similar to the Gray Leaf Spot, especially in the early stages of the infection. There might be a need for more specialized diagnostics to recognize the responsible pathogen. Except for fungicides, very effective methods for these diseases control are residue management (plowing, burning), crop rotation with non-host species, and the use of resistant hybrids.

Common rust (Puccinia sorghi) and downy mildew, are two of the most widely spread diseases globally in cornfields and can cause high yield losses (especially in Asia and Africa) if they are not controlled in time effectively. They spread pretty easily within the crop but can be easily transferred from and into neighboring cornfields, reaching an epidemic’s severity.

Downy Mildew

The disease can survive in crop residues but also in different crop species and weeds growing in/close to the maize field. It is both air and seed born. The crop seedlings can be infected early in the season, and the problem is bigger with warm, humid weather. Depending on the region, the corn variety, and the pathogens causing the disease, the farmer can observe symptoms like limited plant growth, chlorosis in the leaves usually covered by a white powder (fungal growth) on both sides, tassel malformation and as a result reduce or no grain production (7).

The most important and effective measure is the selection of a resistant maize hybrid. Additionally, the farmer can apply crop rotation, use of systemic fungicides, spray, and seed treatment applications as well as planting earlier (4,8).

Common rust

The disease has one of the most characteristic symptoms, which are visible to the naked eye. The symptoms are more abundant in the upper part of the canopy, which is more easily infected by wind transferred fungus spores. In severe infections, the leaves (and other plant parts) are covered on both sides from orange-brown pustules with a powdery surface.

In fields where popcorn or sweet corn varieties are cultivated, it is recommended to apply 2-3 foliar sprays with a suitable fungicide (9) due to the high susceptibility of these plants to common rust. The application should be performed quite early to be more effective. There are also resistant or semi-resistant (tolerant) varieties available in the market.

Head Smut

The disease is caused by the Sphacelotheca or Sporisorium reiliana fungus and is considered to be a disease with high economic importance for maize farmers. This “silent enemy” infects the young maize seedlings during and after emergence from the soil, spreads, and grows systemically inside the plant without showing any symptoms in the early stages. The symptoms appear at the flowering stage of maize (tasseling and silking stage). The reproductive parts of maize plants that have been infected have malformed tassels with tumor-like galls, smut balls, or leafy appearance (9).

The most effective way to avoid any yield loss from this pathogen is to take precautionary measures. The most effective is the use of resistant to Head Smut maize varieties (e.g., Hybrid B840). The fungus can survive in the soil for many years, so using treated seeds with fungicides may be necessary in some cases with known infection field history. Earlier planting and balanced fertilization (phosphorus has a positive effect, while nitrogen can increase the problem with the disease) are also important.

Root Rot (Pythium and Rhizoctonia)

Both funguses infect the root system of maize plants in any developmental stage, leading to rot. The infected plants may have limited growth (dwarf plants), low robust, chlorotic, with discolored roots, and lodge or die. The pathogens are favored by high water concentration in the soil (badly-drained fields) and generally low temperatures.

The crop rotation with legume species resistant to the disease and the use of resistant varieties can offer good results. In fields with a known problem, the farmer can use seeds treated with appropriate fungicides or/and sanitized and take measures to improve the drainage of the field’s soil.

The most important virus diseases in maize

Maize Lethal Necrosis Disease (MLND) or Corn Lethal Necrosis (CLN)

As suggested by its name, the disease is one of the most catastrophic emerging diseases that a corn grower may face. It is a synergetic viral disease caused by 2 viruses: the maize chlorotic mottle virus (MCMV) and one of several viruses from the family Potyviridae (11). It occurs at all growing stages of the plants and can cause limited plant growth, leaf chlorosis (from the base of the leaf) resulting in “dead heart” in younger plants, malformed, little or no ear formation, poor male inflorescences, or even plant death. The viruses can be transmitted by seeds from infected plants and vectors like maize thrips, aphids, and rootworms.

The virus diseases have no treatment. As a result, the farmer needs to take precautions to avoid infections. Firstly, he/she needs to exclusively use certified seeds, especially if the virus has already been reported in the area or if the seeds are coming from such a region. Additionally, there are some resistant hybrids available. Crop rotation with non “host” species in combination with successful control of host-weeds and the insect vectors can be useful. If the farmer observes any suspicious symptoms, he/she needs to remove and burn the infected maize plants.

Crop rotation for reducing the appearance of maize “enemies.”

Monoculture of maize in a large region or from one year to the next can favor maize diseases and pests. The farmer needs to prevent actions for reducing this risk, and crop rotation is an essential tool. The rotation scheme, meaning which crops are going to succeed maize in the field and how many growing seasons will last, has to be designed very carefully. The goal is to choose other crops that are not hosts of maize pests and diseases. For this reason, we need to avoid planting after maize some specific cereal crops (like Sorghum), especially if our field has a known history of Maize Lethal Necrosis Disease. However, wheat, barley, and oat are widely used successfully in maize crop rotation. Beans (soybeans), cowpeas, and peas (plants of the Leguminosae family) are preferable. In crop rotation schemes that last many years, the farmer can also use potatoes or onions. For selecting the best scheme for your rotation, you can advise your local licensed agronomist.

References

Fast Facts and History of Maize

Maize Nutritional Value and Health Benefits

Corn Plant Information and Production

How to Grow Corn for Profit – Maize growing guide

Principles for selecting the best maize variety

Maize Soil preparation, Soil requirements and Seeding requirements

Maize Pests and Diseases

Maize Water Requirements and Irrigation Systems

Maize Fertilizer Requirements

How to successfully control weeds in corn cultivation for higher yields

Yield, Harvest and Post-harvest handling of Maize

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