Corn Disease and Disease Management

Corn plants are attacked by pests and diseases during their entire growth period, reducing yield quality and quantity.

The extent of pest damage varies greatly depending on the plant growth stage, weather conditon, crop management practices and cropping sequence.

Pest and disease detection and identification are, thus, essential to effective pest and disease management and control.

What are Plant Diseases?

Plant diseases refer to a series of harmful physiological processes caused by continuous irritation of a plant by a primary infectious agent called pathogen, exhibited through abnormal cellular activity and expressively morbid morphological and histological conditions called symptoms.

Disease symptoms can be broadly categorized as:

• Necrosis. Disintegration and death of cells, tissues or organs. This is exemplified by spots, wilt, blight and rot.

• Hypoplasia. Underdevelopment or restriction in growth resulting to dwarfing, stunting or chlorosis.

• Hyperplasia. Overdevelopment attributed to abnormal increase in cell number and cell size. This is typified by galls.

Diseases can be noninfectious or infectious.

Fungi, bacteria, viruses, nematodes, protozoa and other parasitic plants cause infectious or biotic plant diseases.

Noninfectious or abiotic plant diseases are due to too low or too high temperatures, lack or excess of soil moisture, lack or excess of light, lack of oxygen, air pollution, nutrient deficiencies, mineral toxicity, soil acidity or alkalinity (pH), toxicity of pesticides and improper cultural practices.

In every infectious disease, a series of more or less distinct events occurs in succession and leads to development and perpetuation of a disease and a pathogen. This chain of events is called a disease cycle.

A disease cycle sometimes corresponds fairly closely to a life cycle of a pathogen, but it refers primarily to the appearance, development and perpetuation of a disease as a pathogen relates to it rather than to the pathogen itself.

The disease cycle involves changes in the plant and its symptoms as well as those in a pathogen. It spans periods within a growing season and from the growing season to the next.

The primary events in a disease cycle are:

• Inoculation, penetration and establishment of infection

• Colonization (invasion)

• Growth and reproduction of pathogen

• Dissemination of pathogen

• Survival of pathogen in the absence of a host or during adverse condtions.

Disease is a function of host, pathogen and environment; all are components of a disease triangle.

Disease triangle is based on an equivalence theorem, which states that the effect of environment, pathogen and host can each be translated into terms of epidemic rate parameter.

A result is that changes in any one of the disease components will all have an equivalent effect on the epidemic.

Common Diseases of Corn

Corn diseases are readily recognized by their symptoms. Various agents, acting either singly or in combination, cause corn diseases. As previously mentioned, these agents can be biotic (living) or abiotic (nonliving). Living disease-inciting organisms are called pathogens.

Pathogens of diseases can either be bacteria, fungi, virus, nematodes and mycoplasma-like organisms. These pathogens can cause visible disease symptoms on the entire plant or on individual plant parts like the leaves, stems, leaf sheaths, ears and kernels.

Corn plants can be infected with a disease at all stages of growth, more so, at seedling stage when the disease often kills the plant, resulting to yield loss. Total yield loss from disease infections may range from 40-100%.

About 112 diseases of corn have been recorded. However, we will only discussed only the most common diseases of corn in the Philippines. Corn diseases in the Philippines can be grouped as either fungal, bacterial or viral diseases. Among these groups, fungal diseases are the most widespread. We will discussed the following disease:

• Downy mildew

• Leaf rust

• Banded leaf and sheath blight

• Bacterial stalk rot

• Corn mosaic

• Corn smut

• Leaf spot

• Ear rot

Downy Mildew

This disease is caused by a fungus, Peronosclerospora philippinensis, which affects corn plants from two-leaf to silking stages.

Symptoms. General symptoms of this disease are:

• White or yellow streaks, first at the base then on the entire leaf blade

• Chlorosis (loss of greenness due to lack of chlorophyll)

• Dwarfing

• Ears and tassels poorly formed in severe infections

Management. Practices found to be successful against downy mildew are:

• Sanitation involving the pulling out and burning infected plants as soon as they are observed in the cornfield

• Synchronous planting

• Crop rotation

• Planting of resistant varieties

Leaf Rust

This disease is caused by the fungus Puccinia polysora, which affects corn plants from mid-whorl to silking stages.

Symptom. Presence of small and circular brown rusty pustules or blisters on the upper and lower surfaces of a leaf. These pustules are rough to touch and later erupt and release numerous rusty spores.

Management. Plant resistant varieties to help reduce occurrence of the disease.

Banded Leaf and Sheath Blight

This disease is caused by the soil-borne fungus Rhizoctonia solani, which affects corn plants from late whorl to ear formation.

Symptoms. General symptoms of this disease are:

• Irregular blotches on the leaves, stalks and ears

• Blotches are grayish-green, with bluish gray or straw colored centers and with distinct brown borders

• Presence of irregular light to dark brown fungus lumps known as sclerotia on the dead tissues

• Infected ears may be wholly or partially rotten showing some seed germination

Management. To be successful against banded leaf and sheath blight:

• Practice sanitation by removing and burning infected leaves and stalks

• Bury the sclerotia by deep plowing

• Avoid planting varieties with very low ear placement

Bacterial Stalk Rot

This disease is caused by the bacterium Erwinia carotovora, which affects corn plants from two-leaf to maturity stages.

Symptoms. General symptoms of this disease are:

• Stalk rot starting from the base progressing upward and eventually causing wilting of lowermost leaves

• Infected inner tissues of older plants deteriorate and become soft with foul odor and later dry up into easily disjointed fibers

• Plants infected before tasseling usually topple over while those infected at post tasseling may remain standing but exhibit wilted leaves

Management. To be successful against bacterial stalk rot:

• Provide good drainage and improve soil condition through proper cultivation

• Practice balanced fertilization. Generally, bacterial stalk rot is severe when there is excessive nitrogen in the soil in relation to potassium.

• Avoid close planting on fields previously infected with the disease

Corn Mosaic

This disease is caused by different strains of the maize dwarf mosaic virus. It is transmitted by corn aphids and infects corn plants from whorl to maturity stages.

Symptoms. General symptoms of this disease are:

• Narrow, pale yellow streaks on leaves parallel to leaf vein

• The pale area later becomes diffused upon a paler green background

• The base of infected leaves produces alternating broad streaks of green and pale green areas

Management. To be successful against corn mosaic:

• Plant resistant varieties

• Remove infected plants as symptoms appear

• Eliminate alternate hosts like weeds

• Plant early to avoid high aphid population

Corn Smut

This disease is caused by the fungus Ustilago maydis. Disease development is favored by high nitrogen level or high quantities of manure. Injuries due to blowing soil or sand particles, cultivation or detasseling may greatly increase the potential for smut infection.

Symptoms. General symptoms of this disease are:

• Galls on aboveground parts, which are glistening white or grayish white tissues

• These galls later turn black due to masses of powdery black spores

Management. To be successful against corn smut:

• Follow a well balanced fertility program based on soil analysis

• Avoid injuring corn plants during cultivation

• Plant resistant varieties

Leaf Spot

This disease is caused by the fungus Helminthosporium maydis.

Symptom. Presence of lesions that vary from small to large elongated or oval tan to brown spots. Under favorable conditions, these lesions coalesce completely burning large areas of the leaves.

Management. Plant resistant varieties to help reduce occurrence of the disease.

Ear Rot

This disease is caused by the fungus Diplodia maydis or Diplodia macrospora.

Symptoms. General symptoms of this disease are:

• Presence of bleached to straw-colored lesions starting from the base of the ear

• Husks tightly pasted together by the white fungus mycelium

• Black pycnidia scattered on the surface of the husks, kernels and cobs

Management. Avoid using highly susceptible varieties. Practice deep plowing and field sanitation.