Diseases in Maize- Identification and Control Measures

Plant diseases are of paramount importance to humans because they damage plants and plant products on which humans depends for food, clothing, furniture, the environment and in many cases the housing. The kinds and amounts of losses caused by plant diseases vary with the plant or plant products, the pathogen, the locality, the environment, the control measures practiced and combination of these factors (Agrios, 2005). Disease is one of the major biotic constraints to reduce crop yield and also deteriorate the quality of product that ultimately reduce the market price. The reason behind the low productivity of most of the crops in Nepal is due to the attack of many plant diseases at different stages of crop.

IDENTIFY AND CONTROL OF DISEASES

Maize is susceptible to many fungal pathogens, to a lesser no of pathogenic bacteria, nematodes and viruses, to least one mycoplasma like organisms and one parasitic like higher plant. Seventy-five fungal and three bacterial species have been recorded pathogenic to maize crop in Nepal (Manandhar, 1997). Several of them are economically important. Besides several nematode species were isolated that are known to be pathogenic in maize (Manandhar, 1997). Some virus diseases have been suspected but have not been verified in Nepal (Manandhar, 1983).

1 . Anthracnose Colletotrichum graminicola

Symptoms

Anthracnose symptoms vary widely depending on numerous factors such as genotype, age of plant and environmental conditions.

– Small oval or elongated water-soaked spots which enlarge up to 15 mm long appear on leaves
– Lesions develop a tan center and red-brown or orange border
– Lesions may coalesce to form large necrotic(dead) patches
– Severely infected leaves on susceptible hybrids may wither and die
– Fungal fruiting bodies develop on dead tissues and may produce pink or orange spore masses
– Top dieback and stalk rot

Cause
Fungus

Comments

Fungus survives the winter on crop debris. Emergence of disease is favored by high temperatures and extended periods of wet and cloudy weather – seedlings and mature plants are most susceptible to the disease.

Management
Plant hybrids resistant to anthracnose; rotating crops and plowing crop debris into soil may help reduce incidence of early season infections.

2. Cercospora leaf spot (Gray leaf spot) Cercospora zeae-maydis

Symptoms

Small necrotic spots with chlorotic halos on leaves which expand to rectangular lesions 1-6 cm in length and 2-4 mm wide; as the lesions mature they turn tan in color and finally gray; lesions have sharp, parallel edges and are opaque; disease can develop quickly causing complete blighting of leaves and plant death.

1. Brown Spots with yellow rings throughout the leaf during the growing period of the Cassava
2. Lesions that are 0.15-0.2 cm in diameter
3. Serious cases can lead to holes throughout the lesions on the leaf

Cause
Fungus

Comments

Disease emergence is favored in areas where a corn crop is followed by more corn with no rotastion; severity and incidence of disease is likely die to continuous corn culture with minimum tillage and the use of susceptible hybrids in in the midwestern corn belt of the USA; prolonged periods of foggy or cloudy weather can cause severe Cercopora epidemics.

Management

Plant corn hybrids with resistance to the disease; crop rotation and plowing debris into soil may reduce levels of inoculum in the soil but may not provide control in areas where the disease is prevalent; foliar fungicides may be economically viable for some high yeilding susceptible hybrids.

3. Charcoal rot Macrophomina phaseolina

Symptoms

Symptoms are usually first apparent at the tasseling stage; plant stalks become shredded and pith is completely rotted with stringy strands of vascular tissue left intact; small, black fungal fruiting bodies are visible in the vascular strands and give the tissue a gray coloration; fungus grows into internodes of the stalk causing the plant to ripen early and causing the stalk to weaken; plant may break.

Cause
Fungus

Comments

Emergence of the disease is favored by warm soils with a low moisture content; fungus overwinters in the soil and can also survive on other host plants which include sorghum and soybean.

Management

There are currently no available fungicides to treat the disease; avoid stressing plants by practicing good water management; rotating crops with small grains may help reduce disease incidence.

4.Common rust Puccinia sorghi

Symptoms

Oval or elongated cinnamon brown pustules on upper and lower surfaces of leaves; pustules rupture and release powdery red spores; pustules turn dark brown-black as they mature and release dark brown powdery spores; if infection is severe, pustules may appear on tassels and ears and leaves may begin to yellow; in partially resistant corn hybrids, symptoms appear as chlorotic or necrotic flecks on the leaves which release little or no spore.

Cause
Fungus

Comments

Disease is spread by wind-borne spores; some of the most popularly grown sweet corn varieties have little or no resistance to the disease.

Management

The most effective method of controlling the disease is to plant resistant hybrids; application of appropriate fungicides may provide some degree on control and reduce disease severity; fungicides are most effective when the amount of secondary inoculum is still low, generally when plants only have a few rust pustules per leaf.

5.Common smut (Boil smut, Blister smut) Ustilago zeae

Symptoms

Tumor-like galls on plant tissues which are initially green-white or silvery white in color; interior of galls darken and turn into masses of powdery dark brown or black spores (with the exception of galls on leaves which remain greenish in color); galls may reach up to 15 cm in diameter and are common on ears, tassels, shoots or midrib of leaves; galls on leaves remain small and do not burst open.

Cause
Fungus

Comments

Fungus overwinters on crop debris or in the soil and can survive for several years; fungus usually enters the plant through wounds; application of nitrogen fertilizer increases incidence of disease, while application of phosphorous fertilizer decreases infection.

Management

Although many practices may be recommended for the control of common smut, the only method that is completely effective is to grow resistant corn hybrids.

6.Downy Mildew disease Peronosclerospora sorghi (Sorghum downy mildew)

P. maydis (Java downy mildew)
P. philippinensis (Philippine downy mildew)
P. sacchari (Sugarcane downy mildew)
Scleropthora rayssiae var. zeae (Brown stripe downy mildew)
Sclerospora graminicola (Graminicola downy mildew or green ear)
Sclerophthora macrospora (crazy top)

Symptoms

Symptoms of all maize downy mildew pathogens are similar although may vary depends on cultivar, age and climate. The disease appear as early from two weeks after sowing resulting in chlorosis and stunting. In older plants the leaves shows mottling, chlorotic streaking and lesions and white striped leaves. Usually the leaves are narrower and more erect when compare to healthy plants and are covered with a white, downy growth on both surfaces.

Cause
Fungus

Comments
The disease is both air and seed born. The pathogen have several alternative hosts.

Management

Grow available resistant varieties and hybrids. Follow crop rotation with non host crops. Use suitable systemic fungicide for both seed treatment and foliar spray. Keep the fields free from weeds. Drying seeds before sowing reduces the disease incidence.

7.Giberrella stalk and ear rot Giberella zeae

Symptoms

Plants wilting and leaves changing color from light to dull green; lower stalk turns straw yellow; internal stalk tissue breaks down; interior of stalk has a red discoloration; black fungal fruiting bodies may be visible on the stalk, often at internodes, and can be easily scraped off; if fungal infection affects the ears, it produces a red mold at the tips of the ear which spreads down; early infection may result in the ear being covered in pink mycelium which causes the corn husk to adhere to the ear.

Cause
Fungus

Comments
Fungus can enter through wounds to stalk or ear; ear rot is caused by the fungus infecting silks and moving down through the ear; fungus survives on corn debris in soil and on debris of other host plants such as wheat.

Management

Stressed plants are more susceptible to Gibberella – providing adequate fertilization and irrigation can help reduce incidence of disease; control insects, especially stem and ear borers; hybrids differ in their susceptibility to the disease and further information is required in order to develop specific control measures.

8.Northern Leaf Blight Exserohilum turcicum

Symptoms

In the beginning we will notice elliptical gray-green lesions on leaves. As the disease process this lesions become pale gray to tan color. Later stage the lesions looks dirty due to dark gray spores particularly under lower leaf surface. The disease can be easily identified in the field due to its long, narrow lesions which are unrestricted by veins.

Cause
Fungus

Comments
The disease mainly spread through rain splash and wind.

Management

Follow proper tillage to reduce fungus inoculum from crop debris. Follow crop rotation with non host crop. Grow available resistant varieties. In severe case of disease incidence apply suitable fungicide.

9.Southern corn leaf blight Bipolaris maydis

Symptoms

Foliar symptoms vary with hybrid and different fungal isolate; lesions on leaves may be tan and elongated and run between leaf veins; lesions may have a buff or brown colored margin; another race of the fungus causes tan, spindle shaped or elliptical lesions with a water-soaked margin that turns into a yellow halo.

Cause
Fungus

Comments

Fungus overwinters in corn debris in soil; disease occurs worldwide but is emergence favors areas with a warm, damp climate.

Management

The most effective method of controlling the disease is to plant resistant hybrids; cultural control methods include plowing crop debris into soil after harvest and rotating crops.

Category : Bacterial

Bacterial leaf blight/stripe Pseudomonas rubrilineans, syn. Pseudomonas avenae

1.Acidvorax avenae subsp. Avenae

Symptoms

Water-soaked linear lesions on leaves as they emerge; lesions turn brown and may subsequently turn gray or white; lesions may have a red border; after the leaves are mature, lesions do not tend to extend any further; no new lesions tend to appear after tasseling; if corn variety is susceptible, mature leaves may shred after maturity.

Cause
Bacterium

Comments
Bacteria can also cause disease in oats, barley, wheat, some millets and sorghum.

Management

Resistant hybrids should be planted in areas where the disease is prevalent; plowing crop debris into soil and rotating crop may not be effective at controlling the disease due to its extensive host range.
Bacterial Leaf Streak disease Xanthomonas vasicola pv.

2. vasculorum
(syn Xanthomonas campestris pv. zeae)

Symptoms

The infected leaves initially shows narrow stripes between the veins. The initial symptoms are generally confused with gray leaf spot disease. But the lesions from bacteria appear brown, orange, and/or yellow when you infected leaves are back-lit. Also in Bacterial Leaf Streak disease the lesions show slightly wavy edges when compared to the smooth, linear lesion margins of gray leaf spot.

Cause
Bacterium

Comments

The bacteria causes gumming disease on sugarcane in several part of the world. First reported on corn in South Africa. Currently this disease is reported in Nebraska (Aug. 26, 2016), Colorado, Illinois, Iowa, and Kansas on corn.

Management
Use healthy and disease free seeds. Remove the infected plant debris and burn them. Follow crop rotation.

3.Bacterial stalk rot/soft rot Erwinia chrysanthemi
Erwinia carotovora

Symptoms

Plants suddenly beginning to lodge (bend to lie along the ground) midway through season; one or more internodes above soil line turning brown, water-soaked, soft and slimy; tissue has foul odor and mushy appearance.

Cause
Bacterium

Comments

Disease is most commonly found in plantations which have overhead irrigation systems or in areas with high rainfall; disease emergence is favored by high temperatures and high humidity.

Management
Plow all crop debris into soil in Fall; plant corn in well-draining soil to prevent waterlogged plants.

4.Goss’s bacterial blight Clavibacter michiganensis

Symptoms

Gray or yellow stripes with irregular margins on leaf surfaces; stripes follow leaf veins and contain characteristic dark green to black water-soaked spots; if infection occurs early then plant may become wilted or withered; it is common to find a crystalline residue on leaves caused by dried bacterial exudate.

Cause
Bacteria

Comments
Disease overwinters in diseased crop debris on, or close to, the soil surface; temperatures below 12°C (53.6°F) and above 40°C (104°F) bacterium grows more slowly and may even be killed off.

Management
Plant resistant sweetcorn hybrids; rotate crop; plow crop debris into soil immediately after harvest.

6.Holcus spot Pseudomonas syringae

Symptoms

Circular or elliptical spots 2-10 mm across near the tips of lower leaves which are dark green and water soaked initially but become cream to tan before turning dry and brown; lesions may have red-brown margins; large lesions may have a yellow halo.

Cause
Bacterium

Comments
Disease can occur anywhere where corn is grown but is not usually very damaging.

Management

Disease is usually not severe but if it does become a problem crops should be rotated and any debris should be plowed into the soil after harvest.

7.Stewart’s wilt Erwinia stewartii
syn Pantoea stewartii

Symptoms

The main symptoms are appearance of water soaked lesions initially. As the disease progress the lesions become long and turn pale yellow with irregular margins running in the length. The pathogen may infect the stem and causes stunting, wilting and death of plant.

Cause
Bacterium

Comments

The pathogen is mainly transmitted by maize flea beetles and to lesser extent by infected seeds.

Management

Grow available resistant varieties. Use certified healthy seeds. Remove the crop debris and burn them. Use suitable insecticide to control flea beetle.

Writer: RASHMI POUDEL (Student, Bsc. Ag 2nd Semester IAAS Paklihawa Campus)