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Disease Management

Diseases

1. Turcicum leaf blight  (Helminthosporium turcicum)

Symptoms
•Symptoms will appear as water-soaked oval spots on leaves.
•These grow into elongated, spindle-shaped necrotic lesions.
•At later stages the spots will become cigar-shaped and 3 to 15 cm long.
•Later the spots become elliptical and tan colour, developing dark areas as they mature that are associated with fungal sporulation. Lesions may coalesce, blighting the entire leaf.
• They first appear on lower leaves and later, on the upper leaves, leaf sheath and cob.

Disease cycle
•Infected seeds, overwintering of mycelium and chlamydospores in crop debris are the primary source of infection.
•Secondary infection occurs mainly through wind-borne conidia.

Epidemiology
•It occurs during May to September. The pathogen requires the temperature range from 17 to 27oC and extended period of dew or leaf wetness.

Management
•Growing resistant hybrids like DHM-1.
•Removal of plant residue of the previous crop.
•Crop rotation.
•Ploughing the field under infected crop residue to reduce the inoculum.
•It is important to control the disease during the period from 14 days before to 21 days after tasseling for optimal management.

2. Maydis leaf blight (MLB) (Bipolaris maydis)

Symptoms


•The pathogen produces lesions that are small (0.6 to 2.5 cm) and diamond-shaped.

•As they mature, the elongated lesions may coalesce, producing a complete
“burning” of large areas of the leaves.
•They vary in size and shape among inbreds and hybrids with different genetic background.
•Race 'O' produced tan, elongated (2-6 x 3-22 mm) lesion between the veins with limited margins, with buff to brown borders, usually attacks only leaves.
 
Maize (Zea mays L.)
 

Disease cycle
•Overwintering plant residues and infected seeds are the main source of infection. Secondary infection is through wind-borne conidia.

Epidemiology
•It occurs during May to September.
•The disease occurs in the presence of free moisture and temperature of 15.5 to 26.5oC.

Management
•Growing resistant varieties - Deccan, VL-42, Prabhat, KH-5901, PRO-324, PRO-339, ICI-701, F-7013, F-7012, PEMH-1, PEMH-2, PEMH-3, Paras, Sartaj, Deccan-109.
•Removal of infected crop debris.
•Crop rotation.
•It is important to control the disease during the period from 14 days before to 21 days after tasseling for optimal control.

3. Bacterial stalk rot (Erwinia carotovora, Erwinia chrysanthemi)

Symptoms
•The stalk near the soil region will present brown and water-soaked appearance.
•Infected stalk will give unpleasant odour.
•Leaves sometimes show signs of wilting or water loss and affected plants within a few days of infection lodge or topple down. Ears and shank may also show rot.
•They fail to develop further and the ears simply hang down from the plant. Infected plants die prematurely.

Disease cycle
•The organism is soil-borne and makes its entry through wounds and injuries made by insects like borer on the host surface.
•The organism survives saprophytically on debris of infected materials and serves as primary inoculum in the next season.

Epidemiology
•High humidity especially overhead irrigation and water-logging, coupled with high temperature (30oC and above)
are conducive for disease development.

Management
•Use of disease resistance varieties.
•Providing sufficient drainage as the disease is more prevalent in poorly-drained soil.
•Application of bleaching powder @ 3 g/10 l of water.
 

4. Pythium stalk rot (Pythium aphanidermatum)

Symptoms
•Usually the basal internodes become soft, water-soaked and dark, causing lodging.
•Damaged internodes commonly twist before the plants lodge.
•Diseased plants can remain alive until all vascular bundles become affected.
•Plants can be affected prior to flowering.

Epidemiology
•It is more common in extended hot (32oC and above), wet weather from June to September.

Management
•Maintain plant population at 50,000 per ha.
•Adequate drainage should be provided.
•Remove the previous crop residue.
•Crop rotation.

5. False head smut (Ustilaginoidea virens)

Symptoms
•It occurs during May to June.
•Only few florets in the tassel get converted into dark green masses of spores causing yield reduction.

Management
•Crop rotation with pulses.
•Destruction of straw and stubbles.
•Uprooting of infected plants.

6. Common Rust (Puccinia sorghi)

Symptoms
•Minute, circular to elongated uredia on both leaf surfaces and also appear sometimes on the husk and floral parts.
•As the crop matures brownish black pustules containing dark, thick-walled, two-celled teliospores develop.
•In severe cases infection spreads to sheaths and other plant parts.

Epidemiology
•It occurs during April to September.
•Cold temperature (16-23°C) and higher relative humidity favor this disease.

Management
•Use of hybrids like Deccan, Ganga-5, Deccan Hybrid Makka-103 and DHM-1 which are resistant to this disease.
•Removal of  alternate hosts like species of Oxalis spp.

7. Downy mildews
Ten downy mildew diseases are known on maize caused by two genera viz., Peronosclerospora and Sclerophthora.
 
Out of these, Sorghum Downy Mildew and Brown stripe downy mildews are more common.

 7.1 Sorghum downy mildew (Perenosclerospora sorghi)
Greatest loss results from systemic infection. This most often results in death or barrenness. Local infection probably does not cause economic loss in maize. Losses have been reported to be as high as 40-60 per cent. Significant loss is usually localized to late planted areas.

Symptoms
•Early maize crops escape infection because by the time conidia are produced they develop resistance on the collateral host. Susceptibility occurs only up to about 15 days of age.
•In susceptible seedling plants, less than 4 weeks after seedling infection becomes systemic.
•All Peronosclerospora spp. induce both local and systemic infection.
•Malformation of tassels can be seen in infected plants. Chlorosis, white stripes, stunting with downy fungal growth on both leaf surfaces are the other characteristic symptoms.

Disease cycle
•The disease has been known to occur through a collateral host Heteropogen centortus on which the fungus perpetuates in the host.
•The role of Heteropogen as a collateral host was proved by the cross inoculation tests and occurrence of the disease in the adjacent fields till both conidial and oospores formed on the collateral host serve as the primary inoculum.
•The breakdown of tissue causes shredding. The oospores either fall to the soil or are windblown, often within host tissue.
•They can remain viable in the soil for 5-10 years. Conidia are formed at night in large numbers. The optimum temperature for production is 20-23oC.

7.2 Brown stripe downy mildew (Sclerophthora rayssiae)

Symptoms
•Sclerophthora rayssiae var. zeae causes only leaf lesions and they are vein limited.
•In early stages of infection the leaves will show narrow chlorotic or yellowish stripes 3-7 mm wide.
•At later stages of infection, the chlorotic stripes take on a reddish or purple color. The disease first appears on the lowest leaves.
•Severely affected leaves may be shed prematurely, but affected leaves remain intact and do not shed, even after severe storms.
•When disease occurs prior to flowering, seed development is adversely affected, and early plant death may result.

Disease cycle
•The oospores present in soil, seed and plant debris serve as source of primary infection.
•They form sporangia and the infection appears in the form of minute flecks (characteristic of zoosporic infections) on the leaves, which grow to form the brown streaks. Secondary infection is about by sporangia formed on the lower surface of infected leaves.

Epidemiology
•It occurs during May to July. Moisture is the most important environmental factor affecting disease development.
•Sporangia require at least a 12 hours moisture period.
•Warm soil temperatures (28-32.5°C) are required for disease development. Young plants are most susceptible to infection with susceptibility decreasing as the plants age.
 

Management for downy mildew diseases
•The  eradication of collateral and wild hosts near maize field and rouging infected maize plants has been recommended.
•Destruction of plant debris by deep ploughing.
•Proper spacing should be maintained.
•Seed should have low moisture content (9 per cent) before sowing.
•Use of resistant varieties like DMR-1, DMR-5 and Ganga-11.
•Early planting is recommended for Sorghum downy mildew.
•Removal of infected crop debris.
•Provision of drainage.
•Weed control to increase aeration within the crop and reduce moisture levels in the soil.
•Avoiding crop rotation with alternate hosts of downy mildew like

Author: Raghavendra Singh, H. Kalita, Chandan Kapoor, R. Gopi, Subhash Babu and S.K. Das