There are so many diseases in maize crop here i m discussing about five diseases of maize crop.1. Bacterial Stalk Rot: Erwinia carotovora, Erwinia chrysanthemi
- This disease occurs in many states including Andhra Pradesh where high temperatures coupled with high humidity develop during the pre-flowering stage of the crop.
- Borer insects play a significant role in initiation of the disease. The organism is soil borne and makes its entry through wounds and injuries on the host surface.
- The organism survives saprophytically on debris of infected materials and serves primary inoculum in the next season.
- High humidity especially overhead irrigation and waterloging, coupled with high temperature (30C and above), are conducive for disease development.
- The basal internodes develop soft rot and give a water soaked appearance. A mild sweet fermenting odour accompanies such rotting.
- Leaves some time 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 hang down simply from the plant
- Use of disease resistance varieties, While no absolute resistance has been identified so far, Hybrids Ganga Safed-2, DHM 103, show significantly
- less disease incidence than other hybrids.
- Avoid waterlogging and poor drainage.
2. Black Bundle Disease and Late Wilt: Cephalosporium maydis, Caphalosporium acremonium
Both the pathogens survive in soil, plant debris and seed. The disease is fovoured by post flowering water stress Symptoms
- The disease kills the plant prematurely after flowering. Infected plants do not show symptoms until they reach to tasseling.
- Wilting generally starts from the top leaves, Leaves become dull green, eventually loose colour and become dry.
- In advance stages the stalk loses its healthy green colour, lower portions become dry,shrunken with or without wrinklings, hardens and turn purple to dark brown which in more prominent on lower internodes.
- When split open diseased stalks, show brown vascular bundles starting in the underground portion of the roots.
- Diseased plants produce only ears with undeveloped shrunken kernels.
- In severe cases affected plants remain abortive causing 100 per cent loss.
- Cephalosporium maydis is primarily soil borne and may infect young maize plants more readily than other plants through roots or mesocotyl. In case of C. acremonium only vascular burdles get blackened.
- Use of resistant varieties like DHM 103, DHM 105, Ganga Safed 2.
- Crop sanitation, crop rotations.
- Avoiding water stress at flowering.
- Seed treatment with Thiram or Captan 3g/kg seed.
3. Charcoal-Rot : Macrophamina phaseolina
Charcoal rot is prevalent in comparatively drier maize growing areas, particularly Andhra Pradesh, West Bengal, Bihar, Uttar Pradesh and Delhi.Epidimeology
- The disease development is maximum during grain filling stage and is favoured by warm temperature (30-40C) and low soil moisture.
- Charcoal rot is found to be prevalent particularly in rabi season when summer temperatures during post flowering period becomes comparatively high (35 - 45C).
- Disease cycle
- The pathogen also attack many other hosts, which helps in its perpetuation. Since the fungus is a facultative parasite it is capable of living saprophytically on dead organic tissues, particularly many of its natural hosts producing sclerotial bodies.
- The fungus over winter as a sclerotia in the soil and infects the host at susceptible crop stage through roots and proceed towards stem.
- The characteristic symptoms of the disease become apparent as the plants approach maturity. The disease generally appears early after flowering.
- Plants affected by M. phaseolina show evidence of pre-mature ripening. The out sides of lower internodes become straw coloured. The pith becomes badly disintegrated.
- The infected stalks may split longitudinally into a mass of fibres.
- A distinguishing character of the disease is the presence of the small black sclerotia in the pith of the affected stalks. Roots are also invaded and show black sclerotia in the disorganised tissue.
- Regular irrigations particularly during flowering time should be provided.
- Use resistant varieties like DHM 103, Ganga Safed - 2 and avoid sowing of suceptable varieties like DHM 105.
- Seed treatment with Carbendazim or Thiram 3g/kg seed is effective.
- Field sanitation, crop rotation should be followed.
4. Common Rust : Puccinia sorghi
- This disease is prevalent in cooler parts of the country but its destructiveness is more apparent in rabi maize in Bihar.
- It is very common in Himalayan region during Kharif season and in South India during rabi season.
- Considerable yield losses were observed in coastal districts in late plantings during rabi.
Cold temperature (16-23°C) and higher relative humidity favor the rust development and spread.Symptoms
- Circular to elongate golden brown or cinnamon brown, powdery, erumpent pustules appear on both leaf surfaces
- 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.
- Plant hybrids like Deccan, Ganga-5, Deccan Hybrid Makka-103 and DHM - 1 which are resistant to this disease to minimise the disease intensity.
- Spray Mancozeb 2.5g/lit or Dithane M-45 spray can be taken (0.4%) as soon as first symptoms are observed and it can be repeated at 10 days interval till flowering.
5. Downy Mildews
- Downy mildews are an exciting group of fungi which attacks many economically important crop plants.
- Ten downy mildew diseases are known on maize caused by two genera viz., Peronosclerospora and Sclerophthera.
- Out of these, Sorghum Downy Mildew and Brown stripe downy mildew are common in our country.
- 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%. Significant loss is usually localized to late planted areas.
- Early maize crops escape infection because by the time conidia are produced they develop resistance on the collateral host susceptibility occurs only upto about 15 days of age.
- In susceptible seedling plants, less than 4 weeks after seedling infection becomes systemic in all growth, subsequent to downward growth of mycelium and colonization of shoot apex (growing point). All Peronosclerospora spp. induce both local and systemic infection.
- Malformation of tassels in infected plants.
- Chlorosis, white stripes, stunting with downy fungal growth on both leaf surfaces are the characteristic symptoms.
- The disease has been know to occur through a collateral host Heteropogen centortus on which the fungus perpetuates of the host.
- The role of Heteropogen as a collateral host was proved by the cross inoculation tests and occurrence of the disease in the fields adjacent till both conidial and oospores, formed on the collateral host, serve as the primary incolum.
- The breakdown of tissue causes shredding. The oospores either fall to the soil or are wind blown, 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 20-23C.
- In India, it has been most severe in Uttar Pradesh, Himachal Pradesh, Southern Rajastan, the Punjab and Hilly parts of West Bengal, especially in areas, which receive 100-200 cm of precipitation.
- A yield loss of 63% was recorded in the Tarai region of Uttar Pradesh
- The Oospores present in soil serve as source of primary infection.
- They form sporangia and the infection appears in the form of minute flecks (Characteristics 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.
- Disease symptoms have been observed only on leaves. They are vein limited.
- Wilting generally starts from the top leaves; Leaves become dull green, eventually loose colour and become dry.
- Chlorotic stripes, 3-7 mm wide will develop and they further extend in parallel fashion and may in severe cases cover the entire leaf lamina.
- Severe infection also incites blotching. The stripes in advanced stage become necrotic with purple or reddish colour and present a burnt appearance.
Crazy top downey mildew : Sclerophthora macrosporaSymptoms
- Disease symptoms first appear as rolling and twisting of upper leaves before mallformation of tassel.
- The important symptoms of the disease is the partial or complete malformation of the tassel which continues until the tassel resembles a maas of narrow, twisted, leafy structures.
- Affected plants show excessive tillering, stunted with chlorotic stripes on leaves.
- Sugarcane downey mildew : Peronosclerospora sacchari
- Downey growth is seen on the both leaf surfaces. Plants may be distorted with small, poorly filled ears with mis-shapen tassels.
- Characterized by local lesions and systemic infection. Initial leasions are small, round, chlorotic spots on the leaves.
- Systemic symptoms appear as pale yellow to white stripes or streaks at the base of the 3 oldest leaves.
- Several streaks may develop on leaf and may extend on the entire leaf.
- High relative humidity with a temperature of 20-25° C favours the disease development. Zinc deficiency predisposes plants to infection
- Control measures for downey 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 and other methods.
- Seed treatment with Metalaxyl at 4 g/kg and foliar spray of Mancozeb 2.5 g/l or Metalaxyl MZ at 2g/l is recommended.
- Use of resistant varieties like DMR 1, DMR 5 and Ganga 11.