Maize: Zea mays Linn.

1. SMUT

The disease is prevalent in all maize growing areas of the world except Australia and New Zealand. The disease is mostly confined to the Himalayas, Kashmir valley and West Bengal.

Symptoms: The pathogen produces as soft tumors or galls on almost all plant parts of above ground. The galls are formed on the ears, axillary buds, tassels, stalks and may be some times on the leaves. The disease is become apparent when galls of different sizes are seen. The size of gall may reach upto 10 cm or more in diameter. The outer tissues of the gall are dull white or grey coloured, much shiny from the surface. The smut produces black powdery mass of spores. The young spores mixed with gelatinous matrix become dry on maturity. The galls on young seedlings may result in extreme dwarfing or death of maize plants.
Causal organism: The smut disease of maize is caused by Ustilago maydis (DC) Corda. The spores are spherical to ellipsoidal, 8-11 µm in diameter, brown or black with spiny ornamentation on walls. The spores produce one or more septate promycelium which bears unicellular ovate sporidia. Disease Cycle: The maize plants are infected through wounds or natural openings from the air borne spores and sporidia. The heterothallic pathogen released haploid sporidia from promycelia, produce infection hyphae and cause weak infection in the host plants. When two compatible haploid hyphae fuse in the host tissues, they produce dikayotic pathogenic hyphae which become diploid at gall formation stage. Teliospores are produced in the galls and overwinter in the soil and maize stubbles. The teliospores are diploid and germinate in next growing season by forming four celled basidium and each cell of basidium bears long basidiospores. The basidiospores are of two types or strains denoted with + and - symbols that infect young maize plants.		(Photographs with courtesy of Leonardo Minaré Braúna)

Disease Management: The control of smut disease by sanitation practices or chemical fungicides is very difficult. The cultivation of smut resistant varieties is the most effective method to control the disease.

2. BROWN SPOT

The disease has been reported in many maize growing areas of the world. The important countries are China, India, Japan and USA. The brown spot disease of maize has minor significance in India but may be severe under favourable environmental conditions. The disease was first time reported in India from Bihar by Shaw in 1910.

Symptoms: The symptoms of the disease occur on leaf blade, leaf sheath and culms. The primary symptoms are water soaked lesions of light green at first and turn reddish brown to brown finally on the leaves. The oval shaped lesions may coalesce to form a large brown blotch. The leaf blade at basal portion has more number of spots and blotches. The affected leaves die prematurely. The same type of symptoms is also present on leaf sheath and stem. The stem tissues are weakened due to severe infection of the pathogen and may breaks at the point of infection. (Photograph with courtesy of Leonardo Minaré Braúna)
Causal Organism: The pathogen causing brown spot disease of maize is Physoderma zeae-maydis Shaw. The coenocytic hyphae of the pathogen produce smooth and brown coloured flat sporangia measures 24-26 x 22-24 µm. The sporangia open through a lid and produce uniflagellate, hyaline, thin walled zoospores measures 5-7 x 3-4 µm. These zoospores, when come in contact with the leaves, germinate and infect host cells. The infected plants produce sporangia inside the host tissues. These sporangia may remain viable for 4-7 years. The primary infection is found through the soil borne sporangia and secondary infection is by spores formed on host surface carried by air. (Photograph with courtesy of Leonardo Minaré Braúna)

Disease Cycle: The fungus is an obligate parasite, persisting inside the host tissue even after crop harvest. At the next cropping season, these sporangia disseminate as air borne spores, releasing zoospores when thin water film is available. The zoospores are attached with leaves and germinate by producing infection hyphae. The hyphae enter into the host tissues and produce characteristic symptoms of the disease. The disease is occurring in severe form if high temperature (27-29 °C) and moisture are present at the early growth period of the host.

Disease Management: The field sanitation practices are useful and reduce the inoculum potential. The long crop rotation is recommended.

3. RUST

The disease is commonly found in warmer countries of the world where maize crop is grown such as Africa, India, South Europe and USA. In India, The disease occurs in southern states and also on hybrid varieties. The dense maize plantation under favourable environmental conditions is affected more severely by the pathogen.

Symptoms: The minute and round to elongated uredosori are present on both the surfaces of leaves in groups or scattered. The rust pustules are yellow coloured at early stage but later they turn brown and surrounded by chlorotic haloes. The entire plant looks unhealthy and pale in severe infection. As the disease advanced, the uredia break off and release reddish brown coloured mass of spores. The dark brown and elongated telia are also present at this stage on the leaves. In the severe form, the leaves dry prematurely causing heavy reduction in grain yield. (Photograph with courtesy of Leonardo Minaré Braúna)
Causal Organism: The rust disease of maize is caused by Puccinia sorghi Schw. The pathogen has heterocious life cycle and it is complete on the two alternate hosts. The uredia and telia are produced on maize and teosinte and the aecial stage is produced on three species of Oxalis. The uredospores are ovoid or elliptic, light brown, minutely spiny, measure 23-33 µm in diameter with four germ pores. The teleutospores are oblong to ellipsoidal, two celled, slightly constricted at the septum, thick walled, round to flattened at the apex, bright chestnut brown coloured, with long pedicel, borne singly on long stalks and measure 30-44 x 11-17 µm. The aeciospores are globose, pale yellow and finely verrucose. (Photograph with courtesy of Leonardo Minaré Braúna)

Disease Cycle: The fungus is an obligate parasite, completing its life cycle on two alternate hosts. The pathogen produces uredia on maize plants which repeat several cycles during the same season. The cultivation of maize crop after maize in the same field or neighbouring fields and the crop is available to the pathogen round the year, facilitating the perpetuation and survival of the pathogen.

Disease Management: The most effective method to control the disease is cultivation of resistant varieties.  

4. LEAF BLIGHT

The leaf blight of maize was first reported from Italy in 1876. The disease has been reported from India, Japan, South Africa and USA. In India, the disease is prevalent in many maize growing states.

Symptoms: The primary symptoms of the disease appear on leaves as small, yellow coloured, round or oval spots which extended along the leaf. The affected leaf tissues gradually become dark coloured and under favourable environmental conditions covered with velvety dark green patches of fungus fructifications. The leaves dried up and showed blight appearance. The growth of plants remains stunted. (Photograph with courtesy of Leonardo Minaré Braúna)
Causal Organism: The leaf blight of maize is caused by Helminthosporium turcicum Pass. The fungal hyphae are present both intra and intercellularly in host mesophyll tissue. The conidiophores arise in groups from stomata situated in diseased leaf surface. The conidiophores are long, narrow, unbranched, slightly curved, septate and yellowish brown. Each conidiophore bears terminal conidium which is olivaceous brown, 3-7 transverse septa, slightly curved, tapering towards the rounded ends and varies in size. (Photograph with courtesy of Leonardo Minaré Braúna)

Disease Cycle: The leaf blight of maize is a soil borne disease. The pathogen survives in the form of conidia on plant debris in the soil. The conidia germinate by germ tube arising from one or both ends. They infect young healthy maize plant leaves and cause leaf blight disease.

Disease Management: The crop rotation and other sanitation practices should be used to minimize the disease inoculum. The disease can be effectively control by spraying of Mancozeb or Zineb.