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Crop Specific
  Disease Management


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Smuts

Galls on stalks and leaves
  • There are two smuts of maize, both worldwide in distribution. They are
  1. Common smut [Ustilago maydis (DC) Cda.] and
  2. Head smut [Sphacelotheca reiliana (Kuhn) Clinton].
Malformed Tassel
  • Common smut is one of the worst diseases of corn in the U.S.A., but S. reiliana occurs only rarely in the country.
  • The two diseases are also known to occur in many parts of Europe, Russia and Australia.
  • In India U. maydis is mostly confined to the Himalayas, Kashmir valley and West Bengal, whereas head smut is found sporadically in other parts also.U. maydis produces gall on the ears, axillary buds, tassels, stalks, and more rarely, on the leaves.
  • The infection first becomes apparent when galls of different sizes.
  • The epidermal tissues of the gall are dull-white or grey in colour, much bulged from the surface and shiny.
Aborted leafybud
  • They rupture to expose the black powdery mass of spores.
  • The spores are mixed up with gelatinous matrix when young, but become dry with maturity.
  • They are spherical, brown in colour, have spiny walls, and measure 7-12 u in diameter.
  • On germination they may produce one or more promycelia, which bear the sporidia.
  • Infection of the host takes place through wounds or natural openings and the initial infections stem from the air-borne spores and sporidia. The sporidia are haploid when released from the promycelium, and produce infection hyphae which cause only weak infection. The fungus is heterothallic and when compatible haploid hyphae fuse in the host they produce dikaryotic pathogenic hyphae, which become diploid at the gall formation stage. The work done by Stakman and his co-workers in the U.S.A. has helped to understand the genetic aspects of the common corn smut fungus. Many biotypes of the fungus have been recognized, which differ in their pathogenicity.
  • It is very difficult to control corn smut, either with sanitation practices or with chemical protection. Growing smut-resistant varieties is the most effective method of control and several smut-resistant hybrid varieties are available for cultivation.
  • Head smut caused by Sphacelotheca reiliana is similar to the one on sorghum, but the two organisms belong to distinct physiologic races. On maize it affects the pistillate and staminate inflorescenes, forming tumours in the place of flowers and the floral parts are covered by a black powdery mass of smut spores. The sori vary in size from one to several inches in length; enclosed in the sori are masses of spores mixed with a network of fibrous remnants of host tissue.
  • The infection is mainly through soil-borne spores which germinate and enter the young seedlings. The fungus grows along the apical tissues of the host, producing the sorus at the time of flowering, on both terminal and lateral floral buds.
  • The disease is checked by adopting careful sanitation and crop rotation measures. Certain varieties of corn are highly resistant to this smut and work on this line for developing smut-resistant varieties suitable for different agronomic regions of India needs to be done.

Brown Spot

Brown diffuse spots on midrib
  • The disease is common in several maize growing regions of the world, including India where it is found in many areas.
  • It was first noticed in Bihar in 1910. Since it occurs sporadically in mild form, causing little or no damage, it is of minor importance.
  • The fungus infects teosinte also.
  • The disease is common in low lying and ill-drained fields.

Symptoms

  • The early symptoms are water-soaked lesions which are light green at first but reddish brown and finally brown. They are oval shaped in the beginning but several spots may coalesce to form brown blotches. These spots and blotches are more concentrated at the basal portion of the leaf blade. Such symptoms also occur on the leaf sheath and stem. When the stem is infected the tissues are weakened and the stem breaks at the infected point, causing severe damage.

Causal Organism : Physoderma zeae-maydis Shaw

Sub. Division : Mastigomycotina

Order : Chytridiales

Family : Physodermataceae

  • The fungus produces coenocytic hyphae and sporangia which are flat, smooth and brown in colour, measuring 24-26 x 22-24 u. On germination, the sporangia open through a lid, liberating uniciliate, hyaline, thin-walled zoo-spores which measure 5-7 x 3-4 u.

Disease Cycle

  • The fungus is an obligate pathogen, persisting inside the host-tissue even after harvest. During the next cropping season the sporangia spread as air-borne spores, releasing zoospores when sufficient moisture is present.
  • The zoospores become attached to the young leaves and germinate to produce infection hyphae, which eater the host tissue to cause the characteristic spots.
  • High temperature (28-29oC) and abundant moisture during the early growth period of the host favour the disease development.

Control

  • Field sanitation practices may reduce the inoculum potential. Though some verietal variations in the reaction to the disease have been reported, because of its minor importance no serious attempt has been made to control it by breeding resistant varieties.

Rust

  • This is a common disease in many countries, including India. The fungus also attacks teosinte. During recent years it has been reported in severe form in some parts of India, particularly on the hybrid maize varieties grown under intensive cultivation practices.

Symptoms

Brown pustules

  • Minute, roundish to elongated uredia occur on both leaf surfaces and sometimes on the husk and other floral parts.
  • The rust pustules are yellowish in the early stages but later become brown, and are surrounded by chlorotic haloes; in severe infections the entire plant becomes pale and looks unhealthy, even from a distance. As the diseases advances, the uredia rupture to release reddish brown spore masses.
  • AT this stage the telia are also found on the leaves.
  • They are dark brown, elongated and scattered all over the leaf blade.
  • In severe cases the leaves dry causing severe reductions to grain yield.

Causal Organism : Puccinia sorghi Schw.

Sub. Division : Basidiomycotina

Order : Uredinales

Family : Pucciniaceae

  • P. sorghi, which is far more important that P. polyspora Underw., produce uredial and telial stages on maize and teosinte and the aecial stage is believed to be produced on three species of Oxalis, i.e., O. corniculata, O. europaea and O. stricta. The uredospores are ellipsoidal, finely echinulate, yellowish brown and measure 23-32 u in diameter.
  • The teleutospores are oblong to ellipsoidal, two celled, thick walled, slightly constricted at the septum, rounded to flattened at the apex, dark brown, with a long pedicel, and they measure 28-45 x 12-17 u.
  • The acciospores are globose to ellipsoidal, pale yellow and finely verrucose. Physiologic specialization in this rust has been reported in other countries. Recent studies in India have indicated the possible existence here of physiologic races, and more studies are needed in this direction.

Disease Cycle

  • The fungus is an obligate pathogen, completing its life cycle on more than one host. On maize it produces uredia which repeat the cycle several times during the season.
  • The role of telia under the conditions prevailing in India is not well understood. Since the uredospores can survive by repeating the life cycle on maize and other hosts, perpetuation of the fungus seems assured.
  • In some areas maize is grown after maize in the same field or neighbouring fields and the crop stands in the area almost all the year round, facilitating the perpetuation and survival of the pathogen.

Control

  • The only practical method of effectively contorlling the rust disease is to grow resistant varieties. Since the disease is gaining importance in India, more work is required to evolve rust-resistant maize varieties. Among the available varieties Cuzeo is reported to be resistant to all biotypes of the rust in India.

Leaf Blight


Lesions on leaf
  • Leaf blight is a common disease of maize, sorghum and other related crops in many parts of the world, including India.
  • The causal organism Helmnthosporium turcicum Pass. belongs to the Deuteromycotina.
  • Its perfect stage is Trichometasphaeria turcica (Pass). Luttrell.
  • Other species of Helminthosporium, i.e., H. maydis Nishik. and Miy., H. rostratum Drechs., and H. carbonum Ullstrup also cause leaf spot or stripe symptoms.
  • The fungal conidia spread through air, and the primary inoculum is from the crop residues.
  • The incidence of leaf blight increases with increasing doses of nitrogenous fertilizers and is less severe when the crop is supplied with a balanced fertilizer.
  • The disease cycle and methods of control are similar to those described under sorghum.
  • Recent studies have revealed that the leaf blight disease on maize could be effectively controlled by spraying either Captan or Zineb.

Symptoms on leafsheath

Several other diseases of maize have been reported in India. They are

  • Seedling blight and wilt Fusarium moniliforme (Sheld) Wine.
  • Seedling blight and top rot : Givverella zeae (Schew.) Petch.
  • Stalk and ear rot Diplodiamaydis(Bert.) Sacc
  • Charcoal rot Macrophomina phasecolina (Tassi)Gold.
  • Bacterial stalk rotPseudomonas syringe pv. lapsa.
  • Bacterial stalk rotErwinia carotovorasub.sp.
  • dissolvens.
  • Bacterial stalk and ear rot E.carotovora sub.sp. zeae.
  • Bacterial leaf spot Xanthomonas campestris pv. maydis.
  • Bacterial leaf stripe X.campestris pv. rubrilineans
  • (Lee et al.) starr and Burk.
  • Leaf spot: Tramatosphaeria maydis (P.Henn.) Rang.et al.
  • Leaf spot Didymella exitials (Mor) Müller.
  • Leaf spot Diplogia macrospora Earle.
  • Leaf spot Hyopchnus sasskhii Shirai.
  • Zonate spot Gloeocercospora sorghi Bain Edg,.
  • Downy mildew Sclerospora phillippinensis Weston.
  • Sclerophthora rayssiae var. zeae Payak and Renfro.
  • Ear rot Cephalosporium acremonium Corda.
  • Rhizoctonia zeae Voorchees.
  • Mosaic, Dwarf-mosaic, rough dwarf, streak and vein evation.
  • The most widely distributed of these diseases is downy mildew, caused by Sclerospora phillippinensis and Sclerophthora rayssiae var. zeae. The former species produces a characteristics downy growth on the leaves, followed by yellow discolouration, browning and necrosis of the blade and stunting of the affected plant (Fig.99). It is also produces numerous hyaline, thin walled, ellipsoidal conidia on dichotomously branched conidiophores. The conidia germinate with germ tubes, which infect the maize plants. The leaf shredding symptoms and the associated oogonial and oospore stage of the fungus are rare in India.
  • S.rayassiaevar. zeaecauses choloratic yellowish stripes on the leavea and the stripes are delimited by the veins. These stripes laterturn brown or straw coloured and become necortyic. The sporangia are hyaline, broadly fusiform or ovate, borne on short sporangiophores. The sporangiophores arise through stomata and bear the sporangia on sympodial branches,. This fungus causes severe damage to some of the hybrid maize varieties like Ganga 3 and VL 54 and also to most local varieties.
  • The various stalk and root diseases, including the seedling blight and wilt caused by Fusarium moniliforme, Gibberella zeae and Macrophomina phaseolina, occur in more severe form in some seasons than in others, especially in ill-drained soils. The stalk rots caused by Pythium aphanidermatum and Diplodiamaydis are favoured by high atmospheric humidity and appears in devastating proportions on young plants soon after heavy rains. In all these cases the fungi are persistent in soil and become active when conditions favourable for infection set in.
  • Five bacterial diseases of maize have been reported in this country. Of these Ps. Syringae pv. Lapsa, E. carotovora sub. Sp. dissolvens and E. carotovora sub. Sp. zeae are relatively important, causing severe damage. These organisms infect the basal portions of t he stem, causing soft rot and sudden wilting of the plant. Also E. carotovora sub. Sp. zeae causes rot of the ear head and shoot. X.campestris pv. Maydis and X. campestris pv. Rubrilineans infect the leaves and are less common, though the latter is reported to cause severe damage on a few cultivars in some parts of North India. All of the bacterial diseases of maize are difficult to control. More work is required to understand the life cycle of the diseases and to work out effective control measures.
  • Of the several leaf spot diseases listed above, Phaeosphaeria maydis causes small pale green or chlorotic lesions, which enlarge to form dry oblong spots with brownish margins; Didymella exitialis produces small elliptic spots which elongate to form whitish or creamish streaks or patches; Diplodia macrosporas produces large greyish to tan lesions which are several cm. Long and about one cm, broad; Hypochnus sasaki produces large discoloured areas alternating with dark bands on leaves, which often droop to touch the ground; the zonate leaf spot caused by Gloeocercospora sorghi is similar to the one caused by the same fungus on sorghum. Every one of these leaf infections could be effectively checked through prophylactic sprays with fungicides such as Zineb or Captan. Since Copper is highly toxic to maize, copper containing fungicides should be avoided.
  • The varies types of infections of the maize ear cause considerable damage. These infections, coming early in the season, cause more damage than later, when the ear is nearing maturity. Both bacteria and fungi seem to be active in this respect. The ear rots caused by Cephalosporium acremonium and Rhizoctonia zeae may infect the ear and continue to cause damage in storage godown, if the ear after harvest is not properly dried before storage.
  • Maize mosaic was first reported in India in 1960, and since then it has been found in several tracts. This virus is reported to be similar to the one causing extensive damage to the maize crop in the U.S.A. The disease causes characteristic mosaic symptoms on the leaves, and the diseased plants produce lighter ear and immature seeds In severe cases maize mosaic may reduce the grain yield up to 30 per cent.
  • At least three different strains of the virus have been differentiated. The vector is Rhopalosiphum maidis. Sugarcane mosaic virus is also known to infect maize, causing characteristic mosaic symptoms.
  • It is common on sugarcane, sorghum and maize, but the damage to maize is much less than that caused by the maize mosaic virus. It is reported that sugarcane mosaic virus gives cross-protection to maize mosaic virus, indicating that the two are closely related. The streak disease is characteristic of chlorotic leaves and stunted plants.
  • The affected plants produce poorly filled heads. The virus is transmitted by Cicadulina mbila. It is identified a Pennisetum strain of maize streak virus.
  • In addition to the pathogenic diseases, there are a few deficiency diseases of maize in many parts of India. Maize, being very sensitive to major and minor element nutrition, reacts very quickly to deficiencies in any of these elements, the most common of which are nitrogen, zinc, phosphorus, potassium, and magnesium.

 
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