Disease of Wheat: Rust

 Black or stem rust - Puccinia graminis tritici

Symptoms 

Symptoms are produced on almost all aerial parts of the wheat plant but are most common on stem, leaf sheaths and upper and lower leaf surfaces. Uredial pustules (or sori) are oval to spindle shaped and dark reddish brown (rust) in color. They erupt through the epidermis of the host and are surrounded by tattered host tissue. The pustules are dusty in appearance due to the vast number of spores produced. Spores are readily released when touched.

As the infection advances teliospores are produced in the same pustule. The color of the pustule changes from rust color to black as teliospore production progresses. If a large number of pustules are produced, stems become weakened and lodge. The pathogen attacks other host (barberry) to complete its life cycle. Symptoms are very different on this woody host. Other spores are Pycnia (spermagonia) produced on the upper leaf surface of barberry which appears as raised orange spots. Small amounts of honeydew that attracts insects are produced in this structure. Aecia, produced on the lower leaf surface, are yellow. They are bell-shaped and extend as far as 5 mm from the leaf surface. 

Brown or leaf rust - Puccinia triticina (P. recondita)

Symptom 

The most common site for symptoms is on leaf blades, however, sheaths, glumes and awns may occasionally become infected and exhibit symptoms. Uredia are seen as small, circular orange blisters or pustules on the upper surface of leaves. 

Orange spores are easily dislodged and may cover clothing, hands or implements. When the infection is severe leaves dry out and die. Since inoculum is blown into a given area, symptoms are often seen on upper leaves first. As plants mature, the orange urediospores are replaced by black teliospores. Pustules containing these spores are black and shiny since the epidermis does not rupture. Yield loss often occurs as a result of infection by Puccinia recondita f. sp. tritici. Heavy infection which extends to the flag leaf results in a shorter period of grain fill and small kernels. 

Yellow or stripe rust - Puccinia striiformis

Symptom

 Mainly occur on leaves than the leaf sheaths and stem. Bright yellow pustules (Uredia) appear on leaves at early stage of crop and pustules are arranged in linear rows as stripes. The stripes are yellow to orange yellow. The teliospores are also arranged in long stripes and are dull black in colour. 

Minor diseases of Maize

 Minor diseases 

1. Bacterial Stalk rot - Erwinia dissolvens

Symptoms

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 and affected plants topple down in few days. Ears and shank may also show rot. They fail to develop further and the ears hang down simply from the plant.

Disease cycle

 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

2. Mosaic - Maize mosaic potyvirus

Symptoms 

 Symptoms appear as chlorotic spots, which gradually turn into stripes covering entire leaf blade. Chlorotic stripes and spots can also develop on leaf sheaths, stalks and husks. Moderate to severe rosetting of new growth is observed. Size of stalk, leaf blades and tassel tend to be normal in late infection. 

Symptom

Pathogen 

 It is caused by Maize mosaic potyvirus. Virions are flexuous, 750-900nm long, ssRNA genome. 

Disease cycle 

 It is transmitted in nature by leaf hopper vector, Perigrimus maidis. 

Disease of Maize: Charcoal rot

 Charcoal rot - Macrophomina phaseolina (Rhizoctonia bataticola)

Symptoms 

The affected plants exhibit wilting symptoms. The stalk of the infected plants can be recognized by grayish streak. The pith becomes shredded and grayish black minute sclerotiadevelop on the vascular bundles. Shredding of the interior of the stalk often causes stalks to break in the region of the crown. The crown region of the infected plant becomes dark in colour. Shredding of root bark and disintegration of root system are the common features. 

Symptom

Pathogen

The fungus produces large number of sclerotia which are round and black in colour. Sometimes, it produces pycnidia on the stems or stalks. 

Favourable Conditions 

• High temperature and low soil moisture (drought) 

Disease cycle 

The fungus has a wide host range, attacking sorghum, pearlmillet, fingermillet and pulses. It survives for more than 16 years in the infected plant debris. The primary source of infection is through soil-borne sclerotia. The pathogen also attacks 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 winters as a sclerotia in the soil and infects the host at susceptible crop stage through roots and proceeds towards stem. 

Management 

• Long crop rotation with crops that are not natural host of the fungus. 

• Irrigate the crops at the time of earhead emergence to maturity. 

• Treat the seeds with Carbendazim or Captan at 2 g/kg. 

• Grow disease tolerant varieties viz., SN-65, SWS-8029, Diva and Zenit. 

Disease of Maize: Head smut

 Head smut - Sphacelotheca reiliana

Symptoms

Symptoms are usually noticed on the cob and tassel. Large smut sori replace the tassel and the ear. Sometimes the tassel is partially or wholly converted into smut sorus. The smutted plants are stunted produce little yield and remain greener than that of the rest of the plants. 

Pathogen 

Smut spores are produced in large numbers which are reddish brown to black, thick 

walled, finely spined, spherical. 

Favourable Conditions 

• Low temperature favours more infection and this fungus also infects the sorghum 

Disease cycle 

The smut spores retain its viability for two years. The fungus is externally seedborne and 

soil-borne. The major source of infection is through soil-borne chlamydospores.

Management 

• Field sanitation. 

• Crop rotation with pulses. 

• Treat the seeds with Captan or Thiram at 4 g/kg. 

Disease of Maize: Rust

 Rust - Puccinia sorghi

Symptoms 

Circular to oval, elongated cinnamon-brown powdery pustules are scattered over both surface of the leaves. As the plant matures, the pustules become brown to black owing to the replacement of red uredospores by black teliospores. 

Symptom

Pathogen 

Uredospores are globose or elliptical finely echinulate, yellowish brown with 4 germpores. Teliospores are brownish black, or dark brown, oblong to ellipsoidal, rounded to flattened at the apex. They are two celled and slightly constricted at the septum and the spore wall is thickened at the apex. 

Uredospores

Teliospores

Favourable Conditions 

• Cool temperature and high relative humidity. 

Disease cycle 

Primary source of inoculums is uredospores surviving on alternate hosts viz., Oxalis corniculata and Euchlaena mexicana. 

Management 

• Remove the alternate hosts. 

• Spray Mancozeb at 2 kg/ha. 

Disease of Maize: Leaf Blight

 Leaf blight - Helminthosporium maydis (Syn: H. turcicum)

Symptoms 

The fungus affects the crop at young stage. Small yellowish round to oval spots are seen on the leaves. The spots gradually increase in area into bigger elliptical spots and are straw to grayish brown in the centre with dark brown margins. The spots coalesce giving blighted appearance. The surface is covered with olive green velvetty masses of conidia and conidiophores. 

Symptom

Pathogen 

Conidiophores are in group, geniculate, mid dark brown, pale near the apex and smooth. Conidia are distinctly curved, fusiform, pale to mid dark golden brown with 5-11 septa. 

Conidia

Favourable Conditions 

• Optimum temperature for the germination of conidia is 8 to 27˚C provided with freewater on the leaf. 

• Infection takes place early in the wet season. 

Disease cycle 

It is a seed-borne fungus. It also infects sorghum, wheat, barely, oats, sugarcane and spores of the fungus are also found to associate with seeds of green gram, black gram, cowpea, varagu, Sudan grass, Johnson grass and Teosinte. 

Management 

• Treat the seeds with Captan or Thiram at 4 g/kg. 

• Spray Mancozeb 2 kg or captan 1 kg/ha. 

Disease of Maize: Downy mildew/Crazy top

 Downy mildew/Crazy top 

Sorghum downy mildew - Peronosclerospora sorghi

Phlippine downy mildew - Peronosclerospora philippinensis

Crazy top - Sclerophthora macrospora

Symptoms 

The most characteristic symptom is the development of chlorotic streaks on the leaves. Plants exhibit a stunted and bushy appearance due to shortening of the internodes. White downy growth is seen on the lower surface of leaf. Downy growth also occurs on bracts of green unopened male flowers in the tassel. Small to large leaves are noticed in the tassel. Proliferation of auxillary buds on the stalk of tassel and the cobs is common (Crazy top).

Symptom

Symptom

Pathogen 

The fungus grows as white downy growth on both surface of the leaves, consisting of sporangiophores and sporangia. Sporangiophores are quite short and stout, branch profusely into series of pointed sterigmata which bear hyaline, oblong or ovoid sporangia (conidia). Sporangia germinate directly and infect the plants. In advanced stages, oospores are formed which are spherical, thick walled and deep brown. 

Favourable Conditions 

• Low temperature (21-33˚C) 

• High relative humidity (90 per cent) and drizzling. 

• Young plants are highly susceptible. 

Disease cycle 

The primary source of infection is through oospores in soil and also dormant mycelium present in the infected maize seeds. Secondary spread is through airborne conidia. Depending on the pathogen species, the initial source of disease inoculum can be oospores that over winter in the soil or conidia produced in infected, over wintering crop debris and infected neighboring plants. Some species that cause downy mildew can also be seed borne, although this is largely restricted to seed that is fresh and has high moisture content. 

At the onset of the growing season, at soil temperatures above 20°C, oospores in the soil germinate in response to root exudates from susceptible maize seedlings. The germ tube infects the underground sections of maize plants leading to characteristic symptoms of systemic infection including extensive chlorosis and stunted growth. If the pathogen is seed borne, whole plants show symptoms. Oospores are reported to survive in nature for up to 10 years. 

Once the fungus has colonised host tissue, sporangiophores (conidiophores) emerge from stomata and produce sporangia (conidia) which are wind and rain splash disseminated and initiate secondary infections. Sporangia are always produced in the night. They are fragile and can not be disseminated more than a few hundred meters and do not remain viable for more than a few hours. 

Germination of sporangia is dependent on the availability of free water on the leaf surface. Initial symptoms of disease (chlorotic specks and streaks that elongate parallel to veins) occur in 3 days. Conidia are produced profusely during the growing season. As the crop approaches senescence, oospores are produced in large numbers. 

Management 

• Deep ploughing. 

• Crop rotation with pulses. 

• Rogue out infected plants. 

• Treat the seeds with metalaxyl at 6g/kg. 

• Spray the crop with Metalaxyl + Mancozeb @ 1kg on 20th day after sowing. 

• Grow resistant varieties and hybrids viz. CO1, COH1 and COH2.

Botanical features of Maize

 Maize is a tall, determinate annual C4 plant varying in height from <1 to >4 metres producing large, narrow, opposing leaves, borne alternately along the length of a solid stem. The botanical features of various plant parts are as follows:

Root: Normally maize plants have three types of roots, i) seminal roots -which develop from radical and persist for long period, ii) adventitious roots, fibrous roots developing from the lower nodes of stem below ground level which are the effective and active roots of plant and iii) brace or prop roots, produced by lower two nodes. The roots grow very rapidly and almost equally outwards and downwards. Suitable soils may allow corn root growth up to 60 cm laterally and in depth.

Stem: The stem generally attains a thickness of three to four centimeters. The internodes are short and fairly thick at the base of the plant; become longer and thicker higher up the stem, and then taper again. The ear bearing internode is longitudinally grooved, to allow proper positioning of the ear head (cob). The upper leaves in corn are more responsible for light interception and are major contributors of photosynthate to grain.

Flower: The apex of the stem ends in the tassel, an inflorescence of male flowers and the female inflorescences (cobs or ears) are borne at the apex of condensed, lateral branches known as shanks protruding from leaf axils. The male (staminate) inflorescence, a loose panicle, produces pairs of free spikelets each enclosed by a fertile and a sterile floret. The female (pistillate) inflorescence, a spike, produces pairs of spikelets on the surface of a highly condensed rachis (central axis, or “cob”). The female flower is tightly covered over by several layers of leaves, and so closed in by them to the stem that they don’t show themselves easily until emergence of the pale yellow silks from the leaf whorl at the end of the ear. The silks are the elongated stigmas that look like tufts of hair initially and later turn green or purple in color. Each of the female spikelets encloses two fertile florets, one of whose ovaries will mature into a maize kernel once sexually fertilized by wind-blown pollen.

As the internodes of the shanks are condensed, the ear remains permanently enclosed in a mantle of many husk leaves. Thus the plant is unable to disperse its seeds in the manner of a wild plant and instead it depends upon human intervention for seed shelling and propagation.

The explanation for each maize male and female floral organ has been given below:

Tassel: inflorescence of male flowers; the tassel consists of several long, in determinate branches bearing short determinate branches (spikelet pairs) that bear two spikelets (compact auxiliary branches of grass inflorescence, that in maize consists of two bracts subtending one reduced male flower).

Stamen: pollen-producing reproductive organs which are collectively referred as androecium.

Stalk: also filament; the part of the stamen on which anther develops.

Anther: the terminal part of a stamen in which the pollen grains are produced.

Microspore: smaller of the two types of spore produced in heterosporous plants; develops in the pollen sac into a male gametophyte.

Male gametophyte: microspores divide twice to produce 3 celled pollen grain/tube (a male gametophyte); two of the cells are sperm; other is called vegetative cell, or tube cell.

Sperm cell: two sperm cells are produced; one sperm cell fuses with the egg resulting in zygote; other sperm cells fuses with central cell giving start to development of triploid tissue called endosperm which surrounds the embryo and serves an absorbtive/nutritive function in seed.

Floral Biology of Maize

 Floral Biology of Maize

Maize is a monoecious plant, i.e. the sexes are partitioned into separate pistillate (ear), the female flower and staminate (tassel), the male flower (Figure 1). It has determinate growth habit and the shoot terminates into the inflorescences bearing staminate or pistillate flowers (Dhillon and Prasanna, 2001). The main shoot terminates in a staminate tassel. Maize is generally protandrous, i.e. the male flower matures earlier than the female flower. Within each male flower spikelet, there are usually two functional florets, although development of the lower floret may be delayed slightly in comparison to the upper floret. Each floret contains a pair of thin scales i.e. lemma and palea, three anthers, two lodicules and rudimentary pistil. Pollen grains per anther have been reported to range from 2000 to 7500 (Kiesselbach, 1949). Within an average of 7000 anthers per tassel and 2000 grains per anther, each tassel could produce 14 x 6-10 pollen grains. Kiesselbach (1949) estimated that 42,500 pollen grains are produced per square inch of corn field. In terms of the ratio of pollen grains produced per ovules fertilized, it appears that since each ear requires about 1000 pollen grains for fertilization, there are about 20,000 pollen grains per kernel in excess of what is actually needed if pollination were 100 percent efficient. The pollen grains are very small, barely visible to the naked eye, light in weight, and easily carried by wind. The wind borne nature of the pollen and protandry lead to cross-pollination, but there may be about 5% self-pollination.

The female flower initially is smooth but protuberances soon form in rows. The basal protuberances are formed first and development advances towards the tip of the ears. The part above the attachment of the carpel develops a single sessile ovule, which consists of a nucellus with two integuments or rudimentary seed coats. The united carpel’s, which will form the ovary wall or pericarp of the mature kernel, grow upward until they completely enclose the ovule. The two anterior carpels, which face the ear tip, form outgrowths, which develop into the style i.e into long thread, known as silks. Silks are covered with numerous hairs, trichomes which form an angle with the silk where pollen grains are harboured. The base of the silk is unique, as it elongates continuously until fertilization occurs. The cobs bear many rows of ovules that are always even in number. The female inflorescence or ear develops from one or more lateral branches (shanks) usually borne about half-way up the main stalk from auxillary shoot buds. As the internodes of the shanks are condensed, the ear remains permanently enclosed in a mantle of many husk leaves. Thus the plant is unable to disperse its seeds in the manner of a wild plant and instead it depends upon human intervention for seed shelling and propagation.