Foliar diseases

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Introduction:

Foliar diseases are the diseases that infect foliar/ leaf part of the plant.

There are so many numbers of foliar diseases. E.g. leaf spot, leaf blight, anthracnose, powdery mildew, downy mildew, sooty mould and rust etc., since these diseases affect the foliar part, the photosynthetic efficiency of the plant get reduced, ultimately the yield get reduced.

The pathogen may be fungal, bacterial or viral diseases.

The inoculums may have superficial growth (e.g. sooty mould) or enter inside the plant cells (e.g. rust, powdery mildew) or may enter systemically (e.g. viral) and affect the whole plant.

The inoculums from leaf surface may spread as airborne (e.g.podwery mildew) or through contact (viral, TMV) or rain splashes (bacterial disease).

To manage the foliar diseases, spraying of the contact insecticides are recommended.

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Late blight:

The disease first appears as water soaked, light brown lesions on the leaf blade.

Symptoms:

If the climactic conditions are favorable with humid and cloudy weather, these lesions spread fast over the entire leaflet and petiole.

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Characteristic lesions are roundish with concentric ring marking on the margin, and generally involve the leaf margin.

The lesions which are dirty brown in the beginning soon turn black.

The severely diseased plants wilt within a few days after the first symptoms are seen on the leaves and in the fields the disease spreads like wild fire, causing severe damage to the crop yield.

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Leaf blight:

Symptoms:

On the leaves, spots of varying size appear.

In severe cases the entire foliage is blighted.

Often several spots coalesce to form large patches, resulting in the leaf blight.

The spots are irregular, brown to dark brown in color, and have concentric lines inside the spots.

Fungicidal sprays, preferably with copper fungicides or zineb given at 15 day interval effectively control the disease.

Control:

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Powdery mildew:

A grayish white powdery growth occurs on the leaf.

Such symptoms also occur on leaf sheath and floral parts.

With the advancement of the disease, the powdery growth may become dark grey and finally black, causing characteristic lesions of varying size on the affected parts, on which minute dark perithecia are seen.

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Control:

While dusting the plants with fine sulphur would reduce the infection, it is expensive and not practical.

Use of one per cent sodium thiosulphate as a foliar spray has been reported to give effective in checking the disease.

Mode of spread:

Warm humid weather and cool nights favor the spread of the pathogen.

Control: The disease is controlled with two preventive sprays of wettable

sulphur, once before the flowers open and a second time after the fruit set.

Two to three dusting with fine sulphur powder also check the disease. Spraying with sulfex (0.2 per cent) or wltasul (0.2 per cent) at

fortnightly interval.

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Rust:

Symptoms:

The fungus produces characteristic rust pustules on the hosts.

The pustules are mostly found on the leaf blade, though often they are found on the petiole and stem of the hosts.

The sori are small, roundish, open, powdery and brown colored.

In severe infections the leaf withers, resulting in considerable damage to the crop.

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Control:

Growing short or long duration crops for such purposes is also useful, and the application of balanced fertilizers to the crop reduces the severity of infection.

Sulphur dust of 400 mesh fineness when applied at the rate of 15 kg per hectare at weekly interval, gives effective control.

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B. Red rust Symptoms:

The alga appears in the form of minute rusty spots on the leaves.

These spots enlarge, spread and often become roundish, increasing to about 10 to 15 mm in diameter.

The spots are characteristically reddish and hairy, occurring often on the upper leaf surface, and spreading to the stem.

It is usually epiphytic on the leaves and may also become parasitic.

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When the leaves are severely infected, they are discolored and if the stem is infected the shoots become pale so as to appear unhealthy even from a distance.

The disease is more or less chronic, as the alga can survive in the stem and old leaves and becomes severe when bright sunlight is available and also when new flush comes out.

Control:

Removal of the infected branches.

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Leaf spot:

Cercospora spp.Symptoms:

Usually water soaked lesions appear on the leaf blade and soon the affected tissues turn brown to reddish brown.

Well defined spots often bound by veins and purplish border develop, the centers of which may turn grey.

Control:The leaf spots usually are controlled by spraying with Mnancozeb or other fungicidal sprays, at 15 days intervals, starting from about a month after sowing and up to fruit set, depending upon the climactic conditions.

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Sooty mould:Symptoms:

The honey dew secreted by the insects on the leaves and twigs of host plants attracts the fungus, which multiplies rapidly, spreading on the plant surface.

Because of the production of masses of black spores, which stick to the leaf surface the sticky honey dew, the foliage appears black and ugly and hence the name , sooty mould.

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Control:

Plants may be cleared of sooty mould by spraying with diazinon or dimecron (0.03 per cent) to kill the insects, followed by spraying with a starch solution, which dries up and come off in flakes, removing the sooty mould growth as it falls away.

Another spray with dimecron would help eradicate the insects, thereby avoiding another attack by the fungus.

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Bacterial leaf spot Symptoms:

The disease first becomes apparent through the minute water soaked lesions, bound the veins, towards the leaf tip. As the disease advances, the lesions appear in groups and are dark brown, surrounded by chlorotic haloes. With age, the lesions become necrotic and dry up in patches.

Control:

Agrimycin -100 and bavistin (0.1 per cent) control the disease.

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Mosaic diseaseSymptoms:

The affected leaves shows mosaic mottling in the beginning, after which the young leaves show severe symptoms, such as distortion of leaf blade, malformation and puckering.

The entire plant becomes stunted and can be recognized from a long distance. Control:

Removal of the infected plants.

The spread of the insect vector may be checked by spraying an effective insecticide.

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YelloAw vein mosaic virus:

Symptoms:

The disease characterized by yellowing of the entire net work of veins in the leaf blade.

In severe infections, the young leaves turn yellow, become reduced in size and the plant is highly stunted.

The damage caused may be anywhere from 50-100 per cent of yield.

Control:

Removal of the infected plant.

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Leaf curl virus:

Symptoms:

Puckering and severe curling of the leaves, together with mottling, excessive branching and stunting and complete sterility of the plant

Control:

Sprays insecticide with roger (0.1 per cent) at fortnightly interval are effective.

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Mycoplasma disease:Little leaf:

The most characterized symptom is the reduction in size of leaves.

As the disease progresses the new leaves produced become smaller and smaller with result that in course of a month the leaves are considerably reduced.

Both the petiole and lamina are involved in the reduction, the leaves becoming almost sessile.

The leaves become thin, soft, glabrous and pale green in color.

In some varieties the thorns disappear.

The growth of axillary buds including buds is stimulated and this is accompanied by the shortening of internodes of the branches.

The plant presents a characteristic bushy appearance.In many affected plants there is no trace of any floral parts being present but whenever found these are modified into green structures.

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Foliar diseases are the diseases that infect foliar/ leaf part of the plant.

To manage the foliar diseases, spraying of the fungicides are recommended.

The common foliar diseases are ,,Late blight, Powdery mildew, downy mildew, rust ,leaf spot, Mosaic disease, leaf curl and Mycoplasma diseases.

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