In this article we will discuss about diseases of plants and its control.
1. Leaf Spot and Blight (Alternaria sp.):
The small dark coloured spots appear on the leaves, which spread rapidly to form circular legions. In humid season, concentric dark rings appear. In severe cases, the cauliflower develops brown colour.
The hot water treatment, as in the case of black rot disease gives a satisfactory control for this disease also.
2. Club-Root (Plasmodiophora brassicae):
The causal organism of this disease lives in the soil and enters the roots by which they enlarge due to characteristic swellings. These enlarged structures are called clubs. Secondary invasion by soft root bacteria follows, forming materials toxic to plants.
As the disease advances, the deformed roots become incapable of supplying the plants with sufficient moisture and nourishment. This results in temporary flagging of leaves on bright days. The affected plants become dwarf, lighter in colour and usually fail to mature marketable curds.
(i) Avoid infected fields.
(ii) Properly lime the soil.
(iii) Treat seedlings at transplanting time with mercuric chloride solution (one part in 500 parts of water) @ 40 cc per seedling.
3. Anthracnose:
Sometimes this disease is very severe. The pathogen mainly attacks the foliage. Black leaf spots and scorched appearance of the foliage are the symptoms. The fungus also attacks the stem and fruits.
Spraying Difoltan or Bavistin (0.1 %) of Dithane Z – 78 (0.2%) at 7 -10 days interval gives an effective control of the disease.
4. Late Blight of Potato:
This disease is caused by a fungus called Phytophthora infestans. It is the most serious disease of potato crop in the North Indian hills, Nilgiri hills, and also in North Indian plains, where it occurs in mild to severe form depending upon the season. The disease affects the foliage, stem and tubers of the potato plant. The disease appears on the leaves as brown water soaked patches which ultimately turn brownish black.
The growing fungus attack the area very fast and soon starts sporulating. On the under surface of infected leaves, a white cottony growth of the fungus develops at the juncture of infected and healthy tissues. The lesions on the stem are purple coloured and sometime 3.5 cm long. The stems get soft and weak at infected points. The affected tubers show light brown patches on the surface and rusty brown necrosis of the underneath flesh.
The disease rapidly multiply under cool temperature (10-25°C) and high humidity (above 80% relative humidity) and the entire crop of susceptible variety may be destroyed in about a week.
(i) Use potato tubers for seed from disease free areas to ensure that the pathogen is not carried through seed tuber.
(ii) The infected plant material in the field should be properly destroyed.
(iii) Grow resistant varieties like Kufri Navtal.
(iv) Fungicidal sprays can be effective, if given properly and timely. Sprays should start a few days before the anticipated time of occurrence of the disease or on the appearance of the initial symptoms. 0 > 01 humid atmosphere favours the disease and such weather can be an indication of disease, in a short time ahead. Spraying should be done with Dithane M-45 or Dithane Z- 78 (2.5 kg per 1000 litres of water per hectare). Spraying should be repeated at 10-12 days \ interval.
5. Early Blight of Potato:
Early blight, caused by Alternaria solani, is another fungal disease of potato prevalent throughout the country. In dry weather with intermittent rain, the disease may appear in an epidemic form both in mid-hills as well as plains and cause severe damage to the foliage. It has been reported to be particularly serious in Maharashtra and in the late planted Katwa (cut seed) crop in Bihar, Uttar Pradesh and Punjab.
The disease first appear on the older leaves of potato plants as necrotic spots of angular, oval or circular shape. These spots are dry and brittle as compared with those caused by late blight. They rarely exceed one centimetre in diameter. A number of these spots can, however, fuse to cover the entire leaf. Under humid conditions, the disease spreads rapidly from the lower leaves to those at the top and may cause premature death of plants.
Spraying the crop with Bordeaux mixture (5:5:50) or Dithane M-45 (2 kg/hec) or Dithane Z- 78 (2 kg/hec) in the plains before the appearance of the disease checks its spread and prolongs the life of the crop. Once the disease has spread, the spraying may not be effective. Crop debris should be invariably destroyed by burning after harvest.
6. Black Scurf of Potato:
This disease is caused by fungus Rhizoctonia solani and is of common occurrence both in hills as well as plains. It is a soil and tuber-borne disease. It attacks many hosts in different stages of growth. In potato, it causes black scurf on tuber, stem canker, aerial tuber and top rosette. The infected tuber bear on the skin chocolate coloured hard bodies called sclerotia.
When the infected tubers are planted, the fungus attacks the growing sprouts or young shoots which may be killed, resulting in gapping stand. Older plants may get diseased near the ground level, producing brown cankers at the collar. This girdling effect results, in wilting and early death of plant.
Checking the downward flow of food manufactured in the leaves results in purpling of the foliage and stunting of the plant.
(i) Good drainage, shallow planting, weed eradication and inclusion of grain crops in long rotation are some of the cultural practices which reduce the one chances of infection.
(ii) The seed tubers (after harvest/before sprouting) should be treated with Agallol 3 (0.5% solution) for 10 min before planting or before keeping in the storage.
(iii) Soil application of Brassicol (PCMB) at the rate of 20-30 kg/hec has been found to be highly effective.
(iv) Only healthy tubers should be used for planting.
7. Common Scab:
This disease is caused by Streptomyces scabies and is present in the plains of Maharashtra and some areas in Punjab. The disease mostly affects the tuber, which may show mere resetting or deep spots up to about 1-2 cm in diameter. These spots or lesions may be differentiated from those of powder scab by the absence of powdery mass released from the ruptured skin.
Since the pathogen is both tuber as well as soil borne, it is necessary to use disease free seed tubers to reduce the incidence. Green manuring before planting potatoes reduces the disease. Irrigation of the crop at regular intervals to keep the soil moist, starting from tuberization can effectively check the disease.
8. Bacterial Wilt and Brown Rot:
This disease is caused by bacterium Pseudomonas solanacearum. This is a dreaded disease of potato in many areas of India. It is endemic in the mid hills, plateau region and Paschim Bangal. The disease has been reported on potato, tomato, brinjal and chilies. Losses due to this disease range from 10-70%. Very often it is associated with root knot nematode and the losses are very severe.
Seed from brown-rot free crop should be used. The plots which show the incidence of brown rot may be put under maize or cereal cultivation preferably for three subsequent seasons. Rain or irrigation water should not be allowed to flow from infested plots to the non-infested or mildly infested plots.
In the plots where only isolated plants show wilting, the soil in the radius of 1.0 m around the infected plant should be treated with 10% Formalin or 0.5% copper sulphate or 0.5% Streptocycline solution upto a depth of 30 cm just after the harvest.
Infected plant material or rotten tubers should never be left in the plots. Collect such material carefully and burn in an isolated comer of the field.
9. Wart Disease of Potato:
This disease is caused by the fungus Synchytrium endobioticum and is localised in Darjeeling district of Paschim Bangal. Due to the quarantine regulations, it has not spread to other parts of the country so far.
All underground parts except roots and basal parts of the stem near ground level exhibit symptoms of the disease. Burls on sterns, stolons and tubers are centres from where abnormal growth activity starts and leads to the development of warts.
Lumps of warts are found attached to the affected parts. Early in the season the warts are green or greenish white if exposed above ground to light and cream coloured when underground. In advanced stages, the warts are darker in colour, sometimes black, and undergo decomposition due to attack of secondary saprophytic organisms. Morphologically the warts consist of distorted proliferated branched structures grown together into a mass of hyper-plastic tissue.
No seed potato should be obtained from Darjeeling hills for planting) the crop. In the affected areas wart immune varieties such as Pimpernel, Kufri Jyoti should be grown.
10. Leaf Roil Disease:
Leaf roll is the most important viral disease of potato, being one of the main causes of potato degeneration. In India, it is estimated to cause a loss of 25 – 50% in yield. It is present throughout the country. The name leaf roil is partly descriptive of the disease. Affected leaves show upward rolling of the margins progressing towards the midrib until the entire lamina is involved. The leaves assume a rigid, leathery texture having a characteristic rattle when brushed with hand. The number of tubers produced per plant and their size is greatly reduced.
(i) Seed tubers should be healthy and certified. Do not plant very small sized tubers, since they are more likely to be from the diseased plants.
(ii) Inspect the field regularly and destroy the plants showing the initial symptoms.
(iii) Spray the crop with Metasystox or Rogor @ 600- -700 ml dissolved in 500-600 litres of water per hectare at 10-15 days interval to check the insects that spread this disease.
11. Potato Mosaic:
These are different types of viral disease. The leaves show green or dark green mosaic pattern on leaves. These may be faint yellowing in patches on the leaves. The plants remain stunted and sick. Size and number of tubers is reduced. In some mosaic diseases the leaves may show necrosis of tissues along the vein. Diseased, small sized tubers are the main source of disease in the field. The disease is spread in the held by insects which carry the virus from sick to healthy plants.
Control measures recommended for leaf roll disease are also effective for checking mosaic diseases of potato.
12. Yellow Vein Mosaic of Okra:
It is a very common disease of okra and limits the cultivation of this crop in some seasons. When the disease appears very early, all the leaves become completely yellow, later on turn brown, dry up and shed. Such plants hardly bear any flowers but tile plant dies prematurely.
If the infection takes place at later stage tile earlier formed leaves on the main stem remain green, whereas the fop leaves and the flowering parts as also the secondary branches show clear symptoms.
On such plants although the fruits are formed in good numbers, they are uniformly yellow at the picking stage and, as such are unmarketable.
At present no definite control measures against yellow vein mosaic are known. Protection of the crop from white flies and other insect vectors by spraying with suitable insecticides may reduce the disease incidence.
Spraying at the initial stage of the crop, just after germination, is most important. If the crop is not sprayed within 20 days after germination, disease incidence remains high. Four to six spraying with systemic insecticides such as Ekalox, Metasystox, Rogor, Dimecron, etc., are recommended.
13. Powdery Mildew (Erysiphe Species):
White greyish powdery coating is seen on the under surface of the leaves. The leaves which are severely affected turn yellow and drop.
Application of 0.2% wettable sulphur at 1 or 2 week interval will control the disease. Benlate (0.1%) or Morestan (0.05%) can also be sprayed at 15 days interval for its effective control.
14. Fusarium Wilt or Stem Rot of Sweet Potato:
This disease is caused by the fungus Fusarium oxysporum var. batatas. This disease of sweet potato is of worldwide occurrence. The fungus causing this disease is restricted to sweet potato although it can reproduce on some other plants such as tobacco, soyabean, maize, etc. The disease causes heavy reduction in the yield of fleshy tubers.
(i) Selection of resistant varieties and avoiding diseased vines for planting.
(ii) Following crop rotations where sweet potato should be grown once in two or three years.
(iii) Before planting, the vines or sets should be dipped in a 0.2% suspension of Areton or Agallol.
(iv) Proper dosage of fertilisers should be given to the crop.
15. Black Rot of Sweet Potato:
This disease is caused by the fungus Ceratocystls fimbricata. This disease probably occurs wherever sweet potatoes are grown. Although primarily it is storage disease, but it can occur in the field as well as in the seed beds.
All under-ground parts of the plants are affected. In the field black brown spots appear on the young sprouts. These eventually girdle the plant. This results in the yellow sickly appearance of the plant or foliage. The blackening extends to roots and also on the stem for some distance. The most conspicuous lesions develop on the roots (tubers). Dark circular depressed spots of varying size appear on the fleshy roots.
The spots are grey black when dry and dark-greenish black when moist. In the centre of these spots or in the older portion of the lesions small black fruiting bodies of the fungus also develop. The shallow, dry decay extends in tile flesh up to the vascular rings. Infected tubers develop a disagreeable bitter taste when cooked.
Control measures are the same as for the fusarium wilt or stem rot.
16. Charcoal Rot of Sweet Potato:
This disease is caused by the fungus Macrophomina phaseolina. The fungus generally thrives as brown mycelium in the tubers. Sometimes the basal parts of the plants may be invaded. Black incrustation or patches may appear on the surface of the tubers.
The fungus is soil-borne and invades young germinating tubers during early stages. The tips of the sprouts turn brown. As the tubers mature, the dry rot appears. Keeping quality of the affected tubers is reduced. This is also considered as a storage disease and it may be associated with cold and wet weather.
To control tills disease, infected tubers should not be used for planting. Treatment of seed tubers with Emesan-6 (0.2%) and Brassicol (0.3%) for 30 min before planting will help control this disease.
17. Soft Rot of Sweet Potato:
Soft rot is a very serious disease of sweet potato caused by the fungus Rhizopus nigricans. The disease begins as a soft, watery rot which progresses rapidly decaying the whole tuber within 4-5 days. When the skin is broken, a straw colour liquid drip out of it.
If the skin is not broken, the affected tubers loose moisture and finally become a shriveled mummy. When the skin is ruptured during progress of the disease, fungal growth can be seen developing in abundance covering the affected surface. The tissues turn brown in colour and a mild odour is noticeable.
Emphasis should be placed on not allowing any wound or bruise on tile tubers during the process of digging or storing. They should be allowed to dry in the sun for one to two hour before storage. The stores should be thoroughly cleaned and walls and floors should be washed with 2.5% solution of copper sulphate.
The temperature of the store should be maintained at 26.50 C – 300C with 90% relative humidity. Sweet potatoes with mud on them are more likely to get soft rot. Therefore, thorough cleaning of tubers becomes necessary.
18. Dry Rot of Sweet Potato:
Dry rot of sweet potato is caused by the fungus Diaporthe phaseolorum. var. batatatis. It is of minor importance in the field but may cause serious damage in the storage.
In the field, the rot usually begins at the stem end of the root. Later it involves the whole tuber. The diseased roots are shrunken and wrinkled. The surface is covered with minute mounds. The internal’ decayed tissues are coal-black.
Control measures recommended for soft rot and black rot including precautions are required for the control of dry rot also.
19. Pink Root:
The root turns pink, shrinks and dies. The causal organism is a fungus which lives in soil. The plant produce new roots but do not complete with the growing season so yield is lowered. It is more serious in hot, dry weather and at the time of bulb formation.
Adopt crop rotation and apply calcium cyanamide at the rate of 2500 litres per hectare.
20. Onion Smut:
It is caused by Urocystis cepulae. The fungus lives in the soil year after year. It is inactive at 30°C or above. The fungus attacks on young seedling plants. Dark, slightly thickened areas on small leaves of younger seedlings occur. Then other leaves are attacked. They are swollen and try to bend downwards. Then, small black pustules appear.
(i) Treat the seed with Thiram 3 g per kg of seed.
(ii) Grow disease resistant varieties like Nebuka, etc.
(iv) Apply fungicide in furrows along die lines of seed sown in nursery.
21. Neck Rot:
It is caused by the fungus Botrytis alii. It is found in all regions where bulbs are stored. The fungus also attacks injured leaves. The lesions appear as sunken, dried areas near the node or the bulb or involve whole bulb. The fungus over-winters in the diseased onions in storage.
(i) Harvest the crop when the tops are completely dry.
(ii) Store the onion at about 0°C and with humidity at about 65 % with good dry air circulation.
22. Downy Mildew:
The disease spreads rapidly during cool and wet weather. Some varieties of onion have the fungus during winter also in storage Dew collects on the plant at night by which the spores germinate and enter the onion leaf.
(i) Grow resistant varieties.
(ii) Spray Zineb, about 6-10 application at an interval of 6 – 8 days.
23. Damping-Off:
This is a fungal disease caused by Phytophthora or Pythium sp. The fungus attack usually starts on the germinating seed spreading to the hypocotyl, basal stem and developing roots. The affected seedlings are pale green and a brownish lesion is found at the basal portion of the stem. The lesion girdles the stem, later extending upwards and downwards. The affected tissues rot and the seedling falls down. The disease is very serious in the nursery.
(i) The disease may be controlled if the nursery soil is sterilised before sowing and the seeds are treated with Ceresan before sowing.
(ii) Hot water treatment (at 51°C for 30 min) of seeds has also been very effective in controlling the disease.
24. Phomopsis Blight:
This disease was first reported from Gujaratin 1914 and since then, from many parts of India, its infestation as seedling blight, leaf spot and fruit rot has been reported. This is a serious disease of brinjal. In seedling infection, it causes damping off symptoms. The foliage is attacked anytime during the season.
When the leaves are infected, small circular spots appear which become grey to brown and have a light coloured centre with irregular blackish margins. The affected leaves may turn yellow and die. Lesions may also develop on petiole and stem, causing blighting of affected portions.
(i) This disease is specific to brinjal and once the seeds are sown or the seedlings are transplanted in the infested soil, the organism becomes active. Therefore, in such fields brinjal crop should not be taken regularly and proper crop rotations should be followed.
(ii) Brinjal seeds should be obtained from disease free plants or fields.
(iii) Use hot water (50°C for 30 min) treated seeds. For self- sown seeds, remove calyx of the fruit, dip for 20 min in mercuric chloride solution (25 g per 35 litres of water). After extraction of seeds, thoroughly dry them in the sun for about a week, then treat with Thiram or Bavistin.
(iv) Spray the plants in nursery beds with Dithane M-45 (0.2 per cent). Repeat spraying in the field at 10 days interval. Difoltan (0.2%) and Captan (0.2%) are also recommended for spraying.
25. Bacterial Wilt:
This disease is caused by the bacterium Pseudomonas solanaceanum. It causes serious problem in brinjal cultivation. The characteristic symptoms of the disease are wilting, stunting and yellowing of the leaves. Afterwards the whole plant collapses.
(i) Proper crop rotations reduce the disease infestation.
(ii) Brinjal crop should not be sown in the wilt affected fields.
(iii) Resistant varieties such as Pusa Kranti and some of the resistant lines developed at the IIHR. Bengaluru should be cultivated.