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In this article we will discuss about the diseases management of some important vegetables with its symptoms.
1. Tomato:
i. Early Blight:
The disease is caused by Alternaria solani. The pathogen can attack the crop at any stage but it is more destructive at maturity. The disease initiates in the month of April but is severe in the end of May. Severe epidemics can result in yield losses up to 78%.
Symptoms:
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The symptoms appear as circular to angular, dark brown to black, concentric ringed spots surrounded by yellow halo on lower leaves. Severe infection leads to premature leaf death, withering, dropping and eventually abscission. The whole leaf turns yellow and dries up when the lesions are more.
The infection also takes place on the leaf petioles and stems as dark black lesions. Symptoms on fruits occur at stem end as dark, leathery, sunken lesions with characteristic target board appearance.
Management:
The left over crop residues of diseased crop should be collected and destroyed. The seed should be treated with Captan or Thiram before sowing. Fungicides like Mancozeb, Captan, Chlorothalonil, Propineb, iprodione, triazole compounds, recently developed strobilurins as spray applications are reported quite effective against early blight.
ii. Late Blight:
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It is a serious problem affecting tomato cultivation throughout the world where the temperature and moisture permit the pathogen to thrive. The disease is caused by Phytophthora infestans.
Symptoms:
The disease affects leaves, stem and fruits of tomato. Lesions produced on leaves and stem are large, irregular and greenish black water soaked patches. As these patches enlarge, they turn brown and paper like. During wet weather, the lesions on the under surface of the leaves have fine white mass around them.
Blightening of the entire foliage may occur during moist conditions within a short period and can cause heavy losses. Green fruits are also attacked and infected fruits are firm which have large, irregular, brownish-green blotches. The surface of the fruit lesion has a greasy rough appearance. In moist weather, white mycelial growth of the fungus develops over the infected parts which are later on followed by rot of the fruit.
Management:
Since the causal fungus survives in infected tubers, these should be destroyed after culling after taking out from cold stores. Kufri Badshaha and Kufri Jyoti have moderate level of field resistance to late blight. Fungicides like copper oxychloride, mancozeb, chlorothalonil, propineb are reported effective as prophylactic applications.
Under heavy disease situations systemic fungicide mixtures such as metalaxyl + mancozeb provide good control of disease. Recently developed strobilurin fungicides have also shown promising activity against both late blight and early blight of tomato.
2. Chilli:
i. Anthracnose, Die-back or Ripe Fruit Rot:
This is one of the serious diseases of chilli in all the chilli-growing areas of the world. The disease is favoured by rainy season. The losses can reach up to 50%. The disease is a major problem on mature fruits.
However, the symptoms on immature fruits have also been found. The disease is caused by Colletotrichum capsici. The pathogen also causes dieback of chilli plants. The fruit rot phase of the disease can cause heavy damage in the field as well as during transit.
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Symptoms:
Dieback:
There is necrosis of the tender twigs from the tip backwards. The entire branch or the entire top of the plant may wither away. The twigs are water soaked to brown, become grayish white or straw coloured in advanced stages of the disease. Large number of black dots called acervuli are formed on the affected twigs.
Ripe Fruit Rot:
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On mature fruits, circular to elliptical sunken spots appear with black margins and marked with concentric rings. Badly diseased fruits turn straw coloured from normal red. On this discoloured area, numerous acervuli are present. The diseased fruits may drop off prematurely.
Management:
The seed should be obtained from healthy fruits. The seed should be treated with fungicides like Captan or Thiram at the rate 2 g/kg seed. The secondary spread can be Checked by spray of fungicides like Dithane M-45, Blitox and Bavistin. Fenapanil and Triazole fungicides like Tebuconazole have also been reported to be effective against this disease.
ii. Wet Rot:
The wet rot is one of the important fungal diseases of chillies, which is severe during the months of August and September. It is caused by Choanephora cucurbitarum.
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Symptoms:
Flower buds and tender young shoots are attacked by the pathogen under favourable humidity conditions. The infected portion shows the tendency of wet rot with luxuriant growth of the fungus on the infected portion, which appears as black pinheads.
The infection spreads downward and the infected portion becomes limply, resulting in the death of the distal parts. The disease also appears on fruits causing soft rot with luxuriant growth of the pathogen with characteristic bead like black conidial heads.
Management:
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Control measures are the same as discussed under anthracnose.
iii. Leaf Curl:
This is a viral disease caused by tobacco leaf curl virus. The disease is very common wherever the chilli crop is grown. It is a major limitation in successful cultivation of both hot and bell pepper. The disease can cause heavy yield losses and reduce the quality of the fruit.
Symptoms:
Initially the leaves become pale yellow in colour, followed by slight curling. The inter-veinal areas are puckered, there is shortening of internodes and reduction in size of the leaves giving the plants witches broom appearance. Severely infected plants remain stunted and assume bushy appearance.
Fruit formation in susceptible cultivars is rudimentary and distorted. The disease is transmitted by whitefly, Bemisia tabaci. This vector spreads the maximum disease during the months of August to October.
Management:
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A combination of use of resistant varieties, barrier/trap crops and insecticides against the vectors has been found successful in reducing the disease intensity. Growing of marigold with capsicum has been reported to cause significant reduction in the incidence of viruses infecting pepper in Mexico.
Valdez and Wolfenberger (1995) reported that the combined use of yellow traps followed by Cypermethrin spray at the rate 110 g a.i./ha reduces the population of Bemisia tabaci.
3. Cruciferous Vegetables:
i. Black Rot:
This disease is wide spread and occurs on both cultivated and wild crucifers where rainfall or heavy dew is plentiful. The pathogen can attack cabbage, cauliflower, turnip, radish etc. and if the favourable conditions prevail cabbage plants are unable to produce marketable heads. The disease is caused by the bacterium, Xanthomonas campestris pv. campestris.
Symptoms:
The first disease symptoms appear, as chlorotic lesions near the leaf margin which progress towards the centre in the form of “V” shaped yellow spots. In the affected portions, the veins and the veinlets turn brown and finally black which is characteristic of the disease.
The vascular blackening of the petiole and stem causes systemic spread of the causal organism. Bacterial ooze may be seen on the surface of the diseased tissue. Curds also get infected and rot.
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Management:
Sprays of antibiotics are not much advocated because of resistance development in the pathogen and other hazards. However, some need-based recommendations are available. For the effective management of the disease, the seed treatment must be followed by three sprays of streptocycline (50 ppm) at transplanting, curd formation and pod formation stages.
In the field, spraying of Streptocycline (100- 200 ppm) has been reported effective against black rot infection. Sprays with copper fungicides at 10 days interval also help in reducing the spread of the diseased. Application of 10-12.5 kg/ha stable bleaching powder as soil drench is also effective.
ii. Alternaria Blight:
The disease occurs on cabbage, cauliflower, radish, turnip etc. When too many spots are formed on the leaves, they die prematurely affecting the plant stand and in addition, the spots predispose the heads to various types of rots during transit and storage. The disease is caused by Alternaria brassicae and A. brassiciqola.
Symptoms:
Initial symptoms of the disease appear as small dark coloured areas, which spread rapidly and form circular spots with concentric rings. In humid weather, the fungus appears as bluish black growth in the centre of the spots.
The cauliflower curds are also infected which show browning; beginning at the margins of the individual flower or flower clusters and later on the whole curd may become brown and rot. Brown to black spots appears on pods in the seed crop of cruciferous vegetables.
Management:
The disease debris after harvest should be destroyed. The seed should be treated before sowing with Captan or Thiram at the rate 3 g/kg seed. Fungicides such as Dithane M-45, Blitox, Rovral and Kavach have been reported to give good control of this disease. Recently Strobilurin fungicides have also shown promising activity against this disease.
iii. Downy Mildew:
The disease occurs on cole crops such as cabbage and cauliflower. In addition, it also occurs on radish. The disease is quite common in young plants and the destruction may reach up to 90%. It causes heavy losses to the seed crop.
The disease is caused by Peronospora parasitica. Under favourable conditions, the pathogen infects 50-60% seed of cabbage. It is also responsible for the post-harvest spoilage of cauliflower heads.
Symptoms:
The disease is characterized by the appearance of purplish brown spots on the under surface of the leaves. The upper surface of the leaf on the lesion is tan to yellow. Fungal growth appears on the under surface of these lesions.
The stem shows dark brown and depressed lesions or streaks. In cabbage, the infection exposes the heads to soft rot during transit. The affected cauliflower curds turn brown at the top and the severely infected curds rot and fail to produce seeds.
Management:
Use of chemicals is imperative once the disease has appeared on the growing crop. With the advent of systemic fungicides, a number of chemicals have been evaluated and recommended for the management of downy mildew. Heat treatment for seeds at 43-500 C for 20 minutes and dressing of seed with systemic fungicide Metalaxyl is helpful in reducing seed borne infection.
Treatment of cabbage seedlings with Captafol at the rate 0.2% or Polyram (Maneb) at weekly intervals significantly reduces the incidence of the disease in nursery. Spraying phosphonic acid or a combination of Metalaxyl + Mancozeb or Cymoxanil + Mancozeb is sufficient to control downy mildew in cauliflower and cabbage respectively.
4. Cucurbits:
i. Powdery Mildew:
The disease is caused by Erysiphe dehoracearum. The fungus attacks the leaves and stem of cucurbits viz. cucumber, pumpkin, bottle gourd, muskmelon and squashes.
Symptoms:
The initial symptoms appear as white to dirty grey spots, which become powdery as they enlarge. This white powdery coating covers leaves, stem and other succulent parts of the plant. Severe infection may lead to premature defoliation and death of veins.
Management:
Fungicides like Carbendazim, Dimethrimol, Pyrazophos, Triademefon and Tridemorph are quite effective in controlling powdery mildew. Strobilurin compounds developed recently also provide effective control of powdery mildew. Application of sulphur dust should be avoided as most of the varieties of cucurbits are highly sensitive to sulphur.
ii. Downy Mildew:
It is an important disease of cucurbits in different countries and affects its successful cultivation. Early infection of downy mildew can cause reduction in crop yield up to 60% whereas late infection is less damaging.
The loss of foliage as a result of early infection precludes normal flower set and fruit development. Maturing fruits fail to get proper colour, are tasteless and look sun burnt. The disease is caused by the fungus Pseudoperonospora cubensis.
Symptoms:
At first, small water soaked lesions appear on the underside of the leaf. Such lesions when viewed from the upper side of the leaf appear as angular spots and grayish spore mass of the fungus is clearly visible on the underside of the spots. The lesions subsequently become necrotic. The severely infected leaves become dry and curl upwards giving blighted appearance to the veins.
The disease appears in epiphytotics form during the months of April to June. Low night temperature (< 200 C) and long dew periods are the most important determining factors for the onset and development of downy mildew. The fungus perpetuates on off-season vines.
Management:
Weekly prophylactic sprays of Chlorothalonil or Mancozeb give good control of this disease but these fail to check established infection. Spraying of Mancozeb and Metalaxyl mixture (Ridomil MZ) starting with the onset of conducive weather with repetition at 10 days interval are very effective.
In recent years, Metalaxyl resistant strains of the pathogen have developed in many countries as a result of continuous use of this fungicide.
Metalaxyl sensitive or tolerant strains of P. cubensis on cucumber and melons can be controlled by Dimethomorph. Cymoxanil + Mancozeb or Chlororthalonil as tank mixture also provide effective control. Recently developed Strobilurin fungicides have been found to be quite effective against this disease.
iii. Cucumber Mosaic Virus (CMV):
Several cucurbits are attacked by viruses causing mosaic like symptoms. The nature of symptoms depends on the host and strains of virus associated with the disease. The losses are high, when the young plants are attacked. Maximum losses occur on squashes, bottle gourd, muskmelon, pumpkin and cucumber.
Symptoms:
The characteristic symptom is yellowish of cotyledons and ultimately wilting of seedlings when the plant is attacked soon after emergence. In older plants, there are alternate green and yellow patches on the leaf. The spots are irregular in shape and enlarge rapidly. The diseased leaves are mottled, deformed, small and sometimes curl downwards.
The veins and veinlets also turn yellow. Young fruits are rough, mottled, deformed and smaller in size. The virus is seed borne and is transmitted by aphids. The disease can also be transmitted through sap and mechanical methods. Some weed hosts serve as source of inoculum.
Management:
Removal of alternate host plants, use of virus free seed and control of insect vectors through the use of insecticides are the possible measures. Neem derivatives could be very suitable for integrated virus disease management as they are primarily feeder poisons for nymphs/larvae of insect vectors of plant viruses.
5. Pea:
i. Powdery Mildew:
It is the most common and serious disease-affecting pea in various countries. The disease becomes more serious during warm and dry days. Early sown varieties escape the disease. The disease is caused by Erysiphe pisi.
Symptoms:
It first appears on leaves and then on older green parts of the plant including the pods. The characteristic symptom is the formation of white floury patches on both sides of the leaves, tendrils, stems and pods.
These patches originate as minute discoloured specks from which a powdery mass radiates on all sides. In the advanced stages of the infection, all the aerial parts of the plant are found covered with white floury patches, which consist of mycelium and spores of the fungus. Seeds shrivel and loose taste.
Management:
Various chemicals have been found effective in the management of pea powdery mildew both as seed treatment and spray. Seed treatment with Triademefon, Bayleton and Contaf was found effective in delaying the first appearance of the disease.
Secondary spread is best controlled by spray of fungicides like Dinocap, Triademefon, Maneb + Tridemorph, Fenarimol, Mildex, Sulfex, Thiovit etc. The fungicides have to be sprayed periodically depending upon the prevailing disease intensity and weather conditions.
ii. Rust:
It is a serious disease of pea arid occurs throughout the pea growing areas of the world. Two species of Uromyces are responsible for causing rust disease i.e. Uromyces pisi, a heteroceous type and U. fabae, an autoceous type. The later occurs in India and causes varying degrees of losses. Under artificial conditions, yield losses up to 30% have been reported. Disease may become severe in K deficient soils.
Symptoms:
The pathogen Infects all the above ground parts of the plant. By the end of February, yellow to orange coloured round or elongated clusters appear on the leaves. These are aecial cups and bear aeciospores. Later, these clusters change to light brown in colour which is the uredial stage.
Finally, the uredosori are replaced by the teliosori, which are dark brown to black in colour. Severely infected leaves may shrivel and shed off. Teliosori are also common on the stem and petiole besides leaves. Under favourable weather conditions, the entire plant may be killed.
Management:
Chemical sprays are particularly useful to halt the early build up of the rust pathogen. In India, seed treatment with mercurial fungicides and Thiram has been recommended but mercurials are banned now. Fungicides like Dithane M-45 (0.2%), Bayleton (0.05%) and Calixin (0.2% a.i.) have been found useful as spray. A total of 3 sprays given at 10 days interval are useful in reducing disease levels.
6. Onion:
i. Purple Blotch:
It has been reported to occur on onion, garlic, leek and other Allium spp. Losses due to the disease are heavy on seed crop. The disease is often aggravated by the thrip injuries. It is caused by Alternaria porri.
Symptoms:
The disease is characterized as small (2-3 mm in diameter) water soaked lesions on the leaves or seed stalks that quickly develop white centres. These lesions enlarge, coalesce, become zonate and brown to purple under favourable conditions and extend both upward and downward.
In moist weather, the surface of the lesion may be covered with black fruiting bodies of the fungus. Similar lesions may form on seed stalks; seeds either do not develop or are shrivelled. The bulbs can also be affected at harvest v-hen the fungus enters through the necks or injuries, causing storage rot.
Management:
The seed should be treated with fungicides like Captan or Thiram at the rate 3 g/kg seed before sowing. Dithiocarbamate fungicides like mancozeb and Zineb are reported effective in controlling purple blotch under field conditions.
Addition of insecticides like Malathion and stickers like Triton in Mancozeb solution has been reported to be more effective against this disease by reducing the injuries due to thrips and providing better adherence of the fungicide to the waxy surface of the plant. Upmanyu (1999) has reported good efficacy of triazole fungicides.
