Everything you need to know about controlling diseases of cowpea.
A. Fungal Diseases
1. Anthracnose (Colletotriclium Lindemuthianum):
The fungus attacks the stem, leaves and pods. Infected portions show dark brown and sunken spots with erased reddish or yellowish margins. Tissues of necrotic spots on leaves collapse, become thin and papery, and produce abundant pinkish slimy mass in humid weather. Infected stems crack and rot. The disease is seed transmissible.
Control:
i. Follow crop rotation.
ii. Use only healthy seeds for sowing purpose.
iii. Treat the seed with 0.125% solution of Captan for an hour.
iv. Dress the seed with Captan @ 2.5-3.0 g/kg of seed.
2. Cercospora Leaf Spot (Cercospora Canescens and Cercospora Cruenta):
The spots generally appear after flowering. Rough, circular, and cherry-red to dark-red spots variable in size are formed by C. canescens. In case of C. cruenta, black mats due to mould growth of fungus are formed. Defoliation occurs in both the cases. At maturity, the infected pods show black sporulation of fungus.
Control:
Spray Indofil M-45 @ 0.2% at 10 days interval, soon after the appearance of disease.
3. Rust (Uromyces Appendiculatus):
The disease is of common occurrence in some parts of the country and may prove destructive by defoliation of plant. The disease mostly attacks the leaves but rarely the stem and petioles. It may occasionally infect the pods. It is more common on the undersurface of leaves. The leaves give rusty appearance, and as a result, the leaves turn yellow, dry, and fall off.
Control:
i. Follow long rotation with non-host crops.
ii. Follow wider plant spacing.
iii. Destroy the disease debris.
iv. Remove weeds to permit aeration.
v. Spray the crop with Indofil M-45 or Daconil @ 2 kg/ha at ten days interval.
4. Ashy Stem Blight (Macrophomina Phaseolina):
The fungus causes brown lesions at collar region. The lesions spread rapidly covering the entire stem portion and killing the growing point. Vascular portion of the roots turns brown and rootlets rot, causing the plants to dry up. The disease is seed borne.
Control:
i. Use disease free healthy seed.
ii. Dress the seed with Captan, or Thiram @ 2-3 g per kg of seed before sowing
5. Powdery Mildew (Erysiphe Polygoni):
The fungus causing this disease attacks all parts of the plant. Grayish-white powdery patches appear on leaves, and then, spread on stem and green pods, and in severe cases, defoliation occurs. The disease generally appears late in the growing season.
Control:
i. Spray the crop with wettable sulfur @ 0.5% as soon as the disease appears.
ii. Spray Benlate or Bavistin @ 0.15% at fortnightly interval.
6. Seed and Seedling Rot (Pythium, Colletotrichum, and Fusarium Spp.):
In northern Indian conditions, the seeds rot in soil, particularly in rainy season. Seedlings are killed before they emerge out of soil, which cause poor stand of the crop.
Control:
Treat the seed with Emison or Bavistin @ 2.5 g or Thiram @3 g/kg of seed.)
7. Dieback (Colletotrichum Capsici):
Twigs and branches dry up from the tip downwards. Small black and dot like structures appear on dried portion and infected pods shrivel.
Control:
Spray the crop with Copper fungicide or Thiram 0.2% at 15 days interval starting as soon as the disease appears.
B. Bacterial Diseases:
1. Bacterial Blight (Xanthomonas Vignicola):
The primary damage is through high mortality of seedlings, especially when the seeds used for sowing are from a severely diseased crop. Infected leaves show light yellow irregular to circular spots with necrotic brown centre, later changing to straw colour. Dark green and water soaked spots of variable shape and size appear on pods, which later become yellow and dry. Affected leaves often fall off early. It has also been observed that the disease increases with an increase in plant population.
Control:
i. Grow resistant cultivars like Pusa Komal.
ii. Use disease free healthy seed for sowing purpose.
iii. Sow the crop sparsely at wider spacing.
iv. The July sown crop contracts less bacterial blight.
v. Treat the seed with hot water at 56°C for 10 minutes.
C. Viral Diseases:
1. Cowpea Mosaic Virus:
The virus is prevalent on all commercial varieties and appears in more severe form during rainy season. The leaves develop a typical mosaic of broad and raised patches along with chlorotic streaks or spots. The diseased plants bear a distinctly pale appearance. The virus is readily transmitted through sap inoculation and is carried in seed to the extent of 4-22%. In nature, the virus is spread by aphids. It is able to infect many other hosts such as beans, sun hemp etc. The disease is seed borne.
Control:
i. Use virus free healthy seed for sowing purpose.
ii. Rogue out the infected plants as soon as observed.
iii. Spray the crop with Rogor or Metasystox @ 0.1% at 15 days interval.
2. Cowpea Aphid Borne Mosaic Virus:
This virus causes severe mosaic of cowpea. However, the severity depends on variety of the host and strain of virus. Infected plants show dark green vein banding or interveinal chlorosis, leaf distortion, blistering and stunting. Plants from infected seeds show bright mosaic symptoms on primary leaves. Pods are twisted and they contain small and few seeds. The virus is transmitted in a stylet-borne non-persistent manner by Aphis craccivora, A.fabae, A. gossypii, A. mediacaginis, Macrosiphum euphorbiae, and Myzus persicae.
Control:
i. Use disease free healthy seed for sowing purpose.
ii. Remove virus infected plants.
iii. Spray the crop with systemic insecticides, e.g., Rogor, or Metasystox @ 0.1-0.2% to keep the vectors that transmit viruses under check.
3. Cowpea Banding Mosaic Virus:
This virus belongs to cucumovirus group and is seed borne in nature. Its symptoms include mosaic mottling accompanied by slight crinkling and reduction in leaf size and typical dark green bands along the veins under field conditions. The diseased plants remain green for larger period and set fewer pods. This virus is transmitted by aphids (A. craccivora).
Control:
i. Use disease free healthy seed- for sowing purpose.
ii. Grow resistant varieties.
iii. Use systemic granular insecticides like Disulfoton and Thimet in combination with paraffin oil.
iv. Follow judicious cultural practices such as time of sowing, mixed cropping, barrier cropping, and density of plant population to check the spread of disease.
v. Follow heat therapy of infected seeds at 65 °C for 15 minutes followed by 2, 4, or 8 days incubation at 30°C.
4. Cowpea (Chavali) Mosaic Virus:
Mosaic pattern, leaf distortion, blisters and reduction in size of leaves, stunted growth, and dark green appearance of plants are the typical symptoms of this disease. This is transmitted by Aphid (A. craccivora) and reported to be seed borne.
i. Use virus free healthy seed for sowing purpose.
ii. Follow heat therapy of seeds at 55°C for 15 minutes.
5. Cowpea Necrosis Virus:
The field symptoms are mild mosaic, severe crinkling, and curling of leaves. The leaf lamina is severely reduced giving a shiny appearance with thickened leaves and veins. It is transmitted by aphid species like A. craccivora, A. fabae and A gossypii.
Use resistant varieties such as Pusa Do Fasli, Pusa Barsati and Pusa Phalguni for cultivation.
6. Cowpea Chlorotic Spot Virus:
Chlorotic spot virus is a member of tobamovirus group. Irregular chlorotic spots, which later coalesce on primary and trifoliate leaves, fine vein clearing in young leaves, reduction in lamina, dwarfing, early defoliation, and poor pod formation are the common field symptoms. This disease is caused by a strain of TMV. The virus is seed borne.
Use virus free healthy seed for sowing purpose.
7. Cowpea Yellow Flecks:
The first symptoms appear in the form of irregular bright yellow flecks scattered on older leaves, which gradually coalesce covering entire leaf lamina. Leaves are reduced in size and defoliation starts early. Affected plants remain dwarf and bear fewer flowers and pods. White fly has been reported to transmit the virus.
Apply systemic granular insecticides.
8. Cowpea Top Necrosis:
This disease has been observed in all cowpea cultivars and incidence varies from zero to 12%. The fruit recognizable symptoms usually appear 3-4 weeks after germination when the plants are in between fourth and sixth trifoliate leaves. Before the final necrosis of apical bud, the leaves develop general mottling and later turn light yellow. Apical growth is completely checked and the petioles appear wrinkled and brownish grey in colour, and finally, the leaves droop down.
During summer season, the plants completely die and dry-up, while during July-October season, axillary buds proliferate with a limited growth giving a bushy appearance to the plants. Later emerged leaves, if any, are dark green in colour and show bright yellow circular to irregular spots widely distributed on leaf lamina. Diseased plants do not flower, and hence, no pods are formed.
9. Cowpea Witches Broom:
Reduction of leaf size, excessive branching, and shortening of internodes and typical witches’ broom appearance of the plant are common symptoms. Plants at later stages trail on the ground and produce phylloid flowers. The disease incidence varies from one to 6% in different seasons.
Apply systemic granular insecticides like Disulfotan.
10. Nematode Diseases:
Parasitic nematodes are becoming not only an independent agencies of causing disease but are also rapidly becoming well known for their being associated with fungal, bacterial and viral diseases. Nematodes cause wilt and root rot diseases.
i. Use trap crops.
ii. Use chemicals, viz. dichloropropane-dichloropropene, and methyl bromide.
iii. Use soil amendments like neem cakes.
iv. Grow resistant varieties.
v. Varietal resistance is the best measure against nematodes. In cowpea, cv. 8991, CVRT 11937 and EC 11910 are highly resistant.