A) Black gram (Vigna mungo) and Green gram (Vigna radiata)

Cercospora Leaf Spot

• Causal Organism: Cercospora spp.
• Host Range: Affects a variety of crops, particularly legumes such as black gram, green gram, cowpea, and soybean.

Economic Importance:

• Yield Loss: Can cause significant yield reduction due to premature leaf drop and decreased photosynthetic area.
• The disease is widespread in tropical and subtropical regions, impacting the productivity and quality of legumes.

Symptoms:

i) Timing: Symptoms appear at any stage of crop growth but are most severe during flowering and podding stages.

ii) Leaf Symptoms:

• • Small, circular to irregular spots with grey or tan centers and dark brown to reddish-purple margins.
  • Spots may enlarge and coalesce, forming large necrotic areas.
  • Shot holing can occur as the dead tissue falls out, leaving perforations in leaves.
  • In severe cases, there is premature defoliation, significantly reducing yield.

iii) Stem and Pod Symptoms: In some crops, lesions may appear on stems and pods, leading to poor grain filling and quality.

Pathogen Characteristics:

• The fungus produces conidia that are:
  • Hyaline (colorless), slender, and multi-septate.
  • Airborne and easily dispersed by wind, rain splash, and contaminated tools.

Disease Cycle:

• Primary Inoculum: The pathogen survives on crop debris in the soil and is also seed-borne.
• Secondary Spread: Occurs through airborne conidia or by rain splash, which infects healthy leaves.

Favourable Conditions:

• Warm temperatures (25-30°C) and high humidity favor disease development.
• Frequent rainfall, heavy dew, or overhead irrigation contribute to infection and spread.

Management Strategies:

i) Cultural Control:

• Field Sanitation: Remove and destroy crop debris to reduce primary inoculum.
• Crop Rotation: Practice crop rotation with non-host crops to break the disease cycle.
• Plant Spacing: Adequate spacing ensures good air circulation, reducing leaf wetness duration.
• Weed Control: Control alternate hosts (weeds) that may harbor the pathogen.

ii) Host Resistance:

• Grow resistant or tolerant varieties where available.
• Use certified disease-free seeds to minimize initial inoculum.

iii) Chemical Control:

• Fungicides:
  • Mancozeb @ 0.25% or Carbendazim @ 0.1%.
  • Chlorothalonil and Thiophanate-methyl are also effective.
  • Apply at the first appearance of symptoms and repeat every 10-15 days.

iv) Integrated Disease Management (IDM): Combine cultural practices, host resistance, and chemical control to achieve sustainable disease management.

B) Anthracnose of Black Gram and Green Gram

• Causal Organism: Colletotrichum lindemuthianum
• Host Range: Primarily affects black gram (Vigna mungo) and green gram (Vigna radiata) but can also infect other legumes.

Economic Importance:

• Significant yield losses occur due to premature defoliation, flower drop, and pod damage.
• Quality and market value of seeds are reduced due to blemishes and shriveling.
• Common in warm and humid regions, especially in South and Southeast Asia.

Symptoms:

• Leaves: Small, circular to irregular brown spots with dark margins. As the disease progresses, the spots enlarge and coalesce, leading to blighting and leaf drop.
• Stems and Petioles: Sunken, elongated dark lesions with a concentric ring pattern. Stem cankers can cause girdling, leading to wilting and plant death.
• Pods: Brown to black sunken lesions with dark concentric rings. Infected pods fail to mature, resulting in shrivelled and discolored seeds.
• Seeds: Infected seeds exhibit dark spots or blotches and may have poor germination.

Pathogen Characteristics:

• The fungus produces: Conidia that are hyaline, one-celled, and curved. Acervuli (fruiting bodies) with setae (hair-like structures), which help in pathogen survival.
• Dispersal: Airborne conidia spread through wind, rain splash, and contaminated tools or seeds.

Disease Cycle:

• Primary Inoculum: Seed-borne pathogen and also survives on infected plant debris.
• Secondary Spread: Occurs through wind and rain-splashed conidia. Insects and human activities can also spread the spores.

Favourable Conditions:

• Warm (25-30°C) and humid conditions with prolonged leaf wetness favor infection.
• Frequent rains, heavy dew, and cloudy weather accelerate disease spread.

Management Strategies:

i) Cultural Control:

• Field Sanitation: Remove and burn infected plant debris to eliminate primary inoculum.
• Crop Rotation: Rotate with non-host crops such as cereals to break the disease cycle.
• Seed Treatment: Treat seeds with fungicides like Carbendazim (2g/kg seed) or Thiram (3g/kg seed) to reduce seed-borne inoculum.
• Proper Spacing and Air Circulation: Maintain optimum plant spacing to improve air circulation and reduce humidity.
• Weed Management: Control alternate weed hosts that can harbor the pathogen.

ii) Host Resistance: Use resistant or tolerant varieties wherever available. Certified disease-free seeds minimize initial inoculum.

iii) Chemical Control: Fungicides: Carbendazim (0.1%) or Mancozeb (0.25%) spray at flowering stage and repeat every 10-15 days. Chlorothalonil and Thiophanate-methyl are also effective.

iv) Integrated Disease Management (IDM): Combine cultural practices, resistant varieties, and chemical control for effective management. Regular monitoring and timely intervention help in reducing disease severity.

C) Yellow Mosaic Disease in Black Gram and Green Gram

• Causal Organism: Mungbean yellow mosaic virus (MYMV)
• Vector: Whitefly (Bemisia tabaci)
• Host Range: Primarily affects black gram (Vigna mungo) and green gram (Vigna radiata) but also infects other legumes.

Economic Importance:

• Severe yield losses up to 85% in susceptible varieties.
• Prevalent in major pulse-growing states of India, including Andhra Pradesh, Tamil Nadu, Uttar Pradesh, Madhya Pradesh, Bihar, Punjab, Haryana, Himachal Pradesh, Rajasthan, and Odisha.
• Affects seed quality and market value due to small and distorted pods.

Symptoms:

• Initial Stage: Small yellow spots or patches on young leaves.
• Progressive Stage: Irregular yellow and green patches on trifoliate leaves, giving a mosaic pattern. Yellow discoloration intensifies, leading to completely yellow leaves in severe cases. Necrosis may occur on affected leaves.
• Growth and Development: Stunted plant growth, late maturity, and reduced flowering. Few flowers and pods, which are often small, distorted, and contain immature seeds. Early infection can cause plant death before seed set.

Disease Cycle:

• Primary Source of Inoculum: Virus survives on alternate weed hosts and other legume crops. Common weed hosts: Croton sparsiflorus, Acalypha indica, Eclipta alba, and Cosmos pinnatus.
• Transmission: Whitefly (Bemisia tabaci) is the primary vector. The virus is not seed-borne but spreads through feeding activity of whiteflies.

Favourable Conditions:

• High incidence during summer season when whitefly population is high.
• Presence of weed hosts and legume crops nearby facilitates virus survival and spread.
• Warm and humid weather favors whitefly multiplication and disease transmission.

Management Strategies:

i) Cultural Control:

• Roguing: Remove and destroy diseased plants up to 40 days after sowing to prevent secondary spread.
• Weed Management: Periodic removal of weed hosts to reduce whitefly habitat and virus reservoirs.
• Crop Management:
  • Increase seed rate to 25 kg/ha to compensate for crop loss due to disease.
  • Cultivate during Rabi season when whitefly populations are lower.
  • Mixed Cropping:
    • Intercrop with maize (60 x 30 cm) or sorghum (45 x 15 cm).
    • For every 15 rows of black gram or green gram, grow 2 rows of maize or sorghum.
  • Border Cropping: Grow 7 rows of sorghum as a barrier to restrict whitefly movement.

ii) Host Resistance:

• Grow resistant varieties to minimize yield loss.
  • Black Gram: Teja, LBG 752, Pant-30, and Pant-90.
  • Green Gram: LGG 407 and ML 267.

iii) Chemical Control:

• Seed Treatment: Imidacloprid 70 WS @ 5ml/kg seed to control early whitefly infestation.
• Foliar Spray:
  • Systemic Insecticides:
    • Dimethoate @ 750 ml/ha or Thiamethoxam.
    • Apply once at 30 days after sowing to manage whitefly vector.
  • Repeated sprays at 10-15 days intervals may be needed during peak whitefly activity.

iv) Integrated Disease Management (IDM):

• Combine cultural practices, resistant varieties, and chemical control for effective management.
• Regular monitoring and timely intervention are crucial for controlling vector population and virus spread.

D) Web Blight of Green Gram and Black Gram

• Causal Organism: Rhizoctonia solani
• Host Range: Affects green gram (Vigna radiata), black gram (Vigna mungo), and other legumes.

Economic Importance:

• Causes significant yield loss, especially in humid and warm conditions.
• More severe in densely planted fields with poor air circulation.
• Affects seed quality and market value due to shriveled seeds.

Symptoms:

• Initial Stage: Water-soaked lesions on lower leaves, stems, and branches. Lesions rapidly enlarge, forming irregular brown patches.
• Progressive Stage: Web-like fungal growth is visible under humid conditions on infected plant parts. Leaves stick together due to mycelial webbing. Shedding of leaves leading to defoliation.
• Pod and Seed Infection: Infected pods show shriveled seeds with poor germination capacity.
• Plant Death: Severe infection leads to drying and death of the entire plant.

Disease Cycle:

• Primary Source of Inoculum: Soil-borne pathogen that survives as sclerotia in soil and plant debris for several years.
• Mode of Spread: Secondary spread occurs through air-borne conidia and water splash. Spread facilitated by dense canopy, high humidity, and poor field sanitation.

Favourable Conditions:

• High humidity (>80%) and temperatures between 25-30°C.
• Dense planting and waterlogging enhance disease severity.
• Continuous cultivation of pulses without crop rotation increases inoculum buildup.

Management Strategies:

i) Cultural Control:

• Field Sanitation: Remove and burn infected plant debris after harvest.
• Crop Rotation: Rotate with non-host crops like cereals (maize, sorghum, or wheat) to break the disease cycle.
• Plant Spacing and Irrigation: Maintain proper plant spacing for good air circulation. Avoid overhead irrigation to reduce leaf wetness.
• Tillage Practices: Deep plowing to bury sclerotia and reduce inoculum load.

ii) Resistant Varieties: Grow tolerant varieties (if available) suitable for local conditions.

iii) Chemical Control:

• Seed Treatment: Carbendazim or Thiram @ 2-3 g/kg seed to protect seedlings from initial infection.
• Foliar Spray: Mancozeb @ 0.25% or Carbendazim @ 0.1% at disease onset. Repeat sprays at 10-15 day intervals for effective control.

iv) Biological Control: Trichoderma spp. (biofungicide) can be applied as a seed treatment or soil application to suppress Rhizoctonia solani.

v) Integrated Disease Management (IDM): Combine cultural practices, resistant varieties, chemical control, and biological control. Regular field monitoring and timely intervention are crucial for effective management.