Wilt of Pigeon Pea (Fusarium oxysporum f. sp. udum)

Economic Importance

• Annual Losses: Estimated at US $71 million in India due to reduced yield and crop quality.
• Affected Regions: Prevalent in the major pigeon pea-growing states of Andhra Pradesh (Telangana and Kurnool districts), Maharashtra, Madhya Pradesh, Uttar Pradesh, and Bihar.
• Economic Impact: Significant yield reduction affecting farmers’ income and the pulse industry.
• Soil Type Vulnerability: More severe in Vertisols (black cotton soils) compared to Alfisols.

Symptoms

• The disease can manifest from early plant stages (4-6 weeks old) until flowering and podding.

Field Indicators: Patches of dead plants in the field, especially visible during the flowering and podding stages.

Characteristic Symptoms:

• • Purple Band: Extending upwards from the base of the main stem, which is a distinguishing feature of Fusarium wilt.
  • Vascular Discoloration: Brown discoloration of the vascular tissues in the region of the purple band.
  • Partial Wilting: Associated with lateral root infection, leading to wilting of branches while the main stem remains intact. This is a definitive symptom that distinguishes Fusarium wilt from Phytophthora blight, which causes total plant death.
  • Total Wilt: Occurs due to tap root infection, leading to the death of the entire plant.

Foliar Symptoms:

• • Loss of Turgidity: Leaves lose rigidity and droop.
  • Interveinal Clearing: Yellowing occurs between the veins of the leaves.
  • Chlorosis: General yellowing of leaves due to disruption in nutrient and water flow.

Pod and Seed Symptoms:

• • Reduced pod setting and poor seed development.
  • Discolored seeds may be observed in infected plants.

Pathogen Characteristics

• Causal Organism: Fusarium oxysporum f. sp. udum
• Mycelium: Hyaline (transparent) and septate (divided by cross walls).
• Microconidia: Small, hyaline (colorless), elliptical or curved. Single-celled or two-celled structures, aiding in rapid spread.
• Macroconidia: Hyaline, thin-walled, linear, curved or fusoid. Pointed at both ends with 3-4 septa (cross-walls).
• Chlamydospores: Thick-walled, spherical or oval, formed terminally or intercalarily. Can occur singly or in chains of 2 to 3. Act as survival structures in soil, persisting for 8-20 years, enabling long-term survival in adverse conditions.

Favourable Conditions

• Crop Duration Susceptibility: Long and medium-duration pigeon pea varieties are more susceptible than short-duration types.
• Cropping Practices: Monocropping and ratooning (growing a second crop from stubbles) increase disease incidence. Early sowing, good weed management, and vigorous crop growth may encourage disease development.
• Soil Type and Temperature: Vertisols are more prone to wilt than Alfisols due to higher moisture retention. Optimal Soil Temperatures: 17°C to 25°C favor pathogen growth and development.

Mode of Spread and Survival

• Primary Sources: Seed Borne: Infected seeds act as a primary source of inoculum. Soil Borne: The fungus survives in infected stubbles left in the field for about 3 years.
• Survival Mechanisms: Chlamydospores in soil remain viable for 8-20 years, making disease management challenging.
• Spread Mechanisms:
  • Primary Spread: Through soil-borne chlamydospores and contaminated seeds.
  • Secondary Spread:
    • Via irrigation water, farm implements, and wind-borne conidia.
    • Human activities, including field-to-field movement of machinery and tools, contribute to disease dissemination.

Management Strategies

i) Cultural Control

• Crop Rotation: Practice long crop rotation with non-host crops like tobacco, sorghum, or castor. Avoid successive cultivation of pigeon pea in the same field to reduce soil inoculum.
  • Mixed Cropping: Intercrop pigeon pea with sorghum to minimize disease spread and severity.
• Soil Solarization: Involves covering soil with transparent polyethylene sheets during summer to elevate soil temperature and kill soil-borne inoculum.
• Field Hygiene: Collect and destroy diseased stubbles after harvest to reduce inoculum load. Avoid movement of contaminated soil, tools, and equipment between fields.

ii) Host Plant Resistance (HPR)

• Resistant/Tolerant Varieties:
  • Asha (ICPL 87119) – High yield and wilt resistance.
  • Maruti (ICP 8863) – Early maturing and wilt tolerant.
  • Lakshmi (ICPL 85063) – Suitable for rainfed conditions.
  • Durga (ICPL 84031) – High resistance and good yield stability.
  • PRG 100, PRG 158, Muktha, Prabhat, Sharada – Adapted to different agro-climatic zones with resistance to wilt.

iii) Chemical Control

• Seed Treatment: Thiram @ 0.3% or Carbendazim @ 0.2% to protect seeds from soil-borne infection.
• Soil Drenching: Apply Carbendazim or Thiophanate-methyl around the base of the plant.
• Fungicidal Sprays: Not very effective in soil-borne diseases but can reduce secondary spread.

iv) Biological Control

• Trichoderma viride: Seed Treatment: Use at 4 g/kg of seed to suppress the pathogen.
  • Soil Application: Multiply 2 kg of T. viride formulation in 50 kg of farmyard manure and apply to the soil.
• Pseudomonas fluorescens: Acts as an antagonist by producing antibiotics and competing with Fusarium oxysporum.

v) Integrated Disease Management (IDM)

• Combine cultural practices, resistant varieties, chemical treatments, and biological control for effective management.
• Regular field monitoring for early detection and rogueing (removal) of infected plants.
• Educational programs for farmers to increase awareness about disease management practices

Sterility Mosaic Disease (SMD) in Pigeonpea

Causal Organism

• Pathogen: Sterility Mosaic Virus (SMV)
• Vector: Eriophyid mite (Aceria cajani)

Economic Importance:

• SMD is a significant constraint in pigeonpea production, especially in India and Nepal.
• Annual Economic Loss: Estimated at US $ 282 million due to reduced grain yield.
• Yield losses can reach up to 95% when plants are infected at early growth stages.

Symptoms:

• Affected Stages: All growth stages of pigeonpea are susceptible.
• Field Appearance: Diseased plants appear bushy and pale green with no flowers or pods.
• Leaf Symptoms:
  • Smaller leaves with a characteristic light and dark green mosaic pattern.
  • Vein-clearing on young leaves is the initial symptom.
  • Severe cases show reduced leaf size, clustering near the tip due to shortened internodes, and stimulation of auxiliary buds.
• Plant Growth: Infected plants become stunted and exhibit complete sterility when infected within 45 days of planting. Partial sterility occurs in plants infected after 45 days, leading to smaller, shriveled, and immature seeds.
• Special Symptoms in Varieties: Certain varieties, such as ICP 2376, show ring spot leaf symptoms, indicating localized pathogen infection while still producing normal flowers and pods.

Disease Cycle:

• Vector Transmission: The disease is transmitted by the Eriophyid mite (Aceria cajani).
• Source of Infection:
  • Self-sown redgram plants and perennial pigeonpea varieties (Cajanus scarabaeoides var. scarabaeoides) act as reservoirs of the virus.
  • Ratooned growth from harvested plants can also harbor the virus.

Favourable Conditions:

• Inter/Mixed Cropping: Higher disease incidence is observed when pigeonpea is intercropped with sorghum or millets.
• Environmental Factors:
  • Shade and humidity enhance mite multiplication.
  • The disease is more prevalent in hot summer weather.

Management Strategies:

1. Cultural Practices: Rogueing: Early removal of infected plants to limit disease spread.
2. Resistant Varieties: Planting tolerant genotypes like: ICPL 87119 (Asha), ICPL 227, JagrutiBahar
3. Chemical Control: Vector Management: Spraying against mite vectors during early disease stages:
   • • Dicofol: 3 ml per liter of water.
     • Sulphur: 3 g per liter of water.