Wilt of Arhar

Causal Organism and its Characteristics

The primary causal agent of Arhar wilt is *Fusarium udum*, a soil-borne filamentous fungus belonging to the *Fusarium* genus. It is a perfect stage fungus, reproducing through conidia. The fungus exhibits several races, contributing to its widespread distribution and varying levels of virulence. *Fusarium udum* survives in the soil as chlamydospores for extended periods, even in the absence of a host plant. Other *Fusarium* species like *F. oxysporum f. sp. ciceri* can also contribute to wilt symptoms, though less frequently.

Symptoms of Arhar Wilt

The symptoms of Arhar wilt vary depending on the stage of infection and the plant's age. Initial symptoms include:

• Yellowing of leaves: Typically starts with the lower leaves and progresses upwards.
• Wilting: Leaves droop and become limp, even with adequate soil moisture. This is often most pronounced during the hotter parts of the day.
• Vascular discoloration: When the stem is split lengthwise, a characteristic brown discoloration of the vascular tissues (xylem) is observed.
• Root rot: In severe cases, the roots may exhibit rot and decay.
• Plant death: Ultimately, the infected plant wilts and dies.

The disease can manifest in two forms: Root rot phase (occurs in seedlings) and Vascular wilt phase (occurs in older plants).

Disease Cycle

The disease cycle of Arhar wilt is as follows:

1. Survival: *Fusarium udum* survives in the soil as chlamydospores.
2. Infection: The fungus infects susceptible plants through the roots, often entering through wounds caused by nematodes or other soil organisms.
3. Colonization: The fungus colonizes the vascular tissues, blocking the flow of water and nutrients.
4. Symptom development: Wilting and yellowing of leaves occur as a result of vascular blockage.
5. Spore production: The fungus produces conidia within the vascular tissues.
6. Dissemination: Conidia are spread to other plants through contaminated soil, water, farm implements, and infected seeds.

Factors Favoring Disease Development

Several factors contribute to the development and spread of Arhar wilt:

• Soil type: Heavy soils with poor drainage are more conducive to the disease.
• Soil moisture: High soil moisture levels favor fungal growth and infection.
• Temperature: Optimal temperature for fungal growth is between 25-30°C.
• pH: Slightly acidic to neutral soil pH (6.0-7.0) favors the disease.
• Susceptible varieties: Certain Arhar varieties are more susceptible to the disease than others.
• Continuous cropping: Repeated cultivation of Arhar in the same field without crop rotation increases the fungal inoculum in the soil.
• Nematode infestation: Nematodes create wounds in the roots, providing entry points for the fungus.

Management Strategies

Effective management of Arhar wilt requires an integrated approach:

• Resistant varieties: Cultivating wilt-resistant varieties is the most effective control measure. Varieties like ICP 8863, ICPL 87, and MAHYC 16 are known for their resistance.
• Crop rotation: Rotating Arhar with non-host crops like rice, maize, or sorghum reduces the fungal inoculum in the soil.
• Soil solarization: Covering the soil with transparent plastic sheets during the summer months can kill the fungus.
• Seed treatment: Treating seeds with fungicides like carbendazim or thiram can protect seedlings from infection.
• Bio-control agents: Using bio-control agents like *Trichoderma viride* or *Pseudomonas fluorescens* can suppress fungal growth.
• Soil amendment: Adding organic matter to the soil improves soil health and suppresses the disease.
• Nematode management: Controlling nematode infestations reduces root damage and infection sites.
• Deep ploughing: Exposing the soil to sunlight can reduce the survival of chlamydospores.