Red rot of Sugarcane

Causal Organism: *Colletotrichum falcatum*

*Colletotrichum falcatum* is an ascomycete fungus belonging to the family Glomerellaceae. It exists in two physiological forms: a virulent form causing systemic infection and a less virulent form causing leaf spot. The fungus produces characteristic sickle-shaped conidia, hence the name *falcatum*. It is a soil-borne and seed-borne pathogen, capable of surviving for extended periods in infected sugarcane setts (seed pieces) and soil debris.

Symptoms of Red Rot

The symptoms of red rot vary depending on the stage of the plant and the severity of the infection:

• Seedling Stage: Reddish-brown lesions appear on the lower leaves, often accompanied by wilting.
• Growing Stage: Internal discoloration of the stalk is the most characteristic symptom. Reddish-brown streaks run longitudinally inside the cane, hence the name "red rot." These streaks are initially small but can coalesce to form larger areas of discoloration.
• Mature Cane: Affected canes become hollow and brittle, leading to reduced juice extraction and sugar content. A characteristic sour smell may be present.
• Leaf Symptoms: Reddish-brown elliptical spots with yellow halos appear on the leaves. In severe cases, leaves may wither and die.

Disease Cycle

The disease cycle of red rot involves the following stages:

1. Inoculum: The primary inoculum is the fungus present in infected sugarcane setts and soil debris.
2. Infection: The fungus infects sugarcane setts through wounds created during planting. It can also infect leaves through natural openings or wounds.
3. Colonization: The fungus colonizes the vascular tissues of the sugarcane stalk, causing internal discoloration.
4. Symptom Development: Symptoms appear as reddish-brown streaks inside the cane and spots on the leaves.
5. Dissemination: The fungus spreads through infected setts, wind-blown spores, and contaminated farm implements.
6. Survival: The fungus survives in infected sugarcane debris and soil for extended periods.

Factors Favoring Disease Development

Several factors contribute to the development and spread of red rot:

• High Humidity: High humidity (above 80%) favors spore germination and fungal growth.
• Warm Temperature: Temperatures between 25-30°C are optimal for disease development.
• Soil Moisture: Excessive soil moisture promotes fungal growth and infection.
• Injured Setts: Wounds created during planting provide entry points for the fungus.
• Susceptible Varieties: Certain sugarcane varieties are more susceptible to red rot than others.
• Monocyclic Crop: Continuous cropping of sugarcane without rotation increases the inoculum load in the soil.

Management Strategies

Effective management of red rot requires an integrated approach:

• Use of Resistant Varieties: Planting resistant or tolerant sugarcane varieties is the most effective control measure. Varieties like Co 86061, Co 89004, and Co 94012 exhibit good resistance.
• Sett Treatment: Treating sugarcane setts with systemic fungicides like carbendazim or propiconazole before planting helps to control the infection.
• Seedling Selection: Using healthy and disease-free seedlings for propagation.
• Crop Rotation: Rotating sugarcane with non-host crops like rice or legumes can reduce the inoculum load in the soil.
• Proper Drainage: Ensuring proper drainage in sugarcane fields to avoid waterlogging.
• Sanitation: Removing and destroying infected sugarcane debris to reduce the source of inoculum.
• Biological Control: Utilizing bio-control agents like *Trichoderma* species to suppress the growth of *Colletotrichum falcatum*.
• Integrated Nutrient Management: Balanced fertilization enhances plant vigor and resistance to diseases.

Control Measure	Mechanism
Resistant Varieties	Possess genetic resistance to the pathogen, limiting infection and disease severity.
Sett Treatment	Eliminates surface and internal fungal inoculum, preventing initial infection.
Crop Rotation	Reduces the pathogen population in the soil by breaking the disease cycle.
Biological Control	Antagonistic microorganisms suppress fungal growth and infection.