Describe the symptoms, causal organism, disease cycle and control measures of red rot of sugarcane.

Symptoms of Red Rot

The symptoms of red rot vary depending on the stage of the plant and environmental conditions. They can be broadly categorized into symptoms observed on seedlings, young plants, and mature plants.

• Seedlings: Reddish brown lesions appear on the coleoptile and first leaves. The seedlings may wither and die.
• Young Plants: Reddish brown streaks develop on the lower nodes and internodes. These streaks gradually extend upwards. The leaves may show chlorotic spots that later turn brown and necrotic.
• Mature Plants: This is where the disease is most conspicuous.
  • External Symptoms: Reddish discoloration of the internodes, starting from the base and progressing upwards. The affected internodes become hollow and brittle. A characteristic reddish-brown discoloration is visible when the stalk is split lengthwise.
  • Internal Symptoms: The vascular bundles show reddish-brown discoloration. The pith becomes reddish-brown and eventually hollows out. A sweet, fermentative odour may be emitted from the affected tissues.
  • Top Rot: In severe cases, the top of the plant may rot, leading to wilting and death.

Causal Organism

*Colletotrichum falcatum* is an ascomycetous fungus belonging to the family Glomerellaceae. It is a hemibiotrophic pathogen, meaning it initially feeds on living cells but later becomes saprophytic on dead tissues.

• Morphology: The fungus produces abundant conidia, which are sickle-shaped (hence 'falcatum') and hyaline. Appressoria are formed on the plant surface to facilitate penetration.
• Survival: The fungus survives in infected sugarcane setts (seed pieces), soil, and plant debris.
• Strain Variation: Different strains of *C. falcatum* exhibit varying degrees of virulence.

Disease Cycle of Red Rot

The disease cycle of red rot involves several stages:

1. Infection: The disease cycle begins with infection of sugarcane setts during planting. Conidia are spread by wind, rain splash, and infected implements.
2. Penetration: Conidia germinate on the sett surface and penetrate through wounds or natural openings.
3. Colonization: The fungus colonizes the vascular tissues, causing reddish-brown discoloration.
4. Symptom Development: Symptoms appear on the plant as described above.
5. Conidial Production: The fungus produces abundant conidia on the infected tissues, which are then dispersed to healthy plants.
6. Overwintering/Overseasoning: The fungus survives in infected setts and plant debris, completing the cycle.

Control Measures for Red Rot

Effective control of red rot requires an integrated approach combining preventive and curative measures.

• Use of Disease-Free Setts: Select healthy, disease-free setts for planting. Treat setts with systemic fungicides like carbendazim or propiconazole.
• Sett Treatment: Hot water treatment (50°C for 30 minutes) can effectively kill the fungus present on the setts.
• Crop Rotation: Avoid continuous sugarcane cultivation. Rotate with non-host crops like rice or legumes.
• Resistant Varieties: Cultivate sugarcane varieties resistant to red rot. Coimbatore 86032, Co 89004, and Co 92004 are some resistant varieties.
• Balanced Fertilization: Ensure balanced nutrient supply, particularly potassium and silicon, to enhance plant resistance.
• Sanitation: Remove and destroy infected plant debris to reduce inoculum levels.
• Fungicide Application: In severe cases, foliar application of systemic fungicides like propiconazole or tebuconazole can provide control.
• Biological Control: Use of bio-control agents like *Trichoderma* species can suppress the growth of *C. falcatum*.