A) Damping Off of Tomato

Causal Organism:

• Pythium aphanidermatum
• (Also caused by other species like Rhizoctonia solani and Phytophthora spp.)

Symptoms:

1. Pre-Emergence Phase:
   • Seedlings are killed before they emerge from the soil.
   • Radicle (root) and plumule (shoot) are killed, leading to complete rotting of seedlings.
   • Seeds may decay, and germination is poor.
2. Post-Emergence Phase:
   • Occurs after seedlings emerge from the soil.
   • Infection at the collar region (ground level) causing water-soaked, soft tissues.
   • Seedlings topple over or collapse due to weakened stems.
   • Infected areas turn brown and shrivel, leading to rapid seedling death.

Mode of Spread and Survival:

• Primary Source: Soil-borne pathogen that survives in soil and plant debris. Persists as oospores or chlamydospores in the soil for several years.
• Secondary Spread: Spread through irrigation water, contaminated tools, and infected soil. Wide host range including other solanaceous crops like brinjal, pepper, and potato.

Favorable Conditions:

• High soil moisture and poor drainage favor the disease.
• Cloudy weather with high humidity enhances pathogen activity.
• Dense sowing leading to poor aeration increases disease incidence.
• Temperature Range: 20-30°C is ideal for pathogen growth and infection.

Management Strategies:

1. Cultural Practices:
   • Raised Seed Beds: Ensure good drainage to prevent waterlogging.
   • Light but frequent irrigation to avoid excess moisture.
   • Avoid overcrowding by maintaining proper plant spacing.
   • Crop Rotation with non-host crops like cereals to reduce inoculum buildup.
2. Soil Solarization: Covering the soil with transparent polyethylene sheets for 4-6 weeks before planting. Helps in killing soil-borne pathogens by increasing soil temperature.
3. Seed Treatment: Trichoderma viride at 4 g/kg of seed for biological control. Thiram or Carbendazim at 3 g/kg of seed to prevent pre-emergence damping off.
4. Chemical Control:
   • Soil Drenching: Copper oxychloride (0.2%) or Bordeaux mixture (1%). Metalaxyl (0.2%) spray during cloudy weather to control post-emergence phase.
5. Biological Control: Pseudomonas fluorescens can be used as a soil application to suppress damping off. Application of Trichoderma spp. in the soil reduces pathogen population.

Preventive Measures:

• Use of Disease-Free Seeds: Always use certified and pathogen-free seeds.
• Proper Field Hygiene: Remove and destroy diseased seedlings and plant debris.
• Balanced Fertilization: Avoid excessive nitrogen which promotes lush growth susceptible to infection.
• Seedling Hardened: Expose seedlings to light and lower temperatures to enhance resistance.

B) Bacterial Wilt of Tomato

Causal Organism: Burkholderia solanacearum (formerly Pseudomonas solanacearum)

Symptoms:

• Rapid and complete wilting of fully grown plants without yellowing.
• Lower leaves may droop before the whole plant wilts.
• Vascular discoloration: Brown or yellow-brown coloration in the vascular tissues.
• Bacterial ooze: When infected stems are cut and placed in water, a white streak of bacterial ooze can be seen coming out of the cut ends.
• Permanent wilting: Unlike other wilts, affected plants do not recover at night.

Pathogen Characteristics:

• Gram-negative, rod-shaped bacterium.
• Motile with 1-4 flagella.
• Prefers high soil moisture and warm temperatures (optimum range: 30-37°C).
• Confined to vascular tissue, but can invade the cortex and pith in advanced stages.

Mode of Spread and Survival:

• Soil-borne Pathogen: Survives in soil for up to 3 years in fallow land and indefinitely in cultivated fields.
• Spread Through:
  • Irrigation water contaminated with the pathogen.
  • Infected seedlings during transplanting.
  • Farm tools, machinery, and human activities can spread contaminated soil.
• Alternative Hosts: Chilli, eggplant, groundnut, potato, and tobacco help the pathogen survive between tomato crops.

Favorable Conditions:

• High soil moisture and warm temperatures favor disease development.
• Injury to roots or stems during transplanting increases susceptibility.
• Clayey soils with poor drainage are more prone to disease outbreaks.

Management Strategies:

1. Cultural Practices:
   • Avoid mechanical injury during transplanting to minimize entry points for the bacterium.
   • Use raised beds for better drainage.
   • Field Sanitation: Remove and destroy infected plant debris.
   • Crop Rotation:
     • Effective rotations include:
       • Cowpea → Maize → Cabbage
       • Okra → Cowpea → Maize
       • Maize → Cowpea → Maize
       • Finger millet → Eggplant
     • These break the disease cycle by avoiding host plants.
2. Soil Treatment: Apply Bleaching Powder at 10 kg/ha to reduce soil inoculum. Soil Solarization: Covering the soil with transparent polythene sheets for 4-6 weeks during peak sunlight.
3. Resistant Varieties: Use bacterial wilt-resistant tomato varieties where available.
4. Biological Control: Pseudomonas fluorescens and Trichoderma spp. can suppress the pathogen in the soil.
5. Chemical Control: Copper-based bactericides can be used to minimize spread but are not fully effective once plants are infected. Streptocycline (streptomycin + tetracycline) as a preventive soil drench in nurseries.
6. Quarantine and Hygiene Measures: Avoid movement of contaminated soil, tools, and seedlings from infested areas. Clean farm tools and equipment after working in infected fields.

Preventive Measures:

• Use disease-free seeds and seedlings from certified sources.
• Avoid growing susceptible crops consecutively on the same land.
• Regular field inspection to remove and destroy infected plants.
• Balanced Fertilization: Avoid excessive nitrogen as it promotes lush growth, which is more susceptible to infection.

C) Early Blight of Tomato

Causal Organism: Alternaria solani

Symptoms:

• Leaf Spots and Blight:
  • Begins as small, dark brown to black lesions on older leaves.
  • Concentric rings within the spots, resembling a bull’s eye pattern.
  • Yellowing of tissue around the lesions.
  • In severe cases, leaves wither and drop prematurely, leading to defoliation.
• Stem Lesions: Dark, elongated spots that can girdle the stem, leading to plant wilting. Common near the soil line in seedlings.
• Fruit Infection: Typically occurs at the stem end, near the calyx. Large, sunken lesions with concentric rings. Infected fruits may rot and become unmarketable.

Mode of Spread and Survival:

• Dissemination: Spread by wind-blown spores and rain splashes. Can also be seed-borne.
• Survival: Survives in infected plant debris in the soil for up to three years. Favorable conditions: High humidity and temperatures between 24-29°C.

Management:

1. Cultural Practices:
   • Crop rotation with non-hosts for at least 2-3 years.
   • Removal and destruction of infected plant debris.
   • Good field sanitation to reduce initial inoculum.
   • Proper plant spacing to enhance air circulation.
2. Chemical Control:
   • Fungicidal Sprays:
     • Mancozeb (0.2%), Chlorothalonil, or Copper-based fungicides.
     • Spray Interval: Every 7-10 days during favorable conditions.
3. Resistant Varieties: Planting resistant or tolerant varieties reduces disease incidence.
4. Biological Control: Use of Trichoderma viride or Pseudomonas fluorescens as biocontrol agents.
5. Seed Treatment: Hot water treatment or fungicidal seed treatment to prevent seed-borne inoculum.

D) Late Blight of Tomato

Causal Organism: Phytophthora infestans

Symptoms:

• Leaf Blight:
  • Begins as water-soaked lesions on leaves, which quickly enlarge and turn brown to black.
  • White fungal growth appears on the underside of leaves under humid conditions.
  • Leaves become blighted, wither, and collapse.
• Stem Lesions: Dark, brown to black lesions on stems and petioles. Stem girdling leads to wilting and plant death.
• Fruit Infection:
  • Water-soaked, greasy lesions that become brown, firm, and rough.
  • White fungal growth may appear under humid conditions.
  • Infected fruits rot and become unmarketable.

Mode of Spread and Survival:

• Dissemination: Airborne spores spread by wind and rain splashes. Spread through infected seeds, seedlings, and plant debris.
• Survival:
  • Oospores (resting spores) survive in soil and plant debris.
  • Survives on infected potato tubers and volunteer plants.
  • Favorable conditions: Cool, wet weather (18-22°C) with high humidity (>90%).

Management:

1. Cultural Practices:
   • Crop rotation with non-hosts (e.g., cereals) for at least 2-3 years.
   • Destroy infected plant debris and volunteer plants to eliminate inoculum.
   • Proper spacing to reduce leaf wetness and humidity.
   • Avoid overhead irrigation to minimize leaf wetness.
2. Chemical Control:
   • Protective and systemic fungicides:
     • Metalaxyl (0.2%), Chlorothalonil, Mancozeb, or Dimethomorph.
     • Spray Interval: Every 7-10 days during conducive weather.
3. Resistant Varieties: Planting late blight-resistant varieties reduces disease incidence.
4. Biological Control: Trichoderma spp. and Bacillus subtilis are effective biocontrol agents.
5. Integrated Disease Management (IDM): Combine cultural practices, resistant varieties, chemical, and biological controls.

Comparison Between Early Blight and Late Blight of Tomato:

Key Differences:

• Early Blight is more common in warm and humid conditions, whereas Late Blight thrives in cooler, wet conditions.
• Early Blight shows concentric rings on leaves, while Late Blight causes water-soaked lesions with white fungal growth.
• Early Blight is caused by a fungus (Alternaria solani), while Late Blight is caused by an oomycete (Phytophthora infestans).

E) Buckeye Rot of Tomato

Causal Organism:

• Phytophthora nicotianae var. parasitica
• Phytophthora drechsleri
• Phytophthora capsici

Symptoms:

• Fruit Rot:
  • Affects green and mature fruits touching the soil.
  • Water-soaked lesions develop on the lower half of the fruit.
  • Lesions are brown to dark brown with concentric rings resembling a buckeye pattern.
  • Infected areas are firm and smooth, without a foul smell.
  • Under humid conditions, a whitish fungal growth may appear on lesions.

Stem and Leaf Symptoms: Stem lesions may occur near the soil line but are less common. No significant leaf symptoms, unlike other fungal diseases.

Mode of Spread and Survival:

• Dissemination: Soil-borne spores spread through rain splashes, irrigation water, and contaminated tools.
• Survival: Oospores (resting spores) survive in the soil for several years. Disease favors warm, wet conditions (25-30°C) with high humidity.

Management:

1. Cultural Practices:
   • Raised beds and staking to prevent fruit contact with soil.
   • Crop rotation with non-host crops (e.g., cereals) for at least 2-3 years.
   • Mulching to reduce soil splash on fruits.
   • Good drainage to avoid waterlogging.
2. Chemical Control:
   • Fungicidal Sprays: Metalaxyl, Mefenoxam, or Copper-based fungicides.
     • Spray Interval: Every 7-10 days during favorable conditions.
3. Resistant Varieties: Planting resistant varieties helps minimize disease incidence.
4. Biological Control: Trichoderma spp. and Bacillus subtilis as biocontrol agents.

F) Leaf Curl of Tomato

Causal Organism:

• Tomato Leaf Curl Virus (ToLCV)
• Transmitted by Whitefly (Bemisia tabaci)

Symptoms:

• Leaf Symptoms:
  • Upward or downward curling of leaf margins.
  • Thickening and brittleness of leaves.
  • Yellowing and mosaic pattern on affected leaves.
  • Reduced leaf size with crinkling.
• Growth and Yield Effects:
  • Stunted plant growth with shortened internodes.
  • Flower drop and reduced fruit set, leading to significant yield loss.
  • Malformed and undersized fruits in severe cases.

Mode of Spread and Survival:

• Dissemination: Transmitted by the whitefly vector (Bemisia tabaci). Not seed-borne or soil-borne.
• Survival: Survives in alternative hosts such as weeds and other solanaceous crops. Thrives in warm temperatures (25-35°C) and low humidity.

Management:

1. Cultural Practices:
   • Rogueing and destroying infected plants to reduce inoculum.
   • Removal of alternate hosts (weeds) near tomato fields.
   • Plant spacing and barrier crops (e.g., maize) to reduce whitefly infestation.
2. Chemical Control:
   • Insecticidal Sprays: Imidacloprid, Thiamethoxam, or Buprofezin to control whitefly vectors. Spray Interval: Every 7-10 days during peak whitefly activity.
3. Resistant Varieties: Planting virus-resistant tomato varieties helps in reducing disease incidence.
4. Physical Control: Yellow sticky traps to monitor and reduce whitefly populations. Reflective mulches to repel whiteflies.
5. Integrated Disease Management (IDM): Combination of cultural practices, resistant varieties, and insecticidal control.