Early Blight of Potato

Causal Organism:

• Alternaria solani
• Sub-division: Deuteromycotina

Symptoms:

• On Leaves: Small, dark brown to black spots with concentric rings (“target spot” appearance). Older leaves are affected first, leading to leaf yellowing and premature defoliation.
• On Stems: Dark, sunken lesions with concentric rings, causing girdling and weakening of stems.
• On Tubers: Dark, sunken spots with leathery appearance. Underneath lesions, dry, corky rot develops, reducing tuber quality.

Etiology:

• Causal Agent: Alternaria solani, a fungus producing conidia.
• Mode of Infection:
  • Conidia spread by wind, rain splash, and irrigation water.
  • Pathogen survives in infected crop debris and volunteer plants.

Disease Cycle:

• Primary Infection: Conidia from infected debris or volunteer plants initiate the primary infection.
• Secondary Infection: Conidia produced on infected leaves spread to healthy plants by wind and rain splash.

Favorable Conditions:

• Warm temperatures (24-29°C) and high humidity.
• Leaf wetness from dew, rain, or irrigation favors conidia germination.

Management:

1. Cultural Practices:
   • Crop Rotation: Avoid planting potatoes or tomatoes in the same field consecutively.
   • Field Sanitation: Remove and destroy infected crop debris.
   • Proper Spacing: Ensure good air circulation to reduce leaf wetness.
2. Resistant Varieties: Plant resistant cultivars to minimize disease impact.
3. Chemical Control:
   • Fungicide Sprays:
     • Mancozeb, Chlorothalonil, or Copper-based fungicides.
     • Start sprays early in the growing season and repeat at regular intervals.

Late Blight of Potato

Causal Organism:

• Phytophthora infestans
• Class: Oomycetes

Symptoms:

• On Leaves:
  • Water-soaked, pale green to dark brown lesions on leaf margins or tips.
  • Lesions enlarge rapidly and become necrotic with a yellow halo.
  • White, downy fungal growth on the underside of leaves during humid conditions.
• On Stems: Blackened, water-soaked lesions leading to stem collapse.
• On Tubers: Irregular, brown to purplish, sunken lesions on tuber surfaces. Corky, reddish-brown rot develops under the skin, causing foul-smelling decay.

Etiology:

• Causal Agent: Phytophthora infestans, a water mold (Oomycete).
• Mode of Infection:
  • Sporangia dispersed by wind, rain splash, and irrigation water.
  • Zoospores released from sporangia infect leaves, stems, and tubers.
  • Pathogen survives in infected tubers, volunteer plants, and cull piles.

Disease Cycle:

• Primary Infection: Infected seed tubers or volunteer plants serve as primary inoculum.
• Secondary Infection: Sporangia spread rapidly by wind and rain, causing secondary infections. Zoospores released in cool, wet conditions initiate new infections.

Favorable Conditions:

• Cool (18-22°C) and humid weather with prolonged leaf wetness.
• Rain, dew, and high humidity enhance sporulation and spread of the pathogen.

Management:

1. Cultural Practices:
   • Use of Certified Seed Tubers: Plant disease-free, certified seed tubers.
   • Field Sanitation: Remove volunteer plants and destroy cull piles.
   • Proper Irrigation Management: Avoid overhead irrigation to reduce leaf wetness.
2. Resistant Varieties: Plant resistant or tolerant potato varieties.
3. Chemical Control:
   • Fungicide Sprays:
     • Metalaxyl, Mancozeb, or Chlorothalonil.
     • Start fungicide application at the first sign of disease or when weather conditions are favorable.
4. Integrated Disease Management (IDM):
   • Combine cultural, chemical, and biological control measures for effective management.

Black Scurf of Potato (Rhizoctonia solani)

Host Plants: Primarily affects potato but can also infect other crops like carrot, beet, and tomato.

Symptoms:

• On Tubers: Presence of irregular, hard, black crusty patches (sclerotia) on the surface, resembling soil particles but firmly attached. These black patches do not penetrate the tuber but affect appearance and market value.
• On Stems and Roots:
  • Stem Canker: Sunken, brown lesions on underground stems and stolons, which may girdle and weaken them.
  • Poor Emergence: Delayed or uneven sprouting due to infection of sprouting eyes.
  • Stunted Growth: Infected plants are stunted with yellowish leaves.
  • Wilting: Due to disruption of water and nutrient uptake.
• On Leaves: Curling and chlorosis (yellowing) of lower leaves.

Pathogen:

• Causal Organism: Rhizoctonia solani (a soil-borne fungus).
• Characteristics:
  • Produces black sclerotia on tubers which are hard, crust-like resting bodies.
  • Mycelium is brown, branched at right angles, and septate (with cross walls).
  • Survives as sclerotia or mycelium in soil and plant debris.

Mode of Spread and Survival:

• Primary Infection: Infected seed tubers are the main source of primary inoculum. Soil containing sclerotia from previous crops.
• Secondary Spread: Through contaminated soil, irrigation water, farm tools, and machinery. Movement of infected tubers during harvesting and storage.
• Survival: Sclerotia can survive in soil for several years, even without a host plant.

Favorable Conditions:

• Temperature: Cool temperatures of 15–18°C favor disease development.
• Soil Conditions: High soil moisture or poorly drained soils. Heavy soils with poor aeration.
• Environmental Conditions: Long periods of wet and cold weather delay emergence and favor infection.

Management:

i) Cultural Practices:

• • Use of Disease-Free Seed Tubers: Essential to prevent primary infection.
  • Crop Rotation: Rotate with non-host crops (e.g., cereals) for 2–3 years.
  • Field Sanitation Removal and destruction of plant debris post-harvest. Avoid volunteer potato plants.
  • Planting Depth and Spacing: Shallow planting and adequate spacing improve aeration and reduce soil moisture around the stems.
  • Irrigation Management: Avoid over-irrigation and waterlogging. Proper drainage reduces disease risk.

ii) Biological Control: Application of biocontrol agents like Trichoderma spp. and Pseudomonas fluorescens. Use of mycoparasitic fungi and beneficial bacteria.

iii) Chemical Control:

• • Seed Treatment: Treat seed tubers with fungicides like Thiram or Captan before planting. Fungicidal dips or dusting to protect emerging sprouts.
  • Soil Treatment: Application of fungicides to the soil at planting time.
• Resistant Varieties: Kufri Swarna, Kufri Giriraj, and Kufri Badshah show some resistance.
• Hygiene and Sanitation: Clean farm tools and machinery to avoid contamination. Store tubers in dry and well-ventilated conditions to prevent sclerotia formation.

Preventive Measures:

• Quarantine and Inspection: Ensure disease-free planting material.
• Avoidance: Avoid planting in fields with a history of the disease.
• Field Monitoring: Regular scouting for early detection of symptoms.
• Destruction of Infected Plants: Remove and destroy infected plants and debris.

Scab of Potato (Streptomyces scabies)

Host Plants: Primarily affects potato but can also infect other root crops like carrot, beet, radish, and turnip.

Symptoms:

• On Tubers:
  • Common Scab:
    • Rough, corky lesions on the tuber surface.
    • Lesions can be shallow, raised, or pitted, depending on environmental conditions and potato variety.
    • Affected areas become brown and cork-like, reducing tuber quality and market value.
  • Pitted Scab: Deep, crater-like depressions on the tuber surface. More severe in dry and alkaline soils.
  • Netted Scab: Fine, net-like russeting on the tuber surface.
• On Roots and Stems: Typically, no symptoms are observed on the above-ground parts of the plant.

Pathogen:

• Causal Organism: Streptomyces scabies (a soil-borne actinomycete).
• Characteristics:
  • Gram-positive, filamentous bacterium resembling fungi.
  • Produces spores that can survive in the soil for many years.
  • Releases thaxtomin, a toxin responsible for scab formation.
  • Prefers neutral to alkaline soils (pH 5.5 – 8.0).

Mode of Spread and Survival:

• Primary Infection: Infected seed tubers or contaminated soil.
• Secondary Spread: Through contaminated soil, irrigation water, farm tools, and machinery. Movement of infected tubers during harvesting and storage.
• Survival: Spores can survive in soil for several years, even without a host plant. Persists in soil organic matter and plant debris.

Favorable Conditions:

• Soil pH: Disease is more severe in neutral to alkaline soils (pH 5.5 – 8.0).
• Temperature: Moderate temperatures of 20–30°C favor disease development.
• Soil Moisture:
  • Dry soil conditions enhance disease severity.
  • Excessive moisture during early growth stages can also favor infection.

Management:

i) Cultural Practices:

• • Use of Disease-Free Seed Tubers: Essential to prevent primary infection.
  • Crop Rotation: Rotate with non-host crops like corn, wheat, or legumes for 2–3 years.
  • Soil pH Management: Lowering soil pH to below 5.2 by applying sulfur or ammonium sulfate reduces disease severity. Avoid using lime in scab-infested fields.
  • Organic Amendments: Application of organic matter like peat moss or green manure can reduce disease incidence.
  • Irrigation Management: Maintain adequate moisture during tuber initiation to reduce infection.
  • Field Sanitation: Removal and destruction of plant debris post-harvest. Avoid volunteer potato plants.

ii) Biological Control: Application of biocontrol agents like Bacillus subtilis and Streptomyces spp. Use of antagonistic microorganisms to inhibit the growth of Streptomyces scabies.

iii) Chemical Control:

• • Seed Treatment: Treat seed tubers with boric acid (3-5%) before planting to reduce surface contamination. Use of thiophanate-methyl or fludioxonil as a seed treatment fungicide.
  • Soil Treatment: Application of PCNB (Pentachloronitrobenzene) in heavily infested soils.
• Resistant Varieties: Kufri Jyoti, Kufri Sindhuri, Kufri Chandramukhi, and Kufri Navtal are moderately resistant to common scab.
• Hygiene and Sanitation: Clean farm tools and machinery to avoid contamination. Store tubers in dry and well-ventilated conditions to prevent scab formation.

Preventive Measures:

• Quarantine and Inspection: Ensure disease-free planting material.
• Avoidance: Avoid planting in fields with a history of the disease.
• Field Monitoring: Regular scouting for early detection of symptoms.
• Destruction of Infected Plants: Remove and destroy infected plants and debris.

Mosaic of Potato

Causal Organisms:

• Potato Virus Y (PVY)
• Potato Virus X (PVX)
• Potato Leafroll Virus (PLRV)

Symptoms:

• Mild Mosaic: Light and dark green mottling on leaves. Slight distortion and crinkling of leaflets.
• Severe Mosaic: Leaf curling and stunting of plants. Yellowing and necrosis of leaves. Reduced tuber size and yield.
• Vein Banding: Dark green bands along leaf veins (common with PVY).
• Rugosity (Wrinkling): Leaf wrinkling and distortion, particularly with PVX and PVY co-infection.

Mode of Spread and Survival:

• Primary Infection: Infected seed tubers.
• Secondary Spread:
  • Aphids: Myzus persicae and Aphis gossypii transmit PVY and PLRV.
  • Mechanical Transmission: Through contaminated tools, hands, and clothing.
  • Contact Transmission: By contact between diseased and healthy plants.

Favorable Conditions:

• Temperature: Moderate temperatures (20-25°C) favor virus multiplication.
• Aphid Population: High aphid activity increases virus spread.
• Cultivation Practices: Use of infected seed tubers and close spacing enhances disease transmission.

Management:

• Cultural Practices:
  • Use of Certified Disease-Free Seed Tubers.
  • Rogueing: Remove and destroy infected plants early.
  • Crop Rotation: Avoid planting potatoes in the same field for consecutive seasons.
  • Isolation: Maintain distance between potato fields to minimize aphid transmission.
• Vector Control: Aphid Management: Use insecticides like imidacloprid or acetamiprid to control aphids. Reflective Mulch: Reduces aphid landing on plants.
• Resistant Varieties: Use resistant varieties like Kufri Sindhuri, Kufri Jyoti, and Kufri Lauvkar.
• Chemical Control: Mineral Oils: Application of mineral oils to leaves hinders virus transmission by aphids.
• Certification and Quarantine: Use certified virus-free seed material. Implement strict quarantine measures to prevent virus introduction.

Leaf Roll of Potato

Causal Organism: Potato Leafroll Virus (PLRV)

Symptoms:

On Leaves:

• • Upward rolling of lower leaves, giving a stiff, leathery appearance.
  • Pale yellow to light green discoloration, starting from leaf margins.
  • Brittle leaves that make a cracking sound when touched.

On Stems and Tubers:

• • Stunting and reduced growth of plants.
  • Tuber symptoms include net necrosis (brown, dry rot inside the tuber).
  • Poor sprouting and low yield.

Mode of Spread and Survival:

• Primary Source: Infected seed tubers.
• Secondary Spread:
  • Aphids: Primarily transmitted by aphids, especially Myzus persicae.
  • Persistent Transmission: Virus persists in aphids throughout their life.
• Survival: Survives in infected seed tubers and overwintering host plants.

Favorable Conditions:

• Temperature: Moderate temperatures (20-25°C) favor aphid multiplication and virus spread.
• Aphid Population: High aphid activity increases disease incidence.
• Cultivation Practices: Close planting and poor sanitation enhance disease spread.

Management:

i) Cultural Practices:

• • Use of Certified Seed Tubers: Ensure virus-free planting material.
  • Rogueing: Early removal of infected plants.
  • Crop Rotation: Avoid repeated potato planting in the same field.
  • Isolation: Spatial isolation of seed and commercial crops to reduce aphid transmission.

ii) Vector Control:

• • Aphid Management: Use of systemic insecticides like imidacloprid and thiamethoxam.
  • Reflective Mulch: Reduces aphid landings and virus spread.
• Resistant Varieties: Kufri Sindhuri, Kufri Jyoti, and Kufri Bahar show some resistance.
• Chemical Control: Mineral Oils: Spraying mineral oils to interfere with virus transmission by aphids.
• Certification and Quarantine: Use certified virus-free seeds and maintain strict quarantine measures.

Preventive Measures:

• Quarantine and Inspection: Ensure disease-free planting material.
• Field Hygiene: Clean tools and machinery to prevent mechanical transmission.
• Field Monitoring: Regular scouting for aphids and early detection of symptoms.
• Destruction of Infected Plants: Remove and destroy infected plants to minimize virus reservoirs.