Black Shank of Tobacco

• Causal Organism: Phytophthora parasitica var. nicotianae
• Host: Tobacco (Nicotiana tabacum)

Economic Importance:

• Common in India, especially in areas with heavy rainfall.
• Causes significant yield losses and affects the quality of tobacco leaves.
• Severe infection leads to wilting and death of plants.

Symptoms:

i)Affects all growth stages of the plant, primarily roots and stem base.

ii) Transplanted Crop Symptoms:

• • Black spots appear on the stem, which enlarge irregularly.
  • Girdling of stem leading to necrotic patches and stem shrinkage.
  • Stem splitting reveals dry, blackened pith in disc-like plates.
  • Wilting and shriveling of the entire plant.

iii) Leaf Symptoms:

• • Water-soaked spots under humid conditions.
  • Spots enlarge, causing blighting and drying of leaves.
  • Bottom leaves dry and wither.

iv) Seedling Symptoms:

• • Black discoloration at the stem base near soil level.
  • Root blackening leading to wet rot in humid weather.
  • Seedling blight under dry conditions with tip withering.

Pathogen Characteristics:

• Mycelium: Hyaline (transparent) and non-septate.
• Sporangia:
  • Hyaline, thin-walled, ovate or pyriform with papillae.
  • Sympodial arrangement on sporangiophores.
  • Zoospores are kidney-shaped and released from germinating sporangia.
• Chlamydospores: Globose, thick-walled, survive under adverse conditions.
• Oospores:
  • Globose, thick-walled, smooth, and light yellow in color.
  • Result from sexual reproduction (oogamous type).

Disease Cycle:

• Primary Source of Inoculum: Dormant mycelium, oospores, and chlamydospores in soil and plant debris. Survives in soil for more than 2 years as saprophyte on organic matter.
• Primary Infection: Oospores and chlamydospores germinate under favorable conditions.
• Secondary Spread: Sporangia or zoospores spread through wind, water, and irrigation. Soil movement, farm implements, and animals aid pathogen dispersal.

Favourable Conditions:

• Frequent rainfall and high soil moisture.
• High population of root knot nematodes (Meloidogyne incognita).
• Poor drainage and waterlogging enhance disease severity.

Management Strategies:

i) Cultural Practices:

• Field Sanitation: Collect and burn infected plant debris to reduce inoculum.
• Selection of Seedlings: Use disease-free seedlings for transplanting.
• Plant Removal: Rogue out and destroy affected plants in the field.
• Drainage Management: Provide adequate drainage in nurseries and fields to reduce moisture levels.
• Soil Solarization: Burn seed beds with paddy husk or groundnut shell (15-20 cm layer) to reduce soil-borne inoculum.

ii) Chemical Control:

• Spot Application at Planting:
  • Bordeaux mixture @ 0.2%
  • Copper oxychloride @ 0.2%
  • Metalaxyl @ 0.2%
• Foliar Sprays:
  • Metalaxyl @ 0.2% – 2 sprays for controlling leaf blight and black shank phases.
  • Copper oxychloride @ 0.2% – 3-4 sprays for effective disease management.

iii) Resistant Varieties: Grow resistant tobacco varieties suited to local agro-climatic conditions.

iv) Integrated Disease Management (IDM):

• Combine cultural practices, resistant varieties, chemical control, and good agronomic practices.
• Regular monitoring for early detection and timely application of control measures.

Mosaic Disease of Tobacco

Pathogen: Tobacco Mosaic Virus (TMV) or Nicotiana virus I

Economic Importance

• Mosaic is the most significant and widespread virus disease affecting tobacco in India.
• It appears in almost all tobacco-growing regions across the country.

Symptoms

• Initial symptoms include light discoloration along the veins of the youngest leaves.
• Leaves develop a characteristic mosaic pattern with alternating light and dark green areas.
• Dark green patches are usually near the veins and later form irregular blisters due to rapid growth.
• Early-season infections cause severe stunting with small, chlorotic, mottled, and curled leaves.
• In severe cases, leaves become narrow, puckered, thin, and malformed.
• Under hot weather, dark brown necrotic spots develop, known as “Mosaic Burn” or “Mosaic Scorching.”

Pathogen

• Causal agent: Nicotiana virus I (Marmor tabaci var. vulgare)
• Structure: Rod-shaped particles measuring 300 x 150-180 µm with a central hollow tube of ~4 µm diameter.
• Composition: Contains RNA molecules encased in a protein coat.
• High resistance: Remains infective in dry form for over 50 years.
• Thermal Inactivation Point (TIP): 90°C for 10 minutes.

Disease Cycle

• Host Range: Infects ~50 species across nine different families, including: Nicotiana species, Tomato, Brinjal, Chilli, and Petunia.
• Transmission:
  • Sap-transmissible: Virus enters host through wounds.
  • Not Seed-Transmitted in tobacco but transmitted through tomato seeds.
  • Contact Transmission: Farm workers involved in topping and clipping can spread the virus via contaminated clothing, hands, and implements.
  • Smoking, chewing tobacco, and snuff can also transmit the virus to standing crops.

Favourable Conditions

• Frequent contact between infected and healthy plants.
• Handling plants with contaminated hands or implements.
• Presence of alternative host plants or weeds harboring the virus.

Management

1. Cultural Practices:
   • Remove and destroy infected plants promptly to minimize spread.
   • Weed management to eliminate alternate virus hosts.
   • Hand hygiene: Wash hands with soap and water before and after handling plants or weeding.
   • Avoid smoking, chewing, or snuffing during field operations to prevent contamination.
   • Crop rotation with non-host plants for two seasons.
2. Biological Control: Spray leaf extracts of Bougainvillea or Basella alba (1 litre extract in 150 litres water) 2-3 times at weekly intervals.
3. Chemical Control: Spray Tannic Acid 1% at 30, 40, and 50 days after planting.
4. Resistant Varieties: CTRI Special (M.R), Jayasree (M.R), Virginia Tobacco 1158, Prabhat, Gautami, Blankat 1, Godavari Special, TMV RR-2, 3, 4, and 6.

Black Root Rot of Tobacco

Pathogen: Thielaviopsis basicola

Economic Importance

• Black root rot is a serious soil-borne disease affecting tobacco worldwide.
• It is prevalent in areas with cool temperatures and poorly drained soils.
• The disease reduces plant vigor, growth, and yield, leading to significant economic losses.

Symptoms

• Stunted Growth: Infected plants exhibit poor growth and reduced vigor.
• Yellowing of Leaves: Lower leaves become yellow and wilt under moisture stress.
• Root Symptoms:
  • Black lesions on roots that gradually enlarge and coalesce.
  • Black discoloration of the root system, leading to rotting.
  • Roots become brittle, and the outer cortex easily sloughs off, leaving a thread-like stele.
• Poor Transplant Establishment: Affected seedlings fail to establish well after transplanting.

Pathogen

• Fungus: Thielaviopsis basicola
• Mycelium: Septate, hyaline, and branched.
• Conidia:
  • Endoconidia: Cylindrical, hyaline, produced in chains within conidiophores.
  • Chlamydospores (Aleuriospores): Thick-walled, dark brown, barrel-shaped, and produced in chains.
• Survival: Chlamydospores can survive in soil for many years, making the pathogen difficult to eradicate.

Disease Cycle

• Primary Infection: Through chlamydospores present in soil or crop residues.
• Secondary Spread: Occurs through soil movement, contaminated tools, water, and infected transplants.
• Soil-Borne Disease: The pathogen remains viable in the soil for long periods, even without host plants.

Favourable Conditions

• Cool soil temperatures (15-20°C).
• Poorly drained soils with high moisture content.
• High soil acidity (low pH).
• Continuous cropping of susceptible crops like tobacco, cotton, and legumes.

Management

i) Cultural Practices:

• • Crop rotation with non-host crops (e.g., cereals) for 3-4 years.
  • Improve soil drainage to reduce moisture content.
  • Avoid excessive soil acidity by applying lime to maintain neutral pH.
  • Use disease-free seedlings for transplanting.
  • Remove and destroy crop residues to minimize inoculum buildup.
• ii) Chemical Control: Soil fumigation with methyl bromide (restricted use). Drenching with fungicides containing thiophanate-methyl or benomyl.
• iii) Resistant Varieties: Use of resistant or tolerant tobacco varieties where available.
• iv) Biological Control: Use of antagonistic fungi such as Trichoderma spp. to suppress the pathogen.