A) Club Root of Cruciferous Vegetables (Plasmodiophora brassicae)

Host Plants:

• Affects cruciferous vegetables such as cabbage, cauliflower, broccoli, radish, turnip, mustard, and Brussels sprouts.

Symptoms:

• Stunting and Yellowing: Infected plants show stunted growth and yellowing of leaves.
• Wilting: Leaves wilt during hot days but may recover at night.
• Root Symptoms:
  • Club-like swelling and distortion of roots.
  • Formation of galls or knobby outgrowths on main roots and rootlets.
  • Reduced secondary root formation leading to poor nutrient and water uptake.
• Plant Decline: In severe cases, plants may become stunted and eventually die.

Pathogen:

• Type: Soil-borne obligate parasite belonging to the group Plasmodiophoromycetes.
• Primary Zoospores: Flagellated, motile, and swim in soil water.
• Secondary Zoospores: Smaller than primary zoospores and infect root hairs.
• Resting Spores: Thick-walled, capable of surviving in the soil for 7–10 years or longer.

Mode of Spread and Survival:

• Soil Borne: Fungus persists in the soil as resting spores for at least 10 years.
• Contaminated Soil: Spread by movement of infested soil through irrigation water, farm equipment, tools, and human footwear.
• Infected Transplants: Disease can be introduced through infected seedlings or transplants.

Disease Cycle:

1. Resting Spore Germination: Occurs in moist, acidic soil (pH < 7) under temperatures of 12–27°C.
2. Zoospore Release: Resting spores release primary zoospores, which swim in soil water and infect root hairs.
3. Root Hair Invasion: Zoospores enter root hairs, form plasmodia, and produce secondary zoospores.
4. Secondary Infection: Secondary zoospores reinfect the same plant or neighboring plants.
5. Gall Formation: Pathogen stimulates abnormal cell division and enlargement, forming characteristic clubs or galls.
6. Resting Spore Formation: Mature galls disintegrate, releasing resting spores back into the soil.

Favorable Conditions:

• Soil pH: Disease severity is higher in acidic soils (pH < 7).
• Soil Moisture: High soil moisture favors spore germination and infection.
• Temperature: Disease development is optimal between 18–25°C.

Management:

i) Cultural Practices:

• • Crop Rotation: Rotate with non-host crops (e.g., cereals, legumes) for 4–7 years.
  • Field Sanitation: Remove and destroy infected plant debris to reduce resting spores in the soil.
  • Avoid Contamination: Clean equipment and tools to avoid soil transfer.
  • Resistant Varieties: Use club root-resistant cultivars when available.
  • Drainage Improvement: Improve soil drainage to reduce moisture levels.

ii) Soil Amendments:

• • Liming: Apply lime (2.5 tons/ha) to raise soil pH above 7, inhibiting resting spore germination.
  • Organic Matter: Reduce high organic matter, as it favors pathogen survival.

iii) Chemical Control:

• • Soil Fumigation: Methyl bromide (1 kg/10 m²) covered with plastic film (restricted in some countries).
  • Soil Drenching: Copper oxychloride (0.25%) for localized treatment.
  • Seed Treatment: Treat seeds with Captan or Thiram (4 g/kg) followed by Trichoderma viride (4 g/kg).

iv) Biological Control: Trichoderma spp. and other antagonistic fungi can suppress pathogen growth.

Preventive Measures:

• Avoid planting susceptible crops in infested fields for several years.
• Regularly monitor fields, especially in acidic soils or regions with a history of club root.
• Maintain optimal soil pH and moisture levels to reduce infection rates.

B) Alternaria Leaf Spot (Alternaria spp.)

Host Plants: Affects a wide range of crops including cruciferous vegetables (cabbage, cauliflower, broccoli, radish), tomatoes, potatoes, carrots, and other leafy vegetables.

Symptoms:

• Leaf Spots: Small, round to irregular, brown to dark brown spots with concentric rings (target-like appearance). Spots may enlarge and merge, leading to blighting of large leaf areas. Yellow halos often surround the spots.
• Defoliation: In severe infections, leaves may turn yellow, wither, and drop prematurely.
• Stem and Petiole Lesions: Elongated, dark brown to black lesions may appear on stems and petioles.
• Fruit Infection: On fruits (e.g., tomato), dark, sunken spots with concentric rings and black moldy growth. Infected fruits may become deformed or fall prematurely.
• Seedling Damage: Damping-off or collar rot in young seedlings.

Pathogen:

• Causal Organism: Alternaria spp. (e.g., Alternaria brassicae, Alternaria brassicicola, Alternaria solani).
• Fungal Characteristics:
  • Hyphae: Septate, branched, pale to dark brown.
  • Conidiophores: Simple or branched, erect, and bear conidia singly or in chains.
  • Conidia:
    • Dark, obclavate or beaked, muriform (transverse and longitudinal septa).
    • Arranged in long chains, giving a “club-shaped” appearance.
    • Often have a tail-like appendage (beak) at the tip.

Mode of Spread and Survival:

• Seed-borne: Pathogen can be carried on or inside seeds, leading to early infection.
• Soil and Plant Debris: Survives in soil and on crop residues as mycelium or conidia.
• Airborne Conidia: Spread through wind, rain splash, and irrigation water.
• Infected Plant Material: Spread through contaminated tools, equipment, and human activity.

Disease Cycle:

1. Overwintering: Infected plant debris or seeds carry the fungus.
2. Conidia Production: Under favorable conditions, conidia are produced on infected debris.
3. Dispersal: Conidia are spread by wind, rain splash, and insects.
4. Infection: Conidia land on leaf surfaces, germinate in the presence of moisture, and infect through stomata or wounds.
5. Colonization: The fungus colonizes leaf tissue, leading to symptom development.
6. Secondary Spread: Secondary conidia are produced on lesions, continuing the disease cycle.

Favorable Conditions:

• Temperature: Optimal temperature for growth is 20–30°C.
• Humidity: High humidity (>90%) and leaf wetness for 10–12 hours favor conidial germination and infection.
• Crowded Planting: Poor air circulation and dense canopy enhance humidity, promoting disease spread.

Management:

i) Cultural Practices:

• • Crop Rotation: Rotate with non-host crops for at least 2–3 years.
  • Field Sanitation: Remove and destroy infected plant debris after harvest.
  • Seed Treatment: Use disease-free certified seeds or treat seeds with fungicides (Thiram or Captan at 2.5–3 g/kg of seed).
  • Plant Spacing: Ensure proper plant spacing to enhance air circulation.
  • Irrigation Management: Avoid overhead irrigation to reduce leaf wetness duration.
  • Resistant Varieties: Use resistant or tolerant varieties where available.

ii) Chemical Control:

• • Fungicide Sprays:
    • Mancozeb 0.25%
    • Chlorothalonil 0.2%
    • Copper oxychloride 0.3%
    • Tridemorph 0.1% (first spray) followed by Mancozeb 0.25% at a month interval.
  • Spray Schedule: Start at the first sign of disease and repeat at 7–10 days intervals, depending on weather conditions.

iii) Biological Control: Trichoderma viride and Pseudomonas fluorescens can be used as biocontrol agents to suppress Alternaria spp.

Preventive Measures:

• Avoid planting in fields with a history of Alternaria infection.
• Practice deep plowing to bury crop residues, reducing primary inoculum.
• Monitor fields regularly, especially under favorable environmental conditions.

C) Black Rot of Cruciferous Vegetables (Xanthomonas campestris pv. campestris)

Host Plants: Affects cruciferous vegetables, including cabbage, cauliflower, broccoli, radish, turnip, mustard, and kale.

Symptoms:

• V-shaped Lesions: Yellow, V-shaped lesions appear on the leaf margins with the base of the V pointing toward a vein.The lesions expand, turning brown and eventually black.
• Vein Blackening: Blackening of veins is observed on the underside of the leaves. Veins in the infected areas become dark and prominent.
• Wilting and Blighting: In severe infections, leaves wilt and drop prematurely. Stunted growth and reduced yield are common.
• Stem Discoloration: Vascular tissues of stems may turn black when cut open.
• Head Rot: In cabbage and cauliflower, the disease can lead to internal head rot, making the produce unmarketable.

Pathogen:

• Causal Organism: Xanthomonas campestris pv. campestris
• Bacterial Characteristics:
  • Gram-negative, rod-shaped bacterium.
  • Produces yellow, mucoid colonies on nutrient agar due to xanthan gum production.
  • Has a single polar flagellum for motility.

Mode of Spread and Survival:

• Seed-borne: Pathogen survives on and within infected seeds.
• Plant Debris: Survives in crop residues left in the field.
• Water and Soil: Spread through irrigation water, rain splash, and surface runoff.
• Mechanical Transmission: Spread by tools, equipment, human activity, and insect vectors.
• Wind-driven Rain: Bacteria are disseminated by wind-driven rain and aerosols.

Disease Cycle:

1. Primary Source: Infected seeds or plant debris in soil.
2. Entry Point: Bacteria enter plants through hydathodes (natural leaf openings), wounds, or insect damage.
3. Colonization: Pathogen moves through vascular tissues, leading to vein blackening and systemic infection.
4. Lesion Formation: V-shaped lesions form, expanding from the leaf margin inward.
5. Secondary Spread: Bacteria ooze from lesions during high humidity and are dispersed by rain splash, wind, and irrigation water.
6. Survival: The bacterium survives on crop residues, seeds, and alternate cruciferous hosts.

Favorable Conditions:

• Temperature: Optimal temperature for disease development is 25–30°C.
• Humidity: High humidity and leaf wetness favor bacterial multiplication and spread.
• Rainfall and Overhead Irrigation: These increase the risk of disease spread through water splash.

Management:

i) Cultural Practices:

• • Use Disease-free Seeds: Use certified disease-free seeds to prevent primary infection.
  • Seed Treatment: Treat seeds with hot water (50°C for 25–30 minutes) to eliminate seed-borne bacteria.
  • Crop Rotation: Rotate crucifers with non-host crops (e.g., cereals) for 2–3 years.
  • Field Sanitation: Remove and destroy infected plant debris after harvest.
  • Avoid Overhead Irrigation: Use drip irrigation to minimize leaf wetness.
  • Plant Spacing: Maintain adequate plant spacing for good air circulation.
  • Resistant Varieties: Use resistant or tolerant cultivars when available.

ii) Chemical Control:

• • Copper-based Bactericides:
    • Copper oxychloride 0.3%
    • Copper hydroxide 0.3%
  • Antibiotic Sprays: Streptomycin sulfate (200 ppm) can be used but should be rotated to prevent resistance.
  • Spray Schedule: Begin at the first sign of infection and repeat at 10–15 days intervals.

iii) Biological Control: Antagonistic Bacteria: Use of beneficial bacteria like Pseudomonas fluorescens and Bacillus subtilis to suppress pathogen growth.

Preventive Measures:

• Sanitation: Disinfect tools and equipment to prevent mechanical transmission.
• Weed Control: Eliminate cruciferous weeds that may serve as alternate hosts.
• Monitoring: Regularly inspect fields, especially during favorable environmental conditions.

C) Downy Mildew of Cruciferous Vegetables (Peronospora parasitica)

Host Plants: Affects cruciferous vegetables such as broccoli, cabbage, cauliflower, radish, mustard, and turnip.

Symptoms:

• Leaf Spots: Yellow to light green irregular spots appear on the upper leaf surface. As the disease progresses, these spots turn brown and necrotic.
• Downy Growth: A characteristic fluffy, white to grayish-purple downy growth appears on the underside of the leaves, corresponding to the yellow spots on the upper side.
• Systemic Infection: In severe cases, the pathogen can invade stems and flower stalks, causing stunted growth and deformation.
• Premature Leaf Drop: Infected leaves may curl, wilt, and drop prematurely, affecting yield and quality.

Pathogen:

• Causal Organism: Peronospora parasitica
• Fungal Characteristics:
  • Obligate Parasite: Requires living host tissue for survival.
  • Conidiophores: Erect, dichotomously branched with long, slender tips.
  • Conidia: Oval, hyaline (transparent), produced at the tips of conidiophores.
  • Oospores: Thick-walled, globose, and yellowish, helping the fungus survive unfavorable conditions.

Mode of Spread and Survival:

• Primary Infection: Through oospores in infected plant debris or soil.
• Secondary Spread: By wind-borne conidia and rain splash.
• Survival: The fungus survives as oospores in crop residues or in contaminated seeds.

Favorable Conditions:

• Temperature: Cool (10–15°C) and moist conditions.
• Humidity: High relative humidity (>85%).
• Leaf Wetness: Prolonged leaf wetness from dew, fog, or rainfall promotes infection.

Management:

i) Cultural Practices:

• • Crop Rotation: Rotate with non-host crops for at least 2–3 years.
  • Field Sanitation: Remove and destroy infected crop residues.
  • Proper Spacing: Maintain adequate spacing for better air circulation.
  • Resistant Varieties: Use resistant or tolerant cultivars when available.

ii) Chemical Control:

• • Seed Treatment: Treat seeds with Metalaxyl (Apron) @ 6g/kg.
  • Foliar Sprays:
    • Metalaxyl (Ridomil) 0.4%
    • Mancozeb 0.25% or Copper oxychloride 0.3% at 10-14 day intervals.

D) Powdery Mildew of Cruciferous Vegetables (Erysiphe cruciferarum)

Host Plants: Affects cruciferous vegetables such as mustard, radish, cabbage, cauliflower, turnip, and kale.

Symptoms:

• Powdery White Coating: White, powdery fungal growth on the upper leaf surface, petioles, and stems. In severe cases, the entire leaf surface may be covered.
• Yellowing and Necrosis: Affected leaves turn yellow, curl, and become distorted. Necrotic spots appear as the disease progresses.
• Defoliation: In severe infections, premature leaf drop occurs, reducing photosynthetic capacity and yield.
• Stunted Growth: Young plants show stunted growth and reduced vigor.

Pathogen:

• Causal Organism: Erysiphe cruciferarum
• Fungal Characteristics:
  • Obligate Parasite: Grows on living host tissues.
  • Mycelium: Superficial, white to gray, with haustoria penetrating epidermal cells.
  • Conidia: Oval or barrel-shaped, produced in chains on erect conidiophores.
  • Cleistothecia (Ascocarps): Dark, spherical fruiting bodies containing ascospores for survival.

Mode of Spread and Survival:

• Primary Infection: Through ascospores from cleistothecia in plant debris.
• Secondary Spread: By wind-borne conidia.
• Survival: The fungus overwinters as cleistothecia on crop residues or as mycelium on volunteer plants.

Favorable Conditions:

• Temperature: Moderate temperatures (15–25°C).
• Humidity: Moderate humidity with dry leaf surfaces.
• Dense Planting: Poor air circulation encourages disease development.

Management:

i) Cultural Practices:

• • Field Sanitation: Remove and destroy infected plant debris after harvest.
  • Proper Spacing: Ensure proper plant spacing to promote good air circulation.
  • Resistant Varieties: Use resistant cultivars when available.
  • Crop Rotation: Rotate with non-cruciferous crops to break the disease cycle.

ii) Chemical Control:

• • Sulfur Dusting: Dusting with sulfur powder at 2.5 kg/ha.
  • Foliar Sprays:
    • Wettable sulfur (0.3%) or Dinocap (0.1%) at 10–15 day intervals.
    • Carbendazim (0.1%) or Hexaconazole (0.1%) can also be used effectively.

Preventive Measures:

• Avoid Overhead Irrigation: Use drip irrigation to minimize leaf wetness.
• Timely Spraying: Start preventive sprays at the first sign of the disease.
• Monitoring: Regularly inspect fields, especially during favorable environmental conditions.