A) Club Root of Crucifers
Causal Organism: Plasmodiophora brassicae
Club root is a severe soil-borne disease affecting cruciferous crops such as cabbage, cauliflower, broccoli, radish, and mustard. It significantly impacts plant growth, leading to reduced yield and poor-quality produce.
Economic Importance
- Club root is one of the most devastating diseases of crucifers worldwide.
- Infected plants exhibit stunted growth, poor head formation (in cabbage and cauliflower), and reduced seed yield.
- Severe infestations can lead to complete crop failure, especially in acidic soils.
Symptoms
- On Roots:
- Swollen, distorted, and club-shaped galls on roots.
- Roots become enlarged, malformed, and appear clubbed, interfering with nutrient and water uptake.
- Secondary roots may be absent or poorly developed.
- Above-Ground Symptoms:
- Wilting during the day, especially under hot and dry conditions, with partial recovery at night.
- Stunted growth and yellowing of leaves.
- Premature senescence and poor development of heads in cabbage and cauliflower.
- Advanced Stages:
- Infected plants become severely stunted and may eventually die.
- Infected roots decay, leading to a foul odor.
Pathogen Characteristics
- Type: Obligate biotrophic endoparasite and soil-borne pathogen.
- Life Cycle:
- The pathogen exists as resting spores in soil, which can survive for 7-10 years.
- Resting spores germinate in the presence of host root exudates, producing primary zoospores.
- Primary Infection: Zoospores infect root hairs, where they develop into plasmodia.
- Secondary Infection: Plasmodia produce secondary zoospores, which infect cortical cells, causing hypertrophy and hyperplasia, resulting in club-shaped galls.
- Mature galls disintegrate, releasing resting spores back into the soil.
Disease Cycle
i) Primary Inoculum: Resting spores in the soil act as the primary source of infection. These spores are long-lived and can persist in the soil for many years.
ii) Secondary Spread: The disease spreads through contaminated soil, irrigation water, farm implements, and infected transplants. Zoospores are motile in water films, facilitating infection during wet soil conditions.
Favorable Conditions:
- Soil pH: Acidic soils (pH below 6.5) favor disease development.
- Soil Moisture: High soil moisture and poor drainage enhance zoospore movement.
- Temperature: Moderate temperatures (20°C to 25°C) are conducive to infection.
Management Strategies
i) Cultural Practices:
- Crop Rotation: Practice long crop rotations (5-7 years) with non-cruciferous crops to reduce soil inoculum.
- Soil Liming: Apply lime to increase soil pH to above 7.0, reducing disease severity. Apply lime several weeks before planting to allow proper soil adjustment.
- Good Drainage: Improve soil drainage to reduce waterlogging, which favors zoospore movement.
- Hygiene Measures: Clean farm implements and avoid soil movement from infested to uninfected fields.
- Resistant Varieties: Use resistant or tolerant cultivars, especially for cabbage and cauliflower. Examples include cultivars like Resistant Danish and Kilaxy in cabbage.
- Biological Control: Use of biocontrol agents such as Trichoderma spp. and Bacillus subtilis to suppress the pathogen. Application of antagonistic microorganisms that compete or parasitize the pathogen.
- Chemical Control: Soil fumigation with chemicals like PCNB (Pentachloronitrobenzene) is effective but costly and environmentally hazardous. Application of fungicides containing fluazinam as a soil drench has shown effectiveness.
- Integrated Disease Management (IDM): Combine cultural practices, resistant varieties, and biocontrol measures for effective management. Regular monitoring and early detection can help in timely intervention.
B) White Rust
Causal Organism: Albugo candida or A. cruciferarum
White rust is a serious disease affecting cruciferous crops like mustard, radish, cauliflower, and cabbage. It can cause significant yield losses due to malformation of floral parts and reduced seed production.
Economic Importance
- The disease is most severe in cool and humid conditions, typically appearing in January.
- It coincides with the occurrence of Alternaria leaf blight.
- Systemic infections can lead to complete sterility of flowers, causing substantial yield losses.
Symptoms White rust exhibits both local and systemic symptoms:
- Local Infection:
- Appears as white or creamy-yellow raised pustules on the underside of leaves.
- Pustules may merge, forming larger patches.
- These pustules rupture, releasing powdery masses of sporangia.
- Systemic Infection:
- Causes hypertrophy (enlargement) and hyperplasia (increased cell number) leading to distortion of stems, leaves, and floral parts.
- Floral parts become swollen, twisted, and malformed, forming structures known as “Stag heads.”
- The inflorescence may become completely sterile.
- Systemic infection is more damaging as it affects the reproductive parts, leading to yield losses.
Pathogen Characteristics
- Nature: Obligate parasite, meaning it requires a living host to complete its life cycle.
- Mycelium: Non-septate (without cross walls) and intercellular (grows between host cells). Produces knob-like haustoria to absorb nutrients from host cells.
- Reproductive Structures:
- Sporangiophores: Short, club-shaped structures that produce sporangia in chains in a basipetal succession (younger spores at the base).
- Sporangia:
- White, powdery structures that are easily dispersed by wind or rain.
- Germinate to produce biflagellate zoospores which infect host tissue.
- Sexual Reproduction:
- Oogonia (female) and antheridia (male) join through a fertilization tube to form oospores.
- Oospores: Thick-walled resting spores that can survive in plant debris and soil.
- Upon germination, oospores release zoospores that initiate new infections.
Disease Cycle
- Primary Infection: Oospores in infected plant debris or soil germinate and release zoospores. Zoospores infect host plants, leading to systemic or local infections.
- Secondary Spread: Zoospores are disseminated by rain, irrigation water, and wind. Sporangia released from pustules spread the disease to neighboring plants.
- Survival and Perpetuation: The pathogen survives through oospores present in crop residues or soil. It also persists on alternate hosts and collateral weeds.
Favourable Conditions
- Cool and humid weather conditions (15°C to 25°C) favor disease development.
- Overcast skies and drizzling rain increase spore germination and dispersal.
- High relative humidity and dew formation are conducive to systemic infection.
Management Strategies
- i) Cultural Practices:
- Field Sanitation: Collect and destroy infected plant debris to reduce the inoculum load.
- Crop Rotation: Rotate with non-cruciferous crops to break the disease cycle.
- Early Sowing: Sowing in the first week of October helps escape peak infection periods.
- Eradication of Alternate Hosts: Remove weeds and collateral hosts like wild crucifers.
- ii) Chemical Control: Seed Treatment: Treat seeds with Metalaxyl (Apron 35SD) @ 6 g/kg seed to prevent primary infection. Foliar Spray: Use Metalaxyl (Ridomyl MZ) @ 0.2% at the onset of disease symptoms. Repeated sprays may be necessary under favorable conditions.
- iii) Resistant and Tolerant Varieties: Resistant Varieties: RC 781, PYSR 8, and PR 10. Tolerant Varieties: Kranthi and Krishna. Growing resistant varieties is one of the most effective and economical control measures.
- iv) Integrated Disease Management (IDM): Combining cultural practices, chemical treatments, and resistant varieties for effective control. Monitor fields regularly for early detection and timely management.
C) Alternaria Leaf Spot of Crucifers
Causal Organisms: Alternaria brassicae and A. brassicola
Alternaria leaf spot is a common and destructive disease affecting cruciferous crops such as mustard, cabbage, cauliflower, and radish. It significantly impacts yield and quality by damaging leaves, stems, and siliquae (seed pods).
Economic Importance
- The disease caused by A. brassicae is more prevalent and damaging compared to A. brassicola.
- It occurs in all rapeseed-mustard growing regions and can lead to significant yield losses under favorable conditions.
- Severe infections can cause premature defoliation, reduced seed size, and poor seed quality.
Symptoms
- On Leaves:
- Circular to irregular spots with concentric rings (target spots). A. brassicae produces grayish spots, whereas A. brassicola forms black, sooty, and velvety lesions. Spots may coalesce, leading to blighting of leaves.
- On Stem: Elongated, dark lesions that may girdle the stem, leading to wilting.
- On Siliquae (Seed Pods): Small, dark spots that enlarge and cause premature shattering. Infected siliquae produce shriveled seeds with poor germination capacity.
Pathogen Characteristics
- Mycelium: Septate and dark brown, growing both inter- and intracellularly in host tissue.
- Conidiophores: Short, dark brown, septate, and irregularly bent. Each conidiophore bears a single conidium at its apex.
- Conidia:
- Shape: Obclavate (club-shaped) with a prominent beak.
- Color: Light to dark brown.
- Septation: Transverse and longitudinal septa (muriform).
- Conidia are produced in chains and are easily dispersed by wind.
Disease Cycle
- Primary Inoculum: The fungus survives in crop debris as dormant mycelium or as conidia on seeds. Infected plant residues act as the primary source of inoculum for the next cropping season.
- Secondary Spread: Airborne conidia are the main means of secondary spread. Conidia are dispersed by wind, rain splash, and insects.
- Favorable Conditions: High humidity (90-95%) and moderate temperatures (20°C to 25°C). Frequent dew, cloudy weather, and light drizzles favor disease development.
Management Strategies
i) Cultural Practices:
- Field Sanitation: Remove and destroy infected plant debris to reduce the inoculum load.
- Crop Rotation: Practice crop rotation with non-cruciferous crops.
- Proper Spacing: Maintain adequate spacing to improve air circulation and reduce humidity.
- Weed Management: Control cruciferous weeds that can harbor the pathogen.
ii) Seed Treatment: Use disease-free seeds or treat seeds with Mancozeb @ 2.5 g/kg seed. This helps in reducing the seed-borne inoculum.
iii) Chemical Control: Fungicide Sprays:
- Mancozeb @ 0.25% or Iprodione @ 0.2% at 10-day intervals starting from the appearance of initial symptoms.
- Multiple sprays may be required under favorable environmental conditions.
iv Resistant Varieties: Growing resistant or tolerant cultivars is one of the most effective and economical strategies. However, breeding for resistance is challenging due to high variability in pathogen populations.
v) Integrated Disease Management (IDM): Combine cultural practices, chemical control, and resistant varieties for effective management.Regular monitoring and early detection can help in timely intervention.
D) Sclerotinia Stem Rot of Crucifers
Causal Organism: Sclerotinia sclerotiorum
Sclerotinia stem rot, also known as white mold, is a devastating fungal disease affecting cruciferous crops such as mustard, cabbage, cauliflower, radish, and other broad-leaved plants. It is a major concern in cool and moist environments, leading to significant yield losses.
Economic Importance
- Affects a wide range of cruciferous crops, leading to reduced yield and poor-quality produce.
- Under favorable conditions, yield losses can reach up to 50% or more.
- The disease impacts seed formation and quality, especially in mustard and rapeseed crops.
Symptoms
- Initial Symptoms: Water-soaked lesions on stems, branches, and leaves. Lesions expand rapidly, becoming light brown with a bleached appearance.
- Advanced Symptoms: Cottony white mycelium develops on affected plant parts under humid conditions. Hard, black, irregularly shaped structures called sclerotia form within the stem pith or on the surface of infected plant parts. Affected plants show wilting, premature ripening, and lodging.
- On Stems and Branches: Infected stems become hollow, brittle, and may break easily. Shredding of stem tissues is common.
- On Pods and Leaves: Pods and leaves exhibit white moldy growth leading to seed shriveling. Leaf blighting and defoliation occur in severe cases.
Pathogen Characteristics
- Type: Necrotrophic fungus.
- Mycelium: Cottony white, septate, and branched.
- Sclerotia:
- Hard, black resting structures that are irregular in shape.
- They can survive in the soil for several years under unfavorable conditions.
- Infection Mechanism:
- Sclerotia germinate under moist conditions, forming apothecia that release ascospores.
- Airborne ascospores infect senescing petals, which serve as a nutrient source for initial colonization.
- The fungus then spreads to other plant parts, causing stem rot.
Disease Cycle
- Primary Inoculum: Sclerotia in the soil act as the primary source of infection. These sclerotia can survive in the soil for 5-10 years.
- Secondary Spread: Airborne ascospores released from apothecia infect senescing petals. Infected petals fall on leaves and stems, leading to secondary infection.
- Favorable Conditions:
- Temperature: Cool temperatures (15°C to 20°C) favor disease development.
- Humidity: High humidity (80-90%) and wet weather conditions.
- Crop Density: Dense canopy and poor air circulation increase infection risk.
Management Strategies
i) Cultural Practices:
- Crop Rotation: Practice crop rotation with non-host crops like cereals for at least 3-4 years to reduce soil inoculum.
- Field Sanitation: Remove and destroy infected plant debris after harvest.
- Proper Spacing and Pruning: Maintain adequate plant spacing to improve air circulation and reduce humidity.
- Irrigation Management: Avoid overhead irrigation to reduce leaf wetness.
ii) Resistant Varieties: Currently, no fully resistant varieties are available, but some cultivars show partial resistance or tolerance.
iii) Biological Control: Coniothyrium minitans: A mycoparasite that attacks sclerotia in the soil. Trichoderma spp.: Effective against Sclerotinia sclerotiorum by parasitizing sclerotia and inhibiting mycelial growth.
iv) Chemical Control:
- Seed Treatment: Treat seeds with fungicides such as Carbendazim or Thiophanate-methyl.
- Foliar Sprays: Apply fungicides like Iprodione, Carbendazim, or Tebuconazole at early flowering and repeat after 10-15 days.
- Chemical Soil Treatment: Application of soil fumigants in heavily infested fields can help reduce sclerotia in the soil.
v) Integrated Disease Management (IDM):
- Combine cultural, biological, and chemical control methods for effective management.
- Regular monitoring and timely application of control measures are crucial for disease suppression.
