A) Mosaic of Soybean

Disease Name: Soybean Mosaic
Causal Agent: Soybean Mosaic Virus (SMV)

Symptoms

General Plant Symptoms

• Stunted Growth:
  • Infected soybean plants are shorter and exhibit delayed maturity.
  • They remain green and unripe even at the end of the growing season.

Leaf Symptoms

• Mosaic Pattern Characterized by light and dark green patches on the leaves, giving a mosaic-like appearance. Vein clearing is noticeable on young leaves.
• Distortion and Puckering: Leaves become crinkled, narrow, and distorted with margins curling downward. Dark green blister-like puckering along the veins in severe cases.
• Texture and Brittle Nature: Leaves become leathery and brittle, leading to premature leaf drop.

Pod and Seed Symptoms

• Reduced Pod Setting: Infected plants show drastically reduced pod formation. Pods are often distorted, undersized, and twisted.
• Seed Discoloration and Malformation: Seeds may show mottling or discoloration, with brown to black patches. Infected seeds are often shriveled and have reduced viability.

Disease Cycle

Primary Source of Inoculum

• Seed-borne Transmission: SMV is primarily seed-borne, surviving in infected seeds. Infected seeds are the initial source of inoculum for the next season.

Secondary Spread

• Vector Transmission: The virus is non-persistently transmitted by aphids, including: Aphis glycines (Soybean Aphid) Myzus persicae (Green Peach Aphid)
• Mode of Transmission: Aphids acquire the virus while feeding on infected plants and transmit it to healthy plants within minutes to hours.

Environmental Factors

• Temperature and Humidity: High temperatures (20-30°C) and moderate humidity favor aphid population growth and virus spread.
• Aphid Population Dynamics: Weather patterns and host plant availability influence aphid outbreaks.

Management Strategies

Cultural Practices

• Use of Virus-Free Seeds: Select seeds from healthy crops to prevent primary infection.
• Rogueing and Sanitation: Rogue out and burn infected plants to eliminate the virus source.
• Crop Rotation and Isolation: Practice crop rotation with non-host crops to break the disease cycle.

Chemical Control

• Vector Control: Use of insecticides to control aphid populations and reduce virus transmission: Monocrotophos @ 1.5 ml/liter Dimethoate @ 2 ml/liter
• Timing of Application: Apply at the early vegetative stage to prevent initial infection.

Host Plant Resistance Resistant Varieties: Planting SMV-resistant soybean varieties is the most effective management strategy. Use varieties with multiple resistance genes for durable protection.

Biological Control

• Natural Enemies: Utilize predators (e.g., lady beetles) and parasitoids to regulate aphid populations.
• Biopesticides: Use of Neem-based products as eco-friendly alternatives.

B) Bacterial Spot of Soybean

Disease Name: Bacterial Spot
Causal Agent: Pseudomonas syringae pv. glycinea

Economic Importance

• Widespread occurrence in soybean-growing regions worldwide.
• Can cause significant yield losses under favorable conditions.
• Affects leaf health, leading to reduced photosynthesis and overall plant vigor.

Symptoms

Leaf Symptoms

• Water-soaked Spots: Small, water-soaked spots appear on the upper surface of young leaves. Spots are angular and are restricted by the leaf veins.
• Necrosis and Yellow Halo: Spots turn dark brown to black and are surrounded by yellow halos. Infected areas coalesce, forming large necrotic patches.
• Leaf Curling and Drop: Severe infections cause leaf curling, distortion, and premature drop.

Stem and Pod Symptoms

• Stem Lesions: Small, dark lesions on stems that may become elongated. Lesions can lead to stem cracking and secondary infections.
• Pod Spots:Small, dark, sunken spots on pods. Infected pods may contain shriveled and discolored seeds.

Pathogen Characteristics

• Pseudomonas syringae pv. glycinea is a gram-negative bacterium.
• It produces toxins that cause cell death, leading to necrosis.
• Bacteria overwinter in crop debris, infected seeds, and soil.

Disease Cycle

Primary Source of Inoculum

• Seed-Borne Inoculum: The bacterium is seed-borne, leading to primary infections in new crops.
• Crop Residue: Survives in infected plant debris left in the field.

Secondary Spread

• Rain Splash and Wind: Rain splash and wind disseminate bacteria to healthy plants.
• Agricultural Practices: Infected tools and machinery contribute to the spread.
• Water and Insects: Contaminated irrigation water and insects aid in transmission.

Favourable Conditions

• High Humidity and Rainfall: High humidity and frequent rain enhance bacterial multiplication and spread.
• Moderate Temperatures: Optimal temperatures between 25-30°C favor disease development.
• Wounds and Injuries: Entry through natural openings (stomata, hydathodes) or wounds caused by insects or mechanical injury.

Management Strategies

Cultural Practices

• Use of Disease-Free Seeds: Use certified disease-free seeds to prevent primary infections.
• Crop Rotation: Rotate with non-host crops (e.g., maize, wheat) to reduce inoculum in the soil.
• Field Sanitation: Remove and destroy infected plant debris to minimize overwintering bacteria.

Chemical Control

• Copper-Based Bactericides: Spray with Copper oxychloride or Copper hydroxide to control bacterial spread. Apply at 10-15 day intervals during wet conditions.
• Antibiotics (Limited Use): In severe cases, Streptomycin sprays may be effective but should be used judiciously to prevent resistance.

Resistant Varieties

• Host Resistance:
  • Plant resistant or tolerant soybean varieties if available.
  • Breeding programs focus on developing disease-resistant cultivars.

Biological Control

• Antagonistic Bacteria:
  • Use of biocontrol agents like Bacillus subtilis to inhibit pathogen growth.
• Natural Predators and Biopesticides:
  • Promote natural predators and use plant extracts with antibacterial properties.

C) Seed and Seedling Rot of Soybean

Causal Agents:

• Pythium spp. (Damping-off)
• Phytophthora sojae (Phytophthora rot)
• Rhizoctonia solani (Rhizoctonia rot)
• Fusarium spp. (Fusarium root rot)

Economic Importance

• Widespread disease affecting soybean production globally.
• Causes poor seed germination, seedling death, and reduced plant population.
• Leads to uneven crop stand and yield losses ranging from 20% to 50% under severe conditions.

Symptoms

Seed Rot

• Pre-emergence Rot: Seeds fail to germinate or sprout but decay before emerging. Infected seeds become soft, mushy, and discolored.
• Post-emergence Rot: Seedlings emerge but collapse at the soil line, showing water-soaked lesions.

Seedling Damping-off

• Pre-Emergence Damping-off: Seedlings fail to emerge or emerge weak and wilted.
• Post-Emergence Damping-off: Seedlings appear stunted, with a brownish lesion at the base. The affected area becomes sunken and constricted, leading to wilting and collapse.

Root Rot

• Root Symptoms: Roots show discoloration, decay, and reduced growth. Infected roots become soft and water-soaked, leading to poor nutrient uptake.
• Stem Lesions: Brown to reddish lesions at the stem base, which may extend upward. Affected stems become brittle and easily breakable.

Pathogen Characteristics

• Pythium spp. Soil-borne oomycetes that produce zoospores capable of swimming in water films. Infects seeds and seedlings under wet and cool conditions.
• Phytophthora sojae Oomycete pathogen causing Phytophthora rot. Produces oospores (resting spores) and zoospores for survival and spread.
• Rhizoctonia solani Soil-borne fungus producing sclerotia (resting bodies). Infects seedlings in warm and moist soils.
• Fusarium spp. Fungal pathogen producing chlamydospores (resting spores). Invades roots and lower stems, leading to Fusarium root rot.

Disease Cycle

Primary Source of Inoculum Soil and Plant Debris: Pathogens survive as resting spores (oospores, sclerotia, chlamydospores) in soil. Also survive on infected plant debris and seeds.

Secondary Spread

• Water and Rain Splash: Zoospores (Pythium, Phytophthora) spread through irrigation water or rain splash.
• Contaminated Equipment: Farm tools and machinery facilitate pathogen movement.
• Infected Seeds: Seed-borne inoculum leads to primary infections in new crops.

Favourable Conditions

• Cool and Wet Soil Conditions: Cool (15-20°C) and waterlogged soils favor Pythium and Phytophthora infections.
• Warm and Moist Soil: Warm temperatures (25-30°C) and high soil moisture favor Rhizoctonia and Fusarium.
• Poor Drainage and Compaction: Poorly drained, compacted soils enhance disease severity.
• High Soil Organic Matter: High organic content provides a nutrient-rich environment for pathogens.

Management Strategies

Cultural Practices

• Seed Selection and Treatment: Use certified disease-free seeds with fungicide seed treatments (e.g., Metalaxyl, Thiram).
• Proper Drainage and Irrigation: Ensure well-drained fields to prevent waterlogging. Avoid excessive irrigation during early growth stages.
• Crop Rotation: Rotate with non-host crops (e.g., cereals, maize) to reduce soil inoculum.
• Planting Practices: Avoid early planting in cold, wet soils. Use optimal planting depth to ensure good seedling emergence.

Chemical Control

• Seed Treatment Fungicides: Metalaxyl, Mefenoxam (for Pythium and Phytophthora). Thiram, Captan (for Rhizoctonia and Fusarium).
• Soil-Applied Fungicides: In severe cases, soil drenching with fungicides may be necessary.

Resistant Varieties Host Resistance: Plant Phytophthora-resistant soybean varieties (e.g., varieties with Rps genes). Select Pythium and Rhizoctonia-tolerant cultivars where available.

Biological Control Biocontrol Agents: Use Trichoderma spp. and Bacillus spp. as seed treatments to suppress pathogens. Organic Amendments: Application of compost and organic amendments to improve soil health and suppress pathogens.