📈 Diseases of Soybean and Sunflower

Soybean (Glycine max) and sunflower (Helianthus annuus) are major oilseed crops in India. Soybean is predominantly grown in Madhya Pradesh, Maharashtra, and Rajasthan, while sunflower is cultivated in Karnataka, Andhra Pradesh, and Maharashtra. Both crops face significant disease challenges.

Diseases of Soybean

Soybean Rust

Causal Organism: Phakopsora pachyrhizi (Asian soybean rust)

Symptoms:

• Small, tan to reddish-brown pustules (uredinia) on the lower leaf surface
• Pustules have a small opening (ostiole) — unlike other rusts that burst open
• Angular lesions limited by veins, starting on lower leaves
• Severe defoliation and yield losses of 10-80%

Favorable Conditions:

• Temperature: 18-26 degrees C with prolonged leaf wetness
• Frequent rainfall and high humidity

Management:

• Grow tolerant varieties
• Foliar spray of Propiconazole 25 EC (0.1%) or Tebuconazole + Trifloxystrobin at first appearance
• Monitor from flowering stage onward

Charcoal Rot

Causal Organism: Macrophomina phaseolina

Symptoms:

• Plants wilt suddenly during pod-filling stage
• Stem pith shredded and filled with tiny black microsclerotia (charcoal-like appearance)
• Epidermal tissue peels off easily revealing dark sclerotia
• Roots show dry rot

Favorable Conditions:

• Drought stress and high temperatures (>30 degrees C)
• Sandy, low-fertility soils

Management:

• Maintain adequate soil moisture during pod filling
• Apply balanced fertilization, especially potassium
• Seed treatment with Trichoderma viride (4 g/kg)
• Crop rotation with non-host crops

Bacterial Pustule

Causal Organism: Xanthomonas axonopodis pv. glycines

Symptoms:

• Small, raised pustules with yellow-green halo on upper and lower leaf surfaces
• Pustules coalesce forming larger necrotic areas
• Premature defoliation in severe cases

Management:

• Grow resistant varieties
• Use disease-free seed
• Spray Copper oxychloride (0.25%) at disease onset

Yellow Mosaic Disease

Causal Organism: Mungbean Yellow Mosaic India Virus (MYMIV)

Vector: Whitefly (Bemisia tabaci)

Symptoms:

• Irregular yellow patches on green leaves; leaves may become completely yellow
• Reduced pod formation, stunted plants
• Major disease in central India

Management:

• Resistant varieties (e.g., JS 335, NRC 7)
• Control whitefly with Imidacloprid or Thiamethoxam
• Avoid sowing near infected mungbean/urdbean fields

Diseases of Sunflower

Downy Mildew

Causal Organism: Plasmopara halstedii

Symptoms:

• Systemic infection causes dwarfing and chlorosis of the entire plant
• White cottony growth on the lower surface of leaves
• Leaves become thick, brittle, and curl downward
• Head may not form, or forms small deformed heads

Favorable Conditions:

Management:

• Seed treatment with Metalaxyl 35 SD (6 g/kg)
• Grow resistant hybrids
• Avoid waterlogged conditions
• Rogue infected plants

Alternaria Leaf Blight

Causal Organism: Alternaria helianthi

Symptoms:

• Dark brown to black, circular to irregular spots with concentric rings on leaves, petioles, and stems
• Spots coalesce causing extensive leaf blight
• Stem lesions may girdle and cause breakage
• Head infection reduces seed set and oil content

Favorable Conditions:

• Warm and humid weather (25-30 degrees C), frequent rains

Management:

• Spray Mancozeb 75 WP (0.25%) or Iprodione at disease onset
• Remove and destroy infected debris
• Crop rotation for 2-3 years

Sclerotinia Head Rot and Wilt

Causal Organism: Sclerotinia sclerotiorum

Symptoms:

• White cottony mycelial growth on the back of the sunflower head
• Head becomes soft, rotted, and disintegrates
• Large, irregular black sclerotia (3-10 mm) form on and inside the head
• Basal stem rot causes sudden wilting

Favorable Conditions:

• Cool, wet weather (15-20 degrees C)
• Dense planting, excessive irrigation

Management:

• Avoid excessive irrigation and dense spacing
• Deep ploughing to bury sclerotia
• Foliar spray of Carbendazim (0.1%) at flowering stage
• Seed treatment with Trichoderma
• Crop rotation with cereals for 3-4 years

Sunflower Necrosis Disease

Causal Organism: Tobacco Streak Virus (TSV) — an ilarvirus

Vector: Thrips (Thrips palmi) — pollen-mediated transmission through the weed Parthenium hysterophorus

Symptoms:

• Sudden necrosis of leaves starting from petioles
• Brown streaks on stems and petioles
• Plants die prematurely; heads fail to develop seeds

Management:

• Control Parthenium weeds around sunflower fields
• Manage thrips populations
• Grow tolerant hybrids

Summary Cheat Sheet