Round-leaf Calathea with Southern Blight? Rare but Treatable

Calathea orbifolia

Susceptibility: Low Severity: High Difficulty: Hard

Southern blight is a devastating soilborne fungal disease caused by Athelia rolfsii (formerly Sclerotium rolfsii), affecting over 500 plant species including vegetables, ornamentals, and fruit crops. The fungus survives in soil through resting structures called sclerotia that resemble mustard seeds and can remain viable for years.

🔍 Symptoms

• Brown to black lesions on stem near soil line
• Water-soaked appearance on lower leaves
• Sudden and permanent wilting of entire plant within days
• White mycelium radiating from plant base onto soil surface
• Spherical tan to reddish-brown sclerotia (1-2mm)
• Yellowing and necrosis of foliage
• Crown and root rot
• On fruits: sunken yellow lesions turning water-soaked

❓ Possible Causes

→ Soilborne fungus Athelia rolfsii surviving through sclerotia
→ High temperatures (25-30°C / 77-95°F)
→ High soil moisture and relative humidity above 90%
→ Acidic soil (pH below 7, especially pH 2-5)
→ Spread through infested soil and plant debris
→ Contaminated tools, irrigation water, and transplants
→ Contaminated mulch or compost
→ Plant stress and stem injuries

💊 Treatment

⚡

Quick Fixes

Step 1

Remove infected plants IMMEDIATELY with roots and soil

Step 2

Apply azoxystrobin drench to adjacent plants

Step 3

Spray mancozeb on base of all plants in area

Step 4

Drastically reduce irrigation

Step 5

Reapply fungicides every 7-10 days for 3 weeks

🌿

Organic Treatment

Step 1

Remove diseased plants and 3 inches of contaminated soil

Step 2

Solarize area with clear plastic for 6 weeks

Step 3

Incorporate compost rich in Trichoderma

Step 4

Adjust soil pH with lime

Step 5

Improve drainage and plant spacing

Step 6

Apply preventive bicarbonate spray biweekly

🛡️ Prevention

✓ Carefully inspect transplants for sclerotia before planting
✓ Avoid planting in fields with history of infestation
✓ Maintain soil pH above 7 (lime reduces sclerotial germination)
✓ Improve plant spacing to increase airflow
✓ Use high-quality mulches from reputable suppliers
✓ Rotate with non-host crops (corn, wheat, barley) for 2-3 years
✓ Avoid overwatering and excessive soil moisture
✓ Disinfect tools between plants
✓ Deep plow (8-12 inches) to bury sclerotia
✓ Remove symptomatic plants and weeds immediately

🌱 Care Guide: Round-leaf Calathea

💧 How to Water

Keep the substrate consistently moist but not waterlogged. Water approximately every 7 days in summer and every 10-14 days in winter. Use filtered or water left to sit for 24-48 hours to avoid mineral damage.

☀️ Lighting

Place in an area with medium to high indirect light, near a window but protected from direct sun. Avoid direct sunlight at all costs as it will burn the leaves.

🪴 Ideal Soil

Requires acidic, well-draining substrate rich in organic matter. Porous composition helps prevent root rot while maintaining necessary moisture.

See more: Round-leaf Calathea → See more: Southern Blight →