Moon Orchid with Southern Blight? Rare but Treatable
Phalaenopsis amabilis
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.
ð Symptoms
- âĒ Brown to black lesions on stem near soil line
- âĒ Water-soaked appearance on lower leaves
- âĒ Sudden and permanent wilting of entire plant within days
- âĒ White mycelium radiating from plant base onto soil surface
- âĒ Spherical tan to reddish-brown sclerotia (1-2mm)
- âĒ Yellowing and necrosis of foliage
- âĒ Crown and root rot
- âĒ On fruits: sunken yellow lesions turning water-soaked
â Possible Causes
- â Soilborne fungus Athelia rolfsii surviving through sclerotia
- â High temperatures (25-30°C / 77-95°F)
- â High soil moisture and relative humidity above 90%
- â Acidic soil (pH below 7, especially pH 2-5)
- â Spread through infested soil and plant debris
- â Contaminated tools, irrigation water, and transplants
- â Contaminated mulch or compost
- â Plant stress and stem injuries
ð 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
ðĄïļ Prevention
- â Carefully inspect transplants for sclerotia before planting
- â Avoid planting in fields with history of infestation
- â Maintain soil pH above 7 (lime reduces sclerotial germination)
- â Improve plant spacing to increase airflow
- â Use high-quality mulches from reputable suppliers
- â Rotate with non-host crops (corn, wheat, barley) for 2-3 years
- â Avoid overwatering and excessive soil moisture
- â Disinfect tools between plants
- â Deep plow (8-12 inches) to bury sclerotia
- â Remove symptomatic plants and weeds immediately
ðą Care Guide: Moon Orchid
ð§ How to Water
Water thoroughly when the potting medium is nearly dry, typically every 7-10 days. Use room temperature water and avoid letting the plant sit in standing water. Reduce watering in winter.
âïļ Lighting
Place in bright, indirect light - an east or west-facing window is ideal. Avoid direct sunlight which can burn the leaves. In winter, supplemental artificial light can encourage blooming.
ðŠī Ideal Soil
Use a well-draining orchid mix consisting of bark chips, perlite, and sphagnum moss. The pH should be slightly acidic (5.5-6.5). Never use regular potting soil as it retains too much moisture.