Fungal Infection Leaf Spot on begonia
What's Happening
Leaf spot diseases in begonias—whether bacterial (Xanthomonas campestris) or fungal (Cercospora, Alternaria)—manifest as circular to irregular brown/black lesions with defined margins. Bacterial spots often show yellow halos; fungal spots may display concentric rings or fuzzy growth in center. Both pathogens enter through wounds, hydathodes (water pores), or stomata when foliage remains wet 6+ hours from overhead watering, condensation, or high humidity with poor airflow.
How to Fix It
- 1
Triage assessment: Inspect all leaves; remove any with >30% coverage or spreading lesions; keep leaves with isolated spots for monitoring
- 2
Sterile removal: Cut affected leaves with sterile scissors 1 inch below visible lesion; seal in plastic bag for trash disposal
- 3
Copper treatment: Apply copper-based bactericide/fungicide covering all leaf surfaces including undersides; repeat every 7-10 days for 4 treatments
- 4
Watering modification: Switch exclusively to bottom-watering; ensure foliage remains completely dry; water only in morning hours
- 5
Airflow enhancement: Position fan 3-4 feet away running 6+ hours daily to accelerate leaf drying and prevent spore germination
- 6
Environmental correction: Reduce humidity to 50-60% if above 70%; increase light to 2000-4000 lux to strengthen tissue resistance
How to Prevent It
Water at soil level only; never splash water on leaves; avoid working with wet plants; sterilize tools between plants; remove fallen debris promptly; quarantine new plants 14 days; maintain strong air circulation; avoid overhead watering even when fertilizing