Althea (Rose of Sharon) Fungal Leaf Spot After Extreme Rainfall

Four 12-year-old blue altheas in Houston, under crepe myrtles with 4 hours full sun + 2 hours dappled, suddenly developed bright yellow leaf spots after two years of excessive rainfall. Previous brown rusty spots were treated with fungicide but recurred. Plants are cut to 24" each spring, reach 6' by June, fertilized with Miracle-Gro, mulched 2–3".

Diagnosis

Severe fungal leaf spot exacerbated by prolonged wet conditions. New leaves emerge clean but develop spots within weeks.

Treatment Plan

1. Fungicide: Chlorothalonil or propiconazole (e.g., Fertilome Systemic Liquid Fungicide), sprayed every 7–10 days until clean new growth appears.
2. Sanitation: Remove all fallen leaves; do not mulch them.
3. Cultural: Hold fertilizer during outbreak; replace mulch with drier medium; improve air circulation by thinning nearby plants.
4. Pruning: Cut back when spraying to improve coverage (optional).
5. Watering: Avoid wetting foliage.

Three weekly sprays should significantly improve the situation.