Geranium Leaf Spot — Brown and Black Spots on Leaves

Spots on geranium leaves can be caused by several different pathogens — both fungal and bacterial — as well as by environmental factors. The spots look broadly similar on first inspection, but the details of their appearance, spread, and the plant's overall condition help distinguish between them. Getting the identification right is important because fungal and bacterial leaf spots require different management approaches.

Fungal Leaf Spot

The most common fungal leaf spot on pelargoniums is caused by Alternaria species. It produces circular brown or tan spots, often with a darker border, that may develop a pale or greyish centre as they age. Spots usually start on lower leaves and spread upward in warm, wet conditions. Overhead watering and crowded growing conditions spread the spores. Remove affected leaves promptly and apply a fungicide containing copper or mancozeb, following label directions. Ensure good airflow and avoid wetting the foliage.

Bacterial Leaf Spot

Bacterial leaf spot (Xanthomonas species) causes water-soaked, dark brown to black spots that often have a yellow halo. The spots tend to be more angular and irregular than fungal spots, and the tissue collapses quickly, sometimes becoming thin and papery. Bacterial infections spread rapidly in wet, warm weather and through contaminated tools or water splash. There are no chemical treatments registered for bacterial leaf spot in most home gardens — management relies entirely on removing infected leaves, improving drainage, and reducing leaf wetness. Severely infected plants are best removed and binned.

Environmental Spots

Not all leaf spots are caused by pathogens. Cold water splashed on warm leaves can cause bleached or brown spots from temperature shock. Sun scorch produces irregular, bleached patches on the upper leaf surface that do not spread. Fluoride or chlorine sensitivity in some sensitive varieties causes tip browning rather than midleaf spots. If spots appear only after rain events or watering and the plant is otherwise healthy, environmental causes are more likely than disease. Switch to watering at the base and avoid misting.

Removing and Managing Infected Material

Regardless of the cause, remove all visibly spotted leaves from the plant and from the compost surface where fallen leaves accumulate. Always dispose of diseased material in the bin, not the compost. After removing leaves, wash your hands and disinfect any cutting tools before moving to another plant. Applying a copper-based spray to the remaining foliage provides both fungicidal and some antibacterial protection and can slow the spread of both types of leaf spot.

Long-Term Prevention

The best prevention for leaf spot in any form is good air circulation, dry foliage, and well-drained compost. Space plants generously — both in containers and in borders. Water at the base of the plant in the morning. Remove spent flowers and yellowing leaves routinely to reduce infection sites. In very wet seasons, preventative copper-based fungicide applications every two to three weeks provide a useful protective barrier. Buying disease-free stock from reputable suppliers and using fresh compost each season reduces the starting pathogen load significantly.