Geranium Botrytis — How to Beat Grey Mould on Pelargoniums

Botrytis cinerea, commonly called grey mould, is a widespread fungal disease that thrives in cool, humid, still-air conditions. It is a particular problem for pelargoniums overwintered indoors or in a greenhouse, and for any geranium grown in overcrowded conditions with poor ventilation. Once grey mould takes hold it can spread from plant to plant within hours in damp weather — but it is entirely manageable with the right approach.

Recognising Botrytis on Geraniums

Botrytis appears as patches of fuzzy grey-brown mould on leaves, stems, and flower petals. Infected tissue softens, turns brown, and may collapse. In still conditions the mould produces a visible cloud of grey spores if disturbed. Unlike rust, which presents as orange pustules on leaf undersides, botrytis forms a diffuse, cottony coating over the surface of dying tissue. It often starts on dead or dying leaves, petals, and soft wounds and then spreads to otherwise healthy tissue nearby.

Remove Infected Material Immediately

The first step is always to remove all infected leaves, stems, and spent flower heads carefully. Place them straight into a bag — do not shake or drop them, as this releases millions of spores onto surrounding plants. Cut stems cleanly just above a healthy node and dispose of the trimmings in the bin, not the compost. After removing all visible mould, allow the remaining plant tissue to dry in good airflow before applying any fungicide. A product containing fenhexamid or iprodione (where available) is effective against botrytis on ornamentals.

Improve Ventilation

Botrytis cannot spread in fast-moving air. Crowded plants in a stagnant greenhouse or on an indoor windowsill are far more vulnerable than well-spaced, well-ventilated ones. Space pelargoniums so that air can move freely around each plant. Open greenhouse vents and doors during the day, even in autumn and early winter — a drop in temperature is less damaging than the humidity that builds when ventilation is closed. Avoid misting leaves or flowers, and water at the base of the plant in the morning only.

Reduce Moisture Around the Plants

Botrytis requires high relative humidity — above about 90% — to germinate spores and infect tissue. Reducing moisture at the plant level is therefore highly effective prevention. Remove dead leaves and petals as soon as they begin to yellow or wilt, before they become infection sites. Do not overwater; allow the compost surface to dry between waterings during winter storage. In a greenhouse, use a small fan on a timer to circulate air overnight when temperatures are cool and humidity tends to peak.

Prevent Botrytis During Overwintering

The most dangerous period for pelargonium botrytis is autumn to early spring, when overwintered plants are stored in low light, cool temperatures, and often crowded conditions. Before storing plants, remove all dead and dying foliage, reduce watering to near zero, and ensure each plant has adequate space. Check plants every two weeks through winter and remove any yellowing leaves promptly. Starting the season with clean, healthy stock is far easier than fighting a full botrytis outbreak in February.