Marigold Botrytis (Grey Mould): How to Stop It

Botrytis cinerea — grey mould — is one of the most destructive fungal diseases marigolds face in cool, damp weather. It spreads with alarming speed on crowded, wet plants, turning flower heads and soft stems into a mass of grey-brown mush covered in dusty spores. Acting fast at the first sign is essential.

How to Identify Botrytis on Marigolds

The first signs are usually tan or brown water-soaked patches on lower leaves or petals. These patches quickly develop a fuzzy grey coating — the spore masses of the fungus. Affected flower heads collapse and turn slimy. In humid conditions the mould can spread from a single flower to neighbouring stems within 48 hours. The spores are airborne, so disturbing infected material in a closed space (a greenhouse, for example) can spread the disease widely in one go.

Remove Infected Material Immediately

At the first sign of grey mould, put on gloves and carefully bag all infected flowers, leaves, and stems without shaking them. Do not compost infected material — it will spread spores throughout your compost heap. Dispose of it in the bin. Cut back to healthy, green tissue, making cuts well below any visible infection. Disinfect your secateurs between cuts with a dilute bleach solution or surgical spirit.

Improve Airflow Around Plants

Botrytis thrives in still, humid air. Remove any lower leaves that are touching the soil, thin out overcrowded stems, and increase spacing between plants if possible. In a greenhouse or polytunnel, open vents and doors during the day to create air movement. Avoid overhead watering — water at the base of the plant in the morning so the foliage stays dry.

Fungicide Treatment

Once botrytis is established, a fungicide helps slow its spread. Copper-based sprays (e.g. copper oxychloride) are effective and widely available. Spray all surfaces — upper and lower leaf, stems, and remaining flowers — in the early morning on a dry day. Repeat every 7–10 days. Note that fungicides will not cure already-infected tissue; they only protect healthy tissue from further infection. A biological fungicide based on Bacillus subtilis is a lower-impact alternative that works in a similar way.

Prevent Botrytis Next Season

Space marigolds correctly (20–30 cm apart for French varieties, 35–45 cm for African). Deadhead regularly so petals do not accumulate on the soil. Avoid high-nitrogen feeds that promote soft, disease-prone growth. Water at soil level and always in the morning. Rotate your marigolds each year to a fresh patch of ground so fungal spores that overwintered in the soil cannot reinfect a new crop immediately.