Botrytis on Dahlias — Grey Mould Identification and Control

Botrytis cinerea — grey mould — is a fungal disease that becomes a serious problem for dahlias during cool, damp, overcast periods. It can destroy flowers, rot stems, and if left unchecked can even threaten the tubers. Understanding when and why it strikes makes it much easier to stay ahead of it.

Identifying Botrytis on Dahlias

The most visible sign is a grey or buff-coloured furry mould that develops on petals, stems, or leaf tissue. It typically starts on old or dying plant material — spent flowers, damaged leaves, or debris lodged in the crown — and then spreads into healthy tissue from there. Infected flower petals go brown and mushy before the characteristic grey spore mass appears. On stems, Botrytis causes a brown, sunken, water-soaked lesion that can girdle the stem and cause the section above to wilt and collapse.

Conditions That Favour Botrytis

Unlike powdery mildew, Botrytis loves wet and humid conditions. It spreads most rapidly when temperatures are between 15–25 °C with high humidity — exactly the conditions common in a cool, wet British summer. Dense, crowded plants with poor airflow, overwatered soil, and beds where plant debris accumulates all create ideal conditions. The fungus overwinters in dead plant tissue and soil, releasing spores in large quantities when conditions warm up in spring.

Immediate Control Measures

Remove all affected tissue promptly and dispose of it — not in the compost heap, as Botrytis sclerotia can survive composting. Cut back to well below the visibly infected area and into clean, firm tissue. After cutting, dust cut surfaces with sulphur powder or apply a copper-based fungicide spray to the wounds and surrounding tissue. Work on dry days when possible, as moving through infected plants in wet conditions spreads spores to neighbouring plants.

Fungicide Options

Copper-based fungicides (such as Bordeaux mixture or copper oxychloride) and those containing fludioxonil or iprodione where available provide reasonable control when applied preventatively during high-risk periods. Spray the whole plant — stems, leaves, and any developing flowers — ensuring good coverage on the undersides of leaves where spores accumulate. Re-apply after rain. Note that fungicide resistance in Botrytis is a real issue; rotating between different chemical classes helps prevent it.

Cultural Prevention Is the Best Long-Term Strategy

Improving the growing environment reduces Botrytis pressure more reliably than any spray programme. Space dahlias generously — at least 60 cm between plants, more for tall varieties — to allow air movement. Deadhead regularly and remove spent petals promptly, as rotting flower material is the most common entry point for the fungus. Avoid overhead irrigation; water at soil level. In late autumn, clear all dahlia stems and leaves from beds before they rot down, as this material provides the fungal reservoir for next season.

Protecting Stored Tubers

Botrytis can also affect stored tubers, particularly if they are stored while still damp, in poorly ventilated conditions, or at temperatures above 10 °C. After lifting, dry tubers thoroughly in a warm, well-ventilated space for at least a week before packing away. Dust with sulphur or treat with a fungicide dust before storing. Check stored tubers monthly and remove any showing grey mould, rot, or softening immediately — Botrytis spreads from one tuber to its neighbours in a closed storage container.

Key Prevention and Control Checklist

• Remove and dispose of all affected tissue immediately — not in compost
• Space plants generously for air circulation
• Deadhead promptly and remove all debris from the bed
• Water at soil level, not on the foliage
• Apply copper or appropriate fungicide during prolonged wet spells
• Dry tubers thoroughly before storage and check monthly over winter