Why Are My Rose Flowers Going Grey and Mouldy?

Rose buds and blooms turning brown, becoming covered in a fluffy grey-brown dusty coating, and collapsing before or just after opening are classic signs of botrytis, also known as grey mould, caused by the fungus Botrytis cinerea. This is one of the most widespread fungal diseases of garden plants worldwide and is particularly damaging to roses during cool, damp, still conditions — which makes it a persistent problem in wet summers or in the autumn flush when nights are cold and dew is heavy.

How botrytis infects roses

The fungus enters through damaged, dead or dying tissue — spent petals that have fallen onto leaves, dead stubs left after pruning, frost-damaged shoots, and the outer petals of flowers as they start to die naturally. From these entry points it can spread into living tissue, killing healthy cells as it advances. In damp, still conditions the fluffy grey sporulation visible on infected material releases billions of spores that spread readily by air, water splash or by contact. The infection cycle is fastest when temperatures are between 15–20°C with high humidity.

Cultural prevention

The most powerful weapon against botrytis is good garden hygiene combined with airflow. Deadhead roses promptly and cleanly, cutting back to a bud rather than leaving bare stubs. Collect fallen petals and leaves from around the base of the plant — do not leave them to accumulate as they become the primary infection points for the next cycle. Prune to open the canopy and improve air circulation, particularly in the centre of the plant. Avoid wetting flowers when watering and water at the base of the plant rather than overhead.

Conditions to watch

Botrytis is most severe after periods of cool, damp weather followed by improved conditions, which trigger a mass spore release. Watch your roses closely in June after wet spells, in August during humid periods, and particularly in September and October when the second flush is vulnerable. Roses in sheltered, enclosed locations — against walls, in courtyards, surrounded by fences — are most susceptible because still air allows humidity to build. A single wet season can cause more botrytis than several dry years combined.

Removing infected material

Cut off affected buds and flowers at the first sign of infection, cutting back into healthy stem tissue. Do not drop infected material on the soil — bag and bin it or burn it. If botrytis has entered a stem, cut back to healthy wood showing no discolouration, sterilising the secateurs between cuts with diluted bleach or rubbing alcohol. Avoid leaving stubs, which are prime infection sites. After clearing infected growth, a liquid copper or Bordeaux mixture spray can help protect remaining healthy growth.

Fungicide treatment

Fungicides containing fludioxonil, iprodione or pyrimethanil are effective against botrytis and can be used to protect flowers in high-risk periods. However, botrytis is well-known for developing resistance to fungicides quickly, so rotate between groups and do not rely on chemistry alone. Organic options include copper-based fungicides and bicarbonate sprays, which raise the pH on the leaf surface to a level that inhibits germination. Neither prevents infection entirely but both reduce the rate of spread.