Botrytis Grey Mould on Sunflowers: Identification and Control

Grey mould — caused by the fungus Botrytis cinerea — is an opportunist. It does not attack healthy, growing plant tissue; instead it establishes itself on dead or damaged material and then spreads from there into living tissue. On sunflowers it is most damaging in the flowerhead and upper stem, particularly in wet, cool summers. Recognising it early is key to keeping it contained.

What Botrytis Looks Like on Sunflowers

Botrytis appears as a dense, grey-brown fuzzy growth on stems, petals, leaves or the developing seed head. When disturbed, it releases a visible cloud of grey spores. Affected tissue beneath the mould is soft, brown and rotting. On sunflowers grown for cut flowers or seed, the flowerhead is particularly vulnerable once petals begin to die — fallen petals trap moisture and provide the perfect entry point. Stem cankers — brownish, water-soaked lesions that girdle the stem — can also develop during prolonged wet weather.

Conditions That Favour Botrytis

The fungus thrives wherever humidity is high, temperatures are cool (15°C to 20°C is its sweet spot), and plant surfaces remain wet for extended periods. Crowded planting, poor airflow, overhead watering and cool, overcast summers all increase risk. Sunflowers with damaged leaves or stems from physical injury, insect feeding or other disease are at far greater risk than undamaged plants.

Removing Infected Material

Speed matters. As soon as you spot grey mould, cut away the affected part of the plant with clean, sterilised secateurs. Cut back into healthy, green tissue — go further than you think necessary. Bag the infected material immediately and put it in the bin. Do not shake the infected stems or leaves as you handle them, as this disperses spores onto surrounding healthy plants. Wipe the blades of your secateurs with diluted bleach or rubbing alcohol between cuts.

Treatment and Protection

After removing infected material, improve conditions immediately. Space plants to increase airflow, remove any dead or dying leaves at the base of the plant, and switch from overhead to base watering. A sulphur-based or copper-based fungicide applied to remaining healthy tissue provides some protection. Potassium bicarbonate sprays are also effective at inhibiting spore germination and are safer around beneficial insects than synthetic fungicides.

Prevention for Next Season

Clear all plant debris from the bed in autumn — botrytis overwinters as hard black resting bodies called sclerotia in plant debris and soil. Rotate sunflowers to a fresh position each year. Space plants generously and water at the base only. If growing sunflowers for cut flowers and deadheading regularly, remove spent flowers before they begin to rot. Keeping tools clean and disinfected prevents inadvertently moving spores from plant to plant during routine maintenance.