Grey Mould (Botrytis) on Butternut Squash — What to Do

Botrytis cinerea, commonly known as grey mould, is a versatile fungal pathogen that can infect almost any part of a butternut squash plant — leaves, stems, flowers, and developing fruit. It is particularly common in damp, cool conditions and in gardens where plants are growing too closely together. Recognising the symptoms early and acting quickly can prevent the mould from spreading to healthy tissue and ruining the harvest.

What Botrytis Looks Like

Grey mould begins as water-soaked, soft brown or tan patches on plant tissue. Within a day or two in humid conditions, these patches develop a covering of grey, fuzzy spore masses — the distinctive characteristic that gives the disease its common name. On squash plants, botrytis most often appears on dead or dying flower petals that remain attached to developing fruit, and then spreads from the decaying flowers into the fruit itself. It can also attack stems, especially at the base of the plant or at wounds.

Why Botrytis Thrives in Cool, Damp Conditions

The botrytis fungus germinates and spreads most rapidly when temperatures are between 15 and 23°C and relative humidity is above 90%. These conditions occur regularly in temperate gardens during overcast summer stretches and in autumn. Dense planting that traps humid air around the leaves and stems creates the ideal microclimate. Once established, the grey spore masses are dispersed by any movement — wind, rain drops, or brushing past the plant — and infect new tissue immediately.

Removing Infected Material

Promptly removing all infected material is the most effective action you can take. Cut away affected leaves and stems using clean, sterilised secateurs — wipe the blades with alcohol between cuts to avoid transferring spores. Remove dead flower petals from the base of developing fruit as soon as the flower closes. Collect all cut material carefully and dispose of it in sealed bags or burn it. Never compost material infected with botrytis.

Treatment and Fungicides

In severe cases or as a preventative in wet seasons, apply a copper-based fungicide or a product containing Bacillus subtilis, a beneficial bacterium that outcompetes botrytis on plant surfaces. These are both permitted under organic standards. Apply to the entire plant, including stems and the undersides of leaves, in dry weather. Repeat applications every two weeks if conditions remain favourable for disease. Synthetic fungicides based on tebuconazole or iprodione are also effective but require careful label reading for appropriate use.

Prevention Through Airflow and Hygiene

Good airflow is the most important preventative measure. Space plants at least 90 cm apart. Remove lower leaves as the season progresses to keep the base of the plant open to air movement. Water at soil level rather than from overhead to keep foliage dry. At the end of the season, clear all plant debris from the bed thoroughly — botrytis overwinters in dead plant material and will reinfect the bed the following year if the debris is left in place.