At a glance
Botrytis, commonly known as grey mould, is caused by the fungus Botrytis cinerea and is one of the most widespread and destructive plant diseases in UK gardens. It affects an enormous range of host plants including strawberries, tomatoes, grapes, lettuces, dahlias, peonies, roses and virtually any herbaceous plant in the ornamental garden. Unlike many fungal diseases that peak in summer, botrytis thrives in the cool, damp conditions of autumn and winter, making it a persistent problem in greenhouses, cold frames and on overwintering plants. The characteristic fluffy grey mould that appears on infected tissue is not just unsightly – it releases enormous quantities of spores that spread the disease to nearby plants rapidly.
Botrytis is classed as a necrotrophic pathogen, meaning it attacks and kills host tissue rather than simply feeding on living cells as biotrophic pathogens do. This makes it particularly destructive because once infection takes hold, the affected tissue dies quickly and the mould colonises the dead material, producing further spores. The fungus can infect through wounds, through natural openings, and directly through intact tissue on stressed or weakened plants. Understanding how it spreads and the conditions it requires is the basis for managing it effectively – this guide covers identification, the plants most at risk, how to treat active infections and the cultural practices that prevent it taking hold in the first place.
Identifying botrytis
The most recognisable symptom of botrytis is the dense, fluffy grey-brown mycelium that develops on infected plant tissue. This grey mould typically appears first on dead or dying tissue – old flower petals, spent leaves, damaged stems or fruit – and then spreads to the surrounding healthy plant. In moist conditions the mould produces vast clouds of grey powdery spores when disturbed. On stems, infection often starts as a soft brown rot at or just below the soil surface, causing the stem to collapse – this is particularly common on young seedlings and is responsible for the condition known as damping off.
On fruits and vegetables, botrytis produces a watery soft rot that collapses the tissue rapidly. Strawberry fruits are particularly susceptible, developing a soft brown rot covered in grey powder that renders them completely inedible. Grapes develop a similar grey rot – the same organism is responsible for the noble rot that concentrates sugars in certain wine grape varieties, though in a UK kitchen garden context it is unwelcome. On ornamental plants, the most visible symptom is often the collapse of flower buds or open flowers, which turn brown and then develop the characteristic grey mould covering. The sclerotia – small, hard, black resting bodies that the fungus produces to survive adverse conditions – can sometimes be seen embedded in infected stems and fruit and are a reliable identification marker.
Most affected plants
While botrytis can affect virtually any garden plant, certain species are consistently more vulnerable than others. In the food garden, strawberries, tomatoes and lettuce are almost invariably affected in wet seasons. In the ornamental garden, dahlias, peonies and roses are the most commonly infected. Under glass or in greenhouses, cucumber, tomato and all types of soft-stemmed bedding plants are at high risk because the enclosed environment creates the high humidity that the fungus thrives in.
Conditions and seasonal pressure
Botrytis cinerea is described as a wound pathogen that is also capable of infecting intact plant tissue under the right conditions. It requires moisture on the plant surface to germinate – spores need free water or very high humidity for 4-8 hours to successfully infect. Temperature also matters: the fungus grows across a wide range from 1-30 degrees Celsius but is most active between 15-25C, which coincides with typical UK autumn conditions. This explains why the disease peaks in autumn and in greenhouses during winter – the combination of cooling temperatures, shorter days with reduced drying time, and high humidity creates near-perfect infection conditions repeatedly through the season.
Treatment – what works
The first and most important response to any botrytis infection is physical removal of all affected tissue. Cut back into healthy tissue, making the cut at least 5cm below the visible edge of any mould or brown discolouration. Remove dead flowers, spent leaves and any other soft plant material from the plant and from the ground around it. All removed material must go in the bin – never the compost heap. This single action – prompt, thorough removal – is more effective than any fungicide treatment when caught early, because it removes the mould producing and dispersing the spores that will infect other plants and other parts of the same plant.
Never compost material infected with botrytis. The sclerotia – the black resting bodies the fungus produces – survive most domestic composting processes and will return to the garden in finished compost. Botrytis spores are also extremely long-lived and can remain viable in soil and debris for several years. Always bag infected material and put it in general waste. This one discipline makes a measurable difference to infection severity in subsequent seasons.
Prevention and cultural controls
Long-term management of botrytis is almost entirely cultural rather than chemical. The fundamental goal is to eliminate the conditions the fungus requires: free water on plant surfaces, dead or dying tissue as an infection point, and stagnant humid air around the foliage. Achieving all three consistently is straightforward but requires discipline, particularly in autumn when the natural dying back of many plants creates abundant potential infection material that needs to be managed promptly rather than left to decompose in place.
In the greenhouse, ventilation management is the single most effective prevention strategy. Opening vents every day during autumn and winter, even briefly, significantly reduces the humidity that encourages botrytis. The practice of opening vents on dry days and closing them before damp evenings – rather than leaving them open or closed regardless of conditions – makes a substantial difference in greenhouses where botrytis is a recurring problem. Avoiding overhead watering entirely during autumn and winter, and watering only in the morning so that foliage has time to dry before temperatures drop, removes one of the key infection triggers. For outdoor plants, clearing dead flower heads and spent stems as soon as they appear rather than leaving the garden tidy-up until spring removes the primary infection points before the cool, damp autumn conditions arrive that botrytis exploits. A garden cleared of dead material in September will almost always show significantly less grey mould than one where cutting back is left until March, because the overwintering spore population is much lower going into the following season.
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