At a glance
Powdery mildew is one of the most widespread fungal diseases in UK gardens, affecting an enormous range of plants from courgettes and cucumbers to roses, dahlias, gooseberries and ornamental shrubs. The white or grey powdery coating that appears on leaves, stems and buds is caused by fungal spores that germinate on dry leaf surfaces and spread rapidly in warm, still conditions with high humidity – the classic British late summer weather pattern of warm days, cool nights and little wind. Unlike most fungal diseases, powdery mildew does not require wet conditions to thrive; it actually grows best when the foliage is dry but the surrounding air is humid.
The good news is that powdery mildew, while unsightly and potentially weakening to severely affected plants, is rarely fatal and can be controlled effectively once identified. Early intervention produces the best results – a plant caught at the first signs of infection responds well to treatment and can continue growing and flowering normally through the rest of the season. Plants left untreated and heavily infected by late summer will drop leaves prematurely, produce poor-quality fruits or flowers and be more vulnerable to secondary infections the following year.
Identifying powdery mildew
Powdery mildew is one of the easier plant diseases to identify because its appearance is so distinctive. The primary symptom is a white or pale grey powdery or floury coating on the upper surfaces of leaves, though it can also appear on the undersides, on stems, flower buds and even developing fruits. In the early stages, the coating appears as small circular patches that grow and merge as the infection spreads. Heavily infected leaves may curl, yellow and eventually die. On some plants such as roses the upper leaf surface develops a powdery coating while the underside shows purple-red discolouration.
It is worth noting that powdery mildew is not a single disease but a group of closely related fungal species, each adapted to specific host plants. The mildew that attacks your courgettes is a different species from the one attacking your roses – they cannot cross-infect between very different plant families. However within one plant family, spores can spread readily between plants of the same type, which is why a garden with multiple rose plants will typically see the disease spread from one to another once it takes hold. This matters practically because treating and removing infected leaves on courgettes will not help your roses, and vice versa. It also means that where one variety in a bed is severely affected, removing it reduces the spore pressure on the remaining plants.
Causes and conditions
Powdery mildew fungi overwinter as spores on fallen leaves and infected plant material in the soil and on the surface of woody plants. In spring, these spores are released and spread by wind to new host plants. The disease typically becomes visible from mid-summer onwards as temperatures warm and the diurnal temperature range – the difference between daytime and night-time temperatures – widens. This temperature fluctuation creates the humidity conditions that favour the fungal spores even when rainfall is low. Plants that are stressed by drought are particularly susceptible, as water-stressed plants have weakened cell walls that the fungal mycelium can penetrate more easily.
Treatment – what works
Treatment is most effective when started at the first sign of infection, before the mildew has spread across more than a few leaves. The first step regardless of treatment method is to remove all visibly infected leaves and dispose of them in the bin – not the compost heap, as composting infected material can perpetuate the disease. Once infected leaves are removed, the remaining plant can be treated with a fungicide spray to halt further spread and protect new growth.
For chemical treatment, fungicides containing myclobutanil or tebuconazole (available in products such as Fungus Fighter) are effective against powdery mildew on ornamentals, and products containing difenoconazole work well on edible plants when used according to label instructions with appropriate harvest intervals observed. Always check the label confirms the product is approved for use on the specific plant and against powdery mildew before purchasing, as not all fungicides are registered for all uses. For an organic approach, a spray of diluted sodium bicarbonate (1 teaspoon per litre of water with a drop of washing-up liquid as a spreader) raises the pH on the leaf surface and inhibits fungal growth; this needs applying every 5-7 days to be effective and works best as a preventative or early-stage treatment rather than a cure for established infection. A milk spray – 40% fresh milk diluted in 60% water – has good research evidence behind it for cucurbit powdery mildew in particular and is a useful organic option for gardeners who prefer to avoid synthetic chemistry.
Never compost leaves or stems infected with powdery mildew. The fungal spores survive in most domestic compost heaps and will be returned to the garden when compost is used as a mulch. Always bag infected material and put it in the general waste bin. This single step significantly reduces the inoculum load in the garden from one year to the next and reduces the severity of infection each subsequent season.
Most affected plants
Virtually any garden plant can develop powdery mildew, but some are significantly more susceptible than others. In the vegetable garden, courgettes, squash and cucumbers are almost invariably affected by late summer – this is so common on cucurbits that most gardeners treat it as an expected part of the growing season rather than a problem to be solved. In the ornamental garden, roses are the most frequently affected shrub, with repeat-flowering modern roses more susceptible than once-flowering old roses. Dahlias, phlox, delphiniums, asters, aquilegias and many annual bedding plants also suffer regularly.
Prevention and resistant varieties
Long-term management of powdery mildew relies more on cultural practices and variety selection than on repeated fungicide applications. Improving air circulation around susceptible plants is the single most effective preventative measure – this means spacing plants further apart than the minimum recommended distance, avoiding planting in sheltered, enclosed corners and pruning to open up the canopy of woody plants like gooseberries and roses. Consistent watering at soil level rather than overhead keeps foliage dry and reduces the humidity around leaves. A potassium-rich rather than high-nitrogen fertiliser regime produces firmer, less sappy growth that is more resistant to fungal infection.
On roses, choosing modern English roses bred with good disease resistance – many David Austin varieties have significantly improved mildew resistance compared to older hybrid tea roses – makes a substantial difference to how much treatment is required through the season. On cucurbits, varieties such as ‘Defender’ courgette and ‘Marketmore’ cucumber have been bred with better mildew tolerance than older open-pollinated varieties, and are worth specifying particularly for growing in polytunnels or greenhouses where air circulation is more restricted. For phlox, the Paniculata types are more resistant than Paniculata maculata, and some modern bred varieties such as ‘David’ and ‘Blue Paradise’ are notably more resistant than the older named varieties.
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