Powdery mildew

What you are looking at

White or pale grey patches on the upper surface of a leaf, starting as discrete circles and merging until the leaf looks dusted with flour. Run a finger across one and it wipes a clear green track, because the fungus grows on the surface rather than inside the tissue. That single test separates it from almost everything else that marks a leaf.

Roses show it differently at first. The young shoots distort, the growing tip curls and takes on a purplish cast, and the white coating follows. On a gooseberry it goes felty and brown as the season wears on. On a courgette it begins as a few coins on the oldest leaves in July and covers the plant by September.

Late in the season, look closely at an old mat of mildew for a scatter of tiny black specks the size of a full stop. Those are the overwintering fruiting bodies, and they are the reason autumn leaf clearance is on the list of things that work.

The mildew on your courgette will not reach your roses

Powdery mildew is a name for a large family of fungi rather than one organism, and almost all of them are restricted to a narrow group of hosts. Rose mildew is Podosphaera pannosa and lives on roses. Oak mildew is Erysiphe alphitoides and lives on oaks. The one on cucurbits is a different fungus again, and it will not cross to a phlox growing beside it.

That matters for what you do next. Isolating a mildewed courgette from a rose achieves nothing. Neither does removing a mildewed oak branch overhanging a vegetable bed. What does help is dealing with the source on the same host, which usually means last year’s fallen leaves and this year’s dormant buds.

Why dry roots and humid air

Most fungal diseases need free water on the leaf to germinate, so they arrive with rain. Powdery mildew works the other way round. Its spores carry enough water of their own to germinate on a dry leaf, and a film of rainwater damages them and washes them off.

What it does need is humid air. Warm days, cool humid nights and dry soil is the exact combination that produces the worst outbreaks, which is why a fine settled August is a mildew year and a wet one is not.

Follow that through and the advice inverts. A plant sitting in dry ground has leaf cells short of turgor, and that is the tissue the fungus colonises most readily. Watering more at the root, and mulching so the moisture stays there, does more against powdery mildew than anything you can put on the foliage. The plants that go first every year are the ones in dry shade at the base of a wall, a fence or a hedge.

What the damage amounts to

On an established plant this is disfiguring rather than dangerous. Coated leaves photosynthesise poorly and drop early, the plant makes less than it should, and it goes into winter with smaller reserves. A rose does it every year for a decade and carries on.

Where it costs you is yield and display. A courgette or cucumber losing its leaves in August stops setting fruit weeks early. A phlox or a Michaelmas daisy coated by flowering time is not worth looking at. Young plants and cuttings can be checked badly enough to stall.

What to do about them

Water the roots properly and mulch, and do it in spring rather than after the white patches appear. Give the plants space when you put them in. Choose a resistant cultivar when you buy, which costs nothing at the point of decision and is the largest single lever on this page.

Once mildew is present, take off the first affected leaves and bag them, and keep watering. Do not strip a plant bare in an attempt to remove all of it. A courgette in September will be white whatever you do, and the leaves that are left are ripening the last of the crop.

On roses, the winter prune is the intervention that pays. The fungus overwinters in dormant buds on the stems, so cutting out thin twiggy growth and any shoot that carried mildew last summer removes the starting population before it moves.

Why there are no products listed here

The biological tier is empty because nothing can be introduced to eat a fungus growing on a leaf. The last tier is empty for a different reason. Amateur fungicide approvals in Great Britain have been withdrawn steadily over the past fifteen years, several of the actives that older gardening books recommend for mildew are no longer approved at all, and the ones that remain give a partial result on a disease that cultural change addresses better.

Milk sprays and sodium bicarbonate solutions turn up constantly in this context. Beyond the thin evidence for either, using a substance as a plant protection product in the UK requires it to be approved for that use, and neither is.

What it gets mistaken for

Downy mildew is a different organism, an oomycete rather than a fungus, and it sits on the underside of the leaf as a greyish or purplish fuzz with angular yellow blotches above, bounded by the veins. It needs wet leaves and cool conditions, so it appears in a wet spring rather than a dry August, and it is much more damaging.

Grey mould is a looser, greyer, fluffier growth that appears on damaged or dying tissue and on fruit, and it puffs out a cloud of spores when knocked. Powdery mildew stays as a flat film.

Hard water deposits and the residue from a foliar feed both leave white marks on a leaf, and both are confined to where the droplets landed rather than spreading outwards in circles. Neither grows.

Woolly aphid and mealybug both look like white fluff at a distance, and both are animals sitting on the plant rather than a film across the leaf blade. Look at the stem joints and the leaf axils, which is where they gather.