Powdery Mildew on Asters: Why the Lower Stems Go Gray and What Actually Stops It

The gray lower half, and how to be sure that is what it is

Powdery mildew shows first on the leaves lowest down and deepest inside the clump, as small dull white or grayish patches that look as though flour has been dusted on the surface. Rub one with a finger and it smears off, which separates it at once from the fixed pale markings of variegation and from the raised orange-brown pustules of rust. Over two or three weeks the patches merge, the leaf yellows around them, browns, dies, and the infection works steadily up the stem.

By flowering time an untreated clump can be gray and bare for the whole lower half of its height, carrying a perfectly good sheet of autumn flower on top of a column of dead brown foliage. The plant rarely dies of it. It looks bad at precisely the moment you grew it for, it goes into winter weakened, and it seeds the infection into every aster around it.

The detail that changes how you handle it: powdery mildew does not need free water on the leaf to infect. It needs high humidity in the still layer of air against the leaf surface, warm days and cool nights. That is why it hits hardest in a muggy settled spell rather than after a downpour, and why a crowded clump suffers badly in a dry summer while a well-spaced one nearby stays clean.

The first patches almost always appear on the innermost lower leaves, where the air is stillest and you cannot see them from outside the plant. If you only ever look at the outside of the clump, you will find the infection a fortnight later than you could have.

Three things beat spraying, and none of them is a chemical

Spacing comes first. The fungus needs a still, humid film of air held against the leaf, and that film only forms where leaves are pressed against each other with nothing moving between them. Eighteen to twenty-four inches (45-60 cm) between crowns, the same distance clear of any dense neighbor, and an open position rather than a corner between a wall and a hedge, all attack the disease at the point where it starts rather than after it has arrived.

Dividing on schedule is second, and it is the one most people skip. A three-year-old clump is a dense ring of many thin stems with a woody hole in the middle, and the inside of that ring is the stillest air in your border. Lifting it every third spring, discarding the exhausted center and replanting only the vigorous outer pieces, resets the clump to an open plant that air can move through. Clumps that mildew every year are very often clumps that have not been split for five.

Water is third, and it runs against instinct. A drought-stressed aster mildews worse, not better. Keep the soil from going dry more than an inch (2.5 cm) down through summer, give one long soak a week rather than daily surface wetting, water at the base in the morning rather than over the top in the evening, and mulch two inches (5 cm) deep in spring to make that watering last.

Only after those three is it worth thinking about a spray, and by then you often do not need one. Where you do, apply it while the plant is still largely clean: a fungicide, or repeated washes of dilute milk or potassium bicarbonate, protects healthy leaves from being colonised. Nothing restores a leaf that is already coated, which is why spray programs started in late summer disappoint everyone who tries them.

1. Space the crowns properly and keep neighbors back Eighteen to twenty-four inches (45-60 cm) between tall clumps, and the same clear of anything dense that will grow into the lower third of the stems. Fill the gaps with annuals for the first two seasons rather than with perennials you will not want to dig up.
2. Divide every third spring without waiting for symptoms Lift the clump as shoots break the surface, throw away the woody middle, replant vigorous outer pieces of three to five shoots each. This is the single most effective thing you can do about mildew on an established plant.
3. Never let it go dry in midsummer Check with a finger at the edge of the clump. Dry to the second knuckle means water, and water enough to wet the top eight inches (20 cm). Base of the plant, morning, once a week rather than a daily splash.
4. Check the inner lower leaves weekly from midsummer Part the stems and look at the leaves nearest the ground in the center of the clump. The first floury patches appear there about a fortnight before anything shows on the outside of the plant.
5. Strip infected leaves the day you find them Pick off every leaf carrying a patch and bag it rather than dropping it on the bed. On an early infection this often halts it outright, and losing the bottom few leaves costs the plant nothing.
6. Clear the ground completely in autumn Once the foliage has yellowed, cut the stems to two or three inches (5-8 cm) and rake up every leaf fragment from around the crown. The spores overwinter in that litter and reinfect the new shoots from below in spring.

Do not compost aster prunings or fallen foliage from a clump that grayed over this year. Returning that material to the beds as compost is the ordinary way an infection carries from one season to the next and from one plant to the rest of the border.

Susceptibility is real, and so is the position

Resistance to mildew varies enormously between asters, far more than between most garden perennials, and it is inherited rather than something you can manage away. Broadly, the plants with narrower, tougher, smaller leaves and an open airy habit shrug it off, while those with broad soft leaves packed on dense upright stems suffer badly. If one clump in your bed is gray every single year while its neighbors stay clean under identical treatment, that clump is genetically susceptible.

The honest response to a plant like that is to take it out. Replacing it costs one afternoon; fighting it costs a fortnight of attention every summer for as long as you keep it, and it acts as a reservoir that infects the clumps around it. Gardeners hang on to these plants for years out of stubbornness, and the border is better without them.

Position is the other half. A clump moved out of a still, enclosed corner into an open bed with morning sun very often stops being a problem entirely, with no change at all in how you treat it. Before you condemn a plant, ask whether you have ever grown it anywhere but the worst spot in the garden.

Keep written notes of which clump grayed over, how badly, when it first showed, and where in the bed it sits. Mildew patterns repeat in the same positions year after year, and after two seasons the notes will point at a corner of the garden rather than at a plant, which is a much more useful conclusion.

The other problems, and when to give up on a clump

Wilt at the stem base is the one that ends plants. Individual stems flag and collapse while the rest of the clump looks fine, and cutting one open near the ground shows brown streaking in the tissue. It spreads slowly through a crown over a season or two. Cut affected stems right out at the base and bin them, and if the clump keeps producing them, lift it in spring, take only clean vigorous outer pieces, and replant those in fresh ground rather than back in the same hole.

Rust shows as raised orange-brown pustules under the leaves. It is much less common than mildew and less damaging, and picking off affected leaves generally contains it. Spider mite turns up in hot dry spells as fine pale stippling with faint webbing on the undersides; the cure is to fix the drought that invited it and then hose the leaf undersides thoroughly.

Slugs graze emerging shoots in spring and can flatten a young plant or a fresh division before it gets going. An established clump with thirty shoots can spare a few and nobody notices, but a first-year plant with four cannot. Protect new plants and divisions through their first two springs.

Give up on a clump when wilt keeps coming back through a crown you have already divided, when it has been badly mildewed three years running despite proper spacing, a division on schedule and clean autumn tidying, or when the woody center has spread so far that the outer ring no longer produces vigorous pieces worth replanting. Note the date and the reason each time, because the same position in the bed tends to produce the same verdict again.