Powdery Mildew on Garden Phlox: Preventing It, Spotting It Early and What Actually Works

Know what you are looking at

Powdery mildew appears first on the lower leaves as small, dull white or grayish patches that look as though someone has dusted the leaf with flour. Rub one with a finger and it smears off, which distinguishes it immediately from the fixed white markings of variegation or the raised orange-brown pustules of rust. Within two or three weeks the patches merge, the leaf yellows around them, then browns and dies, and the infection climbs the stem.

By late summer an untreated clump can be bare-legged for the bottom third of its height with a decent head of flowers sitting on top of dead brown foliage. The plant rarely dies of it, but it is ugly, it weakens the crown going into winter, and it spreads to every phlox in the bed.

Unusually among fungal diseases, powdery mildew does not need free water on the leaf to infect. It needs high humidity around the leaf surface, warm days and cool nights. That is why it strikes hardest in the middle of a still, muggy spell rather than after heavy rain, and why crowded clumps suffer worst regardless of how much it has rained.

The first infections almost always appear on the innermost lower leaves, deep in the clump where air does not move. If you are only looking at the outside of the plant you will find it two weeks later than you could have.

Prevention is airflow, and airflow is thinning

There is no spray as effective as spacing. The fungus needs a still, humid layer of air sitting against the leaf surface, and that layer only forms where leaves are packed against each other with no movement between them. Break it up and the infection struggles to establish in the first place.

Thin every established clump to five or six strong stems in spring, when the shoots are four to six inches (10-15 cm) tall. Space plants far enough apart that mature clumps do not touch, which for border phlox means eighteen to twenty-four inches (45-60 cm) between crowns. Keep tall dense neighbors off the lower third of the stems. Avoid the still corner between two walls or a hedge.

Water the soil, not the plant, and do it in the morning. Do not let a sprinkler play over the foliage in the evening. Keep the plant properly watered, because a drought-stressed phlox is more susceptible, not less, and never feed high-nitrogen fertilizer, which produces soft tissue that infects easily.

In autumn, cut the stems down and clear every scrap of leaf litter away from the crown. The spores overwinter in that debris and reinfect the new shoots from below in spring. Bin the material or burn it; do not compost it and do not spread it back as mulch.

1. Thin to five or six stems in spring As soon as the shoots are four to six inches (10-15 cm) tall, snap out the thinnest at the base until five or six stout, evenly spaced stems remain. You should be able to see daylight through the middle of the clump.
2. 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. That is where the first floury patches appear, a fortnight before anything shows on the outside of the plant.
3. Strip infected leaves as soon as you find them Pick off every leaf carrying a patch and bag it. On an early infection this alone often halts it. Losing the bottom few leaves costs the plant nothing compared with letting the fungus build up spore numbers.
4. Treat only while the infection is small A fungicide, or a repeated wash of dilute milk or potassium bicarbonate, protects clean leaves from being colonised. It will not clear leaves already covered. Applied at first sight it works; applied to a white plant it is wasted.
5. Clear the ground completely in autumn Cut the stems to two or three inches (5-8 cm), then rake up and remove every leaf fragment from around the crown. This breaks the overwintering cycle and is the cheapest control measure available.

Spray programs disappoint people because they are started too late. Once a leaf is coated the tissue underneath is already damaged and nothing restores it. Treat the fungus as something you prevent and catch early, not something you cure.

Choose resistance and place plants to suit it

Resistance to mildew varies enormously between plants of this species, far more than between most garden perennials, and it is genetic. A resistant plant grown in a crowded shady corner will still get mildew, but a susceptible one grown perfectly will get it too. Buying resistance is doing half the work before you plant.

Broad-leaved, thin-tissued plants generally suffer more than those with narrower, tougher leaves. If a particular clump in your garden is white every single year while its neighbors stay clean under identical treatment, that clump is genetically susceptible and the honest answer is to remove it. Replacing it is less work than fighting it every summer for a decade.

The other species of phlox grown in gardens behave differently. Low creeping kinds used at the front of borders and in rock gardens are far less prone to mildew, because the foliage is fine and open, but they are more prone to rotting in a wet winter on heavy soil. Judge each by the position you are putting it in.

Where mildew has been severe for years, move the plant. A clump relocated from a still corner to an open bed with morning sun often stops being a problem entirely, without any change in how you treat it.

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

Stem eelworm is the one that ends the plant. Infested stems are stunted and brittle with narrow, distorted, crinkled leaves, and the growing tips look twisted and wrong rather than merely damaged. There is no treatment. Dig the plant out, roots and all, bin it, and do not plant phlox in that ground again for several years. The nematode lives in the soil and moves on tools and on divisions, which is one more reason to sterilize a blade between plants.

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

Slugs graze the emerging shoots in spring and can flatten a young clump before it gets going. Protect new plants and small divisions in their first two springs; an established clump with fifteen shoots can spare a few, which is one benefit of thinning after the shoots are up rather than before.

Give up on a clump when it has eelworm, when it has been badly mildewed three years running despite good spacing and clean autumn tidying, or when the crown has rotted to the point where the outer pieces are no longer vigorous. Record which clump had what and when, because reinfection patterns repeat in the same positions year after year and the notes will show you which corner of the garden is the real problem.