Impatiens Downy Mildew and Other Problems: What to Look For and When to Give Up

Impatiens downy mildew: the one that ends the plant

This is not an ordinary garden disease and it should not be treated like one. Impatiens downy mildew is a water mold that specifically attacks the common bedding impatiens, and where it has arrived it has made that plant close to ungrowable. It does not disfigure a plant that then carries on. It kills it, quickly, and it does so to every plant in a bed within a week or two of the first one showing symptoms.

The sequence is consistent enough to diagnose with confidence. First the leaves go slightly pale and the plant looks faintly off color, as though it needs feeding. Then the leaf edges curl downward. Turn a leaf over at this stage and there is a fine white or very pale gray bloom coating the underside, even and downy rather than in spots, which is the diagnostic sign and the thing to check for. Within a few days the leaves yellow and fall, then the flowers drop, and you are left with a bed of bare upright green stems with nothing on them. From first pale leaves to bare stems is often five to ten days.

Cool damp nights, morning dew, heavy overnight condensation and crowded plantings are what set it off, so it typically appears in a wet spell in mid to late summer and moves through a planting immediately. The spores travel on wind and splashing water, so a neighboring garden is more than close enough. There is no fungicide available to a home gardener that will cure an infected plant, and no cultural fix that saves one.

Write down the date and the exact bed where you first saw it. The spores overwinter in that soil, and the single most useful record you can keep is a list of the places in your garden where impatiens have failed, because those are the places never to plant the common bedding type again.

What to do about it, which is mostly what to plant instead

Once you have confirmed downy mildew, pull the plants out, bag them and put them in household waste. Do not compost them and do not leave the stems in the bed. Take out the fallen leaves and as much surface debris as you can, because the spores that overwinter and restart the problem next season are sitting in that material and in the top of the soil. Wash your hands and clean any tools before touching healthy plants elsewhere.

Then accept the real conclusion, which is that you should not plant common bedding impatiens in that spot again. Not next year with better spacing, not the year after with a fungicide. The spores persist in the soil, they blow in from elsewhere anyway, and the disease has been widespread enough for long enough that in many regions the common bedding type is simply not a reliable plant any more. Growing it in fresh compost in a container well away from the affected ground gives you a chance; growing it back in the same bed does not.

The honest fix is a different plant. New Guinea types are not affected by this disease and are the direct replacement: larger flowers, thicker darker leaves, more tolerance of sun, and no vulnerability to the mildew that removes the common kind. There are also newer interspecific bedding types bred specifically for resistance, sold as such rather than as ordinary impatiens, that behave like the old plant and survive. If you want the same look in the same shady bed and would rather not grow impatiens at all, wax begonias, coleus and torenia all fill the same role.

1. Check leaf undersides at the first sign of paleness Turn several leaves over and look for an even white downy coating. Doing this weekly through damp summer weather is what buys you the chance to remove plants before they infect the rest.
2. Remove and bag infected plants immediately Pull the whole plant, bag it and put it in household waste. Composting it or leaving stems in the bed keeps the spores in your garden for the following season.
3. Clear the fallen leaves and surface debris Rake off everything that has dropped around the plants. This material is where a large part of the overwintering inoculum sits.
4. Do not replant the same type in that soil Switch that bed to New Guinea types, a resistance bred interspecific type, or a different shade plant entirely. Replanting common bedding impatiens there fails again.
5. Space and water to reduce the risk elsewhere Plant at eight to twelve inches (20 to 30 cm) apart, water at the base in the morning, and never overhead in the evening. This slows the disease down; it does not prevent it.

Spider mites, aphids and thrips

Spider mites are the pest that does the most damage, and they arrive in exactly the conditions impatiens dislike anyway: hot, dry air, and plants under water stress. The first sign is a fine pale stippling across the upper leaf surface, like someone has gone over it with a pin, and the leaves take on a dull bronzed cast. By the time you can see fine webbing at the leaf joints and the shoot tips the population is large. Check the undersides with a hand lens and you will see moving specks.

Because mites thrive in dry air, the first response is water. Spray the plant hard with plain water, undersides included, on three consecutive days, and raise the humidity around it by grouping containers together and keeping the root zone consistently damp. A plant that is never allowed to wilt is markedly less attractive to them. For a heavier infestation, insecticidal soap sprayed until it runs off both leaf surfaces works, repeated at five to seven day intervals for three applications, because it kills only what it wets and does not touch the eggs.

Aphids cluster on the newest shoot tips and the flower buds, leaving sticky honeydew on the leaves below and sometimes black sooty mold on top of that. Ants running up and down the stems are a good indicator. A jet of water three days running clears a small colony; insecticidal soap handles a larger one. Thrips are harder to see and show up as silvered streaking on the petals and distorted, scarred new leaves, and they matter because they spread virus. A plant with mottled, puckered leaves and streaked flower color that does not improve after the thrips are gone is virused, and it should be pulled out and binned.

Rots, gray mold and telling them apart from drought

Impatiens stems are soft and full of water, which makes them easy for rots to get into. Stem rot shows as a brown, soft, sometimes water soaked patch right where the stem meets the soil, after which the whole plant folds over. It comes from the crown sitting wet: planted too deep, mulch heaped against the stems, a container with blocked drainage, or a run of cold wet weather. There is no cure. Remove the plant, take out the soil immediately around it, and fix whatever kept the base wet.

Botrytis, or gray mold, appears as a fuzzy gray growth on fallen flowers that have lodged in the crown, then spreads into the stems behind them. It is a problem of still, damp, crowded plantings, which is why clearing spent flowers out of the center of the plant matters even though impatiens need no deadheading otherwise. Pick off affected parts, bin rather than compost them, and open the planting up so air moves through it.

The distinction that trips people up is rot versus thirst, because both produce a collapsed plant. A thirsty impatiens is flat in the afternoon, standing again within an hour of watering, and completely normal the next morning. A rotting impatiens is limp at dawn as well, does not respond to watering, and gets worse with every further watering. If a plant is limp first thing in the morning with damp soil, look at the base of the stem before you reach for the watering can.

Record which bed or container each planting went into and how it ended. Downy mildew and stem rot both build up in specific places, and a two line note of where things failed is what stops you putting the same plant back into the same soil next season.