Zinnia Powdery Mildew, Leaf Spot and the Pests to Expect

Powdery mildew is not a question of if but when

White dusty patches on the upper surface of the lower leaves, spreading upward until the plant looks as if it has been dusted with flour, is powdery mildew, and it arrives on zinnias in late summer in essentially every garden. It thrives on warm days, cool nights and the heavy dew that comes with them, which is exactly the weather in the second half of the season. Treat its arrival as a scheduled event, not a failure.

Unlike most fungal diseases it does not need wet leaves to germinate. It needs high humidity in the air immediately around the leaf, which is why crowding is so decisive. Two identical plants a foot apart and three feet apart will show mildew several weeks apart, and the wider spaced one keeps flowering while the crowded one is stripping its own lower leaves.

It rarely kills a zinnia. What it does is shut down the lower leaves, cut flower size, and make the plant look wretched from midsummer onwards. Your goal is to delay it long enough that the season is largely over before it takes hold.

1. Space at least 12 in (30 cm) apart Air movement between plants is the single strongest control there is. Wider spacing in a still or humid site is worth more than any spray you can buy.
2. Water the soil, never the leaves Use a can without a rose, a soaker hose or a wand held under the foliage. Overhead watering both raises the humidity around the leaves and splashes spores upward from the ground.
3. Strip the lower leaves early Remove the bottom 4 to 6 in (10 to 15 cm) of foliage once plants reach 18 in (45 cm). Those leaves catch it first and infect everything above them.
4. Remove infected leaves out of the bed Pick off dusted leaves the day you see them and carry them away. Dropping them on the soil under the plant leaves the spores in place for the next splash.
5. Sow a later batch as insurance A sowing made four to six weeks after your first comes into flower as the first block declines, and young clean plants keep the display going into autumn.

Once mildew covers most of the foliage, no treatment restores those leaves. At that point cut the worst plants back by half, water the ground, and rely on the clean regrowth rather than trying to rescue the old leaves.

Leaf spot and gray mold are different problems

Brown or reddish blotches with pale centers, often with a dark ring, that start on the lower leaves and spread upward in wet weather are alternaria leaf spot. Unlike mildew this one does need wet foliage, so watering the ground fixes most of it. It can spread into the flowers and stems in a bad year and it does kill plants, so it deserves more urgency than mildew does.

Gray fuzz in the center of a fully double flower after a wet spell is botrytis. The packed petals hold rain and dew, the center browns, and the mold moves down the stem from there. Cut affected flowers off back to a leaf joint immediately, and take them out of the bed. This is one more reason to pick zinnias frequently rather than leaving old flowers standing.

A whole young plant that collapses at soil level with a pinched brown stem base is damping off or stem rot, and it is a symptom of soil that is too wet or a sowing made into cold ground. There is no cure for the affected plant. Pull it, improve the drainage and sow again once the soil is genuinely warm.

The insects that actually matter

Aphids cluster on the soft growing tips and around the buds, distorting flowers as they open and leaving sticky honeydew that goes sooty black. On zinnias they matter more than the direct damage suggests, because aphids carry the viruses that cause the mottled distorted growth you occasionally see. Squash the first colonies by hand and hose the tips hard, and hoverflies will usually take over from there.

Spider mites are the hot dry weather pest, and zinnias against a warm wall are prime territory. Look for fine pale stippling on the upper leaf surface, then bronzing, then faint webbing in the leaf joints. Hosing the undersides of the leaves hard twice a week breaks the cycle if you start early. Once webbing is obvious the plant is usually past saving.

Slugs and snails take out young seedlings overnight, which is a strong argument for thinning to a spare plant rather than the exact number you want. Japanese beetles skeletonise the leaves and chew the flowers where they occur, and hand picking into soapy water early in the morning is the only method that reliably works on a garden scale. Earwigs ragged the petals of open flowers at night and are best trapped in a rolled newspaper or an upturned pot of straw.

Write down the date mildew first appears each year and which part of the bed it starts in. It reliably begins in the same corner, usually the one with the least morning air movement, and knowing that lets you space that corner wider next season.