Gadenin日本語Get the appPowdery mildew is the defining problem
Late in flowering, forget-me-not foliage goes from green to a dull gray-white, as though it has been dusted with flour, starting on the lower leaves and working upward until the whole colony looks dirty. That is powdery mildew, and on this plant it is close to inevitable. It arrives in the mild, still, humid conditions of late spring, it moves fastest through crowded plantings, and it does not need wet leaves to spread, which is why it appears in a dry spring as readily as a wet one.
What makes it different from mildew on other plants is the timing. It arrives as the plant is finishing anyway, so it is almost never worth treating. A fungicide applied at that stage buys a few weeks of slightly cleaner leaves on a plant that is dying regardless, while the mildew spores it fails to stop go on to reach whatever else in the border is susceptible. The correct response is to remove the plants, and this is the second reason to pull as the flowers fade rather than leaving the patch standing.
| What you see | Cause | Action |
|---|---|---|
| Gray-white dust on upper leaf surfaces | Powdery mildew | Pull the plants; do not spray a dying crop |
| Yellow blotches on top, gray fuzz underneath | Downy mildew | Pull and remove; improve spacing next year |
| Whole plant collapses at the crown | Crown rot in wet ground | Improve drainage; do not mulch over crowns |
| Ragged holes, silvery trails, seedlings gone | Slugs and snails | Protect seedlings only; mature plants shrug it off |
| Tight clusters at shoot tips, sticky leaves | Aphids | Wash off with water; rarely worth more |
| Pale mottling and fine webbing in a hot spring | Red spider mite | Pull affected plants; the season is nearly over |
Reduce mildew next year, not this year
Everything useful you can do about mildew happens twelve months before it appears. Space the rosettes: thin a self-sown colony to six inches (15 cm) apart in autumn, ruthlessly, because a mat of touching rosettes has no air movement between the plants in spring and mildews first and worst. Water at the base rather than over the leaves. Avoid feeding, because soft nitrogen-rich growth is markedly more susceptible. And clear the dead plants away rather than leaving them to break down in the bed, since the debris carries the fungus into the following season.
Position matters as much as spacing. A colony baking against a south-facing wall, where the soil dries out and the air is still, mildews weeks earlier than the same plant in light shade with moisture at the root. Dry roots combined with humid air is the classic powdery mildew situation, and forget-me-not in full sun on thin soil sits in it every spring.
- Thin the colony in autumn to 6 in (15 cm) This is the single most effective measure. Pull surplus rosettes while they are small and the ground is damp, leaving each plant with clear space all round.
- Water the soil, never the foliage The hairy leaves hold moisture for hours. Use a can without a rose, held low, or a trickle hose under the plants.
- Stop feeding the area Do not include the forget-me-not patch in a general spring feed. Lush growth mildews first and flops onto its neighbors as well.
- Pull and remove affected plants promptly Take them out of the garden rather than composting them in a cool heap. Removing them as flowering ends deals with mildew and seeding in one operation.
- Move the colony every few years Let seed fall in a fresh patch and clear the old one entirely. Continuous cropping in one spot builds up both mildew and crown rot.
Slugs matter only at the seedling stage
Slugs and snails will clear a freshly germinated patch of forget-me-not overnight, and because the seedlings are so small the damage often reads as poor germination rather than pest damage. If a shallow sowing simply never appears, check the ground after dark with a torch before you blame the seed. Newly planted autumn rosettes are also vulnerable, particularly in a mild wet autumn when they sit still and do not grow away.
Once the plants are the size of a saucer they are effectively safe. The hairy leaves are unattractive to slugs and the plant grows faster in spring than anything can eat it. There is therefore no case for protecting a mature colony, and any control effort should be concentrated on the few weeks after germination and on newly moved rosettes.
Forget-me-not seedlings are frequently mistaken for weed seedlings and hoed off, which produces exactly the same symptom as slug damage. Mark the sown patch clearly so nobody, including you, weeds it out by accident.
Aphids, mites and the things that are not diseases
Aphids cluster at the shoot tips in a warm spring and leave the leaves sticky, but they arrive so late in the plant's life that they seldom affect the display. A jet of water knocks them off. Red spider mite shows as fine pale mottling and, in bad cases, webbing in the leaf axils, and again appears only in a hot dry spring at the very end of the season. In both cases the plant is within weeks of being pulled anyway, so treatment is rarely justified.
Two common worries are not problems at all. Lower leaves yellowing and dying back as flowering peaks is normal senescence in a biennial spending itself on seed. And a plant that flops sideways after heavy rain has usually simply grown too soft, from feeding or crowding, rather than caught a disease; it will not recover and is not worth staking.
Record what appeared and when
Write down the date you first see gray on the leaves each year, and which part of the patch it started in. Mildew reliably begins in the same place year after year, usually the driest, stillest corner, and having that noted lets you thin harder there in autumn or simply stop growing forget-me-not in that spot. A photograph of the patch on the day you notice it is enough of a record.
Also note the date of any slug damage to seedlings and how much of the sowing survived. If you lose most of a sowing two years running, the answer is to sow in a different place rather than to sow more seed, and only a written record makes that pattern obvious.