Gadenin日本語Get the appPowdery mildew, and when it stops being cosmetic
Powdery mildew appears as a white to grayish dusting on new leaves, shoot tips and, most damagingly, on developing flower panicles. Unlike most fungal diseases it does not need wet leaves. It thrives in warm days, cool nights, high humidity and still air, which is precisely the microclimate of a crowded plant against a wall.
On leaves it is unsightly and causes some distortion and early leaf drop, but the plant carries on. On flower buds it is a different matter: coated buds shrivel and drop without ever opening, and a heavily infected plant simply misses its display. This is the reason mildew is worth acting on rather than tolerating.
The controls are structural before they are chemical. Full sun, generous spacing from walls and other shrubs, and a thinned interior canopy together prevent most outbreaks. Avoid overhead watering in the evening and avoid heavy nitrogen, which produces the soft growth mildew colonises first.
If you do spray, start at the very first white patches, before the buds are involved. A horticultural oil or a potassium bicarbonate spray works on early infection and needs repeating every week to ten days while conditions favor the disease. Once panicles are thickly coated, spraying will not restore them.
Do not apply horticultural oil in the heat of the day or on a plant that is drought-stressed. Water thoroughly the day before, and spray early in the morning or in the evening.
Aphids and the black film they cause
Aphids cluster on the undersides of new leaves and along soft shoot tips, usually first noticed not by the insects themselves but by what falls from them. They excrete honeydew, a sticky sugar solution that coats everything under the canopy: lower leaves, patio furniture, paving and parked cars.
A dark fungus called sooty mold then grows on the honeydew, turning leaves and hard surfaces a dull black. Sooty mold does not infect the plant. It is living on the sugar, not the leaf. But a solid black coating blocks light from the leaf surface, and a badly affected plant loses vigor and drops leaves early.
The important consequence is that you cannot fix sooty mold by treating the mold. Cut off the honeydew supply and the black film weathers away over the following weeks. Wash it off hard surfaces with soapy water, and hose the lower canopy to speed it up.
For the aphids themselves, a strong jet of water aimed up into the canopy every few days knocks most of the colony off and they largely do not climb back. Insecticidal soap or horticultural oil works if you get real coverage on the leaf undersides. Broad-spectrum insecticides tend to make the problem worse over a season by removing the lady beetles, lacewings and parasitic wasps that were holding the aphids down.
The full list of what shows up
Almost everything on this plant falls into one of two buckets: sap-feeders that produce honeydew, and leaf fungi that are ugly late in the season. Learn to tell them apart quickly and you will spend very little time on treatment.
| Problem | First sign | Damage level | What to do |
|---|---|---|---|
| Powdery mildew | White dust on new shoot tips and buds | Serious if it reaches the panicles | Thin for air movement; treat at first sign, not after |
| Aphids | Sticky honeydew under the canopy, curled new leaves | Moderate, mostly through sooty mold | Water jet, insecticidal soap, protect the predators |
| Sooty mold | Black film on leaves and paving | Cosmetic, but blocks light when heavy | Control the sap-feeder above it; wash surfaces |
| Bark scale | White or gray felted crusts on trunks and branches, heavy honeydew | Serious if left, weakens the plant | Scrub trunks with soft brush and soapy water; treat repeatedly |
| Cercospora leaf spot | Brown to purple spots on lower leaves in late summer | Cosmetic; causes early leaf drop | Rake up fallen leaves; thin the canopy; no spray needed |
| Japanese beetles | Skeletonised leaves, beetles on flowers | Cosmetic on an established plant | Hand pick into soapy water in the early morning |
| Root rot in wet soil | Sudden wilting, dark soft roots, no recovery from watering | Fatal | Prevent only: never plant in a spot that holds water |
Bark scale: the one to catch early
Bark scale is the most serious pest this plant gets. It shows as white or light gray felted bumps on the trunk, in branch crotches and along the undersides of limbs. Crushing one leaves a pink smear, which is the quickest way to confirm it. Infestations produce enormous quantities of honeydew, so the whole plant turns black with sooty mold.
It spreads slowly by crawlers and much faster on infested nursery stock and on unclean pruning tools, so inspect the trunks of any new plant before it goes in the ground, and clean your pruners between plants.
Treatment takes persistence and more than one approach.
- Scrub the trunks Wash the trunks and lower limbs with a soft brush and a dilute dish-soap solution. This physically removes a large share of the scale and their egg masses, and takes off the sooty mold at the same time. It is tedious and it works.
- Apply dormant oil in late winter After pruning, while the plant is still bare, spray horticultural oil thoroughly over the trunks and every branch until it runs off. The oil smothers overwintering scale, and complete coverage is what decides whether it worked.
- Consider a soil drench only if the infestation is heavy A systemic insecticide applied to the soil is the most effective option for a badly infested plant. Because it moves into the flowers, apply it after flowering finishes rather than before, so that bees working the panicles are not exposed.
- Re-inspect the following spring Check the trunks as the plant leafs out. Scale populations rebuild from anything missed, so one clean season is not the end of it. Two consecutive years of treatment is normal.
What to record
Note the date you first saw white dust on the shoot tips, the date you first found honeydew under the canopy, and where on the plant it started. Mildew and aphids both arrive at close to the same point in the season every year in a given garden, and once you have two or three years of first-sighting dates you can act a week before instead of a fortnight after.
Also record which side of the plant went first. It is almost always the side with the least air movement or the most afternoon shade, and that is a pruning instruction for the following late winter rather than a spraying problem.