Gadenin日本語Get the appPetal blight: brown flowers that go slimy
Camellia petal blight is a fungal disease of the flowers alone, and it is the reason so many camellias look wonderful in bud and disappointing in bloom. It starts as small brown specks on the petals, spreads until the whole flower is brown, and turns the bloom limp and slippery rather than dry. The flower then hangs on the plant or drops as a brown mass, and the whole display can go over in under a week.
The detail that tells you it is blight rather than frost is where the browning starts and how it feels. Frost browns the outer edges of the petals after a cold night and leaves the tissue papery. Blight starts anywhere on the petal, often as flecks working inward, and leaves it soft and wet. Blighted flowers also usually show a ring of white fungal growth at the base of the petals when you pull one apart.
There is no effective spray for it in a garden setting, and fungicides applied to open flowers do very little. Control is entirely about breaking the life cycle, because the fungus overwinters in hard black structures that form in the fallen flowers and sit in the soil surface under the plant until the following flowering season, when they release spores upwards onto the new flowers.
That means the ground under the shrub is the disease. Clear every fallen flower, do not compost them, and put a fresh layer of mulch over the soil each year to bury anything you missed and block spores from reaching the flowers above. Doing this for two consecutive seasons produces a visible improvement; doing it once does not.
- Pick up fallen flowers weekly Through the whole flowering season, collect every fallen bloom from under and around the plant. Bin or burn them rather than composting, since garden compost rarely gets hot enough to kill the resting bodies.
- Remove blighted flowers still on the plant Pick off any browning flower before it drops. Each one is producing spores while it hangs there, and removing them cuts the load on the flowers still to open.
- Mulch over the root zone after flowering Two to three inches (5 to 8 cm) of fresh composted bark or leaf mold over the whole area under the branch spread. This physically blocks spores released at ground level.
- Thin the shrub to dry the flowers faster Open the center out when you prune. Flowers that dry within a couple of hours of rain get blighted far less than flowers sitting wet inside dense foliage.
- Do the same next year The resting bodies survive more than one season in the soil, so a single clean-up gives only partial relief. Two or three consecutive years of sanitation is what actually brings it under control.
Yellow leaves with green veins mean the pH is wrong
Camellias are acid-soil plants. Above about pH 6.5 they cannot take up iron, however much of it the soil contains, and the leaves go yellow between the veins while the veins themselves stay green. The newest leaves show it first, because iron does not move around inside the plant to be reused. Over a few seasons growth slows, the leaves get smaller, and the shrub declines without ever quite dying.
Test the soil rather than guessing, and take the sample from root depth. Common causes of a creeping rise in pH are years of hard tap water, lime leaching out of a nearby concrete path or house foundation, mortar rubble buried in the ground beside a wall, and mushroom compost used as a mulch.
Chelated iron, watered on or sprayed on the leaves, greens the plant up within a few weeks, but it is a symptom treatment and needs repeating. The real fix is the growing conditions: switch to rainwater, mulch with pine needles or composted bark, feed only with an ericaceous fertilizer, and move a badly affected plant into a large container of acidic potting mix if the ground is genuinely limy.
Do not confuse this with the other yellowing patterns, which the table below separates. Whole leaves going uniformly yellow and dropping, mostly the oldest ones low down, is normal leaf turnover and needs nothing at all.
| Symptom | Cause | Action |
|---|---|---|
| Yellow between green veins, newest leaves worst | Iron chlorosis from alkaline soil | Correct pH, apply chelated iron |
| Whole old leaves yellow then drop in spring | Normal leaf replacement | None |
| All leaves dull yellow, compost stays wet | Waterlogging, roots suffocating | Improve drainage, repot |
| Brown dry leaf edges in late winter | Cold drying wind on evergreen leaves | Provide shelter |
| Pale leaves, weak thin growth | Starvation or a pot-bound root ball | Feed after flowering, repot |
Scale insects and the black film that follows
If the leaves are sticky and a black sooty coating develops on the upper surfaces, look under the leaves and along the stems. You will usually find scale insects: flat or domed immobile brown or white bumps, often lined up along the midrib. They are easy to mistake for part of the plant because they do not move.
The scales feed on sap and excrete honeydew, which falls onto the leaves below and grows a harmless but ugly black mold. The mold itself does no direct damage but blocks light, and a badly covered plant grows poorly. Wiping it off with a damp cloth works on a small shrub; on a large one, deal with the insects and the mold weathers off over a season.
The vulnerable stage is the newly hatched crawlers, which move about before settling and forming their protective covers. Once an adult scale is armoured, contact treatments barely touch it. On a small plant, scrubbing the stems and leaf undersides with a soft brush and soapy water is genuinely effective and is best done in the period just after flowering.
Two other pests worth recognizing. Notches chewed neatly out of the leaf margins are adult vine weevils feeding at night; the damage is cosmetic, but their grubs eat roots and are lethal to container plants, so a potted camellia that suddenly wilts and lifts out of its pot loose has been eaten from below. Silvery mottling on the upper leaf surface with fine black specks underneath is leafhopper or thrips damage, unsightly but rarely serious.
Ants running up and down a camellia are worth a second look. They farm scale insects and aphids for the honeydew and will actively defend them from predators, so a busy ant trail on the trunk usually means there is a sap-feeding colony above it that you have not spotted yet.
Deciding when a plant is not worth saving
Camellias are long-lived and forgiving, so most problems are worth working on. There are two situations where they are not. The first is a plant showing chlorosis on genuinely limy ground: you can keep it alive with iron and rainwater indefinitely, but it will never be good, and the same effort put into a container-grown plant gives you a far better shrub. The second is a shrub with dieback spreading down the branches into the main stems, where cutting back into clean wood would leave nothing.
Everything else is recoverable. A camellia that has lost every flower to blight will flower again next year if you clear the ground. A pot-bound plant that has dropped all its buds will come back after a repot. Even a shrub cut to the ground usually resprouts. Give a struggling camellia two full seasons of correct treatment before deciding it has failed, because the plant works on a twelve-month cycle and rarely shows you the improvement in the same year you make it.
Keep a symptom log, with dates
Every camellia problem is easier to identify from a history than from a single look. Record the date you first see browning on the flowers each year, whether that browning followed a frosty night, and how many flowers were affected. A blight problem starts earlier and spreads through the season; frost damage appears in one hit after a specific cold morning and stops.
For leaf problems, note the date and which leaves were affected, newest or oldest, plus the last soil pH reading and the date of it. For pests, note where on the plant you found them and the date, so that next year you know when to start looking rather than finding them after the damage. A short log kept for two seasons answers most camellia questions faster than any diagnosis from a photograph.