Gadenin日本語Get the appBlackleg: the killer of cuttings and young plants
Blackleg is the one that loses whole trays. The stem goes black and soft right at compost level, the blackening travels upward, and the top of the cutting wilts and collapses over a day or two. By the time the wilt is visible the base is already destroyed and there is no treatment that will save that plant.
It is a soil borne rot and it needs wet compost to get started. Cuttings kept in a sealed propagator, stood in a saucer of water, or pushed into old reused compost are the ones that get it. Geranium cuttings do not need any of those things, which is why the standard advice for this plant is gritty free draining compost, no cover, and no water for the first few days.
If it appears, act the same day. Lift the affected cutting out with its compost and bin it rather than composting it. Do not reuse that compost. Let everything else in the tray dry out hard and move the tray somewhere with moving air. If a valued plant is affected, take a cutting from clean green growth well above the black and start again.
| Problem | How to recognize it | Response |
|---|---|---|
| Blackleg | Stem black and soft at compost level, top wilts | Bin the plant and its compost; dry the rest hard |
| Botrytis | Gray fuzz on old petals and leaves in humid weather | Remove affected parts, improve airflow, water the compost only |
| Geranium rust | Yellow spots above, brown rings of spores beneath | Strip affected leaves, space plants, bin the debris |
| Bacterial leaf spot | Angular dark spots bounded by leaf veins, wedge shaped death | No cure; destroy the plant, do not take cuttings from it |
| Whitefly | Tiny white flies rising in a cloud when disturbed | Wash undersides, use sticky traps under cover |
| Aphids | Clusters on flower stalks and shoot tips, sticky leaves | Wipe or rinse off; watch for distorted new growth |
| Geranium caterpillar | Holes bored into buds, frass inside the flower head | Pick out by hand and remove damaged heads |
| Vine weevil | Notched leaf edges, then sudden collapse with no roots | Tip out and check the compost for white grubs |
Botrytis on flower heads and old leaves
Gray mold is the commonest problem on plants that are otherwise healthy, and it is entirely a matter of humidity and hygiene. It arrives as a soft brown patch with gray furry growth on it, always starting on something already dying: a spent flower head, a yellowing lower leaf, a stub of stalk left after careless deadheading.
It is worst in a still humid spell, under cover, and on plants packed close together. Watering overhead so that water sits in the packed center of a flower head almost guarantees it. Water the compost, never the flowers, and do it in the morning so any splash dries by midday.
The control is mechanical. Take off every spent head at the base, pull off every yellow leaf, and clear all fallen petals from the compost surface each time you water. Bin what you remove instead of composting it. Then thin the plants so air moves between them. In practice a geranium given real airflow and no dead tissue does not get botrytis.
- Remove the source, not just the mold Cut back into clean firm tissue rather than snipping off the visible fuzz. Anything left soft or discolored restarts the infection within days.
- Handle infected material gently Brushing a moldy head releases a cloud of spores over everything nearby. Cup it in your hand as you remove it and drop it straight into a bag.
- Open the plants up Thin crossing stems in the middle of a congested plant and widen the spacing between pots. Airflow does more than any spray.
Under cover in autumn, ventilate on every day that is not freezing. A closed greenhouse with damp geraniums in it is the classic way to lose an overwintering collection to gray mold.
Rust and leaf spots
Geranium rust is specific to this plant and unmistakable once seen. Small pale yellow spots appear on the upper surface of a leaf, and directly beneath each one sits a ring of powdery reddish brown spores. Badly affected leaves yellow and drop, and a plant stripped of foliage stops flowering.
It spreads on splashed water and on air, and it needs leaves to stay wet for several hours to infect. Strip and bin every affected leaf as soon as you see it, water at the base only, space plants so leaves dry quickly after rain, and never take cuttings from an affected plant.
Bacterial leaf spot looks different and is far more serious. The dark spots are angular, bounded by the leaf veins rather than round, and wedge shaped areas of leaf die back from the edge. Stems may blacken from the top downward, unlike blackleg which comes from the base. There is no treatment. Destroy the plant, wash your hands and tools, and do not propagate from it.
The pests that actually matter
Geraniums are not heavily troubled by pests outdoors. The plant is aromatic and tough leaved, and slugs largely leave it alone once it is established, which makes it a reliable choice for beds where other bedding disappears overnight.
Under cover it is different. Whitefly breed continuously in a warm greenhouse or conservatory and are the main nuisance on overwintered plants. Look on the undersides of the youngest leaves for the flat scale like immature stage rather than for the adults. Yellow sticky traps hung at plant height tell you the population is rising long before you would notice otherwise.
Two others are worth knowing. Small caterpillars bore into flower buds and hollow them out, leaving crumbly frass inside a head that never opens properly; pick them out by hand and remove the damaged heads. Vine weevil grubs eat the roots of potted plants and give no warning at all until a plant that looked fine lifts out of its pot with nothing underneath. If you see notched semicircular bites in the leaf edges of a potted geranium, tip the plant out in autumn and check the compost for fat white grubs.
Note the date you first see each problem each year and which plants it started on. Rust and whitefly recur in the same corner of a garden or greenhouse, and a year of records tells you where to look first next season.