Gadenin日本語Get the appPowdery mildew: the standing problem of the genus
Powdery mildew is the disease begonia growers actually deal with. It starts as small circular patches of white dust on the upper surface of the older leaves, low down and towards the middle of the plant where the air is stillest. Left alone the patches join up, the whole leaf goes gray white and then yellow, and it drops. On a bad plant the flower buds are coated too and open distorted or fail to open at all.
The condition that brings it on is warm humid air that does not move, with a wide swing between warm days and cool nights so that moisture condenses on the leaves at dawn. Wet compost is not the cause; a begonia standing dry in a crowded greenhouse mildews readily, while one watered generously in a breezy spot stays clean. That is why crowding, and pushing pots into corners, matter more than watering habits.
Deal with it by changing the air first and spraying second. Take out the worst affected leaves entirely and bin them rather than composting them. Thin any stems that are crossing through the middle of the plant so that light and air reach the center. Space the pots so the leaves of neighboring plants do not touch. Then, if it is still spreading, spray. A dilute solution of potassium bicarbonate, at roughly one teaspoon per quart (about 5 g per liter) with a drop of soap to make it wet the leaf, checks it well and is the mildest thing that works. Sulfur based sprays also work but scorch begonia foliage in hot weather, so they are for cool spells only. Nothing you spray will clean up a leaf that is already half covered; the point of spraying is to protect the leaves that are still clean.
| Sign | What it means | Action |
|---|---|---|
| White dusty circles on older leaves | Early powdery mildew | Remove those leaves, open up the airflow, then spray |
| Whole leaves gray and yellowing | Established mildew | Strip them off; the leaf will not recover |
| Buds coated white, opening distorted | Mildew has reached the flowers | Cut off affected buds and treat the plant |
| Fuzzy gray growth on dead flowers | Gray mold, not mildew | Clear the debris out of the crown; keep water off the leaves |
| Brown crisp leaf edges, no film | Dry air or fertilizer salts | Raise humidity, flush the pot; not a disease |
Rot at the crown, the stem and the tuber
A stem that goes translucent and soft at the base and then folds over, with the leaves above it still looking healthy for a day or two, is bacterial or fungal soft rot. It is nearly always caused by water sitting where the leaf stalks join the stem, or by a tuber left with water standing in its dished upper surface. On tuberous begonias the rot can move from the crown down into the tuber in less than a week, and once the tuber is soft the plant is finished.
Cut affected stems out at the base with a clean blade as soon as you see them, and cut back into firm white tissue. Let the wound dry rather than covering it. Move the plant somewhere with more air movement and stop watering overhead entirely. If the tuber itself has gone soft in one area only, lift it, cut the soft part away with a clean knife until you reach firm flesh, dust the cut with sulfur or leave it exposed to dry for two days, and replant into fresh compost that is barely damp.
Gray mold is a related nuisance that starts in dead material rather than living tissue. Spent flowers that fall into the crown of a plant and stay there through a wet week grow a fuzzy gray coating, and from that base the fungus moves into healthy stems. On a double flowered tuberous begonia the flowers are big and heavy and there are a lot of them to fall, so this needs a weekly clear out through the flowering season rather than an occasional tidy.
- Water at the compost, never over the plant Direct the can to the compost surface at the pot edge. Begonia leaves hold droplets for hours and the crown holds them longer, and that is where rot starts.
- Clear fallen flowers out of the crown weekly Lift the foliage and pick out every spent flower and dead leaf lodged in the middle of the plant. This is the single most effective thing against gray mold.
- Cut rotting stems back to firm tissue Use a clean blade, cut into white unmarked stem, and take the removed material away from the plant. Leaving a soft stump keeps the infection in place.
- Improve airflow before reaching for a spray Space the pots, thin the crossing stems, open a vent. A begonia in moving air rarely needs a fungicide at all.
Bud drop and flowers that never open
Tuberous begonias frequently drop their buds just before opening, and the cause is almost always an abrupt change rather than a pest. Moving a plant that is in bud from a greenhouse into a windy patio, letting a pot dry out completely once, a cold night after a warm week, or a sudden heavy feed will each do it. The plant sheds the buds it cannot support and then carries on, and the next flush usually opens normally if conditions have steadied.
There is one case of dropping buds that is entirely normal and gets misread as a problem. Tuberous begonias carry a large showy double flower flanked by two small single flowers on the same stalk. The small ones are female, they carry a winged triangular seed pod behind the petals, and they are meant to be short lived. Their falling is not a fault. What is a fault is leaving them on, which is covered under pruning.
Flowers that open but are small and pale on a plant that looks otherwise healthy usually means too little light, or a feed too high in nitrogen. Buds that brown and dry without opening, in a dry heated room, means the air is too dry and the bud has desiccated on the stem.
| Symptom | Most likely cause | What to do |
|---|---|---|
| Buds fall off just before opening | Sudden change in temperature, water or position | Steady the conditions; the next flush will hold |
| Small side flowers drop, big ones stay | Normal behavior of the single female flowers | Nothing wrong; remove them early instead |
| Buds dry and brown on the stem | Air too dry, usually indoors in winter | Stand the pot on damp gravel; do not mist the plant |
| Flowers small and pale | Too little light or too much nitrogen | Move brighter, switch to a high potassium feed |
| No buds at all on a leafy plant | Overfeeding with nitrogen, or a short day type out of season | Stop the nitrogen and feed for flowers instead |
Vine weevil, and the pests you will actually meet
A container grown begonia that wilts and does not recover after watering, and then lifts out of the pot with almost no roots left, has vine weevil grubs. They are creamy white, about a third of an inch (8 mm) long, curved into a C shape with a distinct brown head, and they sit in the compost around the root ball eating the roots and, on tuberous types, boring into the tuber itself. The adults are dull black beetles that cannot fly, feed at night, and leave neat notches bitten out of the leaf margins, which is the early warning you get months before the plant collapses.
Check the compost of any suspect pot by tipping the whole root ball out onto newspaper and breaking it apart. Grubs are easy to see against the compost. Pick them all out, wash the roots clean, discard the old compost rather than reusing it, and repot into fresh. A biological drench of parasitic nematodes applied when the compost is above 41F (5C) works well as a preventive on a collection of pots, and needs repeating in late summer and again in spring, which are the two points when young grubs are present.
Everything else is minor by comparison. Aphids cluster on the soft new shoot tips and on flower stalks and are washed off with a firm jet of water or knocked back with insecticidal soap. Thrips cause silvery streaking on the petals and distorted flowers. Slugs and snails eat holes in the leaves of bedding begonias outdoors and leave the tell tale slime trail. Mealybugs appear as white cottony lumps in the leaf joints of indoor foliage types and are dabbed out with a cotton bud dipped in surgical spirit.
Note the date and the pot in which you first find vine weevil grubs. They come back to the same containers and the same corner of a garden year after year, and one written entry is what turns a plant lost every autumn into a nematode drench applied in advance.