Gadenin日本語Get the appSoft rot: what it is and how it gets in
Bacterial soft rot is the one serious disease of callas and it is the reason so many colored tubers never come up. The bacteria responsible live in soil and on plant debris almost everywhere, and they are harmless against an intact tuber. What they need is a wound, and any wound will do: a knife cut from dividing, a bruise from a dropped tuber, a fork strike at lifting, a torn stem base from pulling old foliage away, even a deep scrape from rough handling in a bag.
Once inside, they dissolve the tissue between the cells and the tuber turns to a wet, cream-colored, foul-smelling mush from the inside outward. The smell is the reliable diagnostic: soft rot has a distinct sour, unpleasant odor that no other calla problem has. The rot moves fast in warm wet conditions and slowly in cool dry ones, which is why the disease is a summer and an in-store problem rather than a winter one.
Above ground it shows as leaves yellowing and wilting from the base upward while the neighbors stay green, then as the whole plant collapsing at the crown so the leaves lie flat and the base pulls away wet. In storage it shows as a tuber that gives under thumb pressure and weeps when squeezed.
There is no cure. A tuber that is soft is finished, and the correct response is to remove it, remove the soil immediately around it, and put both in the bin rather than the compost pile.
Everything that prevents it happens before planting
Because soft rot cannot be treated once it starts, control is entirely a matter of not letting it begin. That comes down to three things: keeping the tuber undamaged, keeping the soil warm when the tuber is most vulnerable, and keeping the soil from staying saturated.
The handling side is straightforward once you take it seriously. Buy or select firm tubers with no soft spots. Do not drop them, do not force them into a tight hole, and do not tidy them with a knife. If you divide, do it several days ahead so the cut faces dry into a hard corky callus before the tuber meets soil, and use a blade cleaned between tubers so you are not carrying the problem from one to the next.
The soil side is a matter of patience. Planting a colored calla into cool wet ground gives the bacteria the conditions they want and gives the tuber nothing, and this single mistake accounts for more losses than everything else combined. Wait for 60F (16C), plant into free-draining soil or a container, and do not water at planting. Where the ground is heavy clay, do not fight it; grow them in pots.
Rotation helps too. Do not plant colored callas in the same ground where a tuber rotted, for several years. The bacteria persist in soil and debris, and repeating a planting in infected ground reliably repeats the loss.
| What you see | Likely cause | What to do |
|---|---|---|
| Nothing emerges; tuber soft and foul when dug | Soft rot from damage plus cold wet soil | Remove tuber and surrounding soil; plant warmer and later next time |
| Leaves yellow from the base up, plant collapses | Soft rot in the tuber, or waterlogging | Lift and check; destroy if soft, improve drainage if not |
| Stored tuber weeping and sour-smelling | Soft rot spreading in the box | Discard it and everything touching it; check the rest weekly |
| Brown papery leaf edges, plant otherwise fine | Too dry at the root, or scorching sun | Water more; shade the colored kinds from hot afternoon sun |
| Pale mottled streaks on leaves, distorted flowers | Virus spread by aphids | Destroy the plant; control aphids; do not divide it |
| Sticky leaves with small green or black insects | Aphids on the young growth | Rinse off with water, repeat every few days, check leaf undersides |
| Fine webbing and stippled dull leaves in hot dry air | Spider mites, usually indoors or under cover | Raise humidity, rinse foliage, isolate the plant |
| Ragged holes chewed in leaves near ground level | Slugs and snails on soft new growth | Clear debris, patrol after dark in damp weather |
| Gray fuzzy mold on dying flowers in wet weather | Botrytis on spent tissue | Remove spent spathes promptly; improve air movement |
The problems that look like disease and are not
A large share of calla complaints are watering mistakes rather than pests or pathogens, and telling them apart saves throwing away a plant that would recover. The three most common are worth knowing by sight.
Yellow lower leaves on a plant standing in wet soil with a firm base is waterlogging, not rot yet. Stop watering, improve drainage or move the pot somewhere sheltered from rain, and it often recovers. The same symptom with a soft crown that pulls away wet and smells sour is soft rot, and it does not.
Brown crisp leaf margins on a white calla is thirst, plain and simple, and the answer is more water or a wetter position rather than any treatment. The same symptom on a colored calla in strong sun is usually scorch, and the answer is afternoon shade.
Clear droplets at the leaf tips in the morning alarm people regularly and are entirely normal. The plant is shedding surplus water through pores at the leaf margin, which is what a plant with plenty of water at the root does overnight. It is a sign of a well-watered plant, though on a colored calla it is worth checking that well-watered has not become waterlogged.
- Check the base of the plant before anything else Almost every serious calla diagnosis is made at soil level. Squeeze the base of the stems gently: firm means the trouble is cultural, soft and wet means the tuber is going and the plant should come out today rather than next week.
- Use your nose Soft rot smells sour and unmistakably unpleasant. No amount of overwatering, scorch or nutrient trouble smells like anything. If there is a smell, the plant is not going to be saved.
- Isolate before you investigate Move a suspect pot away from the others before you start digging into it. Soft rot spreads readily on hands, tools and splashed water, and a few minutes of separation costs nothing.
- Bin, do not compost Affected tubers, the soil directly around them and any collapsed foliage go in the household waste. A home compost pile does not reliably reach the temperature needed, and spreading that compost puts the problem back on the beds.
- Clean tools between plants Wipe blades with alcohol between tubers when dividing and after cutting anything suspect. This is how soft rot travels through a collection, and it is the easiest transmission route to shut down.
Handle all of this in gloves. Calla sap contains needle-shaped calcium oxalate crystals that burn skin and are severely irritant in the eyes or mouth, and the plant is poisonous to people and to pets. Wash your hands after working on a calla, whether it is healthy or not.
What to record about losses
Note where each tuber was planted, the soil temperature you planted it into, and whether it emerged. When something fails, dig it up and write down what you found: soft and foul, dry and hollow, or simply absent. Those three findings have different causes, and after two seasons the pattern in the notes usually points straight at the culprit, most often a bed that is too cold and wet in early summer.
Record the count of tubers discarded at lifting and at each winter check, alongside the storage temperature. A batch that goes into the box sound and comes out soft points at the storage; a batch that was already suspect at lifting points at the season. Keeping the two numbers separate is what makes the record worth having, because the fixes are in different halves of the year.