I have the large variety of begonias and planted dozens of plants. In different pots, outdoor terrace. One-by-one something is destroying the stem close to the earth and an otherwise beautiful, healthy-looking plant just falls over with the stem totally rotted/eaten across at ground level – everything else about the plant is fine. Some of the plants do have signs of something perhaps eating the leaves/brownish edges, but nothing major. I don’t see any insects. I did notice that in a few pots, there are mushroom-like things growing – from what I read it might be a kind of fungus, so I tried pouring copper powder on everything, leaves, soil – not helping. Pots range in size from four inches deep to 2 feet deep – not affecting the problem. Soil is a mixture of soil from previous seasons and new potting soil from Home Depot. Temperature has been cool, some rain – no problem about having dried out, but not soaked either. Sun is direct more or less and around 4-6 hours a day – not affecting the problem either. Help please before they are all wiped out.
I’m so sorry to hear of your begonia troubles!
Assuming your plants may have a fungal disease remove & discard all affected plants and the soil in which they were growing. Do not reuse the soil or compost it. Do not take cuttings from these plants.
If some plants seem healthy but were in the same pot as the ones that were affected, transplant them into a new pot, using fresh, sterilized potting mix that drains well.
Don’t mix these “old” plants with new ones – as the latter might be in the early stages of infection. Keep the old plants at a distance from the new ones for a few weeks – essentially, quarantine them.
Clean your garden tools and pots well to avoid cross-contamination and get rid of any soil that adheres to these.
When buying new plants, inspect them carefully for yellow, wilted lower leaves, and make sure the surface of the soil looks healthy (no odd lumps that might be fungus). Consider going back to the place you purchased the begonias to ask if fungal infections have been a concern this season – you may have purchased contaminated plants.
Do not water the plants from overhead, as you should avoid wetting the leaves (which would promote fungal growth). And avoid overwatering – permit the top layer of soil to dry between waterings, so that the plants are not constantly in contact with wet soil. And if some plants look like they are wilting, don’t water them until you are certain that this is due to dehydration (as it may indicate a new infection).
Give the plants some air – don’t plant them too close together – good air circulation will help avoid further fungal problems.
And tend to the begonias regularly – remove any dead/dying flowers and leaves. Keep on the look out for insects as this may be a secondary problem as you report some leaf damage. If you spot something, send us a photo and we can help you with identification.
If you would like to do some research on Begonia diseases I have included some good resources, just follow the link:
From your description, the plants may have a fungal infection that has caused crown or stem rot. A few possible culprits come to mind:
- Rhizoctonia Crown Rot. Rhizoctonia, which is naturally present in soil, can be introduced into soil mixtures by contaminated hands, tools, transplants, among other things. Rhizoctonia fungus results in webbing and, at the surface of the soil, sunken dry brown rotted areas of stem.
- Pythium fungus, whose spores love wet soil. Begonias infected by Pythium will have small roots that are discoloured, and the stems will blacken and look water-soaked.
- Botrytis blight, which attacks weakened plants and is at its worst during cool, wet weather (like we’ve had). Botrytis will cause a soft brown rotting of the stems, and grey/fuzzy Botrytis spores should be visible in the rotting tissue.
Unfortunately, once a fungal infection has taken hold, fungicides are not very effective. But you may want to apply the fungicide once you have planted new plants/transplanted old ones that appear healthy – as this may prevent an infection.