I brought in two potted begonias from my porch when the weather turned cold, about six weeks ago. They have recently developed white spots on the leaves. Could you tell me what this is and what to do? Are other houseplants at risk for catching it?
The begonias are sitting about 2 feet from a SW facing bay window, near some orchids and a Christmas cactus.
This sounds like powdery mildew, a type of fungus, which generally infects a specific plant species — this means that it should remain on the begonias and not spread to your other plants. Powdery mildew is often present where there is too much moisture (e.g., from excessive watering or humidity) and/or poor air circulation.
Isolate the affected plants from other begonias. Cut off all infected parts of the plants, cutting into the healthy tissue a bit, to ensure no diseased plant material remains. Destroy the diseased materials – don’t compost them – put them in the garbage. Clean up any debris under or around the plants and remove the top 2.5 cm (1 inch) of soil, replacing it with fresh potting soil. Prune the healthy parts of the plants to ensure good air circulation. You can also consult a gardening centre for a household fungicide to use on houseplants – make sure this is one that can be used indoors.
See an earlier post on the Ask A Master Gardener site, Dying begonias, which provides a lot of helpful strategies to help keep your begonias disease- and pest- free, e.g., avoid watering from above (as wetting the leaves could promote growth of the fungus), and avoid planting the begonias too close together.