Begonia Diseases

Few plants can top begonias (Begonia spp.) for variety. They come in dozens of different types, with leaves in many shapes and colors, and they have flowers that can be extra-colorful and sometimes large and showy. Usually grown as annuals or as houseplants, begonias can also grow outdoors in U.S. Department of Agriculture plant hardiness zones 9 or 10 through 11, depending on the variety. But begonias are also susceptible to several plant diseases. You can prevent or control most of these if you identify the problem and deal with it quickly.

Begonias can develop one of several diseases caused by growth of fungal organisms that thrive in moist, humid conditions.

Overgrowth of certain types of fungus can cause rotting of parts of a begonia plant, including foliage, stems or roots.

Botrytis blight can start as tan to brown leaf spots, followed by rotting of the entire plant at its crown, with powdery gray areas on infected areas.

Rhizoctonia crown rot is a fungal disease that causes rotting of the plant at the soil surface. Eventually the entire plant collapses and dies.

Pythium rot can kill begonia seedlings and, on larger plants, leads to shiny black areas on stems near the soil line. 

Prevent these fungal diseases by providing good air circulation for begonia plants, keeping them well-spaced from each other and not close to other, larger plants. Watering at the soil line also helps by keeping leaves and stems dry. If you grow a potted begonia, only use commercial planting media that's been pasteurized to destroy fungi and always use containers that have drainage holes in them. It also helps to clear away dropped flowers and leaves regularly, because these can harbor fungus.

If you catch one of these diseases early, you might save the plant by cutting away affected stems, using sharp shears to cut into healthy plant tissue behind the infection. Wipe your blades with rubbing alcohol between cuts to prevent spreading the problem. To protect a healthy plant or prevent a recurrence, you can also spray the begonia with a fungicide such as a copper-based product, which is available as a ready-to-use liquid, repeating every 10 days as needed.

Don't spray copper-based fungicides near water gardens, ponds or streams as it can kill aquatic life. Follow all safety precautions on the product label.

Some fungal diseases make themselves known by changes in a begonia's leaves. These include powdery mildew, which causes fluffy white deposits on leaves. It can also spread to flowers and stems, and damage the entire plant if untreated.

Another fungus called rust might also cause problems; it produces orange-colored pustules on the reverse sides of leaves, with the entire leaf eventually turning yellow and dying. Both powdery mildew and rust are best treated by spraying with a fungicide such as one containing copper, in the same way as for diseases that cause rot.

A begonia might also develop a disease called bacterial leaf spot, which is caused, as its name suggests, by growth of bacteria. Signs include round, blisterlike spots on leaves where the blisters join and form large areas, with leaves eventually falling. Remove affected leaves for a minor problem but, if this spreads to most of the plant, it's likely to die and it's best to discard it.

A begonia might also develop a viral disease that causes spots surrounded by rings on leaves and results in a stunted, poorly growing plant. A viral disease isn't treatable once it develops, so examine begonias carefully for signs before you buy them and discard any infected plant.