Begonia flowers are susceptible to a variety of fungal infections that may cause the crown and stems of the plant to rot. These diseases often cause plant death if left untreated.
Three fungal diseases commonly attack the crown of a begonia plant: botrytis blight, rhizoctonia crown rot and stem rot fungus.
Botrytis blight causes established begonias to rot at the crown. Plant stems become covered with dusty gray fungi as a result. Rhizoctonia crown rot causes plants to rot at the soil line. Stem rot fungus causes a soft rot at the begonia’s crown, and the stems eventually collapse.
The majority of crown rot diseases are extremely difficult to control. For botrytis blight, managing humidity is important, as is spacing plants far apart in the garden. Trifloxystronbin, mancozeb and fludioxonil fungicides effectively treat botrytis blight. For rhizoctonia crown rot, applying tthiophanate methyl or etridiazole to healthy plants helps prevent symptom development. No known treatment exists for stem rot fungi.