Begonias (Begonia spp.) are annual or perennial bedding plants that are also widely grown in pots. The plants are natives of tropical and mild tropical regions with nearly 900 varieties around the world. The waxy, herbaceous flower clusters bloom from summer to fall in a range of colour including orange, red, salmon, yellow or white. Stem rot is among the pathogenic disorders of begonias.
An infection from Phythium fungi leads to root and stem rot in begonias. Stem and root rot in begonias may also be caused by the Rhizoctonia solani fungus. Both fungi affect a host of other flowers besides begonia and are characterised by similar symptoms.
The Phythium fungus causes water soaked discolouration to appear on the stems. There is damping off in seedlings. Affected roots become discoloured and plants become stunted in growth. Diseased plants are highly prone to sunburn. Early symptoms of Rhizoctonia infection include wilting and yellowing foliage that are stunted in growth. The base of stems develops a soft rot with sunken lesions. A brown fungal growth is visible in the infected areas during wet weather.
Both fungi are soil-borne and survive in contaminated soil for extended periods of time in the form of fruiting bodies or spores. These spores are activated in excessively moist soils and during the warm spring weather, rapidly infecting healthy plants. The diseases also spread with contaminated tools or by propagation of sick plants with seed or cuttings.
Avoid overwatering plants and grow in well-drained soil. The disease is very hard to control once plants are infected. Taking preventive measures is one of the best control strategies. Practice rotation of plants in any area, and clear the site of weeds and debris prior to planting. Use disease-free seeds and healthy transplants from reliable sources. Create raised beds in areas that have poorly draining soils. Solarise soils in areas of known contamination. Chemical control options include the use of iprodione and thiophanate methyl. Start use of fungicide at the first sign of disease.