This disease is caused by bacteria (Xanthomonas campestris pv. begoniae) that infect tuberous and fibrous begonias. The slimy substance that oozes from infected lesions is composed of bacterial cells that can live for 3 months or more. The bacteria are spread by splashing water, contaminated equipment, and infected transplants. Infection is favored by high humidity. Localized leaf infection causes early leaf drop. If the plant''s water-conducting tissue is infected, the whole plant softens.
Cut off and discard infected plant parts. Disinfect tools after working with diseased plants. Remove and destroy severely infected plants and discard the soil immediately surrounding them. Avoid wetting or splashing the leaves. Space plants far enough apart to allow good air circulation.
Begonias are susceptible to several fungi that cause leaf spots. Some of these fungi may eventually kill the plant or weaken it so that it becomes susceptible to attack by other organisms. Others merely cause spotting that is unsightly but not harmful. These fungi are spread by splashing water, wind, insects, tools, and infected transplants and seed. They survive the winter in diseased plant debris. Some leaf spot organisms affect a large number of plants. Most of these fungi do their greatest damage during mild weather (50° to 85°F). Infection is favored by moist conditions.
Clean up and destroy infected leaves and debris. Water early in the day to allow the leaves to dry by nightfall. Apply a fungicide labeled for leaf spot.
Several species of this common insect feed on begonias. Mealybugs damage plants by sucking sap, causing leaf distortion and death. The adult female mealybug may produce live young or may lay eggs in a white, fluffy mass of wax. The immature mealybugs, called nymphs, crawl all over the plant and onto nearby plants. Soon after they begin to feed, they produce white, waxy filaments that cover their bodies, giving them a cottony appearance. As they mature, they become less mobile. Mealybugs cannot digest all the sugar in the sap, and they excrete the excess in a fluid called honeydew, which coats the leaves and may drop onto surfaces below the plant.
Separate infested plants from those not affected. Apply an insecticide labeled for mealybugs. If only a few bugs are present, wipe them off with a damp cloth or with cotton swabs dipped in rubbing alcohol. Inspect new plants thoroughly before putting them in the house.
Powdery mildew on begonia is caused by a fungus (Erysiphe cichoracearum). The powdery patches on begonia are composed of fungus strands and spores. Air currents carry these spores, which are capable of infecting leaves, stems, and flowers of the same or nearby plants. The disease thrives in dim light and warm days with cool nights. Older leaves are more susceptible than new leaves. Plants in dry soil are more susceptible. Severe infections cause yellowing, browning, and leaf drop.
Remove infected leaves. Move plants to locations with more light. Keep plants out of cool drafts and in rooms with temperatures as even as possible. Do not crowd plants together.
This disease is caused by several different fungi, also known as water molds, that persist indefinitely in the soil. These fungi thrive in waterlogged, heavy soils. Some of them attack the plant stems at the soil level, while others attack the roots. Infection causes the roots and stems to decay, resulting in wilting, yellowing leaves, and the death of the plant. These fungi are generally spread by infested soil and transplants, contaminated equipment, and moving water. Many of these organisms also cause damping-off of seedlings.
Allow the soil around the plants to dry out. Remove and discard severely infected plants. Avoid future root-rot problems by planting in well-drained soil.
This insect is a common pest of begonia. The four-winged adult lays eggs on the undersides of leaves. The larvae are the size of a pinhead, flat, oval, immobile, and semitransparent, with white, waxy filaments radiating from the body. They feed for about a month before changing to the adult form. The larvae are more damaging because they suck more sap from the plants than do the adults. They cannot digest all the sugar in the sap and they excrete the excess in a sugary material called honeydew, which coats the leaves and may drop from the plant.
Remove heavily infested leaves. Vacuum plants to pick up adults. Apply an insecticide labeled for this pest, following label directions. Inspect new plants when you bring them home.