Stem and Root Rot (fungi – Pythium spp., Rhizoctonia spp. and others): Affected plants wilt and break over at the soil line due to decayed spots being formed on stems. This problem is often severe when cuttings are placed in beds for rooting. Fungi responsible for this condition may be introduced on cuttings or be present in soil. Use sterilized soil. Fungicides may be used as a drench. Use cuttings from healthy plants only.
Botrytis Blight and Stem Rot (fungus – Botrytis cinerea): This fungus is most severe when temperatures are cool and moisture levels are high. Affected plants decline rapidly with stems and leaves developing brown, water-soaked lesions. In advanced stages all tissues may be penetrated by the fungus. This disease is especially severe under greenhouse conditions where begonias are propagated. Growers should be sure to start with disease-free cuttings, use a sterilized medium and keep the growing area free of any type of weak or decaying plant material that might serve as a food source for the fungus.
Leaf Spots (fungi – several): Brown spots appear on foliage and reduce plant vigor. Most leaf spotting fungi thrive under high moisture conditions. Change the location of potted plants if conditions are overly wet and use appropriate protectant fungicides.
Powdery Mildew (fungus – Erysiphe cichoracearum): Affected leaves have a white powdery substance on the upper surfaces. This problem is occasional in nature but may occur when environmental conditions are ideal for disease development.
Bacterial Spot (bacterium – Xanthomonas campestris, pv. begoniae): Small blister-like spots appear on leaves. These become clear with age and may run together to form larger spots. Affected leaves may shed prematurely. Chemical control may be only partially effective.
Root Knot (nematode – Meloidogyne spp.): This nematode causes knots to form on roots. This problem can be prevented by using nematode-free planting stock and a sterilized potting medium.
Rust (fungus – Coleosporium solidaginis): Orange-red pustules form on the underside of leaves. In severe cases, foliage turns yellow and dies. Alternate host is pine where it produces a blister rust on the needles. Use rust resistant varieties when available. Fungicides should be used at the first sign of disease and continued on a 7-14 day schedule.
Aster Yellows (mycoplasma): First symptoms are observed as a yellowing or chlorotic appearance along the veins of young leaves. As the chlorosis becomes severe, defoliation occurs. Affected plants do not wilt or die, but have a spindly type growth which detracts from the plants overall appearance. Yellows may attack only a portion of the plant. Secondary shoots are formed profusely on infected plants. The disease is spread by leafhoppers. Control is obtained by insect control and removal of diseased plants.