Pests and diseases
All the pests that can affect large flowered begonias can also affect these plants though tarsonemid mite is not such a problem Mealy bug can be a problem especially with large plants with dense foliage. Dabbing the white areas with surgical spirits or meths can remove small scale infestations. In theory systemic insecticides should control mealy bug and similar sucking pests but few effective chemicals are now available to amateur growers. Provado is the best but not everyone finds it totally effective. This is also the best treatment to prevent or at least limit the spread of vine weevil.
Most begonias are prone to fungus diseases of which forms of mildew are the most common. Mildew spores are abundant and control should start by keeping the plants growing vigorously and well ventilated. Plants under stress due to low temperatures, too dry or too wet conditions are more likely to succumb. Anti-fungal sprays are often used but these need to be applied before the infection has taken hold. It is best to remove affected leaves as they will never look good. Botrytis can also affect begonias and is dealt with in the same way as mildew
Stanley R.D. da Prato