Powdery mildew is a fungal disease that attacks a wide variety of plants, including all begonias.
The damage due to infection by the fungi can be slight to severe; affect some plants and not others; and be worse in some seasons than in others.
Usually the first symptoms of the disease are white to pale gray fungus growths-similar to small white stars-which appear on the leaves, stems or flowers. Young plants and those under any stress, such as one that is badly in need of water, tend to be the first affected and more severely damaged than older, healthy ones.
Generally speaking, mildew is more prevalent in the fall when high humidity is rapidly converted to moisture by the onset of cooler evenings. Moisture allows the spores to germinate. In a conservatory or greenhouse, adequate air circulation is vital, as is control of the humidity.
Shade houses have the advantage of better ventilation, but jamming plants close together will be counterproductive, as it will reduce the all-important airflow around them. In the open garden, good spacing will reduce the chances of infection. Spraying and watering is best done in the early morning, with care taken not to wet the foliage when watering.
Organic remedy: One old remedy, for those who prefer not to use toxic sprays, is to use 2 tablespoons of baking soda to 1 gallon (4 L) of water applied as a spray. Another natural product one can use is milk.
In an experiment carried out in South America, it was found that milk diluted with ten parts of clean water and sprayed once a week was very effective in controlling powdery mildew on vegetables.
An option in areas particularly mildew-prone is to consider adopting a preventive spray program from the beginning of the season. Before buying the spray, check the information given on the container for the following names, as these are the present-day chemicals supposedly best suited to controlling powdery mildew: proaconazole, myclobutanil, triforine, tridemorph, penconazole and fenarimol. In some instances, copper fungicides can also be effective. Sulfur, both as a spray or a dust, is also effective and is now available as a soluble powder. As opposed to all other sprays, mildew does not become resistant to sulfur.