Posted: 15/08/2015 at 20:54
Powdery Mildew can be kept in check by a monthly spray of 'Systhane' (available from most GCs) applied from June until the end of flowering.
Dry conditions and lack of air circulation are the main causes of mildew in SPs.
Poor air circulation is usually as a result of planting too densely. Dry conditions can be helped by watering the foliage regularly.