The best times for this is before spring growth begins, which is usually after the plant has flowered. However, one or two common problems may be encountered. The first sign of trouble is usually a black sooty mould. This is caused by a fungus growing on 'honeydew' which is excreted by the insects. It is easily controlled by spraying with Malathion, making sure to spray the underside of the leaves where these pests collect.
Double strength is used than is recommended for greenfly and blackfly.
Chlorosis Yellowing of the leaves, known as chlorosis, can be caused by several factors. In spring particularly, this may be no more than old leaves dying off. However, the cause could well be excessive alkaline conditions, locking up necessary trace elements, iron, boron and manganese. The solution is to feed with chelted iron or Epsom Salts for manganese deficiency. Another cause of yellowing leaves could well be a virus, but rarely do these harm the plant.