Several species of aphids infest plants in warm, settled weather. The commonest are the black bean aphid and the peach-potato aphid. Young growth is distorted and weakened. The leaves are covered with sticky honeydew which later becomes covered with sooty mould. Keep the plants well watered in dry weather. Spray with a contact or systemic insecticide as soon as colonies start to appear.
Red Spider Mite
If leaves develop an unhealthy bronze colour, look for tiny spider-like mites on the underside of the leaves. The presence of fine silky webbing is a telltale sign. In hot settled weather spraying may be necessary, use Liquid Derris.
Long winding tunnels are eaten in the leaf tissue by small grubs. At first the tunnels appear white, later they turn brown. Chrysanthemum and Calendula leaves are commonly attacked in this way. The carnation fly behaves rather differently producing blotches on the leaves and sometimes killing the plant. Pick and destroy mined leaves.
Leaves and shoots sometimes wilt badly even though the soil is moist. If the plant is an Antirrhinum, Aster, Sweet Pea, Carnation, Chrysanthemum, Lupin or Poppy then the likely cause is a soil-borne fungus. Tissue inside the stem will probably be stained brown. There is no cure. Remove diseased plants. Do not grow susceptible plants on the same spot.
Viruses may be carried by insects, tools or fingers. There are many different symptoms of virus infection, leaves may be yellow, covered with yellow spots or patches ('mosaic'), crinkled and distorted or white-veined. Stems may be covered with brown stripes ('streak') or stunted and distorted. There is no cure, but fortunately annual plants are rarely bothered. Buy healthy stock and keep aphids under control.
Grey Mould (Botrytis)
This is a destructive disease in wet seasons. The fluffy grey mould appears on the leaves and the stems are attacked. Remove mouldy leaves and badly infected plants immediately. Spray with a systemic fungicide.
Look for the telltale sign of coloured swellings on the leaves and stems. These raised spots may be yellow, orange or brown. It is a common disease of Antirrhinum, Hollyhock, Carnation, Chrysanthemum and Sweet William. Pick off and burn diseased leaves. Chemical control is not practical.
Leaf spot is a family name for a wide group of diseases which appear on many types of flowering plants. Leaf spot (round or oval coloured spots) is an important disease of Pansy, Phlox, Polyanthus, Poppy and Sweet William. Ring spot (dark concentric rings of spores) is common on Carnation, and leaf blotch (irregular-shaped spots) affects Larkspur. Pick off diseased leaves. Fungicides are available but spraying is not worthwhile.
Powdery and Downy Mildew
The main symptom of powdery mildew is a white mealy growth on the leaf surface. It is encouraged by overcrowding and lack of soil moisture. This is the disease commonly seen on Larkspur, Chrysanthemum, Nigella and Verbena. Plants may be crippled. Spray with a systemic fungicide at the first sign of disease and again one week later. Repeat if disease reappears.
Downy Mildew is less likely to be troublesome although Antirrhinum, Sweet Pea, Poppy and Wallflower are often affected in damp weather. The upper leaf surface shows yellow or dull patches, greyish mould growth occurs below. Plants are crippled by a severe attack. Spray with a fungicide at the first sign of disease and repeat at fourteen day intervals.