Pests and Diseases Affecting Dahlias Flowers
Aphids (greenfly and blackfly), capsid bugs, caterpillars, earwigs, red spiders, slugs, thrips and wasps are the main pests which attack, while wireworms can damage the roots in the ground and woodlice can damage them while they are in store.
Aphids normally attack the young foliage and cause twisting and distortion, and, like thrips (another sucking type of insect which lives on plant sap), they spread virus diseases from one plant to another.
Capsid bugs, which are also sucking insects, feed on the foliage and flower buds and distort them. A routine spraying every fortnight with B.H.C., malathion or derris will control aphids and capsid bugs, while caterpillars are eradicated by D.D.T. Earwigs attack the leaves and flowers and can be controlled by dusting round the plant with B.H.C. Red spider may be a very serious pest in hot, dry weather, causing the leaves to wither, yellow and fall, and can only be controlled by malathion and derris.
Slugs can cause a lot of trouble to young plants, but are easily kept in check by the use of a pre-prepared metaldehyde and bran bait, or the more persistent pellets which are thrown on the ground at planting time and afterwards. Thrips usually attack late in the season, causing mottling of the leaves and flower petals, and can be controlled in the same way as aphids and capsid bugs. Wasps may do considerable damage to the stems but can be controlled by B.H.C. Or derris dust applied to the nest.
Wireworms and woodlice are controlled by a dusting of B.H.C. In the infected areas.
Apart from certain virus infections, diseases are not as troublesome to dahlias as pests. Spotted wilt is the most serious of these infections. It causes a mottling and spotting of the foliage, occasional black spots on the stem, weak stems and poor flowers. Remove and burn immediately any plants showing some or all of these symptoms, as thrips and aphids may carry the infection to healthy plants.
smut, which usually attacks the plants in cool, damp weather, causes light coloured spots on the leaves, which eventually turn brown. Spraying with colloidal copper or Bordeaux mixture helps to control this trouble.
Crown gall is the least serious disease and causes a-like growth at the base of the stem and the top of the tuber. It is not highly infectious. The best control is to burn all the infected plants.