Plant Pests and Diseases affecting Dahlia Plants
Dahlias are not troubled by a great many pests and diseases, but there are a few important ones which must be kept in check. I would suggest that spraying commences when the plants are in their early stages of growth, and is continued regularly throughout the growing season, at least once every four weeks. During long spells of wet weather it may be better to apply dusts as these are not washed off the plants so easily as liquids. During reasonable weather, I much prefer to use sprays, as it is easier to obtain a complete coverage of the plant with the material.
Also, be very careful to use really clean pots and to keep the greenhouse scrupulously clean, and in general make sure that the garden and greenhouse are kept clear of rubbish and diseased plant material.
Plant Pests affecting Dahlia Plants
Aphids. The two main types of aphids which attack dahlias are greenfly and blackfly. They can be seen in large groups, chiefly on the younger foliage and tips of the shoots, but also on older leaves, and they suck the sap of the plants. In a bad infestation this will cause the plant to be considerably weakened, and wilting will result. They are also the chief carriers of virus diseases.
They may even attack cuttings and young plants in the greenhouse, so I would suggest fumigating the house to ensure that the young dahlias remain clean. For this purpose BHC smoke generators are suitable, or if preferred BHC may be applied as a spray.
In the garden it will be necessary to spray either once a week or every fortnight with BHC.
Capsid Bugs. This is an insect that we very rarely see unless we look carefully for it, because any slight movement or noise will make it drop to the ground. About 1/4in. in length, it is green in colour, almost of the same shade as theleaves, and therefore very difficult to detect. These insects pierce the stems, young leaves and young buds. If the young leaves have been attacked, small pinholes will start to appear in them as they grow, and in a severe attack the leaves will be completely riddled. In this case it is best to remove the affected leaves. If growing tips are attacked, the resultant growth will be distorted. BHC should be applied regularly throughout the season.
Earwigs. These familiar insects are slightly more difficult to control than capsids, as they hide in some awkward places, such as hollow bamboo canes which may be used to support the plants. Damage is done to the dahlia leaves, and if the earwigs eat the growing shoots and young buds then subsequent growth will be distorted.
One way to help prevent these pests infesting the plants is to seal the tops of canes with putty, so depriving them of a hiding place. Earwigs can be trapped by placing upturned flower pots filled with hay on the tops of the stakes. The traps must be checked often, and the earwigs can be shaken out of these into paraffin, or killed by treading on them. Earwigs are more likely to attack if tubers were planted, as these will have a portion of the old stem attached which makes an ideal hiding place for them. Dust the young shoots with gamma-BHC to control them. Regular spraying with BM should also effect control.
Eelworms. These microscopic creatures sometimes attack dahlias and cause the leaves and shoots to become distorted and discoloured — seemingly for no apparent reason. It is not possible to give dahlias hot-water treatment as you can with, and the only thing to do is to dig up the affected plants and burn them.
Slugs and Snails. The underground slug plays more havoc than the one that lives above ground, as it eats and tunnels its way into the dahlia tubers. A bad attack will result in a severe check to plant growth and if the feeding roots are damaged then the plant will certainly die.
As soon as the dahlias are planted it is advisable to water thearound them with a liquid slug killer, which contains metaldehyde. This must be kept up at regular intervals throughout the season. For slugs that live above ground, metaldehyde slug pellets placed around the base of the plants will help to prevent attacks.
Snails can do a lot of damage by eating leaves and growths and these pests crawl up into the plant. They may be picked off and destroyed; alternatively, metaldehyde slug baits or pellets placed around the plants will keep them at bay.
Plant Diseases affecting Dahlia Plants
Botrytis. For description and control measures see Garden Diseases affecting Bulbs, Corms and Tubers
Mildew. This disease is more prevalent in some parts of the country than it is in others — usually in high-rainfall areas. In a wet season it may cause trouble. It can be seen as white patches on the foliage, stems and buds, and may be controlled by spraying weekly or fortnightly with a fungicide based on dinocap.
Smut. For description of Dahlia Smut and its control see Garden Diseases affecting Bulbs, Corms and Tubers
Viruses. There are two main virus diseases which affect dahlias — Dahlia Mosaic and the Tomato Spotted-wilt Virus. The symptoms are more or less identical, and it is almost impossible, except by laboratory tests, to differentiate between them. The first signs of infection are stunting or dwarfing of the plants. In the early stages yellow areas can be seen spreading from the main leaf veins, and the veins may also appear yellow — this is known as ‘vein banding’. Gradually the leaves become spotted or mottled with yellow, and may be contorted.
If these symptoms are seen on cuttings, started tubers, young or large plants, then they should be pulled up and burned. Another important point is to keep the plants free from aphids and other sucking insects, because they can transfer viruses from plant to plant.