DAHLIA PESTS AND DISEASES
Cause: Earwigs seem to be compulsively drawn to to dahlia petals
Symptoms: Characteristic ragged holes and bite marks in the petals, usually occurring during the summer months. The typically hide within the flowers during the day, coming out to feed after dark.
Treatment: A popular method is to place traps for them. This can be done by placing scrunched-up paper or dry straw firmly into a flower pot, and placing it at flower height on the top of a cane. Alternatively, try spraying a contact insecticide either late in the evening or just after dark when the earwigs are at their most active.
Cause: Slugs are the number 1 garden pest, and are particularly active in the spring and over wet summers.
Symptoms: Slugs are particularly drawn to new growth creating irregular holes in and along the sides of the leaves. This damage will coincide with characteristic slime trails.
Treatments: There are a number of treatments both chemical and organic.
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Cause: Aphids are a well known pest insect that can quickly colonize the soft tissue parts of your plant. They damage and weaken the plant by sucking the sap out of pressurised parenchyma cells just below the leaf cuticle.
Symptoms: Clusters of these small insects are readily identifiable, normally at the plants tips or on the underside of their leaves. In severe cases, the infected parts can begin to wither due to the quantity of sap being removed from that area. The foliage can become sticky and may show signs of a harmless black mould called sooty mildew.
Treatment: There are many chemical treatments available including a number of organic, but all of these must be applied at the first signs of infection to achieve the best results.
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Cause: Dahlias can be cross infected by the cucumber mosaic virus which is transported from plant to plant by aphids.
Symptoms: This can be identified by a mosaic, yellow leaf mottling, often appearing on severely stunted growth. You may also see browning of the veins.
Treatment: Although its not a serious problem in the garden, the plant should still be lifted and destroyed to prevent further spread of the virus.
FOOT ROT, CROWN ROT, ROOT ROT AND BROWN CORE
.Cause: These rots are caused by a wide range of fungal attacks making exact identification almost impossible. However their action and treatment are all very much the same.
Symptoms: The earliest signs of this fast acting rot are indicated by the production of smaller leaves which turn yellow before wilting. Flowering will be reduced and the plant will be loose in the pot or ground due to its decaying root system.
Treatment: Any infected plants should be removed and destroyed. Sometimes, improvements in drainage, nutrient availability and changes in the soil pH can indirectly improve the situation but there are no effective chemical cures.
GREY MOULD and DOWNY MILDEW
Cause: This group of fungi can affects many plants and unfortunately they are not host specific. This means that they are able to cross infect plants from a variety of different host plant families.
Symptoms: These moulds can easily be identified by a white or grey powdery coating that can appears on the leaves, stems or flowers of your plant. As the infection progresses this coating will spread to envelope the rest of the plant, eventually killing it. It can often appear in the flowers themselves or even earlier causing the buds to shrivel.
Treatment: There are a number of effective chemical treatments that can be applied here, but infected plants will need fortnightly applications during the Summer - particularly during prolonged dry periods. Sulfur is a popular choice, particularly with organic gardeners although it can damage sensitive plants. A more reliable alternative is to use a general systemic insecticide.