Dahlia Disease Guide

Insect Pests:

There are a large number of insect pests associated with dahlias, the largest of these insect groups is the APHID group, comprising thousands of different species of sap sucking and predatory insects. It would be impossible to go into all the different species here so for simplicity we will concentrate on the main common types that a grower of dahlias will come across. In the Aphids the 3 main types are Greenfly, Blackfly and Whitefly.

Greenfly, Aphids & Whitefly

The most common group of aphids generally green in colour usually found at the top of the plant close to buds on the softer parts of the plant. Size approximately 1 mm in length six legged flightless insect. Causes damage from sucking the plant’s sap and producing sugary excrement that coats the leaves and stem providing ideal conditions for sooty mould to gain a hold. Also a source of transmission of plant viruses.

Any treatment for control of greenfly should be backed up with a fungicide treatment to prevent secondary infection of sooty mould.

Active chemicals (always read manufacturers instructions)
Sybol (Primiphos methyl)
Intercept/Provado (Imiadracloroprid)
Xl All (5% nicotine/soap)
Nicotine Shreds (90% nicotine smokes)
Malathon/sugar (Malathon with added sugar syrup at 1pint/1ounce sugar/gallon spray solution)
Bilogics (Biological controls)
Fingers (Squash them)

Spider Mites.

The two spotted spider mite is prevalent in hot dry conditions in the UK, Prevention is easier than control. Although there are some very effective miticides available to control this pest. To prevent a build up of this pest it is important to keep growing plants humid, a spay of water under the leaves should prevent the pest somewhat.

Active chemicals

Dynamec, very expensive but a little goes a very long way (our dilution rate is 15,000 :1) may be available from your local nursery, you will have to ask nicely! This chemical is highly effective against mites with 100% kill rate of adults and immatures. The chemical is highly potent even at extreme dilution rates but must be used with extreme care!

Nicotine, This is a poison and may only be available in highly dilute form mixed with soap.

other active chemicals against mite, Dicofol, Fenpyroximate, Aldicarb, Ambamectin, Majestyk.


These are the tiny brown specs of dust that fun all over the light coloured flowers, usualy seen on the lighter flowers because the barker colours mask them. By tiny in size we are talking about an eighth the size of a pin head! color is light brown. An easy way to check for presence of thrip is to take a piece of white paper to the open bloom on a warm day and place the paper under the flower and tap the bloom to dislodge the thrips. if you see brown dust scuttling all over the paper the you’ve got thrips.

Active chemicals,

Decis, Dichlorovos, Majestyk, Ambamectin, Conserve.

Wasps and Bees

Have you ever seen the damage wasps can cause to the stem of a dahlia plant? they strip off the outer cuticle of the stem to get at the sugar laden sap the plant produces. They are a difficult pest to deal with and care should be taken when tackling them, they Sting!. It is difficult to recommend one chemical or otherwise that will get rid of them as they are very resistant to many of the chemicals listed. The best solution i can come up with is to trap them in sugar solution mixed with insecticide and placed in a bottle fixed to a cane in amongst the dahlias.

Known active chemicals, Dichlorovos (lethal to honey bees), Sybol 2, Majestyc.

Wire Worm

A small burrowing grub that eats dahlia tubers from the inside out, prevalent if potatoes have been grown on the dahlia beds in previous years. Also to be found in recently incorporated turf when preparing new ground from grassland.

Controls. Sterilisation of growing beds. Steam sterilisation or application of Dazomet.


Pretty self explanatory these pests will devour laves and leave plants looking like the proverbial net curtains.

Controls; Decis

Fungal Diseases of Dahlias

Powdery Mildew

Visible on the surface of the underside of the leaf with little or no yellowing of the corresponding surface.
May cause a fine black speckling on leaves and stems giving colonies a grey appearance.
Infection is favoured in warm and humid conditions

Controls: copper amonium carbonate (Fungex), Karathane, Dorado, Carbendazin


Visible as grey brown spots attacking leaf margins and as a grey mould attacking stems. Usually seen
as an attack on damaged plant tissue it can affect undamaged tisues when in contact with infected material.

Sourced from dead or dying plant debris.

Controls:Benomyl, Propiconizole(tilt), Thiram


An infection of dahlia stems, initial symptom is a wilting of stems where closer inspection shows a grey
mould eating the lower part of the stem. The stem will have black fruiting bodies within it of a similar size to
vermin  faeces.

Sources often airborne from brascicas or oilseed rape, Burn affected stems

Controls; Propiconizole, Filex

Dahlia Smut

Initial symptoms are a pale spotting on the lower leaves of the plant, this enlarges and the symptoms move

up the leaves to higher leaves on the plant. Eventually holes appear in the centre of the spots leading to
necrosis and leaf die back.

The disease is prevalent in moist humid conditions and is spread by water splash and contact.

Controls; Bordeaux mixture, Copper ammonium sulphate or any copper based fungicide, Fosetyl aluminium, Propicanizole.

Viral Disorders

Dahlia Mosaic Virus