Pests
and Diseases
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Basics Corking Mealy Bugs Mildew and other fungal diseases Red Spider Mites Root Mealy Bugs |
Root Rot Sciara fly or Fungus gnats Tip Die-Back Tobacco mosaic virus White Fly |
Euphorbias are not only very attractive plants, but they have a big advantage in that in general cultivation is easy and they are less susceptible than many other succulent plants to pests and diseases. They are therefore suitable for greenhouse cultivation or the window-sill.
Whether or not a plant is flourishing is not always apparent, but as you get more experience with euphorbias, you get to know their growing and rest periods and then more easily spot changes which might indicate a problem. A reliable sign is change of colour, in particular pale or deformed sections indicate pest damage or a lack of nutrients. Also lack of growth can point to a problem – the main growing season for most species is spring and autumn, with many plants taking a mid-summer rest. The dropping of flowers usually means too much water, or a poorly drained soil, whereas the drying off of flowers, especially with the Madagascan species, indicates a lack of water.
Corking means a change in the outer layer of skin (epidermis) of the plant. Most commonly this is a brown colour accompanied by a thickening of the tissues. The cause can be strong sunshine on previously shaded plants, but corking is also a natural phenomenon in habitat with aging plants. Although we may think corking is unattractive, it is a plant’s natural reaction and needs no treatment.
Normal corking with the
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Euphorbia excelsa |
Euphorbia mammillaris |
Euphorbia polyacantha |
Mealy bugs are the commonest pest on euphorbias. They are up to 3 mm long, oval, similar to wood-lice but whitish, easily distinguished by the white, cotton-like substance they produce. The adults prefer inaccessible areas and so may be difficult to find unless plants are very carefully examined. In an infestation of E. flanaganii at the shoot tips, the plant reacts by the whole branch drying back and being shed. Infected plants should be isolated to prevent spread of the pest. As First Aid, the cocoons containing oval, yellow, shiny eggs, the small yellowish larvae, and the whitish adults, can be removed from individual plants with a pin. However, since the mealy bugs prefer inaccessible areas, even in the soil around the plant’s neck, generally this only achieves a reduction in numbers.
Treatment is with regular spraying with pyrethrum, nicotine solution or mineral-oil based insecticides. Also regular applications or sprays with soft soap and spirit helps against the drought loving mealies.
Mealy bugs |
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Normal form on |
Long haired form on |
Mildew and other fungal diseases
Mildew appears as a white, powdery film on parts of the plant. The disease can be caused by too high humidity, insufficient ventilation, too little light, plants placed too close together, or low resistance to infection from lack of nutrients. So it follows that the best protection from mildew is improved cultural conditions.
If however mildew strikes, the infected plants must be immediately isolated, and in some cases the infected parts must be removed and destroyed. Regular cleaning of the infected parts can help, otherwise chemicals need to be used. As there is no specific fungicide on the market for mildew on euphorbias, you will need to choose one for mildew on roses or ornamentals.
One disadvantage of a systemic fungicide is that is cannot be used in the rest period when no water is being taken up by the plant. At these times contact fungicides need to be used such as sulphur or sulphur preparations, or sprays. Foot sprays against fungal infections are very effective and harmless to humans.
Some euphorbia species produce abundant nectar in the flowering season. If this is not collected by hover flies or other insects, mould can also develop on it. This does not directly damage the plant as it disappears as the nectar dries, but it can interfere with pollination. Untreated, the mould can recur every year in some plants. It can be dealt with in the same way as mildew. Finally, there are other types of fungi living in the soil which can spread and block air spaces. In this case depotting is the answer, cleaning the roots and repotting in fresh soil. Frequent applications of 1% Chinosol will also work.
Please read carefully the manufacturer’s safety instructions before using any chemicals.
Mildew |
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Monadenium pseudoracemosum |
Euphorbia viguieri |
Euphorbia guiengola |
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Soil fungus |
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View through a draining hole |
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Red spider mites are about 0.5 mm, light brown to reddish in colour. The winter form is terra-cotta. Under a lens, two dark spots can be seen on the back which differentiates it from the 1-2 mm red spider, with which it is often confused due to the similarity of the name. The true red spider is beneficial and eliminates small insects including red spider mites. The mites prefer to feed on young growth. An attack is usually seen by white or yellow mottling on the infected plant. The web, protecting the youngsters, is not so easily seen.
Infected plants should be isolated and the plants repeatedly doused with lukewarm water. The mites can be treated chemically with so-called acaricides. An alternative is biological treatment with a parasitic mite, which feeds on the juvenile stage of the red spider mites. As red spider mites multiply very quickly, the parasites need to be introduced at the first sign of an attack in order to put a stop to the rapid spread.
Spider mites |
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Traces of spider mites |
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Euphorbia hedyotoides |
Euphorbia muirii |
Root mealy bugs look just like normal mealy bugs and produce the same white cottony substance, but feed exclusively on the roots, where they can sometimes remain undetected until repotting takes place. They are smaller than mealy bugs, whitish, covered in white waxy threads, and very mobile, enabling them to spread to other pots. Root mealy bugs can be eliminated by a very thorough washing of the roots, and a final dip of the soil-free root ball in a solution of contact insecticide. Especially some monadeniums seem sensitive to root mealy bugs.
Root mealy bugs |
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Roots of Monadenium stapelioides |
Roots of a Euphorbia squarrosa |
Root rot or wet rot can affect euphorbias if the soil remains too wet for a long time, or if too much water is given too quickly after the root ball has dried out completely in the rest period. Weakened plants can easily succumb to fusarium fungus, leading to root rot. The disease manifests itself with brown discolouration of the plant neck, spreading to the whole plant, the inside of which turns to a soft, colourless jelly.
The only cure for root rot is to cut off the still unaffected parts of the plant, but generally this only works during the early stages of the disease.
An attack of sciara fly can be noticed when the 2-3 mm long black flies come up when the plant is moved. The adults do not damage the plants, but the larvae do. They are around 5 mm long, glassy, legless and with a black head. They are most dangerous in seed trays, as the young plants need constantly moist conditions which are ideal for the larvae.
If there is an outbreak in a seed tray, the best solution is to remove undamaged seedlings, taking care no soil goes with them and replant afresh, for safety watering with a contact insecticide. New egg-laying can be avoided by covering the surface of the soil with a thin layer of sand. The sand dries quickly and discourages the female sciara from laying her eggs in the soil. Adults can be trapped using the yellow sticky strips.
Fungus gnats |
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Trapped by a yellow sticky strip |
The drying back of shoot ends is not a disease, but the plant’s natural reaction to excessive dryness or lack of nutrients. If it occurs, plants should be repotted into a more moisture-conserving soil, or watering should be judiciously increased. In some species, tip die-back is quite normal, E. gariepina for example, or of outer branches, E. tridentata for example.
Kept too dry |
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Euphorbia septentrionalis |
Normal drying back |
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Euphorbia gariepina |
Euphorbia tridentata |
The tobacco mosaic virus (TMV) is a virus that infects plants causing a characteristic mottling and discolouration of the leaves (hence its name). Normally TMV in euphorbias has little effect on the growth of the plant, aside from the typically discoloured or sometimes distorted leaves, and the plants can survive a long time. As the plant sap contains the infectious virus, there is a danger of passing it to other plants, and so infected plants are best removed from the collection.
Tobacco mosic virus |
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Euphorbia viguierei |
Euphorbia bupleurifolia |
This is a name given to various pests which can be found on euphorbias. The adults, which are about 1.5 mm and look like tiny flour-covered moths, and the yellow-green larvae, 0.3-0.7 mm long, prefer the under-side of leaves, the adults usually occupying the upper leaves and the larvae more mature plant parts. They suck plant sap and excrete honey dew which can lead to black mould. In serious infestations the plant can turn yellow and dry back.
Infected plants should be isolated where possible and must be treated. Yellow sticky strips can be used to catch the adults, but it will also be necessary to spray at least 4 times at 5-10 day intervals with pyrethrum, soft soap or insecticide (including systemic). A leaf-shine spray also helps against the larvae. As the eggs are unaffected by these methods, repeat sprays will be necessary.
White fly can also be dealt with biologically with the parasitic wasp, Encarsia formosa.
