Plant Pests and Diseases affecting Chrysanthemum Plants
Plant Pests and Diseases
Chrysanthemums are not prone to a great many plant pests and diseases, and in some seasons, in fact, they are hardly troubled at all. However, it is wise to keep an eye open for the following pests and diseases and to take early steps to prevent them from becoming established.
Plant Pests affecting Chrysanthemum Plants
Aphids. I would class these as very troublesome pests of, so precautions must be taken even when the plants are in the cutting stage. If they are noticed in the greenhouse then fumigate with BHC smokes. Outside I would recommend spraying about every 10 to 14 days with gamma-BHC or nicotine. Particular attention should be paid to the undersides of the leaves and the growing tips.
Capsid Bugs. These may also prove troublesome on chrysanthemum plants. A spraying every 10 to 14 days with gamma-BHC or DDT should effect control.
Earwigs. I have dealt with these pests under dahlias (see Plant Pests and Diseases affecting Dahlia Plants) and the same methods of control are advocated for chrysanthemums. As an alternative to dusting or spraying with BHC, nicotine or DDT will give satisfactory control. Eelworm. This is another serious pest of chrysanthemums and can probably be classed as one of the most injurious of them all. It is a microscopic, colourless, slender worm which generally lives within the tissues of the leaves. A single leaf may contain up to 15,000 eelworms. They are most troublesome in rainy seasons, as the eel-worms then travel from leaf to leaf in the surface film of moisture. Therefore, when the plants are wet, the spread of these pests is most rapid. Heavy mists in late summer and autumn also provide ideal conditions for eelworm movement.
At the end of the flowering season the eelworms will remain in the stools, and if cuttings are taken from infested stools then these also will contain the pest. Unless stools are given warm-water treatment, immersion in water at 47.2°C (115° F) for 5 minutes, the pest will be carried over from one season to the next.
The first signs of attack are yellow-green or purplish triangular blotches between the leaf veins. These blotches later turn brown or black and eventually cover the whole leaf which dies and shrivels up. These symptoms start at the base of the plant and gradually extend upwards until the whole plant is affected. It will be noticed that there is a gradual transition between healthy and affected leaves as the eelworms progress up the plant. The blooms may also be attacked, in which case they may be deformed. The amateur should not confuse ‘natural die-back’ of the leaves at the base of a plant with an eelworm attack. Die-back normally occurs towards the end of the growing season when some of the basal leaves may turn yellow. In this case, however, there is a sharp transition from yellow leaves to healthy ones.
Leaf-miner. This, I should say, is a very serious pest of chrysanthemums, and, as the name suggests, it infests the foliage. The adult fly lays its eggs in the leaves and when the grubs hatch they start tunnelling inside the leaves, eating the tissues as they go. This has a considerable weakening effect on the plant if a great number of leaves are infested. It is not difficult to tell when the plant is being attacked as silvery coloured, wavy lines appear all over the leaves. Leaf-miner starts early in the year and may even be found on young plants in the greenhouse. Spraying should, therefore, commence early, before any sign of attack is seen, because once the grubs are in the leaves they are difficult to eradicate. The most efficient insecticide I find is gamma-BHC which should be applied every 10 to 14 days.
Slugs and Snails. I do not have much bother with these two pests on my chrysanthemums although I believe that some people have many of their plants severely damaged by them. They are usually more troublesome in a wet season. The control measures I recommend in my post concerning Plant Pests and Diseases affecting Dahlia Plants would be equally suitable for chrysanthemums.
Thrips. These pests do not trouble chrysanthemums a great deal except maybe during hot, dry weather. They probably cause more trouble in a greenhouse than they do outside. The flower buds and blooms are attacked and the pests make them discoloured, together with shrivelling of some of the petals.
A regular spraying with gamma-BHC or DDT will keep them at bay in the open garden, but for the greenhouse I would recommend fumigating with either of these materials.
Plant Diseases affecting Chrysanthemum Plants
Botrytis. This disease will make the stems and leaves of cuttings rot off if they are over-watered and if the greenhouse atmosphere is constantly damp. Avoid watering except when really necessary, and turn the glass of the propagating case at least once a day, preferably twice. When the rooted cuttings are placed on the staging of the greenhouse ventilate the house as much as the weather will allow, and maintain a gentle heat to keep the atmosphere dry.
Botrytis also causes damping of the blooms of the greenhouse chrysanthemums, so to prevent this give as much ventilation as possible and keep the temperature around 10° to 13° C (50° to 55° F) to keep the atmosphere dry.
Powdery Mildew. This disease, although it appears during the summer, is usually more troublesome towards the end of the flowering season of the early chrysanthemums. When the late-flowering varieties are brought into the greenhouse they should be watched very carefully for any signs of. It is normally more severe in a damp season, although plants may still be infested in a hot, dry summer and autumn.
Mildew is seen as a greyish-white powdery substance on the leaves, including the undersides. In most instances it will first of all start on the lower leaves and then gradually work its way up the plant. Every effort should be made to prevent it from starting.
Sulphur may be used, applied either as a wettable formulation or as a dust. Regular applications will be necessary throughout the summer and autumn about every 10 to 14 days. Continue to spray or dust the lates when they are in the greenhouse, up until the time the buds start showing colour. Dinocap, one of the new fungicides can also be used, but some varieties may be damaged and, the manufacturer’s instructions should be consulted before use.
Rust. Generally, this disease does not cause much trouble nowadays although the occasional outbreak may be experienced, therefore it is wise to have some idea of the symptoms. It appears as raised spots of a powdery orange-brown dust on the undersides of the leaves of the plants. These are the reproductive bodies or ‘’ and may be rubbed off on the fingers. A preventive application of sulphur, as a spray or dust, should be made every two or three weeks to be on the safe side.
Virus Diseases. There are a number of virus diseases which affect chrysanthemums, but undoubtedly the most serious one is that known as aspermy. This causes great distortion and dwarfing of the blooms and thereby renders them useless. It also produces a slight mosaic mottling of the leaves. The second most serious virus iswhich makes the leaves mottled or spotted with concentric rings or irregular markings. These two are well-known viruses of and they affect chrysanthemums just as badly.
If any of these symptoms are seen on chrysanthemums then the plants should be destroyed by burning. It is most important to control insect pests, especially aphids, capsid bugs and thrips, as they can transfer a virus from plant to plant, often over considerable distances.
Some chrysanthemum suppliers are applying heat therapy to their stocks of plants, so ensuring that all the plants they sell are as free as possible from virus diseases. This is a highly complex business and beyond the scope of the amateur gardener, but even so it is good to know that the commercial growers are doing their best to supply us with clean plants.
General Hygiene for Chrysanthemum Plants
Apart from spraying regularly against plant pests and diseases, I feel that I should mention one or two other points which are very often overlooked by some gardeners.
First of all, the question of keeping the greenhouse scrupulously clean. Before the late varieties are taken inside, I would recommend scrubbing the glass, woodwork, walls, staging and so on thoroughly with a disinfectant. This will help to check pests, diseases and algae, and will ensure that the house is kept in a generally fresh condition. The house should also be fumigated against pests and diseases. This job can be done again after the greenhouse varieties have finished flowering, so that the house is clean when the cuttings are taken.
Another important point is always to use clean pots, preferably washed in hot water. Always ensure that the greenhouse is kept free of rubbish, dirty pots and so on, as it is this sort of thing which encourages a build-up of plant pests and diseases. The same applies of course to the open garden — the more rubbish and weeds that are allowed to accumulate the more trouble you will have with pests and diseases.