Pests and Diseases of Cacti and Succulents

PESTS

Succulents are attacked by the same pests as most greenhouse plants, but these are quite easily dealt with provided excessive infestation is not allowed to occur. Routine individual examination and treatment are best, and all-over spraying or fumigation should only be a last resort.

Ants These may disturb and damage roots, and sometimes introduce greenfly. Control: The best thing is to find the nest and destroy it with boiling water or by pouring in carbon disulphide, or at least find and block their entry into the greenhouse. Otherwise use a proprietary ant-killer or derris dust.

Aphids The ordinary greenfly may attack succulents: a watch should be kept when there is a garden infestation. Control: Spray with dimethoate or malathion.

Mealy bug This is the commonest pest. A relation of the aphid, it is white, woolly and about 2 in. long. Control: Pick off individual insects with a small paint-brush dipped in soapy water. Spray with an insecticide such as dimethoate or malathion. Red spider If plants have white or yellow rnarkings, a webbed appearance, or begin to shrivel, red spider should be suspected. These tiny red mites can only be seen under a lens. Control: Spray with derris, dimethoate or malathion, or fumigate with azobenzene.

Root mealy bug Similar to mealy bug, but infests roots, making white woolly patches. If the plant looks unhealthy, examine the roots. Control: The soil must be shaken off (and destroyed) and the pests removed as much as possible. The roots should then be dipped into dimethoate or malathion solution. As a deterrent, the plants may be watered with a malathion solution, or with a prepared nicotine emulsion, or a few crystals of paradi-chlorbenzene or flakes of naphthalene may be placed among the crocks.

Scale insects Small waxy mounds about 2 in. across are the covering of scale insects, a kind of aphid that settles down in one place. Control: Spray with dimethoate or malathion. It is best to use a paint-brush soaked in the insecticidal solution to remove the scale bodily, or to move each scale with a blunt stick, taking care not to damage the plant.

Slugs and snails Considerable damage can be done by these creatures, and regular inspection should be made. Control: Proprietary meta-and-bran baits.

Thrips Small grey or white marks on plants may be caused by these. Insects, which are very thin, about 2 in. long, and jump. Control: Spray with dimethoate or malathion. Woodlice These pests may attack plants, especially seedlings. Control: Use BHC dust, or attract the pests by placing a scooped-out turnip or potato nearby.

Note: It is important that all insecticides are handled with great care. Follow maker’s instructions at all times., especially when fumigating. Do not spray in full sunlight; early in the day is best.

Dimethoate is a systemic insecticide which passes into the sap of the plants.. Other materials mentioned are contact poisons and remain on the leaf surfaces. Do not spray crassulas or kalanchoes with malathion.

DISEASES

The diseases of succulents are perhaps more difficult to deal with, because their main cause is faulty cultivation.

Damping-off

Seedlings of succulents may suffer from damping-off, when they rot at the base and fall over. Care in watering, adequate ventilation and sterilised compost will help to avert this; as an additional pre-caution, Cheshunt compound should be used on the seed-pans — say every month. Dry rot This is a mysterious disease, in which the plants shrivel and parts become dry and withered. Even if such parts are removed, the rest of the plant usually dies. It is com-monest in the resting period, and the only thing to do is to try to force the plant into growth, slowly, with extra warmth and additional watering.

Frost

Frost damage is superficially similar to soft rot, though in bad cases the result is a slimy mass and the plant is finished. Slightly frosted plants should be thawed out slowly, any damaged part removed, and dusted as suggested for soft rot.

Mildew There is a kind of mildew which attacks cacti, producing the usual greyish film on the surface. It occurs in over-damp, airless. Conditions; the plant can be cleaned up with thiram, dinocap or a colloidal copper spray. Soft rot Rotting, usually at the base, may be due to various causes. The most common is over-watering, especially if associated with a heavy soil or bad drainage. Rotting at the top of a plant is often due to water lodging there. Rot may also follow soft growth caused by unbalanced feeding, or by fermenting material in the soil such as partly decayed manure. Excessive salt in the soil or an excess of any single chemical may have the same effect. Some plants dislike full sun and may be scorched on a cloudless day; such scorch may become a seat of rot. Scorch may also occasionally follow careless watering, which leaves drops of water on the plants that act as lenses in the sun. Careless removal of cuttings or offsets may result in rot.

All these sorts of decay must be checked by removal of the infected area. In a cactus the flesh is often streaky round the soft area; these streaks must be cut right out. Sometimes the damaged area can be treated by dusting with powdered charcoal or flowers of sulphur; usually the plant is so much mutilated that only a healthy upper part is left. Fortunately most succulents root readily as cuttings, and this is the thing to do with such upper parts. If the root is unaffected, it should be dusted and may sprout afresh.

Corky, brown patches and orange or brown spotting, the latter especially on opuntias, indicates a past severe check, such as a chill, or physical damage.

Starvation

When a plant has been in a pot too long the soil becomes replaced with roots. Starvation and drying out result, and the plant may begin to wither, go yellow and finally rot.’ Turn the plant out, remove the dead roots, cut back the top to healthy tissue and repot.

Infrequent repotting and the starvation which results are probably the most usual causes of bad health among succulents.

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