While the cactus grower is fortunate in not having many pests and diseases to deal with in his collection, there are several which can do considerable damage if they are not checked early. Constant cleanliness is the best way to prevent these outbursts.

Watch plants that are not healthy, and if parasites cannot be detected on the plant body, then uproot it. Check the roots to see if they are healthy and growing well. If they are rotted or broken, they should be cut off cleanly, and the plant re-rooted. Or there might be mealy-bugs on the roots. Insects may be the cause of a weak-growing plant. It is very important to watch your plants, and keep them healthy and vigorous, and never too crowded. It is much easier to prevent infestation than to cure it. Window collections are ideal places for insects, and it is therefore natural that pests increase and multiply to an amazing extent when no control measures are adopted. In nature, these pests are kept within bounds by their natural enemies, but in confined areas, this is not possible. In these close conditions, the pests multiply rapidly so that a plant is thickly covered and quickly drained of nourishment.

Mealy Bug
The commonest of all cactus pests which can cause serious damage to young plants and to new growth. The insects are small, and covered with a fuzzy white secretion. They are found usually in stems, and in the new woolly growth. They are not active in the cold weather, but they come to life with sudden, renewed activity in spring. They excrete a honey dew, similar to aphids, and this is eagerly sought by the ants. Ants also carry mealy bugs from plant to plant.

TREATMENT: Individual mealy bugs can be picked off with tweezers. Whilst this may save an individual plant from immediate damage it is however best to follow up by spraying the whole collection on the assumption that for every insect found, there may be others. Such sprays as systemic insecticides are good for large collections since they are taken up by the roots of the plants into the sap and are therefore ready to kill any sap sucking insects as soon as they start feeding. It is no use applying them except at a time when the plants are growing and therefore can absorb them. The mealy bugs will die, but it may be some time afterwards before they drop off. It is necessary to repeat the insecticide periodically. There are some plants which are more susceptible to mealy bugs than others �these usually are the more woolly types.

Root Mealy Bug
Closely related to the above and equally if not more dangerous. It is very difficult to eradicate and is especially dangerous during the winter months. The actual bug is seldom seen above ground, so if a plant looks particularly unhealthy in early spring, knock the plant out of the pot and have a look at its root system. If the roots are covered in a whitish grey mass, then you know that you have got root mealy bug.

TREATMENT: Knock all the soil off the roots of the plant and burn it, and thoroughly wash the pot itself �if it is a clay pot destroy it. Keep it away from other pots and from compost until it has been sterilized. Trim the roots of the plant, cutting right back to the base of the plant if necessary �be thorough �burn the discarded roots, etc., also. Pay particular attention to the root crown and collar. Then either treat the plant as a cutting if all the roots have been removed or then repot it carefully in fresh potting compost and do not water for a few days. Always check new plants brought into the collection for mealy bug and root mealy bug as you can quite easily bring them in this way and once you have got them they can spread rapidly.

Scale
Scale is a troublesome insect which may be difficult to eradicate once it gets a foothold in the collection. It spreads quickly and Opuntias seem specially liable to its attacks. The insect punctures the plant and sucks the sap out.

TREATMENT: Can be removed by means of a stiff brush  or pointed stick. But you must make sure that all the scales are properly removed, as the insects will not die until the round, scale-like covering has been removed. Where the infestation is too severe to deal with in this manner, then painting with a nicotine spray with "Cleansol" is recommended. (A wetting agent is needed since the scales are greasy and water repellant.)

Red Spider Mite
Often found in collections where the composts are too dry. It is difficult to see it with the naked eye. It attacks the skin of cacti, leaving yellowish-white spots which scab up and leave scars. Treatment is by spraying with a nicotine spray.

Ants
Ants are carriers for mealy bugs. They also block up drainage holes and build in the composts. They can best be dealt with using a good killing bait or spraying with a 2% Chlordane. A collection free from these pests is the mark of a grower of clean plants.

Slugs and snails
They are very partial to young cacti, especially the new shoots of many Mammillarias, Cerei and Opuntias. At the first sign of them put small piles of slug bait at intervals on your staging near the snail tracks.

Rot or fungus disease
A black rot sometimes appears near the base of the stems of cacti. This is caused by infection entering through a skin break, for it is here that faults due to bad watering are most likely to occur. Roots which are not properly trimmed, or a carelessly removed offset or cutting are some of the causes of rotting. It is a good idea therefore in winter to check the collection as the fungus spreads quickly and valuable plants may quickly be lost. If you do find a soft spot starting, scrape it out carefully until all the soft material is removed, then dust it with a little fine sulphur. It also may be a good idea to repot the plant and withhold watering for a while until the wound has scarred over. Well grown plants are sometimes strong enough to form a skin within themselves and in such a way seal off the affected area. Often the whole root stock is destroyed and the plant must be cut back until there is no discoloration visible at all. The remainder of the plant may be treated as a cutting or, if the remaining piece is too small, it can be grafted.

Water dripping from leaks in the roof is another way rot will set up. Such hairy plants as Cephatocereus senilis; Oreocereus and hairy Mammillarias are susceptible to rotting.

Diseases mostly occur in plants which are not properly looked after. In any case, a cactus is a remarkably hardy plant.