PESTS AND DISEASES OF CACTI AND SUCCULENTS
Although most newcomers to the hobby of growing cacti and succulents regard their plants as exotic, and perhaps feel concern that the pests that attack them will also be unusual in some way, they need have no worries. The majority of pests likely to be encountered are the types to which most pot-plants are susceptible, and there are well-tried remedies to apply.
The location in which the plants are grown will have a bearing on the pests likely to appear, and also on the treatment to be adopted to eradicate them. It is unlikely that a collection housed in a living room will be attacked by snails, slugs or caterpillars but these can wreak havoc among leaf succulents housed in a greenhouse or frame. In the latter situation it is often possible to locate the offenders by eye, and there is a peculiar satisfaction in administering a quick coup de grace to the creature that has destroyed a cherished plant. If visual inspection fails to reveal the pests, the use of conventional garden compounds will usually do the tick. Take care to keep pets and children away from these and all other chemicals used in the garden.
Aphids such as the familiar greenfly and black fly may be attracted to the juicy young flower stems of succulents, but are easily eradicated by spraying with one of the conventional aphid killers. Whitefly is a more persistent pest, but fortunately does not often attack. If it does, use a specific insecticide.
The four pests most likely to be encountered in a succulent plant collection are mealy-bug, root mealy-bug, and spider mite and sciarid fly.
This pest is not so common nowadays as it was some years ago, (someone should tell the ones on my plants this!!) but the grower should be on his guard against it, particularly as it may be introduced on plants brought into the collection. A period of quarantine for all acquisitions is a good policy to adopt.
The mealy-bug is a small sap-sucking insect which looks rather like a miniature wood louse with a waxy white covering. In the early stages of infestation it tends to live a solitary life, quietly extracting its food from the tender tissue near the growing point of the plant.
As the infestation develops nests like tiny pieces of cotton wool may be found, with an insect busily engaged in motherhood out of sight of prying eyes. Eventually large numbers of insects may congregate, particularly at the growing tip of the plant, which is rapidly destroyed by their feeding.
In small collections where it is possible to give each plant a close inspection at frequent intervals, mealy-bugs are easily dealt with by using a fine paintbrush to dab a little methylated spirits onto them. This is by far the safest treatment for the plants grown indoors. If the infestation is heavier, the collection is larger or the affected plants themselves have a waxy coating, which would be marked by the methylated spirits, a systemic insecticide may be used. This is diluted and sprayed onto the plant or watered onto the compost. The plant absorbs the insecticide, which makes its sap poisonous to the mealy-bug. It should be noted however, that using the same insecticide time and time again might allow the pest to build up immunity to it.
Red Spider Mite
This minute creature, all but invisible to the naked eye, is not a spider but a sap-sucking mite. Because of its small size it can build up to plague proportions before coming to the attention of the grower. The pest delights in the warm dry conditions found in a cactus collection, and growers should always be on their guard against it. The presence of the mite is usually betrayed by a browning of the plant body near the growing point, but by then it normally too late to avoid permanent scarring Close inspection with a 1Ox lens may reveal the pest, but it is quite likely that the marauding army will have moved on to lusher pastures.
Since the mite is a sap sucker, the use of systemic insecticide is the best means of defence. It seems to be very resistant to sprayed insecticides. Do ring the changes with the systemic insecticide though, to prevent a build up of immunity.
This, the mushroom fly, has moved into our collections as peat based composts have grown in popularity. The adult fly is tiny and has a weak erratic flight, so tends to be found in close proximity to the plants. The insects even run about on the surface of the compost and will quickly conceal themselves when danger threatens. It thrives in moist conditions, laying eggs at the base of the plant stem. These hatch out to become tiny white larvae, which feed in the compost. Unfortunately they feed not only on decaying humus but also on plant roots. The conditions required for seed germination suit them very well, and an infestation can rapidly destroy a crop of young seedlings.
Spraying with a pyrethum-based insecticide is a safe way of dealing with the adults, but the larvae in the compost are a more difficult proposition. Insecticides containing permethrin or diazinon, which can be mixed with or watered on the compost, are the most suitable line of attack.
In general, fungal attacks are relatively uncommon among succulent plant collections, thanks to the dry conditions in which the plants are grown. There are however occasions when the spores, which are floating in the air, find conditions to their liking and the grower must then act quickly.
Botrytis may attack young seedlings, and in the warm moist conditions essential for germination can rapidly spread. It is a common experience to notice a few sickly seedlings one day, and find the whole batch keeled over within another couple of days. Fortunately this so called damping off disease can be avoided by treating the compost to destroy the spores before sowing seeds, or by watering with a copper based fungicide if it shows up among germinating seedlings.
During the dull days of winter, particularly if the collection is kept at a low temperature, care must be taken when watering. Water trapped among the leaves or steps may remain there long enough to induce rot, and if this occurs the only remedy left is to amputate and re-root. Drips from the roof of a greenhouse, or from overhanging shelves, can also collect on the plants beneath and lead to rotting.
One group of plants in particular, the stapeliads, tends to be susceptible to a fungal disease. This manifests itself as patches of black on the stems, steadily spreading until the whole plant is infected. There is no reliable cure for this complaint: removing and destroying the infected plant to prevent the spread of the disease is a drastic but necessary treatment. The use of a systemic fungicide several times a year may build up resistance within the plant to keep the disease at bay.