Pests and diseases of succulent plants
Precautions against pests and diseases
Quarantine
When new plants are acquired, from whatever source, it is a good idea to keep them separate from the rest of the collection for a few weeks so that obvious pests can be spotted. This allows time for eggs of pests to hatch and the progeny dealt with. Many growers repot newly acquired plants into their favourite growing medium, and this is a good occasion to examine the general condition of the roots and check for pests such as root mealy bugs.
It may seem over-cautious, but many people like to treat their
new plant with systemic insecticide while re-potting. This doubtless helps to avoid introduction of new pests into the collection. Repotting with "sterilised" compost which has been heated sufficiently to kill insects, larvae and eggs is a good idea.
Inspection
a regular check on the condition of your plants, perhaps while you are watering them, will help you to spot the early signs of pests and diseases which are best treated early before serious damage is done to the plants, or before they can spread through the collection. However, never assume that just one plant in a collection is affected. Pests may well have spread to other specimens nearby, even if you can not see them. Ants "farm" mealy bugs” for their honeydew secretions and may help to spread them through the collection.
Also watch how well individual plants are growing. Poor growth, a sudden change in condition or a limp plant which fails to take up water can be a warning sign of damaged roots caused by e.g. root mealy bugs, vine weevil, or roots rotting as a result of over-watering.
Hygiene
Cleanliness is an important measure in preventing outbreaks of pests and diseases. Always remove dead leaves and flowers as soon as possible. Leaf litter provides an ideal hiding place for pests. If wet by watering, dead plant material is a breeding ground for fungi and production of their spores. Treatment of the ground under the staging, walkways and areas of paving with a household disinfectant solution, which may be smelly but effective traditional way of discouraging pests and diseases.
General comments on pest control measures
Many common pests can be controlled by use of systemic insecticides, contact insecticides, insecticidal soaps and, in some cases, natural predators. Systemic
Insecticides are very effective as they are absorbed by the plant, making its sap poisonous to the pests. However, they are also toxic to people and absorbed through the skin in the same way. Dimethoate (
Contact insecticides such as Malathion (Endocel, Maltocx) can also be effective, but only at the time of application and all parts of the plant must be covered. Unfortunately, Malathion is toxic to Crassulaceae and some other succulents.
A range of insecticidal soaps are also available, and some people swear by spraying with diluted washing-up liquid (a few drops in a liter), which at least is fairly harmless.
It is worth noting that repeated use of insecticides can select for resistant insects among any survivors (evolution in action !). It is not yet clear whether resistance will develop to the new insecticidal soaps. This can be avoided by ensuring that treatments are as thorough as possible, so there are no survivors and by using more than one insecticide in rotation.
Mealy bugs
A very common pest of cacti and succulents. There are many species of mealy bugs, but these insects are all small and hard to identify by amateur growers. Their host-plant range and individual sensitivity to control measures are poorly characterized. There are probably several species of mealy bug going around collections. From time to time one certainly sees mealy bugs which "look different".
The insects are small and grey or light brown and so difficult to see among the spines of cacti. Their general appearance is reminiscent of tiny woodlice about 2-3 mm long. A squashed mealy bug leaves a characteristic red. Mealy bugs often accumulate to feed on the tender tissues at or near the growing point. Very often, when nesting, they hide around the base of succulent plants, just below soil level or under the old dried leaves.
The first sign of a problem is often small balls of white fluff on the plant, on cactus spines or around the base or under the rim of pots. These are actually where the females are nesting up inside the white fluff and producing young, which may be either born live or produced from eggs. There may also be some sugary honey
dew produced by feeding mealy bugs, which can encourage black mould. Ants "farm" mealy bugs for their honeydew secretions and may help to spread them
through the collection, so it is a good idea to discourage any invading ants even though they are not intrinsically harmful to the plants.
Control of mealy bugs: If there are only numbers of mealy bugs to be dealt with, dabbing a little methylated spirit (industrial alcohol, denatured alcohol) will
kill them. Some people also spray their plants with methylated spirit diluted at least 1:3 with water. If you try this, remember that the fumes are potentially toxic
and flammable and the liquid could harm the epidermis of delicate plants.
For large or widespread infestations, use regular applications (weekly for several weeks) of insecticidal sprays (read the label to find pests controlled, use and precautions). Wash off as many of the mealy bugs as possible with a high pressure water jet from a sprayer, and treat the plant with a contact insecticide such as malathion (not for Crassulaceae) or a systemic insecticide containing dimethoate, taking precautions to keep the insecticide off your skin and avoiding inhaling the spray. Adding a drop of washing-up liquid may help the insecticide to wet waxy surfaces and penetrate into all crevices. A single application will often not be sufficient to eliminate all the insects and their young. In a bad case, total immersion of the plant in a bucket of insecticide with a couple of drops of washing-up liquid to help wetting of the soil, will get the majority of the mealy bugs including root mealy bugs. The plant will need to be carefully dried out after a soaking and is at risk of rotting where damage from mealy bugs has occurred below soil level, so although effective, this can be a "kill or cure" method.
Root mealy bugs
These are also very common pests of cacti and succulents, but are found only on the roots of infested plants where they do considerable damage. This may lead
to the plant rotting where the damage allows fungal or bacterial infections to enter the plant tissues. They produce powdery white or white fluffy deposits in the soil which may sometimes also be seen underneath the pot. Their general appearance is reminiscent of tiny pinkish-brown woodlice (2-3 mm long) in the soil and roots.
Control of root mealy bugs: Use regular applications (weekly for several weeks) of insecticide (read the label to find pests controlled, use and precautions) watered into the soil, or immerse the plant pot up to the top of the soil in a bucket of insecticide with a couple of drops of washing-up liquid to help wetting of the soil. The plant will need to be carefully dried out after a soaking, especially if treatment must be carried out in cold weather.
As a preventative measure, ground up moth balls added to the potting mix seem to discourage infestation by root mealy bug, and probably discourages
other insects. However, the chemicals in the moth balls can cause damage to plastic plant pots and are best used with clay pots. Root mealy bugs also seem
to prefer peat-based mixtures to soil-based composts, although not exclusively.
Red spider mite
The mites are exceedingly small and a strong magnifying glass is needed to see them clearly. An early sign of their presence is the appearance of brown dots where the plant epidermis has been damaged, merging into confluent scarring and sometimes webbing on the plants. The harmful microscopic red spider mites, which damage plants, should not be confused with a commonly seen, much larger red mite 2-3 mm across which is a harmless predator.
Control of red spider mite: The reddish-brown mites thrive in hot dry conditions and dislike humid conditions, so overhead watering and spraying plants may discourage mite attack. They are affected to some extent by insecticides (check labels), but a miticide (Quinalphos) (Ecolux,Quniatox, Ekalux) is really needed to control them properly.
Tiny mites causing abnormal, irregular bumpy growth on all parts of Aloes
Control of Aloe mite: Cut out and burn all diseased parts in the early stages, or the whole plant in the case of a serious infestation.
These insects have the appearance of flat or slightly mounded waxy, brown scales on leaves and stems. The insect under the protective scale feeds on the plant sap and can transmit virus diseases between plants. A sugary honeydew may be produced and encourage black mould. Scale insects do not seem to be common, but appear from time to time, particularly on plants put outside for the summer.
Control of Scale Insects: Use regular applications (weekly for several weeks) of systemic insecticidal sprays like Phosphamidon (Dimecron. Sumidon, Chemidon).
SnailsWhile these are less traditional pests of succulent plants, they can be extremely destructive if they get into a collection. although unlikely to invade the house, beware of carrying them indoors on the sides of a pot or hidden under a plant. Snails find easy access to greenhouses and cold frames via vents, cracks in the structure and doors left open. The fleshy succulents are an obvious target for snails, but they even seem to be able to cope with spiny cacti. Having dealt with the spines, they relish scooping large chunks of tissue out of the plant body.
Control of Snails: Generally, careful inspection will allow the culprits to be
picked off plants and pots by hand. If you dislike killing these handsome creatures, they must be removed to a distance of at least 200 yards or, being
territorial animals, they may return. If hand picking fails, a range of pelleted insecticide like Phorate (Thimet, Foratox, Granutox) available this can be sprinkled between the pots and are very effective.
Sciarid fly
These tiny black flies are also known as mushroom flies or soil midges and lay their eggs in moist soil. The flies are weak fliers and generally cluster around infested plant pots and may rise up if disturbed. The translucent white larvae, up to 1 cm long generally eat dead and decaying vegetable material, but may also continue into living roots and upwards into the stem of succulent plants. The conditions used for seed raising also suit the flies and their larvae which will then destroy the developing seedlings.
Control of sciarid fly: The most effective remedy is to use soil-based mixtures and avoid peat. This also reduces the incidence of root mealy bug. Contact insecticides containing Pyrethrum or its derivatives are effective against the adults but eliminating the larvae is difficult. Mixing insecticides such as Pyrethrum when mixed with the compost may be helpful. Use of DDVP (Nuvan, Chemovan, Vapona) is also recommended.
Vine Weevil
Large gray beetle-like insects seen in the late summer lay eggs in the soil which hatch into fat white larvae with brown heads. These eat the roots and up into the base of stems of plants which then collapse suddenly. The adults nibble leaves and the notches left in the leaves are a warning sign of their presence.
Control of Vine Weevil: When repotting, look out for larvae in the potting mix, and replace all the soil if any are seen. Adding gamma benzene hexachloride (BHC) (avoid inhaling dust) to the potting mix will discourage the larvae. A new insecticide has recently become available for controlling vine weevil for a whole season by mixing with the potting compost.
A common pest of leafy succulents. People growing fuchsias and cabbages will be familiar with these pests. Small (2 mm) white flying insects, a little like tiny moths can be seen flying around infested plants and a cloud of these insects rises if the foliage is disturbed. If the under-side of foliage is examined, the immature non-flying nymph stage will be seen, and these must be eliminated to control whitefly, in addition to the adults. The nymphs secrete a sugary honeydew substance that encourages black mould on plants.
Control of whitefly: Use regular applications (weekly for 4 weeks) of insecticidal
sprays of Chlorpyriphos (CPP, Coroban, Drusban, Ruban). A single application will not be sufficient to eliminate all the development stages of these insects.
The common greenfly and blackfly seen on garden plants may occasionally invade the greenhouse and start a colony on a leafy succulent. The majority of aphids is female and produces a rapid succession of live young. In the autumn, males appear and fertilise the females so that eggs can be produced to survive the winter. As with other sap feeders, aphids produce honeydew which in turn encourages black mould.
Control of Aphids: Spraying with most insecticides like Endosulphan or Monocrotophos (Monocil, Balwan, Chrophos) is usually effective. A repeat treatment may be applied after a few days to eliminate any survivors.
Diseases of succulent plants
A range of fungal and bacterial diseases affect succulent plants, some of which can collapse and die very rapidly, once the disease has taken a hold. The world abounds with fungal spores, which are opportunists, waiting for the correct conditions for germination. Generally, fungi do not affect cactus and succulent plant collections because of the relatively dry conditions used by most growers. Damp conditions are a universal requirement for activation of fungal spores, and many of the problems with fungal infection of succulent plants arise from failure of excessive watering or condensation to evaporate, because of unexpected or seasonal cold weather. Damage from insect pests, which penetrate the plant's epidermis to feed on sap, may provide a route for entry of fungi into the nutrient-rich inner tissues. Hence, unexpected collapse of a plant is often the final symptom of a mealy bug infestation which has gone unnoticed. On the other hand, some fungi provide their own mechanisms for penetrating the epidermis.
Seedlings are especially susceptible to fungal attack of the lower stem which causes damping off. Once the seedling has wilted, it is usually too late to save it and preventative measure is a better option.
Some Fungal Diseases
Black or Sooty Mold: A ubiquitous fungus which is often seen on plants covered with honeydew from whitefly, mealy bugs etc or on plants with nectar-producing glands such as certain Ferocacti. Generally, sooty mould is more unsightly than harmful on otherwise healthy plants. However, it will attack seedlings following mechanical damage or excessively wet conditions and other weak or damaged plants.
Basal Stem Rot: Cold or damp conditions may lead to rotting of stems, often just around the soil level where damp soil may be in prolonged contact with the plants
stem. The rotten tissues may go black or reddish brown depending on the plant and organism attacking it. If the stem is cut well above the rotten part, it may be possible to re-root or graft the healthy tissues and save the plant. Many people support the basal stems of difficult plants with a layer of grit above the potting medium, so that there will be little water retention against the stem in this critical region.
A range of brown or gray spots on leaves and corky brown marks on stems of are undoubtedly due to fungal attack following damage or prolonged contact with drops of water. Others may reflect poor cultural conditions or the natural development of corky or woody stems as the plant matures. In many cases, fungal attack and poor culture are linked. Improving ventilation, temperature control, watering and application of fertiliser may help to prevent all sorts of problems.
Growers of Asclepiads will be familiar with black spots developing on the stem.
Control of Fungal Diseases:
Once a plant has collapsed or the stems have started to become soft and rotten it is often too late to save it. However, an attempt may be made to save part of a valuable plant by cutting away the infected tissues with a clean knife, sterilised with methylated spirits. A wide margin of apparently sound tissue should be removed as the infection will almost certainly have spread further than is apparent. The remainder can be painted or dipped in a systemic fungicide such as Mancozeb, Zineb or Captan or dusted with sulphur and rooted as a cutting or grafted onto a compatible stock.
Botrytis or damping off in seedlings can be avoided by lightly spraying the potting mix with a systemic fungicide such as Benlate or Nimrod T. Spraying with a copper sulfate solution is a traditional remedy, but copper fungicides may accumulate in the soil with potential copper toxicity to plants. Any seedlings that become infected should be removed promptly before more spores are produced, the remaining seedlings sprayed with fungicide and surface moisture allowed to evaporate.
While these notes focus on pests and diseases of cacti and succulents, incorrect cultural conditions are a major cause of poor growth or loss of house plants in general. The single commonest cultural problem is over-watering, with the roots left wet for excessively long periods resulting in rotting. Other growth problems
are related to insufficient light and too low or high a temperature. Most cacti and succulents are expected to flower when they reach the mature size, or even before, and failure to flower may indicate unsatisfactory growing conditions.
Over-watering: is probably the single most common cause of failure of succulent plants to thrive. The plant may appear to do well at first, its leaves plump up and new growth produced. However, the roots may be suffering in wet soil and begin to rot unseen. The plant still looks well as the few remaining roots are able to take up sufficient of the plentiful water. As the roots continue to die in the stagnant soil, a point is reached at which they are unable to supply sufficient water and the plant appears to be suffering from lack of water. If more water is supplied, the situation gets worse and the rot may spread upwards into the basal stems or plant body. Eventually the plant body is observed to be soft and discoloured, perhaps yellow or grayish, by which time it is usually too late to save it. The moral is that if a plant appears to be failing to take up water, knock it out of its pot and examine the condition of the roots before supplying more water.
Other reasons for loss of roots include pest damage and dormancy. Watering a
Under-watering: If insufficient water is provided for the prevailing temperature and stage in the growth cycle, leafy succulents stop growing and may shed their leaves and the apical tip of stems may die. This is followed by die-back or self-pruning of stems and branches. Cacti may shrink back into the potting mixture and possibly take on a reddish or purple hue because of production of coloured stress pigments. In some cases, shrinkage of a cactus during drought produces
Irreversible folds in the plant body which never fill out again. However, careful watering usually reverses the effects of drought on succulent plants. Small amounts of water should be given to water-starved plants at first, in case some of the roots have been lost.
Poor light: Plants kept with insufficient light grow with pale or yellow sometimes stunted leaves and elongated relatively thin stems with long spaces between the leaf joints. This is known as etiolating. Cacti become soft and elongated with weak spination. The condition can generally be reversed by providing stronger light, although elongated growth in cacti will always remain as a record of the change in growing conditions. Succulent plants can often be pruned to restore their shape.
Cacti and some succulents will not usually become etiolated in dark conditions if kept cool and absolutely dry, and some growers allow their plants to become dormant for winter storage.
Scorch and heat damage: Scorch can affect plants if there is a sudden exposure
Scorching can be avoided by the timely application of shading to the greenhouse, improved ventilation and air circulation within the growing area to even out air temperatures. When moving plants into direct sunlight or putting them outside for the summer, harden them off gradually in diffuse sunlight or put them under mesh shading for a few days to acclimatise.
Cold damage: although many cacti and succulents are surprisingly cold-hardy if kept absolutely dry during the winter, some species from perpetually tropical climates (e.g. Madagascar) can suffer damage to the soft tissues at their growing points, and scarring and collapse of their stems leading to fungal attack and death of the tissues. The only solution is to maintain higher temperatures for susceptible plants.
Please remember that all insecticides, fungicides and some other horticultural chemicals are very toxic to people, and handle with extreme caution - rubber gloves, face mask and goggles are advisable.