Indexaloe mite aphids cultural problems diseases fungal diseases hygiene inspection mealy bugs mice over watering pests poor light precautions quarantine red spider mite root mealy bugs scale insects sciarid fly scorch and heat snails under watering vine weevil whitefly
Pests & diseases
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When new succulent plants are aquired, 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. A good way to do this is to maintain a "quarantine" window ledge separate from other plants. Many growers repot newly aquired 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.
A regular check on the condition of your succulent 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.
Cleanliness in the greenhouse is an important measure in preventing outbreaks of pests and diseases. Always remove dead leaves and flowers as soon as possible. Some growers like to remove flower stalks of e.g. Adromischus before they flower and drip nectar on the plants, potentiating moulds. 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. Tidy up debris in the green house left from re-potting and propagation. Treatment of the ground under the staging, walkways and areas of paving with a disinfectant solution e.g. Jeyes Fluid is a 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 may be toxic to people and absorbed through the skin in the same way.
Dimethoate is an effective ingredient of a systemic insecticide if you can find it, but unfortunately it has generally been withdrawn from the non-professional market in EC countries.
The best systemic insecticides currently available are based on Imidacloprid. In the UK this is marketed as "Provado Vine Weevil Killer," but is effective against a wide range of insects including mealy bugs. In the USA Bayer market a formulation containing Imidacloprid. Plants should be watered with this insecticide during the growing season. The substance is taken up into the plant which then becomes toxic to insects. A single treatment lasts for several months. Imidacloprid is not very effective against red spider. It is relatively low in toxicicity to most animals other than insects and application without spraying minimises contact. It is unlikely that bees would be harmed by use of Imidacloprid indoors.
Contact insecticides such as Malathion can also be effective, but only at the time of application and all parts of the plant must be covered. Unfortunately, Malathion and its derivastives are 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 litre), 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 Imidacloprid or 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.
Biological controls are available for some pests but are incompatible with insecticides. Use one or the other. It is difficult to obtain a predator/prey balance that allows long-term protection in a small collection, and I would question the efficacy of biological controls in a small glasshouse, but it may be worth experimenting with these if you dislike using insecticides.
Above: Mealy bugs on an
Opuntia Pad in Madeira.
Photo: J. Ridler.
Above: Mealy bug on a Cycad leaf.
Palm House, Kew. Photo: RJ Hodgkiss
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 of Mesembs such as Lithops
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. Small numbers can be removed carefully with a pin, but it is hard to spot them all.
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. Watering with Imidacloprid (Provado Vine Weevil Killer) seems to be very effective against mealy bugs and has not so far shown any sign of toxicity to a wide range of succulent plants.
Some fumigant smoke cones are also effective against mealy bugs, and have the advantage of being a dry treatment, but require repeated use to be really effective. Give the cone a good shake before igniting to reduce the risk of poor burning, place on a non-flammable surface and retire promptly after lighting the blue touch-paper fuse, before smoke emission begins. I like to do a preventative fumigation in the Spring and Autumn when it is too cold to spray or water the plants with systemic insecticide, but it is getting hard to find effective types of smoke cone in the UK.
Biological control of mealy bugs: Introduce the predator Cryptolaemus montrouzeri, which requires temperatures of at least 70°F (21°C). It is difficult to obtain a predator/prey balance that allows long-term protection in a small collection.
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 are a better option.
Some Fungal Diseases
Aloe rust is a fungus that causes round brown or black spots on leaves of Aloes and Gasterias. It is of some importance in commercial cultivation of Aloe vera. The black colour is caused by oxidation of phenolic substances in the sap which seals of the affected area. Once formed, the black spots are permanent and can be unsightly, but do not usually spread. Fungi can be discouraged by spraying with a systemic fungicide, but prevention is the best option. Do not allow water to lie on the leaves for long and avoid excess damp in cool weather. Arrange for plenty of air circulation and sunlight.
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 spots on leaves and corky brown marks on stems of are undoubtably 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 which spread and develop into sunken patches of dead tissues. This fungal infection can spread to the whole plant unless the affected part is removed promptly or treated with fungicide. Usually this happens after overal-liberal water, perhaps where water droplets fail to evaporate because of unexpectedly cold conditions.
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 Nimrod T 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 alllowed 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 succulent plant at the wrong time of year when it is dormant can cause rotting as effectively as can also happen if the roots have been eaten by insect pests.
Under-watering: If unsufficient 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: more about light and lighting
Natural sunlight is the best way of illuminating your plants, many of which are native to arid hillsides under scorchingly bright sunlight. Succulent 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 etiolation. 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 succulent plants if there is a sudden period of sunshine after the dark winter days, or even after a prolonged cloudy period during the summer. Sunken brown or white patches develop down one side of a plant where the tissues have effectively been "cooked" and the green chlorophyll destroyed. Sometimes a glasshouse plant loses all its green pigment through excessive heat alone, even though it may not have been in the direct sunlight.
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.
Some species such as Echincactus grusonii which are otherwise easy to grow, can develop unsightly brown marks which spoil a specimen plant, if temperatures are too low.
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