Cactus pests and diseases

Presence of mealy bugs (Diaspis echinocacti) on a cactus

Once in a while cactus collections can present problems related to pests, diseases or inadequate growing conditions, which can be avoided taking the suitable precautions.
This problems are described next, along with some ideas to fight against them. It is important to read carefully the prospect of any product used, following the instructions of each manufacturer and to avoid the contact with pesticides and other chemicals. We must remember that pesticides, fungicides and other chemicals are toxic for people and must be handled with precaution. When applying these products it is recommended the use of rubber gloves, masks and goggles.

Pest and disease prevention

Quarantine
Whenever we acquire new plants, it is a good idea to keep them separated from the rest of the collection for a few weeks, so that possible pests can be spotted. This gives time to eggs of the insects to hatch and allow us to get rid of them preventing this insects from infecting other plants. If the plant came in a pot, many collectors discard the soil and replace it by its favorite. This one is a good occasion to examine the conditions of the roots and for looking for pests like root mealy bug.
Much people treat the new plant with systemic pesticide when repotting. This will avoid to introduce pests to the collection. Potting in sterilized soil by heat, is also a good idea.

Inspection
A regular revision to the condition of the plants, perhaps while they are watered, help to identify early signs of pests and diseases, and the soon treatment will avoid a serious damage to the plants. In any case, never we must assume that only one plant in the collection is affected. Pest or diseases can propagate to other near specimens even when we can't see them.
Symptoms like lack of growth or a sudden change in the state of a plant that seems not to assimilate the water correctly, can be a sign of damage in the roots caused, by example, by root mealy bug or root rotting due to over watering.

Hygiene
Cleaning plays an important role in the prevention of pests and diseases. The rests of leaves or dry flowers in the soil constitute an ideal hiding place for pests. If they are soaked by watering, these elements will transform into the ideal growing environment for fungi and the later production of their spores.
The regular cleaning of the place where the plants are growed, with disinfectants solutions helps to avoid the appearance of pests and diseases.

Pests

Most of pests can be controlled by using contact or systemic pesticides or soaps. The systemic pesticides are very effective since being absorbed by the plant, they make this sap poisonous for pests. Nevertheless also they are toxic for people since they are absorbed by the skin in the same way. Dimetoate is an effective ingredient in a systemic pesticide, but it is highly toxic for the people reason why it must be handled with care, following the instructions of the manufacturer. Also, its use is forbidden in some countries, so you must check your country regulations first.
The plants can be watered twice a year with systemic pesticide (at the beginning and at the end of the growing cycle, by example), like a preventive measure.
The contact pesticides can also be effective, but only at the time of the application and only if all the affected parts are covered.
Some collectors assure that pulverizing detergent diluted in water (1 or 2 drops by liter) it is also effective and harmless to people besides.
It is important to mention that the repeated use of insecticide can produce resistant insects which will be immune to them (evolution in action!). This can be avoided providing complete treatments, so there are not left any survivors. Using more than one pesticide in rotation also helps to prevent this.

Opuntia specimen affected by mealy bugs

Mealy bugs
There are many mealy bug species, but these insects are small and difficult to identify.
Generally they are of gray or clear brown color and they look like tiny cockroaches of between 2 and 3 mm.
Mealy bugs often group to feed on tender tissues or near the growth point.
Quite often, when they nest, they hide near the base of the plants, right under the level of the ground. The first sign of this problem is the appearance of small cotton white ball shaped formations on the plant or near the edge of pots. These small balls are the place where females nest and produce their young, which can be born alive or from eggs.
If the presence of mealy bugs is small, pulverizing the plant with a little industrial or denaturalized (diluted in 3 parts of water) alcohol, will do the trick. We must remember that the emanations can be toxic and flammable, and the liquid can damage the epidermis of the delicate plants.
For bigger infections we must use pesticides. Treat the plant with a systemic or contact pesticide. Please follow the instructions and take the precautions indicated in the prospect.

Enhanced view of Rhizoecus cacticans (root mealy bug)

Root mealy bug
It can be found in the roots of infected plants where they produce a considerable damage. This can lead to the rotting of the plant since the damage allows that fungi or bacterial infections penetrate the plant.
Symptom of its presence is the appearance of spongy white deposits (or white dust) in the soil and some times under the pot. They look like tiny cockroaches (2-3 mm. long) of pinky brown color. They can be on the roots and soil.
Regular applications of pesticides provided with waterings will help to control them. Like a preventive, some growers add to the soil mix grounded moth balls. This apparently helps to avoid other infections also. Nevertheless, the chemicals presents in mothballs can cause damage plastic pots and it is better to use them in clay pots. It is important to mention that, in some countries, chemicals must be used only according to manufacturers instructions, and any other kind of use could be illegal.

Red spider mites
They are tiny and a magnifying glass is needed to see them clearly. A symptom of its presence is the appearance of brown or reddish points where the epidermis of the plant has been damaged. These harmful insects do not have to be confused with a bigger spider (2-3 mm) harmless to the plants.
This small spiders appear when the climate is very dry and warm. Since they hate the humidity, vaporizing the plants with water will discourage these insects from invading the plant. Also a great number of pesticides fights them.

Diseases

Sometimes cactus may be affected by bacterial or fungal diseases. Airborne fungi spores are waiting the ideal conditions for germination all the time. Fortunately fungi do not affect generally cactus collections due to the relatively dry growing conditions which are used in general. The excessive humidity is ideal for the activation of fungi spores, and many of the problems with fungal infections in cactus occur because excessive watering, or the non evaporation of the water due to sudden or seasonal cold climate. The damage produced by insects which penetrate epidermis of the plant to feed themselves with their sap, can make easy the access of fungal infections to internal tissues. Therefore, the unexpected collapse of a plant is often the final symptom of an infection by root mealy bugs, not taken care of on time. There are also fungi provided with mechanisms to penetrate the epidermis of the plants.
The young seedlings are specially susceptible to fungi attack in the low part of the stem, which causes damping off. Once the seedling has wilted, usually it is too late to save it and a preventive measure is a better option.

Some fungal diseases

Espostoa lanata specimens affected by fusarium rot

Fusarium rot
Several cryptogamic diseases caused by Fusarium sp. fungi are known this way. The penetration generally occurs by the roots, reaching the sap vases, to invade all the plant. Also a smooth and black rotting in the apical zone could appear. Sometimes black or dark brown with a yellow edge could appear. It constitutes slow propagation necrosis. In some cases, young plants can decay in 2 or 3 days.

Neck rot
Cold and/or excessive humidity in the soil, can often lead to the rotting of the stems at the level of the ground, where the soaked soil has been in prolonged contact with the stem of the plant. The rotted tissues can become reddish black or brown, depending on the plant and the organism that is attacking it. If the stem is cutted over the affected part, it can be possible to root or graft the cutting to save the plant.
It is useful to put on the top of the soil a layer of gravel, heavy sand, etc., which will help to avoid excess of humidity in this one delicate zone, besides of providing additional support for the plant. The presence of brown, gray spots or brown spots with cork texture in the plants, is a sign of the presence of fungi due to the prolonged contact with drops of water. Others can reflect bad growing conditions or the natural development of some stems as the plant matures. In many cases, the presence of fungi and bad growing conditions are closely related. Improving ventilation, control of temperature and watering will help to prevent all kind of problems.

Fungal disease control
In the worse scenario, once the plant has collapsed or the stems have rotted, it is often too late to save the plant. Nevertheless, as an extreme measure, it is possible to try to save part of the plant cutting the infected tissues with a sterilized blade. A good portion of apparently healthy tissue must also be removed since the infection can be extended far more from which is apparent. The rescued tissues can be treated by immersion of its base during some minutes in systemic fungicide. Damping off on seed trays or Boytris produces decoloration and viscosity in tissues. It is important to mention that temperatures over 20º C make difficult their propagation. It can be avoided slightly vaporizing the soil with a systemic fungicide. Any infected seedling must be removed to avoid the production of spores.

Growing problems

Next to the pests and diseases, bad growing conditions of cactus are the cause of poor growth and loss of plants. The most common problem is over watering, being the roots in contact with the water during long periods, can have as a result root rot. Other problems of growth derive from poor lighting and too low or too high temperatures. The lack of flowering in mature cactus is a clear symptom of incorrect growing conditions.

Over watering
Probably the most common cause for the plants do not prosper. The plant seems to be well in the beginning and we observed new growth. Nevertheless, the roots can be suffering in the soaked soil and start to rot without we notice it. If these conditions continue, a point is reached in which the roots have been deteriorated in a degree so that they cannot provide sufficient water to the rest of plant and this one starts to look as if it needed watering. If we provide more water, the situation will be worst and the damage will be bigger. Possibly the body of the plant looks soft and decolorated, perhaps yellowish or grayish. At this point is generally too late to save the plant. The advice is: if the plant seems not to assimilate water when enough water have been provided, we would have to remove it from its pot and examine the condition of the roots before continuing watering. Other reasons for loss of roots can be damage by pests or dormancy. Watering the plant at the wrong time of the year can cause as much damage to the plant as if the roots have been eaten by insects.

Under watering
If we not provide enough waterings when cactus are in active growth, plants will stop growing and apical meristem (growth point) of the stems could die. The plants can shrink and be half buried in the soil and possibly take a reddish or purple shade due to the production of pigments by stress. In some cases, the shrinking of a cactus during a period of prolonged drought produces a fold in the body of the plant that never will fill again. Little amount of water to the plants that have undergone these conditions must be provided at first, in case that some roots are lost.

Etiolated Rebutia specimens

Poor lighting
Plants growing under insufficient light, often present thin, elongated stems. This is known as etiolation. The cactus become soft, extended and with weak spination. This condition can be reverted offering to the plant intense lighting, but elongated growth will stay as a remainder of the change in the growing conditions. Cactus will not etiolate in dark conditions if you keep them cold and dry. Some collectors find this useful so their cactus enter dormancy and then store them during winter in positions not so cold.

Burn and damage by heat
Cactus could be burned if there is an sudden period of sun after the dark days of winter, or after dark periods during summer. Sunk patches of brown or white color appear in the part of the plant where tissues have been burned due to the destruction of chlorophyll. Some times a plant loses all its green pigment due to excessive heat, even when it has not been exposed to a strong insolation. The burns can be avoided adding mesh shading to the conservatory and improving ventilation and circulation of air. If we want to move plants that have been in the interior to direct sun, we must make it gradually during some days to allow them become acclimated.

Cold damage
Even when many cactus are very cold resistant if you keep them dry during winter, some tropical climate species can suffer damages in the soft tissues in their point of growth. The only solution is to keep temperatures a little higher for the sensitive species.