Growing cacti is not really difficult and can be quite rewarding. With the sufficient
lighting, the right soil mix, and proper watering, the cacti will do all the work. However,
there are a few problems that can plague your plants and take some of the fun out of growing
This page is designed to help you diagnose your cactus illness, treat the problem, and
learn how to prevent future problems as much as possible.
To help sort through the various cactus sickness that you might encounter, we'll break them into
Under each heading below, you will find a description of the symptoms with images to help you diagnose
your plants problem and information on treating and caring for plants with that specific illness. Finally,
even though this information is specific to cacti, much of it can be applied to other succulent plants
and even some non-succulent plants.
Over-watering Etiolation Desiccation
Sun Burn Frost Damage Pot Bound
Scale Mealybugs Cochineal Insect
Red Spider Mite Slugs & Snails Nematodes
Rodents & Birds Worms/Caterpillars Aphids
Thrips Other Bugs
Corking Physical Damage Phototoxicity
Photo: Gregory Whitney
Succulent plants store water in their tissue. That, by definition, is what
makes it a succulent plant. Cacti are succulents and use the water they store in their tissue to
help them survive long, dry periods. Since cacti as a general rule grow in dry places, they take up
water through their roots very quickly and efficiently. On the other hand, they do not have the
ability to quickly get rid of excess water. While this all works out fine in the wild, when cacti are
grown in cultivation, people can supply more water than the cacti needs. This is actually a common
situation and the plants in cultivation look plump and happy compared to their habitat counterparts.
But the real problems start when people don't just give their cactus more water than it needs, but
more water than it can handle. This is over-watering. When a cactus is over-watered, it will swell up
more and more and often times the cactus stem will become so saturated that it splits open in one or more places.
While a cactus can usually recover from this, the other result of over-watering (especially when combined with
cold temperatures) is rot. Sometimes just the roots will rot off, but often times the whole plant will
turn to mush.
The first thing to do with an over-watered cactus is stop watering it! In most cases,
repotting would also be a good idea. When the plant is unpotted, you can let it stay unpotted to dry
out for a week or two and then repot it. If the plant body has split in any place, the damage is
permanent and there is nothing you can do but let it scar over. If the plant has started to rot, saving
the plant will depend on how far the rot has set in.
Photo: Gloria Labbe
In general, cacti require lots of very bright light to be healthy. This level
of light is extremely difficult to provide for plants grown indoors. South-facing windows (North-facing in
the southern hemisphere) with no obstructions can give a cactus enough light to prevent etiolation, but
even the brightest window in a house still does not capture as much light as the plant would like. Plants
grown in windowsills will also lean towards the light as the inner side will be getting much less light than
the side facing the sun. Artificial lighting doesn't stand a chance against the power of the sun. It is
possible to use HID lights, but these are expensive to purchase and operate and not something many
people would like to have in their living space.
The CactiGuide.com glossary of terms defines etiolation as "Pale, sickly, excessive growth caused by insufficient light,
incorrect feeding, or overcrowding." New cactus growers may not know what normal growth should look like
or may have inherited a plant that is already etiolated. The images here demonstrate typical etiolation on
cactus plants. The growth is often a very light green or yellow-green color. It forms a point almost like
pulled taffy and is usually quite weak and flimsy.
Once a cactus has put on etiolated growth, the effect is permanent. While you can
almost always bring a plant back to full health with a change of conditions, it will be forever misfigured
where the etiolation occurred. To correct etiolation, you need to get your cactus into stronger light. This is
usually more sunlight or sometimes supplementing existing sunlight with artificial lighting. In either case,
it is important not to simply take a plant out of the window sill and stick it out in full sun the next day.
It must be gradually introduced to stronger light to prevent sunburn. Even healthy plants can get sunburn from
a dramatic increase in light; in etiolated plants this could be fatal. If you have a plant with poor lighting,
you can reduce etiolation by keeping it cooler and not watering as often. Ideally, having nights that are around
20 degrees cooler than the day temps would improve the situation for the cacti, but again, it would be less than
ideal for their human roommates. The best option for locations that are too cold to leave cacti out year-round is to grow
outside during the summer and then put the plants in a cool spot (45ºF-50ºF) with no water for the winter.
The cacti in these conditions will sit dormant and can be kept with no light at all. While some species won't
tolerate this, most will do fine or even better with this dormant period.
It is a myth that cacti don't need water. In fact, cacti are very fond of water.
While it is not hard to over-water a cactus, too little water will eventually kill a cactus off. Cacti
store water and keep it and they can wait patiently for a long time between watering and so it is better to
under-water than to over water. As you can see in the picture at left, this cactus plant has desiccated from
lack of water, but once it got water again, it put on new growth. Temperature has a big effect on the rate that
desiccation occurs. The hotter it is, the faster it dries up. Typically, when temperatures are high, cacti want water,
light, and will do their growing under these conditions. When the water runs out (or the heat gets too high), then
the plant will shut down and sit dormant until conditions improve. If conditions don't improve eventually, it will die.
Well the solution that would seem obvious to fix the problem of desiccation, would be to
give your plant a bunch of water. However, it is much better to water only a little bit at first and gradually
increase the watering over many weeks. This is largely due to the fact that desiccated plants will have lost many
of their roots and will only be able to take up a little water at a time. As the roots grow, it can take in more
water. If too much water is given and the roots can't take it all in, the plant will start to rot from sitting in
Photo: Andy Cook
Plants that have been just moved from a shady location to direct
sunlight are likely to get sunburn. Sunburn on a cactus shows the worst damage at the top and side
that is facing the sun. Ridges are more likely to burn than valleys along the cactus stem. Sometimes,
cacti have wide ribs and little spine protection and then sunburn will occur in between the ribs. A
light burn will just be a whitish discoloring on the exposed areas. More severe burns will become
a hard brown scar along the plant in whatever areas did not get shaded by spines or other parts of the stem.
If you catch a sun burned cacti at the whitish discoloring stage, you can undo the
damage by getting it some shade protection. If the plant gets to the brown
scaring stage, then the damage is permanent and the plant will have to grow out of it in time. Again, prevention
is the best defense with sunburn. If you purchase a plant that has been sitting inside a store for some
time, you will need to gradually move it into full sun. Do this by giving it full sun for a short period
of time each day, and then regularly increase the length of time in the full sun over the course of a couple weeks.
Keep in mind that some species do not ever want full sun all day. Also note that "full sun" in England is
much different than "full sun" in Arizona. The stronger the sun where you live, the more careful
you have to be to keep your plants sun burn free.
More info in this article: The Sunburned Cacti
Photo: Tony Marino
Photo: Byron Fan
It might seem to be a statement of the obvious that to have frost damage, one must have frost from freezing
temperatures. However, some cacti are sensitive to cold temps that are not freezing temps while most cacti
will not tolerate even cool temps if they are wet. Even so, there is a difference in the type of
damage from frost as opposed to cool conditions. Cold-sensitive cacti or wet and cold cacti will not
be damaged by the cold itself, but will succumb to other problems as a result of the cool temps. Typically,
wet and cold cacti results in rot. On the other hand, frost damage will actually destroy plant cells
that are exposed to the freezing temps. This is brought about when the ice crystals within the cell cause the
cell wall to rupture. Often, the damage is not immediately apparent, but shows up a day or two later, when the
destroyed cells turn black and either start to rot or dry up.
To the surprise of many, there are many species of cacti that can survive freezing temperatures. While most of
these must be dry and cannot tolerate "hard frosts", there are in fact a handful of species that
grow in the Northern US and into Canada that can take temperatures as low as -35ºF (-37ºC). These
cold-hardy cacti grow low to the ground and in the fall they shrivel up as they lose any excess moisture in their
cells. However, even the toughest of these cacti need a good draining soil that does not allow them
to be excessively wet.
Once a cactus is damaged by frost, there is nothing that can be done to reverse it. Often, the best
thing to do is to cut the damaged area off the cacti to prevent rot in that area. Typically, frost
occurs from the top down and the growing point will be destroyed. When this occurs, the cacti will produce
offsets out of one of the surviving lower areoles. These offsets can be removed and rooted to produce a new
"normal" looking plant.
To prevent frost damage, cactus plants must be protected from the frost in the first place. If they are potted
they can be brought indoors. Depending on the severity of the frost, a simple cover can be used. On a small
cacti, a styrofoam cup can be used to cover them for the night. Patio heaters can sometimes be setup over
an entire collection. Some use an ordinary light bulb in combination with some sort of covering.
Photo: Ian Nartowicz
Plants that are "pot bound" or "root bound" are plants that have grown
in a pot for so long and to such a size that their roots completely fill the entire pot in a tight mass. In time, the
mass of roots becomes so tight that even water has a hard time penetrating it. This of course is one of the main
problems with a pot bound cactus. If you water such a plant the way you would normally water a cactus, it is unlikely
that the plant will have enough water to sustain it. As a result, the plant will start to dessicate or wither from
lack of water. You could, soak the plant in water, but then you would have the opposite problem. Once the root mass
is fully saturated, it will take a long time to dry out and you will likely end up with an overwatered plant! A second
problem with pot bound cacti is that nearly all the nutrients in the soil will be gone by this time. If you put fertilizer
in your water, that would solve the problem except that you are back to the first problem; how to get the water in the root
mass without soaking it.
The solution to this problem is simple - repot it! Repotting is something you should be doing anyway
to check on the roots, making sure they are fine and that they are pest free as well as supplying nutrient rich soil for
the roots to move into in the new pot. When you have a severely pot bound root ball, you will need to break it apart
before repotting. This can be a very difficult task! The best method is to dig in with your bare hands and work at it.
You can use a water hose too to help loosen things up, but mostly you will wear your hands out pulling the roots apart.
You could use a trowel, screwdriver, knife, or similar tool, but then you will damage many of the roots. You will
damage roots with your hands too, but not nearly as much.
Photo: Gregory Whitney
According to Wikipedia there are over 8000 species of scale insects. The most likely species to be found on
cacti are those in the Diaspididae family which is also known as "armoured scale". This name comes from the scale-like
protective covering that these pests hide under. The attachment to the plant tissue is surprisingly strong and when
a scale insect is removed, it leaves a little scar behind where it was attached and sucking the plant juices. One small
scale insect could hardly affect a cactus that has passed the seedling stage. However, scale insects multiply quickly
and can completely cover the surface of a cactus in just a few days. This gives the entire plant a fuzzy or frosted
appearance. Scale seem to prefer shade and as seen in the pictures here, they may occupy every open space on the
shady side of the plant and leave the sunny side untouched. Cacti with dense spination seems to provide sufficient
shade for the scale and they will cover all parts of these plants.
Left untreated, scale will spread to other nearby cactus plants, but they seem to prefer certain species over
others. I am not sure if some species of cacti have skin that is too tough for the scale or if they just don't
taste good. I suspect the former, however, as scale will often be found only on tender new growth and not elsewhere
on a cactus.
The first thing to do with a plant that is infected with scale is to physically remove
the scale from the plant. This can be done using your fingernail or a tweezers, but the most efficient method
is with a spray nozzle on your garden hose. There is a good deal of satisfaction to blasting the little "blood-suckers"
off the infected plant, but be sure that your water jet isn't strong enough to actually damage the cactus. Also take
care to get down in the cracks and under the spines.
Once you have removed them in this way, you can be sure that they make their way back. To prevent repeated outbreaks,
you should treat your plant with a systemic insecticide. This is a poison that the plant will take in through the roots
and into the plant tissue making the plant itself poisonous to the insect. For a systemic to work like this, the cactus
must be growing and taking in water an nutrients at the time. A good practice is to apply the systemic once at
the onset of the growing season. This means you may have to keep an eye on the plant and physically remove the scale
again while you wait for the systemic to take affect.
Mealybugs are the most tenacious of cacti pests. Not only are they a plague to nearly
every grower, but they also affect different parts of the plant. There are mealybugs that affect the plant tissue and
like to hid out in between ribs and tubercles, there are root mealybugs that live under the soil sucking on the roots,
and there are even mealybugs that live on the spines of a cactus and suck the plant juices through the spines.
Aside from being the most common of cacti-eating pests, mealybugs are also quite difficult to get rid of and is
nearly impossible to do so without the use of a systemic pesticide. Contact insecticides will kill mealybugs, but do
have to be added in fairly high concentration due to the protective covering that the insect makes for itself.
Also, contact insecticides are risky due to their oily makeup. This can cause the plant epidermis to burn
in the sunlight. This is called phototoxicity.
For best results, after physically removing the mealybugs with a toothbrush or high pressure water spray, it is best to
treat the plant with a systemic insecticide. This should be applied only when the plant is growing or it will not
be taken into the plant tissues. It is also not a bad idea to treat you cactus at the onset of the growing
season to prevent mealybugs from getting a start.
Since mealybugs affect different areas of the plant, unlike scale and other pests, it is always wise to
check the entire plant when mealybugs are detected. In other words, if you have spine mealybugs, be sure to check
down in the joints of the plant and unpot the plant and inspect the roots.
This pest does not attack all kinds of cacti, but is specialized on the Opuntia genus. This insect attacks Opuntia
and Nopalea species, including Cylindropuntia. Its common name is "cochineal insect". The insect family it belongs
to is Dactylopiidae and the genus is Dactylopius. There are five species in the U.S. The specie D. coccus
Costa has been used in Mexico and still is for the production of carmine red pigments. It has even
been mentioned that the red pigments at times are added to the juice of pomegranates to enhance its color.
The females are red in body color and up to about 3 mm in length. Once they are mature, they are dried and
the red pigment is then extracted. This pigment was commercially important until 1875, when aniline dyes
were introduced. The insect pictured at left is immature, measuring only 1 mm in length, but otherwise
resembles a female.
The first picture shows how the insect looks before I disturbed it by removing the cotton candy like
waxy fibers covering its body. As you can see, it resembles its relative the mealybug a lot.
-Above text by Harald
On infested plants, typically the entire underside of the stems are covered completely with the waxy-white cotton-like
fibres. This material is produced by the insect and helps to protect it as it sucks out the plant juices.
While this insect can certainly hinder the growth of its host plant, it seldom seems to actually kill the plant.
It is rare to have an attack of the cochineal insect on indoor plants. Typically these are found on Opuntia
species that are grown in the ground. Much like the treatment for Mealybugs and Scale, it is
best done in two steps. First by physically removing the pest. This is easiest done with a strong spray of water.
And secondly by treating the plant with a systemic insecticide.
Red Spider Mite
Photo: John Balcom
Photo: Neville Morton
Photo: Neville Morton
Photo: Neville Morton
Red spider mites are not actually spiders, but they are mites. They get that
name because they are red in color and they make a spider-web like silk on whatever plant they are
attacking. The spider mites themselves are extremely small and depending on your eyesight, you might
not be able to even see them. As real spiders often make webs on cacti, don't panic the first time
you notice a silky web. However, it is worth taking a close look at your plant if you do see one.
If the web belongs to spider mites, you will see the plant turning dry-brown especially wherever there
is new growth at the apex of the plant. Spider mites love the fresh new growth and will always attack
the tender parts first. Eventually they will eat the entire outer layer off your cactus which will
kill it if left untreated. After spider mites are killed off, the plant will continue to grow from the
top and as it does the area damaged from the mites will "move down" the plant. The damaged
areas will never look healthy again and you will have to wait for the plant to grow out of it. If you
get a new plant that is dry-brown around the bottom, it is likely one that has survived a spider mite
attack and started to grow out of it. Sometimes, this is just natural corking, however.
If you even suspect spider mite damage on your cacti, quarantine all infested
plants immediately. Treatment with some form is pesticide is the only cure. It is best to look for
a pesticide that specifically states control of spider mites on the label. Multiple treatments will
be required as the eggs will not be killed by the pesticide. Typically, reapplication will need to be
done every week or so depending on temperatures. At higher temperatures, the mites reproduce faster and
more frequent treatment will be necessary. Since most pesticides will create a phototoxic reaction on
the cactus epidermis, it is best to keep your plants out of direct sunlight after treatment for several
Because spider mites are difficult to detect until they have done a lot of damage and because of their small
size, a systemic pesticide is typically not a useful treatment. Too much damage will be done before
the systemic could take effect. Using a systemic as a preventative measure is a good idea, but mites
are not insects and are not controlled as easily with insecticides. Finally, it is also a good idea
to spray the areas surrounding the infected plant and to pay close attention to uninfected plants. The
chances are good that the spider mites have been on the move before you noticed them.
Slugs & Snails
Photo: Darryl -Corona Cactus
Photo: Ian Nartowicz
Photo: Ian Nartowicz
Slugs and snails are soft-bodied mollusks called gastropods. If conditions are optimal, they can exist in
your garden in very large numbers. It is rare to have slugs or snails indoors. Because they are prone to
drying out, they wait until night to come out of hiding. As they move along the ground or on your cacti,
they leave a tell-tale slime trail behind them. In the daytime, this is usually dries out and glitters in
light. Even though these animals are very soft, they can crawl unharmed right across the top of the spines
on your cactus to get to the part they want to eat.
Snails and slugs prefer the softer new growth of cacti and seldom bother with the older growth. On the
cactus stems, they scrape off the top layer of the tissue as they go. This leaves behind a rounded patch
of exposed tissue that quickly scabs over. Flower buds are particularly enjoyed by snails and slugs and they
will almost always eat the flower buds before eating on the stem at all.
If only a plant or two is being munched on by nails or slugs, you can most likely
find them and squish them. On potted plants, lift the pots and check underneath them. Any cool/shady spot
is a good hiding spot and they don't seem to go too far from where they had their last meal. Going out at
night with a flash light, is another way to physically eradicate these pests.
On a larger scale, particularly with plants in the ground, snail/slug bait seems to work best. Using a
saucer of beer may kill some, but is not terribly effective. Snails and slugs are repelled by
copper and could be kept off with strips of it around the base of the plant, but obviously isn't
practical or economical. On the other hand, there are liquid treatments that can be squirted on the ground
in a circle surrounding each plant or pot. Snails will start to cross this and die soon afterwards. As soon
as the sun comes up the slugs and snails will quickly dry up and nothing will be left but their shells.
Snails seem to be seasonal and are only a real problem when conditions are right - warm and moist. Treatment will
need to be continual during these times as neighboring snails will move in to replace the last batch you killed.
Nematodes live below the soil and therefore are not noticed until they've done significant damage. Slowed or reduced growth
above ground, often signifies trouble below. Nematodes are not likely to be found in potted plants, but in those planted
in the ground. As a result, the plant must be dug up to inspect the roots. Digging up an established plant is not desirable, but
if no other symptoms are visible, it is sometimes necessary. As illustrated at left, the presence of the nematodes is apparent
by the knobby tuber-like bumps on the root system. Other images show the roots dissected with nematodes inside. The brownish areas
are plant tissue that has begun to rot from the damage.
The first three images below show a plant parasitic nematode and its feeding tool called a stylet. It is a spear-like
structure that is used by the nematode to penetrate plant cell walls. One image shows the stylet actually
protruding a little from the head of the nematode. The other shows it inside its head area. The other magnified images
show common nematodes that also feeds on plant roots, with the common name "root knot nematode".
Root knot nematodes effectively block the movement of water through the plant and also the uptake of nutrients.
These animals feed on plant roots and cause them to become malformed.
This kind of nematode secretes a chemical with its saliva. This chemical causes the plant to produce abnormal cell
growth. This is known as "giant cells". The nematode thus lives almost in a warehouse and can remain attached
for life feeding as much as it likes. The tumor-like growth of the root system interferes with the movement of both nutrients and
water. That is the reason why plants infested with these eelworms will look unthrifty or die even if they are pampered with water
and fertilizer. When the roots start to rot because of their feeding the plant will slowly succumb too.
The plant parasitic nematodes cause damage of 10 billion $ in the U.S. alone and can be found anywhere on
a plant, from the seed to leaf, to wood and roots.
-Above text by Harald
Nematodes are not insects, so insecticides are not likely to affect them. There are
nematicides but, we have no further info at this time on availability and application. The best treatment is a fumigation of course, but plants in general
will be killed or severely stunted, if for example methyl bromide is used with them still being in the ground. For annuals it is the preferred
and most effective treatment.
Rodents & Birds
Photo: Tony Marino
Photo: Tony Marino
Photo: Tim Nevis
Photo: Tim Nevis
Photo: Tim Nevis
Photo: Gregory Whitney
You might end up with a mouse in your house, but
rodent and bird damage is something you would usually look for on plants grown outdoors. The damage from rodents
and other small animals looks just like something has been eating on your cactus. This makes a lot
of sense, however, bird damage is a little less obvious. Since birds peck at things with their
pointed beaks, bird damage usually looks like someone chopped up your cactus and spread it around. In some
cases, especially with smaller plants, the entire cactus may disappear overnight. It may have been eaten all in
one sitting or carried off. It also seems that one of these critters is content with a small nibble here and
there as opposed to munching down the whole plant. Also, spines don't seem to be as much of a deterrent as would be
Rodents will work at night while birds will probably be doing their damage in early
morning. Covering your plants with a wire mesh of some sort is an easy non-lethal method of preventing this
sort of damage, but is not very attractive. For rodents, poison can be set out, but should be used with
caution. Mouse traps can also get the job done. Once a rodent has moved into an area, it will systematically
keep munching your plants. Birds seem to move from one area to another and usually are more isolated attacks.
Clearly, you can't do anything about damage that has already occurred. Once an animal intruder is discovered
munching your plants, you can only fight to prevent future damage.
Photo: Gregory Whitney
Photo: Gregory Whitney
There are a few moth species whose caterpillars will eat some cactus species. The most notorious of these is
Cactoblastis cactorum which feeds on Opuntia species. Typically a moth will lay its egg in a protected spot
somewhere on the cactus. Then, when the egg hatches, the caterpillar starts eating its way through the plant.
In some cases, the caterpillar will be eating on the inside of the stem unseen by the cactus grower. When these
are discovered, it is often too late.
Since there are usually only a few caterpillars on a plant, it is easiest to simply
squish them when detected. Continual inspection of your plants is important even if you've never had any of the
pests listed on this page. The only good thing about cactus moths is that the caterpillars are easy to spot and
kill. Of course, since they are larger, they can also eat more of your cactus in less time.
"Purple Borer"(Phycitinae sp.) By Harald
Most everyone is familiar with the little green monsters known as aphids. While there are many different kinds of aphids, their
overall appearance and behavior is fairly consistent. They congregate on soft plant tissue in masses and suck the plant's juices.
On cacti, these are rarely found on the plant body and usually only affect the flower buds and flowers themselves. They can be found
on very soft, new growth. If you have ants on your cacti, be on the lookout for aphids as they are often brought in by the ants.
Aphids are among the more easily disposed of pests and can be blasted off with a high pressure water hose. It
may be neccessary to do this several times. If the aphid problem persists, spraying the infected parts with soapy water will be a stronger
deterrant. If this still doesn't work, then a systemic insecticide is recommended, but it will have to be taken up into the plant tissues
before it is effective.
Photo: Aaron Kewin
There are over 5000 species of thrips known with diets that include plants as well as other insects or even fungus. Some thrips are reportedly used as a biological
control of red-spider mite. While many people prefer "natural" pest control, obtaining and applying the predatory thrips is not very practical. On
this page, our concern is with the thrips that feed on cacti. There is one primary species that does this, the Western Flower Thrips (Frankliniella occidentalis). These
appear as elongated, small, orange/yellow moving things on your cactus plant. They are only 1/20th of an inch (1.3-1.4 mm) long and less than half as wide. In addition
to their small size, they also prefer to hide down in the cracks and crevices of cactus ribs, tubercles, or the base of flowers.
Fortunately, thrips do not seem to be as successful as other pests such as mealy bugs or scale and therefore, their presence is not all that common. Of
course, this is little solace to you if your plants are one of the few under attack by thrips. When thrips are encountered, they can be found on indoor plants, in the greenhouse or outdoors. They tend to be found outdoors only in summer months, but can be found year round inside.
Again biological controls, while favored by many plant growers are not very effective. Not only are predators of thrips few, but many common controls such as Ladybugs are
ineffective due to the thrips small size and ability to hide in tight places. As a result, pesticides are required for complete eradication of thrips. As with other pests
using a contact insecticide that is sprayed right on the plant is not desirable for a number of reasons. Most problematic with this method is the potential burning of
plant tissue due to phototoxic reactions. Contact insecticides also can wash off the attractive waxy coating or wool on many plants. Finally, contact treatments are
limited in effectiveness because you must be sure to saturate down into even the smallest cracks and this still may not kill the eggs.
Therefore, preventative treatment with a systemic insecticide at the onset of the growing season will give you the best results. However, if you are reading this, chances
are it is too late to prevent the problem. Since systemics take time to work, a manual removal of pests should be performed using tweezers, cotton swabs, or a jet of water on
plants without delicate wool. This process may need to be repeated until the systemic finally takes hold. Infected plants should be quarantined until the thrips are
positively gone. Nearby plants should be carefully monitored and treated with the systemic as a precaution.
Photo: Dale Denham-Logsdon
Aside from the creepy-crawly pests mentioned above, there are a number of less frequently encountered insects that feed on cacti. Some feeding
exclusively on a single type of cactus and others that eat just about everything that is green. These pests are typically found as individuals
or in relatively small numbers. Opuntia seem to be a favorite of the insects and it is even less common to find these on other types of cacti.
If you notice any "bug" on your cactus plant, aside from true spiders, they are probably up to no good and should be removed. These larger
pests are most likely to be found on plants grown outdoors and least frequently on indoor plants. Often times these bugs will go unnoticed for
quite some time. Despite their presence, few of them will cause any critical or life-threatening damage to your plant. Most likely, your plant
will simply suffer some unsightly bite marks.
A regular treatment of systemic insectcide will keep the larger insect pests away as well as prevent scale and mealie infestations. If you have
not treated your plant and find an insect munching away, you may simply remove it manually. Squishing it is optional, but not doing so will
likely mean their return. A systemic insecticide will be useless at this point as the insect will have moved on long before the systemic has
taken effect. Contact insecticides will kill the pests, but this is not recommended due to phototoxicity which would cause even greater
damage to the plant.
Fungal attacks are typified by a rust-colored or black spot surrounded by dried brown
sections on the stem of a cactus. Fungal attacks seem most common in epiphytic cacti and do not occur very often
on other cacti. Rot is sometimes a fungus "eating up" the dead plant tissue, but in that case
something else triggered the start of tissue death. In a true fungal attack, there is a fungus growing inside
the plant tissue that kills it as it goes. This is what makes it turn brown and dry.
Fungal attacks are extremely difficult to stop. The best option is to find any uninfected stems and re-start
a new plant from them and throw the rest of the plant away. Fungicides are available that could slow
down the fungus attack, but the fungus rarely is eliminated in this way.
Photo: Gregory Whitney
Rot is very likely the number one growing problem encountered by new growers.
Cacti are susceptible to rot because they are mostly water-filled fleshy tissue. This is just the type of
environment that bacteria and fungi thrive in. In proper conditions, rot does not occur in cacti because
the moisture in the environment is low. In the wild, most cacti grow in rocky, fast-draining soil, and
high temperatures. While the amount of moisture a cactus can tolerate varies from one species to another, as
a general rule rich soil with a high organic content that is continually damp will rot pretty much any cactus,
especially if the temperatures are low. The low temperatures exasperate the problem because the cacti are
dormant at low temps and not using any of the available moisture and it takes longer for the soil to dry out.
Rot usually starts from the bottom up. Very often the roots will rot off first, but the plant body will not
be rotting. This might happen where top-dressing keeps moisture away from the plant body. Sometimes,
portions of the cactus higher up might rot first. This usually occurs with over-watered plants. In
this case, the cells will fill with water until they burst open like a water balloon. After the cell wall
is ruptured, the cell dies. If a group of cells die from this, it is the perfect place for rot to set in.
Rot can vary in appearance from red to black. Rotted parts of cacti are mushy, slimy, and typically have
a bad odor. Rot may not be visible on especially spiny plants even after the plant is long dead. Additionally,
since rot usually works from the bottom up, by the time the problem is noticed, the plant is too far gone.
Note the picture at left of a Gymnocalycim on top and underneath. From the top, the plant looks fine, while
underneath it is completely rotted and beyond saving.
Rot is usually a bacteria, but can be fungal as well. Plants in dry environments can sometimes
get rot if they have been damaged in some way to let infection in.
As mentioned above, once rot is detected, it is usually too late to save the plant. This
isn't always the case. Sometimes rot is caught just when it starts or only a portion of the plant has
started to rot. To save a cactus from rot, the rotted portions must be removed entirely. This might mean
cutting the plant in half or more. The remaining healthy portion of the plant should be light green,
firm like an apple, and not have any spots of discoloration in it. If you can still see some spots, you
must keep cutting. If you cannot get to blemish-free, healthy tissue, then it is too late and the
plant is doomed.
This page says often that prevention is by far the best cure for any
problem. To avoid rot in your cactus, be sure that you have a good soil that does not sit damp for
long periods when you water it. Also avoid watering when your cactus is not growing. If the temperature
is low, your cactus will not continue to grow and any moisture will start to rot it quickly. If
you live in a cold climate, give the last water to your plants 2 to 3 weeks before the temps start
to drop in the fall. As a rule, when cacti are dormant, they don't need any water at all.
Left: John Chmielewski, Right: Bruce
Photo: Tim Nevis
Photo: Tim Nevis
Many people who are new to growing cacti get worried when they see parts of their cactus plant
turning brown and dead-looking. While this could certainly be a problem if it is caused by a pest or
some other factor, often times it is just a natural aging process of cacti called Corking. Because so
many people do think something is wrong, a description of this process and images are included to help
those distinguish from a real problem and the natural aging process. As the cactus
matures, the lower parts or base of the plant turns from smooth-green skin to a tough, brown, bark-like
appearance. As seen in the pictures, corking starts from the base of the cactus and works up the plant.
If a cactus is turning brown from the top down, or in spots on the side or in the middle, then it is
not corking, but a real problem such as spider mite or sun burn.
Since corking is a natural part of cactus growth. There is nothing to treat. If
your cactus plant has corking around the base, then simply enjoy the beauty of a maturing plant.
Sometimes when moving a cactus or weeding around them, a grower
may scrape, poke, smash, or otherwise damage the cactus skin without even realizing they did so. One very
common way this happens is when multiple potted cacti or cuttings are transported together. Often-times
the spines from one cactus will puncture or scratch the cactus next to it. The immediate appearance might
only be a slight discoloration in that area. After a few days, when the cactus begins to heal, the damaged
area will turn into a brown scar. This can leave behind undesirable markings on the cactus. And because
many times the damage wasn't noticed when it happened, many people often suspect a disease or other pest is
One important thing to watch for is spreading. Unless you have a really high level of activity
around your cacti, physical damage should be a rare thing. If you notice scar tissue continuing to spread on
an on-going basis, you probably have another problem such as snails or insects.
If the plant is damaged in this way, there won't be anything you can do to reverse it. The
scar will remain for the life of the cacti. The only way to treat this sort of damage is to avoid it in the
first place. When moving multiple cacti, take special care not to let them touch each other. Protect the plants
with thick pieces of styrofoam or plenty of newspaper. Watch your weeding tweezers when you go after that
pesky Oxalis. If your plants are in the ground. Lizards, birds, bugs, falling branches, etc. may be the cause
which will take your control of the situation away entirely. It is perhaps best to tell yourself that this is
how plants in the wild would look and just don't plan on entering it in any cactus shows.
Damage on cacti due to phototoxic poisoning is looks very much like Sunburn and
in a sense is sunburn. It is caused when chemicals -usually insecticides or fungicides- are applied to kill some
other infestation and the plant is afterwards exposed to direct sunlight. In this way, a cactus that has been in
strong sun, but shown no previous signs of damage is suddenly burning in the sun. Not all chemicals cause this
reaction, it is usually those that cointain some sort of oil. Since this problem is brought about by the grower's
own action, it should be fairly easy to diagnose. If the damage appears shortly after chemically treating the plant,
then it is likely the cause. If no chemicals were used, then phototoxicity can be ruled out.
Once again, prevention is the key. If you must use chemical controls on your plants,
be sure to keep them out of direct sunlight until you've had a chance to wash it off later. If you are reading this
after the fact, then you can't undo any damage that has been done. Get your plant out of the sun immediately if it
has just recently been treated with a chemical.
Note: In order for this type of damage to occur, the chemical must be sprayed on the plant. Systemic insecticides that
are applied to the soil to be taken up by the roots will not burn the plant in this way. However, if you splash the systemic on the
plant tissue while watering it, you may experience problems. Always take care when applying toxic chemicals to your plants.