(Editor's Note: This article was originally published on August 4, 2010. Your comments are welcome, but pleae be aware that authors of previously published articles may not be able to respond to your questions.)
Having grown over 5000 plants in the last 20 years and having far less than that to show for it, I have learned a few things about plants dying. One of the ways my plants have died, and even more often, been injured, sick or mangled, has been by damage from other life forms. These life forms include insects, arachnids, mollusks, fungi, bacteria, mammals and birds. However, 99% of the time when I encounter one of my unfortunate damaged captives (that is how I refer to my plants, nearly all which are being forced to survive in an environment that is certainly not that similar to their own and is often relatively marginal compared to what they really should be growing in... so I think of them as unwilling captives) I rarely get to see the damage actually taking place. So I have learned to be a bit of a detective and sleuth out the cause of my hapless plant's misery. Sometimes it is obvious and other times the problem remains a mystery. But the plant's appearance, even sans bad guy, provides important clues as to who to blame for all the problems. Toni Leland wrote an excellent article recently along similar lines, but with the more commonly grown plants most others grow as the stars of the story. This story stars some of the less commonly grown succulents and tropicals which I waste my time growing. The following situations include many of those cases in which I think I know at least part of the answer concerning the victim's struggle or demise, having had so many of them attack my own plants in my back yard. This article by no means even comes close to covering all the bugs and damage that one can encounter in tropicals and succulents. Rather it is a more personal account of damage I see in my Mediterranean climate of southern California.
The most common killer in my yard is certainly me. I am at least in part responsible for most of the deaths and injuries to my floral prisoners. Often it is because I provided too much or too little water, sun, or simply the entire situation I dumped them into was just too wrong. So when I see something look sad, sick or dead, I automatically feel guilty and wonder what I did this time. There are no specific clues for my own decimating contributions to my plant collection. Just a dead or dying plant. However, I am not the only one contributing to illness and death in my weird plant collection.
Aside from myself, I think my primary nemesis has been of the microscopic variety. Fungus is my main enemy and is probably the number one killer of palms, cycads, succulents and other ornamentals aside from myself. The signs of fungal disease are many, though often they are hidden until it is far too late for me to intervene. It still amazes me how long it can take me to actually come to the conclusion a plant may be struggling due to fungus despite my seeing it happen time and time again. A palm that is just sitting there, not growing, for months on end, particularly during or soon after winter, should always prompt me to yank on its spear (un-opened new leaf). Eventually I do, often finding the entire thing simply pulls out with a sickening ‘schlop' sound and with that tell-tale look and odor of something rotten going on in the growing center of the palm. Or a cactus that is just starting to lean a tad can be grasped and completely collapse in my hand, either spouting a stinky ooze in all directions, or if I am really unobservant, come apart from its ground attachment as a hollow shell of its former self (having completely rotted out and since dried up). Fungus can look like black spots on leaves, discoloration of tissues, a softening of structure or an odor of sickly sweetness. But there are not too many pathognomonic symptoms of this complication. In many succulents it is just a secondary complication to the presence of other bugs (eg Mealy Bugs). I don't mess around much with the more common plants like roses or other flowering things, but from what I understand, fungus is common among those types as well. Fungi love moisture, and I imagine it might be much more of a problem for those growing plants in humid climates. So for our lack of humidity here in southern California, I at least have to be grateful.
just one manifestation of fungus (black spots on this Gasteria)
The black stuff one can often see on unhappy palm leaves and other tropical foliage that has been overrun by ants is also a fungus- black sooty mold. The mold itself is not really the problem... the problem is the ants and their minions, the mealy bugs and aphids. I fortunately don't have much problems with sooty mold as I don't have too many ants in my yard (no idea why, but I am keeping my fingers crossed for now). See below for more on aphids and mealy bugs.
Black Sooty mold on my hibiscus, secondary to aphid attack
Bacteria, like many fungi, are omnipresent, and always secondary invaders. I rarely encounter too many problems with bacterial infections in my own garden, but one can find thousands of references to bacterial plant infections on line. Pink rot is a common infector of palms, but I rarely see it as I have almost no humidity. Anyway, the symptoms are rarely apparent until it's too late.
Moving up a bit in size, the next offender is mites. Spider mites are the most common mites one might encounter in one's plants, but this is primarily a problem when tropicals are grown indoors, in a low light, low wind, low humidity situation. When I had a greenhouse I would often see the pale leaves, very finely dotted and webbed with the tell-tale sign of spider mites (the mites themselves are pretty hard to see). Palms and bamboo were primarily the afflicted plants in my greenhouse.
I have a particularly bad problem with Aloe mite, an unusual mite in that it 1- has only 4 legs (most mites have 8) and 2- lives within its own galls it stimulated the plants to form. These galls are extremely disfiguring to the plant are often referred to as aloe cancer. This is certainly one bug that can literally only be identified by the symptoms the plant displays as finding the bug itself is a real challenge, without a sharp knife and microscope. See this article:
Aloe mite damage on leaves on an aloe (left) and the flower tip of another (right)
Aloe mite damage may kill this aloe (left); typical 'gall' with solid center where the mites live and breed (right)
A double attack of aloe mite and snails (left); right is a photo I took of a microscopic shot of an Aloe Mite found in one of my aloes
though probablyl not the same mite, there are mites that attack Agaves, too- this shot shows my Agave guadalajarana with mite damage on the leaves... I have never found this mite microscopically, though
The next larger bug I have problems is a bad one- scale. Scale is a tiny, often-armored insect that is the sap-sucking equivalent of the plant barnacle. There are dozens of kinds of scale and most are hard to get rid of. And even if you kill the armored types, their barnacle-like shells often remain, making your plant continue to look infected and making you wonder if the scale is all dead or faking it. At least scale is easy to see, so there is little mystery. Still, an Opuntia cactus pad that is withered and whitish is not a healthy look, live scale or not. Many of my cacti get that ugly white-spot look and then they start looking unhappy as well. Typically scale attacks cacti getting too much shade (often only on the shady side of the plant if in partial sun). I get scale on cycads, too. Though there are usually few symptoms on my cycads and fortunately we don't have much in the way of that horrific white Aulacaspis scale here in California
Hard scale on Zamia cycad (left) and a smaller white species on Opuntia (right). That sort of scale on cycads is not nearly as bad as the more infamous Aulacaspis scale, but it is still annoying. Symptoms of scale damage from this scale are uncommon. However, the cactus scale is much more prolific and can quickly damage the cactus surface or suck the life of it
two more examples of cactus scale, with the poor Fairy Castle on the left being so badly damaged that secondary fungal infection has pretty much killed the entire plant
Severe case of scale on this Pachycereus, with snail just using the cactus as a hiding place (left); right shows a large colony of Stapelias completely wiped out by scale attack and secondary fungal invasion
Aulacaspis scale on a Cycas (not in my garden, thankfully) (left) and dying plant (right) from severe infestation
Scale on a Kentia palm... note the faded, sick-looking leaves on the left. Right shows scale on petiole
Left scale is attacking Haworthia in too little light, allowing secondary fungus to attack; right shows one of the soft species of scale (on a palm in my garden)- this is Cottony Cushion Scale
By far the biggest problem I have in my collection of weird plants is mealy bug, which I am convinced breeds by spontaneous generation (just shows up magically wherever it is not wanted). This is actually a form of scale though a lot easier to squash, and is a really evil little creature that quickly gangs up on plants (mostly my plants) and sucks the life out of them. The main symptoms in succulents are gunky looking whitish stuff in the growing centers of the rosettes, and a sticky goo when touched. The main symptoms on tropicals are whitish spots of cottony gunk along the petioles and undersides of the leaves, and, again, a sticky goo. Another main symptom on tropicals is the presence of black sooty mold. The main symptom on bamboo is of dark purply gunk growing in all the crevices where the branches come off the main culm. The main symptom on cacti are clumpy white spots on pads (often confused on Opuntias with Cochineal insects) or tiny globs of white spit or dandruff on the spines of globular cacti.
Mealy bugs on my Beaucarnea (left) and Agave (right)
Damage at the leaf bases of this agave are from mealy bug, which can be seen on the right as 'snow' in between the leaves
This is a very common problems between the leaves and in the crown of aloes, particularly if not getting enough direct sunlight. Right is a leaf rotted off an aloe secondary to mealy bug damage, and the bugs can be seen mid leaf sort of blending in the with spots
Buningia cactus with mealy bugs on spine tips (look like spit)
Cochineal insects, a close relative to the mealy bugs (and also a form of scale insect), are a pest of Opuntias only (as far as I can tell) and can completely swarm over the pads giving the plants a white, frosted look. These are gooey to the touch and leave a purple stain when smashed. Unlike most other forms of scale I see in my garden, these grow in full sun on Opuntias while the other scale and mealies almost always prefer the shady sides of the plant. Opuntia covered with this stuff also look weak and dehydrated, and eventually dead.
Cochineal insect attack on Opuntia (photo Xenomorf)
Root mealies don't have a typical symptom other than a weak or rotting plant. But if you dig the plant up, you can see flakey white areas in the soil, particularly in soil with lots of peat in it.
Aphid attacks are generally identified by seeing the orange or black spots (aphids) all over the flowers or new stems of my succulents or tropicals. Probably the primary victims of aphid attacks are Echeveria flowers, which seem almost to spontaneously generate populations of orange or black aphids all over them, or the Hibiscus flowers which quickly get covered with black or dark green dots. These bugs do not do extensive damage like most of my other pests, but are icky and annoying and do decrease the number and health of the flowers. Also, particularly on hibiscus, they allow the growth of sooty mold making the leaves look blackish.
Aphids covering Graptoveria in my yard (left) and on Echeveria flower (right)
Aphids all over Strawberry tree (left) Aphids attacking unopened Hibiscus flower (right)
Some of the tropicals in my yard occasionally have white beards growing on the undersides of their leaves (Hibiscus, Cannas, Bananas and Begonias primarily). This is due to a white fly infestation, one of the hardest parasites to keep away from my plants. Earlier on the affliction can be detected by the presence of white circles or swirls on the darker side of these leaves. Following uninterrupted white fly problems will be more black sooty mold, so leaves will not only appear hairy, but also a dirty black as if the smog was being deposited on them.
Hibiscus leaves covered with white flies, and beard of long-standing infestation (left)
typical swirl pattern on underside of a begonia leaf (left); 'frosted' leaves and sooty mold on badly infested hibiscus plant (right)
Thrips are a problem for several tropicals I like to have around, though their evidence varies from plant to plant. On palms these seem to suck the life out of new leaves in the bud, making it look like the plant is on its way out as green leaves are replaced with spotted or shredded brown ones. But on Indian Laurel Figs (Ficus microcarpa) one sees folded leaves that are often blackened and spotted. This infection rarely seems to amount to much on these Ficus trees. On these trees the offending bugs are pretty easy to find (just unfold the leaves). I never see them on the palms.
Thrips damage to Chamaedorea palms
Typical thrips damage to Ficus microcarpa leaves showing mature and immature insects, and some eggs (left); typical look of curled leaves due to thrips infestation (right)
The main eaters of succulent plant material in my yard are the snails and slugs. Though only present early in the mornings (or in winter, sometimes all day), these leave telltale shiny slime trails and also devastated leaves, stems and flowers. They literally have eaten entire plants of mine and seem especially fond of Haworthias, Aloes, Agaves, Crassulaceae (Echeverias, Graptopetalums and Crassulas) and a few palms (Chamaedoreas and Majesty Palms). When feasting on Agaves, which have much tougher leaves, the tell-tale evidence are trails and craters made in the upper leave's surface, permanently disfiguring these leaves, but they not leave holes completely through them, as they do with softer-leaved plants. I often find these creatures on the cacti and many of the tropicals, but they, with a few exceptions, ignore these plants and use them mainly as mating sites and hiding places. Some soft-surfaced cacti get damaged, though.
Snail damage on Agave attenuata from snails (left) and Agave 'Joe Hoak' (right)
Snail and slug damage on this Opuntia (left); Snails ate the top off this Copiapoa (right)
Aloe with crown eaten out by slugs (left); Haworthia with large chunks taken out by snails (right)
But whatever the snails and slugs ignore in the way of tropical foliage, the grasshoppers and katydids concentrate on. Very few of my plants escape all forms of biologic predation. Grasshoppers also eat all day long while the slugs and snails feast at night or dusk and dawn. These are some of the only plant predators that I actually understand how they get into my yard (they fly in). I have tall walls all around the property and often wonder how all the other bugs get in. Large gaping holes or munched foliage with curved edges signifies their feeding evidence. Though for all their relatively enormous size, they do relatively little serious damage to my plants, at least compared to mollusks and caterpillars.
Chewed up palm leaflets (left) and the offending chewer (right)
Katydid damage on Kalanchoe leaves (left); immature katydid and munched Euphorbia leaves (right)
Telling the 6 legged insect damage from caterpillar damage is not always easy, but the latter tends to leave a lot their fecal matter as their calling card. Also grasshoppers tend to eat one leaf, and then move to another plant far away. Caterpillars (usually of the ‘inchworm variety') eat one third of one leaf and then move onto the next nearby leaf. On some plants, like the Brugmansia, they leave a lot of holes, as if they were only sampling the leaves and not really meaning to cause a lot of damage. However, it is amazing how a tiny, slow, inch-long insect can devastate a entire plant in short order.
Brugmansia damage and tell tale caterpillar poop (left); the offending inch worm (right)
Pasion vine damage (soon to be complete anihilation)- left; offending Gulf Fritillary caterpillar (right)
In more tropical climates, there are dozens in not hundreds more insects and other small creatures that damage tropicals and succulents, but fortunately I have not seen most of these borer beetles, sucking bugs, leaf skeletonizers etc. in my yard. One can write dozens of articles about all these bugs, but this at least covers most of the miniature pests I have personally encountered in my yard.
Bird damage is strictly limited to fruit damage in my yard. Since I do not grow much in the way of fruits, except passion fruits, there is little more so say. Holes pecked in fruits are signs of their predation.
Mammal predation is indeed significant, though in my current yard which is in the center of a larger rural area completely surrounded by tall brick walls I do not have much in the way of mammalian plant damage. Tree squirrels munch on bark and eat fruits, but they pretty much ignore the plants I care about. I used to have problems with ground squirrels, particularly with them eating the bottom halves of the lowest fruits, but they were pretty much too skittish around my dogs to hang around long. But that same could not be said of rabbits or gophers. The latter was by far the worst plant killer in terms of completely killing and/or eliminating plants from my collection. I lost more money in terms of plant replacement costs from these varmints than to all the other plant killers combined (myself not included). Yet their immediate predation often would go unnoticed. The first sign that anything might be wrong would be a plant actually beginning to sink into the landscape, or to fall over sans roots. Sometimes bananas would just appear thinner (would turn out to be completely hollowed out with only the thin outer leafy-stems left to support the green foliage). Rabbit predation was easier to detect by the near complete absence of entire leaves (usually of smaller palms). Usually the newest, softest leaves would be missing.
The following are some links that might prove helpful if more information about the above parasites and pests is needed.