The Ultimate Guide to Houseplant Pests & Diseases, by Jim Hole

Caring for Seasonal Plants

Holiday plants are hard to resist. Not only are they readily available, but their beauty and great value make them perfect guests in our homes. Yet the holidays are hard on plants, too. One of the reasons that holiday plants take such a beating is that most people think of them as disposable—and, for the most part, they are. Of course, we all know people who manage to keep a poinsettia from one year to the next, but the bragging rights that come with that coup are usually more impressive than the plant itself.

The secret to keeping a holiday plant looking healthy isn’t really much of a secret. Simply make time to water and deadhead. It comes down to recognizing that you don’t need to grow the plant—you simply need to maintain the status quo. What I mean is that a holiday plant is on holiday. It has spent months working and growing to its peak of perfection, and now all that it has to do is retain its beauty. This fact makes your job relatively easy—putting the plant in the right environment and watering properly is often all that is required. It does take a bit of effort and patience on your part, but no more than it does to be nice to well-meaning relatives and friends who overstay their holiday welcome.

Pests & Diseases

Dealing with pests and diseases is difficult because the process involves considerable diagnosis. It’s natural for people to focus on the remedy that promises to ‘cure’ a sick plant, but the cure is only one piece of the puzzle. To put the whole puzzle together, you have to know what kind of a plant you have, the kind of insect that’s attacking it, the treatments available and the frequency of those treatments. If you miss even one of these pieces, you lessen your chances of solving your problem. With that said, don’t lose hope! Knowledge is the key to identifying and dealing with pests and diseases, and you’re about to acquire it.

Pests

The joy of purchasing a beautiful houseplant quickly fades when the first ‘pet’ is discovered on a leaf. The pet I am referring to, of course, is
not the four-legged, furry kind, but rather the six (or more!) legged
variety that loves plants as much as we do.

When most people try to take on the insects and pests that attack houseplants, their goal is usually to annihilate them—to destroy every last one! While I agree that eradication is the ultimate goal, I have yet to see an instance where anyone has successfully implemented a program without first having a thorough understanding of the enemy.

The good news about insect and insect-like enemies is that of the millions of insect species in the world, only a handful are pests of houseplants. The bad news is that handful is a bit like the Dirty Dozen—a tough bunch to kill. Many of the pests we battle have adapted extremely well to our attempts at controlling them, pesticide resistance being a prime example. But, by far, the number one reason I see pest control fail is due to a failure to understand the pest.

Part of understanding pests is understanding their place in nature. It’s hard to look past the fact that pests cause a lot of frustration, but they are part of the ecosystem—not extraneous to it. In nature, pests are kept in balance by a well-functioning system of predators and prey. Of course, household environments can’t support the diversity required to keep plant pests in check, so the inevitable result is an exploding pest population and a trail of dead and dying plants.

So beyond seeking to help you identify your pest problems properly, I hope to inspire in you an unlikely appreciation for the pests themselves. Now, I didn’t say love; I said appreciation…as in new-found respect. After all, as much as their destructive tendencies drive us crazy, they are fascinating enemies and amazingly successful creatures—despite our best efforts to obliterate them.


Spider Mites

Physical description: Spider mites—just one of the many types of mite that attack plants and probably the worst of the lot—are generally 5 mm long and oval shaped, varying in colour from yellowish to greenish and even reddish to brownish. Spider mites are difficult to see, so they usually go unnoticed until they become a full-blown problem.

The damage and the signs: Spider mites are polyphagous (meaning they eat many different plants) and spend most of their time piercing plant tissue and feeding on sap. The first indication of a problem is usually mottled leaves. Heavy infestations produce frail, silky webbing that can cause plants to die.

Where to look for them: Spider mites live on the underside of leaves.

Seriousness of the problem: Severe if not controlled early. One of the reasons mites spread quickly is that they produce threads that act as crude parachutes, enabling mites to glide on drafts from one plant to another. They can also hitch a ride on our clothing.

Preferred environment: Spider mites like hot, dry conditions, so they are well suited to living indoors. They prefer plants with soft easily pierced tissues and tend to avoid those with foliage that is waxy.

Treatment and prevention: Spider mites can be challenging to get rid of. Because the eggs are resilient to most sprays, treatments must be applied at five- to seven-day intervals. For best results, use a good insecticidal soap and cover these pests thoroughly.

Spider mites have truly amazing fecundity (reproductive rates) that is second to none
in the arena of houseplant pests. The typically male-to-female ratio of offspring is roughly three females for each male. But don’t let the shortage of males fool you—spider mites don’t need to mate to produce offspring. Fertilized females produce both males and females, while unfertilized females produce only males.


Scale Insects

Physical description: Scale insects earn their name because their protective shells look like scales. They are difficult to see without a magnifying glass, but even when you can see them, you’re not seeing their bodies per se but rather their shell-like, waxy protective coverings. Most scale insects are 1.5–3 mm in diameter and round or oval shaped. To the naked eye, scale looks like brown scabs.

The damage and the signs: Scales cause discoloration, stunting and even leaf drop if the infestation is bad enough. The greatest problem is usually the honeydew (the clear, sticky secretion the scale leaves behind) and the sooty mould that grows on it. Unfortunately, the scales tend to stick to the plant long after the insect is dead.

Where to look for them: Scale insects can be found on stems, leaf veins, undersides of leaves and leaf joints.

Seriousness of the problem: Scale problems are often overlooked for two reasons. First, they look like part of the bark, and second, they are immobile for a good part of their lives and conceal themselves in the plant.

Preferred environment: Scale insects don’t like extreme weather and prefer warm, humid, shady conditions.

Treatment and prevention: Females can lay a huge number of eggs—up to 3000 over several weeks. The crawlers that emerge from the eggs are most sensitive to sprays because they haven’t developed their protective shell-like armour. Scale can be removed with your fingers when the insects are adult size, but you’ll have the best chance of controlling the problem if you apply a thorough coating of insecticidal spray or horticultural oil on the young crawlers every 7–10 days until all signs of these pests are gone. As hard as scale is
to battle, you can get rid of it.


Mealy Bugs

Physical description: Mealy bugs have soft, slender, waxy bodies that are 12 mm long and dusty looking. Many mealy bugs are covered with waxy threads. Females are wingless and tend to move very little.

The damage and the signs: Mealy bugs suck the sap from leaves and leave behind a trail of sticky honeydew. Other signs of damage are yellowing or dropping leaves.

Where to look for them: Mealy bugs can live on the undersides of leaves, on leaf joints, below the surface of the soil or on newly pruned stems.

Seriousness of the problem: A mealy bug’s waxy coat keeps it well protected from predators and pesticides, so controlling the problem is a serious issue. Because pesticides are often absorbed through the exoskeleton, the waxy coat on a mealy bug acts like a safety suit, shucking off the spray and rendering many pesticides useless.

Preferred environment: Mealy bugs tend to inhabit leaf axils (the spots where leaves meet stems) and leaf veins. They reproduce quickest in warm, humid environments but manage to survive in less than ideal environments.

Treatment and prevention: Mealy bugs are easy to deal with on a bug-by-bug basis, but if neglected they can become a big problem. Simply picking off offenders when you notice them
and following up by dabbing rubbing alcohol directly on the insects will often do the trick. For bigger infestations, aim for a thorough application of horticultural oil or insecticidal soap, every 7–10 days for as many applications as needed.

Although mealy bugs are notorious pests, one species produces bright-red pigments that have been used as a dye. In fact, I’m willing to bet that if you have ever had strawberry ice cream, chances are good that you have eaten mealy bugs—the pink colour of the ice cream was often intensified with the dye extracted from the cactus-loving cochineal mealy bug.


Thrips

Physical description: Thrips are insects from the order Thysanoptera, which means fringed wings, and are generally just a few millimetres long and a fraction of a millimetre wide. They are also very mobile and able to run, fly and jump. Most thrips are tan or dark coloured.

The damage and the signs: Thrips feed on foliage, buds and flowers by scraping and piercing the cells of plant tissue and drawing out the nutrient-rich contents. Infested leaves have a silvery, stippled appearance that resembles the damage caused by mites. An easy way to distinguish a thrip infestation from a mite infestation is to look for the brownish specks of excrement that thrips leave behind where they feed. Besides causing leaves to shrivel, thrips transmit viruses that can cause more damage than the pests do.

Where to look for them: Thrips can be found on leaves, buds, flowers and under plant
debris. Females make slits in plant tissue and lay the eggs within. When larvae mature, they often fall into the soil to pupate before emerging as adults.

Seriousness of the problem: Thrips aren’t a problem for tropicals with tough, waxy leaves, but they love to live and feed on flowers. Although they feed on both pollen and petals, petals are at greatest risk of becoming stippled from the thrips’ rasping mouths. Yellow flowers are particularly prone to attack.

Preferred environment: Thrips prefer direct sunlight and very warm temperatures. These pests are thigmotactic, meaning they love to be in tight spaces, literally ‘hugged’ by plant tissue.

Treatment and prevention: Prevention is the key to controlling this pest. Once thrips get into the flowers, there are few options for control other than snipping off the damaged flowers and disposing of them. Thrips are far worse when the weather is hot, so growing plants in cooler temperatures can slow thrip damage substantially.


Fungus Gnats

Physical description: Fungus gnats are generally 3–4 mm long; the adults resemble small fruit flies.

The damage and the signs: The lifespan of an adult fungus gnat is only about three weeks. Adult fungus gnats are unable to feast on plants, but the larvae eat the young tender roots, allowing the entry of root rot. Pull plants out of their pots and check the soil for chewed roots and small, white larvae with black heads. Damage is usually restricted to the plant’s roots, but tender stems can also be attacked.

Where to look for them: Fungus gnats live in the soil.

Seriousness of the problem: Although fungus gnats do minimal damage to plants, they are notorious for flying around and annoying people.

Preferred environment: Damp soil is a fungus gnat’s favourite place to live.

Treatment and prevention: Fungus gnats love moist soil surfaces, so avoid infestations by not overwatering. Pesticides are also available and are very effective
if applied to the soil surface.


Aphids

Physical description: Aphids are pear shaped and usually green, but watch for them in shades of black, brown, grey, yellow, red and purple. It’s interesting to note that aphids come with and without wings. Wingless aphids are called apterous, and winged adults are called alate.

The damage and the signs: Aphids seek protein and, therefore, feed on plant sap, but because sap is sugar rich and protein poor, aphids must extract large quantities to get enough protein. Aphids rid themselves of the excess sugar by excreting it in the form of visible sticky honeydew. Aphids may also transmit viruses to plants through their saliva, sometimes causing malformation of leaves. Another sign to watch for is the white skins that aphids shed as they grow.

Where to look for them: Aphids prefer to cluster on stems and under leaves.

 Seriousness of the problem: Aphids have a huge capacity for rapid reproduction, and young aphids reach maturity in only six
to seven days.

Preferred environment: Aphids react to plant leaf colour and prefer those that are yellowish-green. They also prefer to live on new growth. Some species are very specific feeders, while others are polyphagous (eat many different species of plants), like the green peach aphid.

Treatment and prevention: An insecticidal soap works best. Apply the soap thoroughly and often until the problem is under control. If the soap doesn’t cover the aphids’ bodies, it won’t kill the pests.


Whiteflies

Physical description: Whiteflies have wedge-like wings and are commonly described as looking like tiny bits of ash. When infested plants are disturbed, clouds of winged adults swarm into the air.

The damage and the signs: Whiteflies feed by sucking juices from leaves. The leaves often become covered in a sticky honeydew, turn yellow and fall off. Leaves may also appear covered in sooty mould. Because whiteflies need a lot of protein for development, they consume large amounts of plant sap. Thus, like aphids, whiteflies secrete a large amount of
honeydew on which sooty mould often grows.

Where to look for them: Look for wingless nymphs on the underside of leaves and for adults on stem ends and the tops of plants.

Seriousness of the problem: Whiteflies can transmit plant
viruses. They are similar to aphids and mites in that their populations can explode. Although they are slow moving, whiteflies spread to nearby plants quite effectively.

Preferred environment: The optimum relative humidity for whitefly is 75–80%. Eggs can survive for only a few days at slightly freezing temperatures, but even moderately sub-freezing winters will kill whitefly in any of its developmental stages.

Treatment and prevention: Thorough applications of insecticidal soap, neem oil and botanical insecticides can be used repeatedly to spot-treat heavily infested areas until the infestation is under control. Yellow sticky traps are also useful for trapping and monitoring the presence of adult whitefly populations.

Whiteflies aren’t really flies at all. In fact, they are more closely related to aphids. As their name suggests, whiteflies have a waxy coating on their bodies that give them a white appearance.


Diseases

When it comes to plant diseases, prevention, not cure, is the operative word. This is the reason most treatments for plant disease usually involve protecting plants before they get a disease rather than trying to eliminate a disease once it has established itself.

There aren’t many products available to control diseases, so the principles of disease control are rather simple: always start with healthy, disease-free plants, don’t introduce disease to those plants and always grow your plants in a healthy, stress-free environment.

Once again, knowledge is key to identifying and dealing with

disease problems, so take the time to learn about what you’re battling. It’s important to know what you’re up against and whether you can beat it.

Biotic Diseases:

Powdery Mildew

What is it? Powdery mildew is a fungus that forms a white powder-like growth on leaf and stem surfaces. It also commonly appears as dry, brown, papery leaf spots. There are numerous powdery mildew species that infect a wide range of plants, but as unattractive as they are, they are rarely fatal. Large amounts of powdery mildew can, however, impair photosynthesis. The one good thing about powdery mildew fungi is that they are fairly host specific, meaning the powdery mildew on your umbrella plant isn’t going to spread to your African violet.

What causes it? Powdery mildew is caused by fungi that survive in dead and decaying plant materials. Spores are often carried to plants by both wind and insects. Contrary to popular belief, powdery mildews aren’t caused by a wet environment; however, they do require high humidity for spores to germinate and invade plant tissue. Grouping plants too closely together and depriving them of adequate air circulation will also encourage a powdery mildew problem.

Control and prevention: Powdery mildew loves thin, weak, leaf tissue, so providing the best growing environment possible goes a long way to preventing an invasion. Unfortunately, fungicides are effective only prior to the mildew becoming established, so stay on top of the problem and remove infected growth as soon as you notice it.


Root & Crown Rot

What is it? Root and crown rot is a catch-all term for a series of plant diseases that attack plant roots and the transition zone between roots and stem, known as the crown. Fungi are usually the culprits, but bacteria can also cause the damage. As the name suggests, root and crown rot causes stems, crowns and roots to turn brownish-black and become soft and mushy. Lesions will normally form on stems near the surface of the soil. When it comes to African violets, the symptoms are slightly different. Watch for older outer leaves that show signs of drooping and for younger inner leaves that are stunted or are turning brownish-black.

What causes it? Root and stem rot diseases are caused by micro--organisms that live in the soil. The disease organisms are, by and large, various species of fungi, but they need the right environment to do their dirty work, such as soil that doesn’t drain easily or has been over-compacted. In fact, I would say 90% of root rot problems can be eliminated by choosing high-quality potting soils and watering properly.

Control and prevention: Your best means of control and prevention is to use high-quality pasteurized soilless mixes, to avoid over-watering and not to repot plants too deeply.


Gray Mould

What is it? Gray mould (or Botrytis sp.) is a fungal disease that forms a fuzzy, grey mould on young foliage and flowerbuds—old, tough, waxy leaves are rarely attacked. For gray mould to get established, it must find foliage that’s had free water sitting on it for a few hours. If there is no free water, it is impossible for gray mould to attack plant tissue. African violets and begonias are susceptible to gray mould. Stem and crown rot may also occur as a result of this disease.

What causes it? The gray mould fungus requires water sitting on susceptible plant matter for an extended period of time. Over-watering, extended periods of cool, damp conditions and improper maintenance (such as lack of deadheading) can all lead to a problem with gray mould.

Control and prevention: Water your plants close to the soil level. Letting water shower over the leaves creates the moist environment in which the Botrytis fungi loves to live—dry leaves are a poor environment for the mould. Let the soil dry out slightly between waterings and space susceptible plants to allow for adequate air circulation. Removing infected leaves and
tissue won’t get rid of the disease, but will slow its spread. Use a proper fungicide. Remember
that the fungi tend to thrive on weakened or injured tissue, so keep plants healthy by deadheading and maintaining an appropriate fertilizing and watering schedule.


Trunk Gall

What is it? Gall looks something like a brown knot and is usually found growing on the tree’s crown or trunk. Most galls won’t kill a tree, but they will disfigure it.

What causes it? Gall is caused by the soil-borne bacteria Agrobacterium tumafaciens, which enters the tree through wounds in the crown or stem. In most cases, gall is not lethal. If the gall cannot be cut out without extensively damaging the tree, I suggest leaving it alone.

Control and prevention: The best way to prevent gall is to use high-quality, pasteurized soilless mixes and to take care not to nick or wound the crown or trunk of trees.


Environmental Diseases (Abiotic):

Edema

What is it? When plant cells absorb more water than they can transpire, the cells literally burst, leaving cork-like swellings on the leaves and stems.

What causes it? Edema is caused by high humidity, over-watering and cool air temperatures.

Control and prevention: Improving light levels so that plants photosynthesize at an increased rate can help prevent edema. Increasing air circulation and allowing plants to dry out slightly between waterings will also help.


Chlorosis

What is it? Chlorosis is a nutrient deficiency that causes leaves and stems to look pale. When nutrients are lacking, chlorophyll, DNA, RNA, proteins and lipids cannot be manufactured. As a result, enzymes are less able to carry out important chemical transformations. Growth is slowed and susceptibility to disease may increase. Infected flowering plants may become dwarfed and produce fewer flowers.

What causes it? Chlorosis may be caused by a lack of nutrients in the soil, poor functioning roots, cold temperatures and water-logged soils.

Control and prevention: Maintain plants in the healthiest condition possible, and be sure to fertilize when needed. Chlorosis is often caused by iron deficiency and soil pH that is too high, so have a soil test done if problems persist.


Leaf Drop

What is it? Leaves dropping off plants.

What causes it? All plants shed leaves, but if a plant suddenly drops healthy green leaves, it is usually an indication of low light. If the dropped leaves are brown or yellow, you might be dealing with soil that is too dry or too wet, natural shed or even disease or insect problems.

Control and prevention: Give the soil a scratch before you water it. Soil that appears dry on the surface may have plenty of moisture below. Green leaf drop is common with plants that are brought indoors after having spent the summer outside. Plants may take a while to adjust to the lower light levels indoors (usually two to three weeks). Remember also to repot plants when necessary—a pot that is too small won’t hold enough soil to support proper leaf development.


Leaf Scorch

What is it? Leaf scorch results when intense sunlight hits leaves that aren’t adapted to high light conditions—such as when low light plants are suddenly moved to high light areas. The result is patches of dead tissue on the areas of leaves that are most exposed to the sun.

What causes it? During hot, dry weather, a plant’s stomata or pores close up and heat builds up. Strong sunlight can destroy chlorophyll in leaves. Prolonged exposure to these environmental factors, combined with an insufficient amount of water, will cause leaves to become scorched and dried out.

Control and prevention: Be vigilant about watering plants during hot, dry spells. If you take plants outdoors for the summer, be especially mindful of leaf scorch. Mulch the soil surface to improve its water-holding capability, and use a high-quality potting mix. Provide diffused light for your plants during months when sun is most intense. There is no ‘cure’ for leaf scorch. When tissue turns brown, it is dead.


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-Jim