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Common pests & diseases on trees and how to treat them

Your newly planted tree is our priority. We want to see it grow and flourish, but sometimes it can be susceptible to pests and diseases due to the new environmental conditions it has been exposed to in its new home.
We’re here to help, so that’s why we’ve put together a list of common pests and diseases on trees and how to treat them. Please note that this is only a guideline and other underlying factors may also cause your tree to deteriorate. 

“In 95% of the case, the reason for the tree’s deterioration, is that the watering recommendations set by us have not been followed correctly. We urge clients to install a drip irrigation system for their trees and follow the recommended duration and frequency.”

Human error! Filling up above root collar 
This is the most common cause of tree death after insufficient watering

The filling up of soil / compost mix on top of the original root ball of the tree will smother your tree. This will cause an infection and rotting of the cambium layer of the tree.

Symptoms can include abnormal browning patterns of the leaves and loss of leaves as well as decay and some rot around the root collar.


Aphids are classified as sucking insects (Aphids, scale, spider mites & white fly), meaning that they suck nutrients out of the leaves and stems of the plant in order to survive and reproduce.

You will notice little yellow spots on the tree leaves when aphids are present and feeding on the tree.

New leaves will present as deformed as they develop and attempt to grow to full size.


There are many different types of Aphids that can be found on trees and some so small that it is difficult to see them with the naked eye.

What we commonly find on the trees are the green, brown & woolly aphids.

The woolly aphid generally appears as white fluff on the underside of the tree leaves and can sometimes be seen on the stem as well where trees have recently been pruned back.

The green or brown aphids are more difficult to spot, you will often see a collection of these aphids on soft new growth.



Natural predators, such as ladybirds, can keep the infestation under control if there’s only one or two aphids present on the tree. 

However, the aphid population can explode rapidly, so the tree can be infested within a week.

If the tree however is struggling with an infestation (such as many yellow and deformed leaves), the natural predators cannot keep the population under control and further treatment options can be considered.

We recommend a contact insecticide spray such as Garden Ripcord. The active ingredient is Cypermethrin (Pyrethroid).

Once the pest has been eliminated, we recommend using insecticide granules to prevent any future outbreaks. 

Aphids on Conifers

They occur as soon as the temperatures start cooling down and can then rapidly increase in number.

The first signs of an infestation would be yellowing of foliage as they suck the sap from the tree branches. You will discover them on the main stems and branches of the tree and not on the new soft growth (like with other aphid species), making them more difficult to spot. The aphids secrete a honeydew substance which makes the tree’s branches sticky. The honeydew in turn then attracts ants that protect the aphids from their natural predators in exchange for honeydew. Sooty mould can also start growing on this sticky substance, turning the branches black.



Conifer aphids do have many natural predators like ladybirds, certain wasps and chameleons that can keep their population in control. If an outbreak occurs there are a few methods that can be used to decrease the population:

1. Spraying the tree down with a strong stream of water to wash the aphids out.
2. Controlling any ant populations surrounding the trees.
3. Spot spray with organic Pyrethroid based insecticide. It is best to try and let the insecticide just run down the stems and branches to avoid spraying and killing any of your natural predators.
4. If a large infestation does occur, do a once off treatment with organic insecticides combined with organic horticultural oils. Look for products that will not kill your natural predators.
5. If you are not an organic gardener, regular treatments with insecticides formulated for the control of aphids are recommended.


Sucking insects (Aphids, scale, spider mites & white fly) excrete a clear sticky, sugary substance called Honeydew.

As aphids are very vulnerable insects they use the Honeydew to attract ants who collect the substance as food and in exchange protect the aphids against predators.

Unfortunately it is not possible for the ants to collect all of the honeydew available.

This then results in the tree leaves, branches & stems becoming sticky.



The treatment of the aphids above will also deter the ants. Or you could treat for the ants so that the aphids are exposed to the natural predators to control them. There are various treatments for ants available on the market.

Sooty mould

This mould is a result of a by-product, honeydew, that comes from aphids, leafhoppers, mealybugs, scale insects and psyllids.

Sooty mould appears as a black substance growing on the honeydew.
It looks unsightly in the landscape, but is not harmful to trees.

It is most commonly found in periods during the year where we are experiencing high temperatures and the tree is experiencing stress due to a lack of moisture.



The only way to control the outbreak of this fungus is to control to population of the aphids with the appropriate chemical treatment, thus reducing the amount of honeydew that is excreted.

Leaves of the tree can also be washed to dilute the sugar-syrup as well as to remove the black mould.

Red Spider Mites


They are tiny insects (hardly visible to the naked eye) related to spiders and ticks. They are the same as the aphids mentioned above in the sense that they feed by sucking the chlorophyll out of the leaves.

They generally occur during the hot and dry conditions (Mostly in summer) and multiply very quickly. Spider mite population can spread via dust during dry conditions



For small outbreaks:

  1. Remove and dispose the infected leaves.
  2. Wash down the tree with water to remove some of the dust, spider webs as well as the mites.
  3. Keep trees moist.

If the insect population increases to such a degree that it cannot be controlled mechanically, a chemical control will need to be applied:

  1. Look at using insecticidal soaps and horticultural oils.
  2. Liquid sulphur spray or miticides can also be considered.
  3. Spray above and below the leaves of the tree with the chemical treatment

For best results, the spray needs to physically come into contact with the pest.


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Erythrina stem borer


This type of borer is generally present when the trunk is too moist. It feeds on the outer cambium of the bark which causes the bark to become rotten. 



Very little research has been done on the treatment methods for this stem borer. The below treatment is based on our limited knowledge of what we have experimented with.


Cut away any of the infected bark.

Clean the wound and let it DRY OUT. Do not add a sealant or paste.

Erythrina TwigBorer – Terastia meticulosalis


The activity of this beetle causes the growing tips to die off, BUT the result AFTER tip die-back, is the flushing of new leaves and twigs in a much greater abundance than before tip die-back. Hence this forms part and parcel of new and expanding growth of the canopy of this species.


The young larvae of Terastia meticulosalis are found inside the stems of Erythrina herbacea, where their feeding produces a characteristic dying-off of the tip of the host plant. The infestation rate by Terastia meticulosalis can be relatively high. As the availability of undamaged shoots of Erythrina herbacea declines, neonate larvae bore and feed inside the leaf stalks and later make their way inside the stems.

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No treatment necessary.

Erythrina Gall Wasp

The Coral tree is the most popular host for this wasp which is present in varying degrees on more than 90% of all Coral trees. The activity will vary from year to year and within the year, from season to season, but it is likely to always be present on the trees. Except for severe infestations, the wasp does not affect the health of the tree in any significant way.


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No treatment necessary.



Vachellia xanthophloea canker


This is very common to the fever tree. It is generally seen as a fungus infection. When the pathogen is active, it occurs on a wide distribution of fever trees in a short period (1 to 3 months). Infection is encouraged / enhanced by maintaining a moist / dark environment in the infected area. The black is where the cambium has died up to the level / depth of the xylem (heartwood) and the colouring to black of the heartwood is normal and a GOOD sign (self-sealing to prevent further distribution of infection into the stem).



Keep infected areas as dry as possible – drip irrigation as opposed to sprinklers.

Remove all plants casting shade on the base of the stem – the more sun on the areas, the better.

Effective deep watering is the most important treatment

Leaf scorch due to very cold winds

In the Cape Town winter, we can receive very cold winds from the Atlantic Ocean that can dry out a tree’s leaves and the soil around it so quickly that it can’t replenish the moisture fast enough. This results in what is known as leaf scorch in evergreen trees.


Scorch shows up in a number of ways;

  • Scorched leaves have brown, desiccated edges or may be entirely brown and dry
  • Wind scorch may be worst in exposed areas or on the windward side of the plant
  • Large-leaved evergreens are affected worse than small-leaved plants, and pot plants are at particular risk



Prevention is better than cure so we recommend the following steps if you know that your site is prone to a cold wind blowing in winter.

  • Set up a windbreak. Preferably a screen that can filter out 50-60% of the wind. Erecting a solid wall is not ideal as it can deflect the wind over the top and may cause damaging turbulence only a short distance from the barrier.
  • Reduce water loss from the soil by adding mulch.

And once your trees have leaf scorch… 

Cut out the scorched foliage in the spring when new growth starts. Conifers will often fail to regrow from badly damaged shoots and should not be pruned.

To encourage recovery, feed plants in spring with general-purpose fertiliser.


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Black Leaf Spot on Aloes

The following information comes from Sunbird Aloes website:

This may be caused by a variety of fungal or bacterial infections, and is more likely when plants are under stress due to excessive moisture or high temperatures combined with high humidity. Spots are more likely to appear on older leaves. Some aloes have a genetic susceptibility, and often a few spots is unavoidable. This is no cause for concern as it does not affect the plant’s health or flowering. If the spots are really unsightly (e.g. covering more than 10% of the leaf surface) spraying with fungicide may clean up the new growth.




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