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  • Writer's pictureShelby Pietersen

THE BASICS OF COMMON TREE DISEASES IN THE WESTERN CAPE

Tree diseases are common, yet we have found that most homeowners lack knowledge in this area. Here is some information on the most commonly found reasons for decreased health in trees.


Choking the Tree

The most frequent reason for tree death following inadequate watering is being planted too deep. Your tree will be choked if the soil or compost is piled too high above the original root ball. This will infect the tree's cambium layer and cause it to decay.

Symptoms can include unusual leaf browning patterns, leaf loss, deterioration, and possible rot along the root collar.


POLYPHAGOUS SHOT HOLE BORER (PSHB)

The Polyphagous Shot Hole Borer was recently declared a major threat and danger to trees in the Western Cape of South Africa.

However, the path of damage it has left behind has left academics, biologists, horticulturists, tree growers, and landscape architects searching desperately for a remedy.

The issue is that there isn't much that can be done to save a tree once it has been attacked by PSHB. The only thing left to do is to cut down the entire tree, properly dispose of the deadwood, and pray that the insect has not spread to any other trees. The PSHB (Euwallacea fornicatus) can infect more than 200 types of trees and has a wide range of hosts throughout the world.


APHIDS and ANTS

Aphids, spider mites, and white flies are sucking insects that expel honeydew, a transparent, sticky, sweet fluid. They draw nutrients from the plant's leaves and stems in order to survive and procreate.


When aphids are present and eating on the tree, you will see little yellow spots on the tree leaves. As new leaves develop and try to reach their full size, they will appear malformed.

Due to their vulnerability, aphids employ honeydew to entice ants, which use the liquid as food and in return defend the aphids against predators.


Unfortunately, the ants can't gather all of the honeydew that is available. As a result, the tree's leaves, branches, and stems start to become sticky and start to deteriorate.

When aphids are present and eating on the tree, you will see little yellow spots on the tree leaves.


As new leaves develop and try to reach their full size, they will appear malformed.


Treatment:

If there are only one or two aphids on the tree, natural predators like ladybirds can keep the issue under control. The aphid population may grow quickly, the tree could get infected in just a week.

The ants will not return if the aphids are treated (we recommend planting trap crops, like nasturtiums). Alternatively, you may try to remove the ants in order to expose the aphids to their natural predators, which would help control the infestation. On the market, there are numerous anti-ant medications.

The natural predators, however, cannot keep the population under control if the tree is battling an infestation (such as many yellow and malformed leaves). In this case, further treatment measures can be thought about.


APHIDS ON CONIFERS

As soon as the temperatures begin to fall, they appear and might then multiply quickly. As they ingest the sap from the tree branches, the first signs of an infestation would be the yellowing of the leaves.

They are harder to notice since you will find them on the main stems and branches of the tree rather than the new soft growth (as with other aphid types). The honeydew material that the aphids emit makes the tree's branches sticky. In exchange for honeydew, the ants that are drawn to the honeydew defend the aphids from their natural enemies. On this sticky substance, sooty mould may begin to grow, turning the branches black.


Treatment:

Natural predators of conifer aphids include ladybirds, certain wasps, and chameleons, which can keep their population under control. There are a few techniques that can be utilized to reduce the population if an outbreak occurs:

  • Spraying a forceful stream of water down the tree to flush the aphids out.

  • Eliminating any ant colonies around the trees.

  • Applying acetone to the infected areas.

  • Use a spot insecticide with an organic Pyrethroid basis. To prevent spraying and killing any of your natural predators, it is better to try to let the insecticide simply run down the stems and branches.

  • If a significant infestation does occur, perform a one-time application of organic pesticides and horticultural oils. Consider purchasing things that won't harm your natural predators.

  • Regular applications of insecticides designed for the control of aphids are advised if you do not garden organically.

RED SPIDER MITES

They are small, almost invisible to the human eye insects that are linked to ticks and spiders. They feed similarly to the previously mentioned aphids by draining the chlorophyll from the leaves.


They typically appear in hot, dry conditions (mostly in the summer), and they grow swiftly. When it's dry out, dust might help spread the spider mite population.


Treatment:

Small-scale outbreaks:

  • Eliminate and discard the diseased leaves.

  • To get rid of some of the dust, spider webs, and mites, wash down the tree with water.

  • Keep your trees and plants wet.

If the insect population rises to the point where mechanical control is no longer possible, more drastic measures will have to be used:

  • Consider applying horticultural oils and insecticidal soaps.

  • Another option is to use miticides or liquid sulfur spray.

  • Spray the chemical treatment both above and below the tree's leaves.

  • The spray must come into direct touch with the spider mites for optimal results.

SOOTY MOULD

This mould is a byproduct of honeydew, a substance produced by aphids, leafhoppers, mealybugs, scale insects, and psyllids, among other pests. A black substance that appears to be developing on the honeydew is sooty mould.

Despite being unattractive in the landscape, it doesn't harm trees. It is most frequently observed during times of the year when temperatures are high and the tree is under stress from a lack of moisture.


Treatment option:

The only approach to stop the spread of this fungus is to use the proper chemical treatment to reduce the aphid population, which will reduce the amount of excreted honeydew.

The tree's leaves can also be cleaned to remove the black mold and dilute the sugar syrup.


VACHELLIA XANTHOPHLOEA CANKER

The fever tree experiences this disease frequently. It is typically thought to be a fungal illness. When the pathogen is active, it happens quickly on a large number of fever trees (1 to 3 months).


By keeping the diseased region moist and dark, infection is fostered or accelerated. The heartwood's normal and positive colouration of black indicates that the cambium has perished up to the level or depth of the xylem (heartwood), where the black is located (self-sealing to prevent further distribution of infection into the stem).


TREATMENT OPTION:

  • Drip irrigation should be used instead of sprinklers to keep diseased areas as dry as possible.

  • Remove any plants that cast shade on the stem's base; the more sunlight that reaches the area, the better.

  • The most crucial treatment is effective deep watering.

If your tree is infected with any of the above-mentioned pests and diseases, please contact Overberg Arborists for a consultation today.


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