The Basics of Common Tree Diseases in the Western Cape

Tree problems are common in the Western Cape. What we find though is most homeowners do not know what they are looking at until the tree is already stressed. Here are a few of the most common reasons we see behind poor tree health and what to watch for.

Choking the tree

One of the most common reasons a tree dies, even when it is being watered, is planting it too deep. This happens when soil or compost is piled up over the original root ball. The trunk base stays damp, the cambium gets infected, and decay starts.

Symptoms:

• Unusual leaf browning

• Leaf drop that does not match the season

• General decline and dieback

• Rot or soft tissue around the root collar

Polyphagous Shot Hole Borer (PSHB)

The Polyphagous Shot Hole Borer has become a major threat to trees in the Western Cape. The damage left behind has had academics, biologists, horticulturists, growers, and landscapers searching for answers.

The hard truth is this. Once a tree is properly attacked by PSHB, there is often not much you can do to save it. In many cases, the only responsible option is to remove the tree, dispose of the wood correctly, and reduce the chance of spread.

PSHB (Euwallacea fornicatus) can infest more than 200 tree species and has a wide host range worldwide.

Aphids and ants

Aphids, spider mites, and whiteflies are sap-sucking insects. They feed off leaves and stems and leave behind honeydew, a clear sticky sugary liquid. That honeydew attracts ants. Ants then protect the aphids from predators so the infestation gets worse.

What you will notice:

• Small yellow spots on leaves

• New leaves that look twisted or misshapen

• Sticky leaves, branches, and stems

• Overall weakening of the tree

  
  

Treatment: If you only have a few aphids, natural predators like ladybirds can keep it under control. The problem is aphids multiply fast and a tree can look worse within a week.

If you treat the aphids, the ants usually stop coming back. Trap crops can also help. Nasturtiums are a good option. You can also target the ants so the aphids lose their protection. There are many anti-ant products available.

If the tree is already showing heavy symptoms like lots of yellowing and malformed leaves, predators alone often cannot keep up and stronger treatment may be needed.

Aphids on conifers

Conifer aphids often show up as temperatures start dropping and they can multiply quickly. They feed on sap in the branches and stems. They are harder to spot because they sit on the main stems and branches rather than soft new growth.

Signs to look for:

• Yellowing foliage

• Sticky branches from honeydew

• Ant activity around the tree

• Black sooty mould forming on the sticky areas

  
  

Treatment: Natural predators include ladybirds, certain wasps, and chameleons. If the population spikes, these steps can help reduce numbers.

• Spray a strong stream of water down through the tree to flush aphids out

• Remove or treat ant colonies nearby

• Apply acetone to infected areas where appropriate

• Use a spot insecticide with an organic pyrethroid base and let it run down stems and branches instead of blasting everything

• If infestation is serious, a once-off application of organic pesticides and horticultural oils can helpIf you do not garden organically, regular aphid-control insecticides can be used as directed

Red spider mites

Red spider mites are tiny, often almost invisible, and related to ticks and spiders. They feed like aphids by draining chlorophyll from leaves. They love hot dry conditions and they multiply fast. Dust also helps them spread.

Treatment:

For small outbreaks:

• Remove and discard badly affected leaves

• Wash the tree down with water to remove dust, webbing, and mites

• Keep plants well-watered to reduce stress

  
  

For bigger outbreaks:

• Use horticultural oils and insecticidal soaps

• Miticides or liquid sulphur spray can also be used

• Spray needs to hit both sides of the leaves

• The chemical must make direct contact with mites for the best result

  
  

Sooty mould

Sooty mould is the black coating that grows on honeydew left behind by pests like aphids, leafhoppers, mealybugs, scale insects, and psyllids. It looks bad, but the mould itself usually does not harm the tree. It is most common in hot conditions when trees are stressed and moisture is low.

Treatment:

• Control the pest causing the honeydew with the correct treatment so the honeydew stops

• Wash leaves and branches where possible to remove the black coating and dilute the sticky residue

Vachellia xanthophloea canker

This disease is commonly seen on fever trees and is usually linked to fungal infection. When active, it can move fast, sometimes affecting large numbers of fever trees within 1 to 3 months.

Moist shaded conditions around the stem can encourage infection. A black staining in the wood often indicates the cambium has died down into the heartwood, and the tree is trying to seal off the spread.

Treatment:

• Use drip irrigation instead of sprinklers to keep the stem as dry as possible

• Remove plants that shade the base of the stem so sunlight and airflow can reach it

• The most important support is correct deep watering

If your tree is showing any of the signs above, get it checked early. Contact Overberg Arborists for a consultation.

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