Dirk Bellstedt, James Deane and Faye Crankshaw a.k.a. “The Tree Musketeers”
Now that spring is a thing of the past and summer is finally with us, a clear look into the sky instead of a look through the leafy branches of an oak tree is an indication of the devastation that the Polyphagous Shot Hole Borer (PSHB) and its associated fungus has wreaked in Somerset West over the past year.
In September 2019, I took two photographs of the historic oak trees in Harewood Avenue with their beautiful spring leaves.
Now this registered heritage site is no longer, the trees have recently been removed by the City of Cape Town’s Invasives team and their leaves and trunks have been put through the shredder, in an effort to destroy the beetle.
The effect that this beetle has had on the trees of Somerset West, is vividly illustrated by this example, but there are many, many more English oaks or European oaks, that have been removed permanently from the gardens and streets of Somerset West over the last couple of months.
Known by the Latin name of Quercus robur, they were originally brought to the Cape from the grachten (water canals) of Amsterdam by Simon van der Stel, who owned the Vergelegen Estate that Somerset West residents know so well.
Although there are many other oak species that are also grown in the Cape, the English or European oak trees are the ones that are simply referred to as oaks and are synonymous with the old Cape Dutch farmsteads of the Cape.
One wonders for how much longer these homesteads will be accompanied by them? Unfortunately, oak trees have for as long as most of us can remember been not entirely healthy.
They always suffered from powdery mildew which causes their leaves to go grey and fall off prematurely in autumn and suffered from a fungal wood-rot infection of the centre of the tree.
Many of the older oaks were hollow, yet they could survive because the outer ring of wood and bark was still able to transport enough water and nutrients to their branches.
Sadly, the shot hole borer specifically drills into and multiplies in the first few centimetres of wood under the bark of oak trees.
Clearly this is the last straw for many of these trees and they remained as bare skeletons, whilst other trees sprouted their green leaves in this year’s spring.
Stem treatment of oak trees has saved them in some instances, but often with considerable die back. However, severe cutting back of the tree has managed to save a few of them.
Many cork oaks are also planted along the streets of Somerset West, such as those along Old Stellenbosch Road, and they are also heavily infested although they are not dying yet.
Other oak trees such as pin oaks are apparently almost completely resistant to the borer beetle, in spite of the fact that they are listed as a susceptible species by the Forest and Agriculture Biotechnology Institute (FABI) of Pretoria University.
It would therefore appear that there are differences in the sensitivity of tree species to the borer beetle in different regions of the country.
We have also observed that plane trees, of there are two types, the London plane and the American plane, are also heavily infested but have not succumbed like the oak trees and somehow seem to be holding their own.
The infestation on these trees can be easily seen as small holes oozing sap over their grey and brown bark. In the USA, plane trees are called sycamores and due to a similar infestation by the borer beetle of sycamores in California, they have died there in huge numbers, yet here, so far, they have survived.
It therefore makes sense to consider treating your plane tree by spraying its trunk and applying symbiotic bacteria, fertilizer and especially water to the soil under these trees.
The borer beetle infestation has certainly sensitized us much more to the fact that trees, just like other plants require TLC in the form of water and fertilizers as well.
The removal of paving to allow better watering may even save the life of your tree!
Another tree that according to the FABI list should not have survived is the box elder, also known as the boxelder maple, Acer negundo, that comes from North America.
Of the first infestations by the borer beetle that were observed in Somerset West were on this tree species, and many of the trees were badly affected and were removed to curb the spread of the borer beetle.
Now many of the trees that were treated have recovered and have resprouted healthy foliage and, in some places, even unsprayed box elders have survived.
Another tree that shows heavy infestation is the beefwood (Casuarina cunninghamiana) which is often planted as a windbreak between fruit orchards.
This tree is classified as an invasive species and its sensitivity to the borer beetle is perhaps a blessing in disguise as the borer beetle may bring its spread under control.
However, the fact that the beefwood is planted as a windbreak between orchards means that it played a role in the deciduous fruit industry.
The borer beetle can, in fact, infest plum, peach and nectarine trees and urgent research on these trees is currently being conducted on Vergelegen and Lourensford Estates in an effort to curb the effect of the borer beetle on the deciduous fruit industry, which is a major source of employment in the Western Cape.
Similar to beefwoods, the invasive poplar tree, Populus canescens, which grows in many of the river courses in parks in Somerset West, becomes heavily infested, but so far, no dying off of poplar trees has been observed.
Graciously, many other tree species appear to be spared of the borer beetle infestation.
Trees such as white stinkwoods, the previously mentioned pin oaks, and all conifers remain uninfected. Only time and another season will tell which trees will survive and which will not.
For those of you interested in following the latest news on possible treatments and control we can advise that you join the “Stop Borer Beetle” whatsapp group, which has many anti-borer members!
