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Saturday, July 18, 2015

Look For Clues to Spider Mite Damage on Cypress

Q. I have a group of Italian Cypress in my back yard.  I am completely new at this (growing trees and gardening) because I have lived in apartments and condos until recently.  I planted them, they looked very happy, but in early summer my gardener pointed out they were being attacked by spider mites and said I should do something quickly or they will die.  Well, they survived and did not die, and the new growth on top seems to be green but the growth on the bottom is still pretty brown.  I asked at a nursery and was told it will never green up and I have never seen needles fall from the tree, so I suspect he is right (though I am suspicious because nurseries sell trees).  My wife on the other hand is certain that, given a couple of years the problem will go away.  I was going to replace all the trees (they are like 15 feet tall and 3 years old, so they grow very fast) but that is upsetting my wife.  I would like to know who is right, the nursery or my wife (who did grow up around trees).

Readers Italian Cypress
A. I am in the middle on this one. I did look at the original pictures enlarged and it certainly could be spider mite damage. I would be looking for the needles having a "dusty" appearance (tens of thousands dead, cast "skins" of spider mite making a surface "dust" on the foliage. I would also look for perfectly round eggs with a magnifying glass or dissecting scope). Without seeing the branches in person and the extent of the damage it is hard to judge. If the damage has extended all the way into old growth in the interior they will not recover but be permanently brown. If the damage is light and there is some green growth remaining it is possible for them to slowly recover. 

Electron micrograph of spider mite feeding
Take the branches with dried “needles” and bend them strongly. If they are dry and snap they will not recover. If they are supple and do not snap like a dry twig they may recover. That is the best I can tell you. It may not have been spider mites. I would have to see the foliage very closely and inspect it for mite “residues”. Not all spider mites make webbing and some webbing is caused by spiders so seeing webbing alone does not guarantee it is spider mites.
Webbing like this may or may not indicate spider mites are present. Look for other clues too.
During the heat of the summer I would hose down the Italian cypress once  a month or so or after any kind of “dirt” storm. Dirt or dust on the leaves interferes with the natural control of spider mites by predatory mites and beneficial insects. There is a natural “ecosystem” at work on cypress that keeps the bad critters under control. Spraying insecticides or miticides can interrupt this natural control system by killing off the beneficials.

Do not irrigate Italian cypress too frequently. If watered often it can cause root damage and also create the same kind of browning. These are Mediterranean plants and do not tolerate wet soil during warm weather. Water them no more than about once a week if you have a “normal” soil but give them a thorough watering when you do.

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