The Often Misdiagnosed Oak Leaf Phylloxera and its Effects on English Oak

November-December 2002

ENGLISH OAK (QUERCUS ROBUR), both the regular and pyramidal variety, is affected by an insect called the Oak Leaf Phylloxera (Phylloxera sp., Phyllox-eridae). This problem is often misdiagnosed in the late summer as fungal Anthracnose. The key to recognizing this pest is to follow the progression of symptoms from spring to fall.

Unfortunately, there is very little written information about this pest. Apparently, the insect overwinters on English oak in the egg stage, which are laid in bark crevices. I have looked for the eggs but have never been successful in spotting them, even on a tree with a high population. The eggs should hatch near the time of leaf emergence. The nymphs walk to the leaves and start sucking sap from the lower side of the leaf. The insect is yellow and oval with a maximum size of approximately 2 mm. They often start sucking at the leaf edge and cause a very characteristic leaf roll. Each Phylloxera feeding site will result in a yellow spot showing on the upper side of the leaf. This damaged tissue will remain yellow for a period of time and then die, turning brown.

When the second generation is produced, the nymphs appear to gather around a large Phylloxera (Mom?). They then will scatter around the leaf or move to newly forming leaves. There are an unknown number of generations produced per year.

Oak Anthracnose (Apiognomonia quercina) is the typical misdiagnosis for Phylloxera. However, the two should not be confused.

As the summer progresses, yellow spots turn brown, new yellow spots are formed and brown spots coalesce into brown blotches. The tree may put out a summer flush of leaves, which will become quickly infected with the Phylloxera. The newly formed leaves will be deformed by the sucking action of the insects while the leaves are undergoing growth.

Oak Anthracnose (Apiognomonia quercina) is the typical misdiagnosis for Phylloxera. However, the two should not be confused. Anthracnose symptoms usually show up in spring just after leaf emergence. Wet spring weather promotes the germination of spores and tissue quickly shows irregular brown blotches. Infection is greatest on lower branches where tissue remains moist. Leaf drop of heavily infected leaves may occur. As the weather dries, new leaf growth may be formed and these leaves should not be affected and show symptoms.

You should determine whether control of the insect is warranted. If the trees show well-defined, small spots on the leaves by the fall, the population should be tolerated. If a large percentage of the leaves are brown by fall, you may wish to attempt control of the insect.

Phylloxera is a season-long drain on the resources of an oak. In the past, I have treated a heavily infected English oak in the middle of the summer. The tree quickly produced over 1 foot of new growth on most tips, which to my thinking was in response to the energy produced and no longer siphoned from the tree.

Ladybird beetles, larvae and adults can usually be found on the infected trees by summers end. However, they may not provide enough population control to reduce symptoms to acceptable levels for most people.

Since the insect is expected to overwinter on the plant in the egg stage, one application of dormant oil in late fall or April should reduce the population significantly. For success, complete coverage of the tree is required. Your sprayer should have enough power and water flow to soak the tree up to the top; non-motorized backpack sprayers should not be used.

If the problem is noticed during the growing season, a contact insecticide can be sprayed on the lower side of the leaves; good coverage is essential. I have had excellent results with insecticidal soap. The soap kills by dehydrating the insect. Every leaf must be covered, as soap has no residual activity; once it dries, it will not kill insects that walk over its surface.

Nursery and Landscape Plant Production (OMAF Publication 383) does not list this pest or insecticides that can be used. Read the label of a registered insecticide; if the label allows the use of the pesticide on generic sucking insects on generic oaks, it can be used on Phylloxera legally.

— Patricia Thomson, B.Sc.F. Consulting Arborist, Kelly’s Tree Care Ltd. with thanks to Alex Bykov, City of Toronto Forestry, for his help in fact-checking my observations.

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