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UC Cooperative Extension Ventura County
669 County Square Dr. Suite 100
Ventura CA 93003
Phone: 805.645.1451
Fax: 805.645.1474

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Walnut Blister Mite

The blister mite (Phytotus tristriatus) is microscopic in size, and you need at least a 10 x magnification to see the mites crawling among the hairs in the blister on the undersurface of the leaves. The mites are creamy white and cigar-shaped. The feeding activity of the mite causes an irritation to the leaf tissues, which results in the plant producing the blister-like structures. The damage is primarily aesthetic as it only impairs the visual beauty of the tree. The mites can also infest the green husk of the nuts causing a russeting or blackening of that tissue.
The infestation will continue to spread until most or all the leaves on the tree show symptoms. The mite survives in the twigs and buds on the tree over winter when no leaves are present. So if you wish to control this mite, I suggest you spray the tree during the winter (January/February). Spray the dormant twigs to “run-off,” so that all twig, branch and trunk surfaces are thoroughly covered with the spray. The dormant spray may have to be repeated for several winters to clear up the problem, but a marked reduction in infested leaves should be noticeable in the spring after the first dormant spray.