Natter’s Notes
Blister Mites: Grapes; Pears; Walnuts
Jean R. Natter, OSU Master Gardener
Blister mites are running wild this season. It seems that the perfect conditions combined for population explosions of these tiny, elongated mites which noticeably disfigure the leaves of grapes, pears, walnuts, and more. Among pears, even newly planted trees are victims. The symptoms are similar on all three genera but the specific causal agents – also referred to as eriophyid mites or simply eriophyids – vary.
Blister or eriophyid mites reside on the undersides of the leaves, injecting fluids into the leaf tissue as they feed. Those fluids cause the blistering on the top surface with the associated white “fuzz” (enlarged plant hairs) in the concave areas on the reverse. (So, no, it’s not fungal growth.)
Blister mites are far different than the more common spider mites. Adults are microscopic, light in color, cylindrical, tapered at the posterior end, with two pairs of short legs just behind the head. Nymphs are the same but are smaller.
Grape Erineum Mites (Colomerus vitis) are microscopic, wormlike, with 2 short pairs of legs at the head end, and white-yellow in color. They overwinter between the outer bud scales and bud tissue and feed on leaves during spring and summer. Feeding from the undersides of the leaves produces a blistered appearance on the top of the leaves. At the same time, the corresponding depressions on the underside are filled with enlarged light-colored leaf hairs which shelter the mites from natural enemies and pesticides. In spite of how extensive and nasty-looking the infestation is, blister mites seldom affect grape health or production.
As the season progresses, the enlarged leaf hairs progress from white to yellow and, finally, brown. Then, from mid-August until leaf drop, the mites return to their overwintering sites beneath the bud scales.
Home management for grape erineum mites: Sprays aren’t needed. “Dormant-season oils and insecticides used for other pests and sulfur applications for powdery mildew usually control this pest.”
Pear Leaf Blister Mites (Eriophyes pyri) feeding causes reddish to yellowish green blisters on the top surface of the leaves, often in 2 lengthwise rows, one at each side of the main vein. With time, the blisters turn brown or black. Leaves may drop prematurely. Loss of excess foliage weakens trees, reduces shoot growth, and interferes with fruit maturation and fruit bud formation. Feeding on fruit causes irregular, russeted spots which feel rough and somewhat scaly. PNW Insect says “Eriophyid mites move from tree to tree, perhaps by wind or carried on birds or insects.”
Home Management for Pear Leaf Blister Mites: Lime sulfur applications in the fall can significantly reduce populations of these mites the following year. (The trick, though is to obtain it in small home-use packaging.) Applications before bud swell can also be effective. Or apply superior oil as buds begin to swell.
Walnut Blister Mites (Aceria erineus)
Just as do the previous two mites, walnut blister mites overwinter beneath bud scales. When springtime temperatures rise, the mites feed among the leaf hairs on the undersides of the leaves. Several generations occur during the summer, which attack new foliage as soon as it unfurls.
Home Management of Walnut Blister Mites: Naturally-occurring predator mites almost always keep mites under control if broad-spectrum insecticide applications are avoided. Heavy rain and cold weather also suppress mite numbers.
Broadleaf weeds like mallow, bindweed, white clover, and knotweed enhance mite numbers. Avoid excessive nitrogen applications, as this encourages mites.
Horticultural oil is the only spray suggested for home-use.
Resources
Eriophyid Mites: https://utahpests.usu.edu/uppdl/files-ou/factsheet/eriophyid-mites2010.pdf
Grape Erineum Mites: https://pnwhandbooks.org/insect/small-fruit/grape/grape-grape-erineum-mite
Pear Leaf Blister Mites: https://pnwhandbooks.org/insect/tree-fruit/pear/pear-eriophyid-mite
Walnut Blister Mites: https://pnwhandbooks.org/insect/nut/walnut/walnut-walnut-blister-mite
(Click the link below for PDF containing the above text and all the images.)
