Grape erineum mite—Colomerus
vitis
This eriophyid mite (family Eriophyidae) can affect grapevines wherever they grow. The mite can be common on vines not sprayed with sulfur to control powdery mildew or other pests because sulfur also controls the mites. There are at least three strains of the mite (bud strain, leaf-curl strain, and erineum strain), which may be different species. Note that numerous species of mites can occur on plants.
Identification
Because the mites are minute, their presence is generally diagnosed by their characteristic damage symptoms. Feeding by the erineum strain causes roundish, warty swellings on the upper surface of grape leaves and discolored, felty, roundish patches (erinea) in depressions on the underside of leaves. The plant symptoms from the bud strain and leaf-curl strain of the mite can be less obvious as described below under “Damage.”
The mites are carrot or wedge shaped, pale yellow to whitish, and about 1/100 inch (0.25 mm) long. Deutonymphs (first-stage immatures) are about one-half the length of adults. Protonymphs (older immatures) are about the same length as adults. Adults and nymphs have 4 legs at the head (wider) end of their body. The oval eggs are translucent to white and about one-fourth the length of adults.
Life cycle
Grape erineum mite develops through four life stages: egg, protonymph, deutonymph, and adult. The mites of all three strains overwinter under bud scales and in crevices of terminal twigs. Unlike the erineum strain that moves from overwintering buds to feed on young leaves, the bud mite strain and leaf-curl strain remain feeding in buds throughout the year. Because the mites actively move, they can be found on any plant part. They readily spread as contaminants on animals (e.g., insects), equipment, and hands that contact grape and by splashing rain and wind.
During warm weather, development of eggs into reproductive, adult females occurs in 2 to 3 weeks. Grape erineum mite has several generations per year.
Erineum strain. When newly flushing leaves develop in spring, the mites move from buds to feed in groups on the underside of succulent, immature leaves. Mite feeding causes the plant to produce masses of enlarged leaf hairs (erinea) in a depression. As the mites reproduce and increase in abundance, some move to new locations on the same leaf or other leaves. Erinea form 10 to 30 days after the mites start feeding, varying with the rate of leaf development. The mites are unable to induce formation of new erinea once leaves mature and harden. From mid-August through leaf drop, the mites move to bud scales where they overwinter.
Damage
This species feeds only on grapes. Although feeding of grape erineum mite alters the appearance and growth of infested grapes, infestation by these mites may not reduce the yield amount or quality of grapes. The mite may also vector Grapevine pinot gris virus, which causes leaf deformation and mottling and stunted shoot growth. Whether the virus is of importance in residential grapevines is uncertain.
Bud mite strain. This strain of C. vitis feeds in buds and does not induce the production of erinea. Infested buds can be killed, so no shoots emerge from them. Lateral shoots may proliferate from where the terminal bud was killed. If tissue grows from the infested buds, it can be blisterlike. There can be scaring of twig bark, flattened canes, and shortened internodes that give growth a bushy (witches' broom) appearance.
Erineum strain. Hemispherical, warty growths on the upper leaf surface corresponding to the location’s felty masses on the leaf underside are the main damage symptoms of feeding by the erineum strain of the mite. For years this was the only recognized mite strain and type of damage caused by grape erineum mite. The leaf swellings early in the growing season are initially green, then turn yellow and by August become brown. The erineum patches on the underside are initially whitish, then turn yellow and finally reddish brown. Some infested leaves may only partly expand and can die and drop prematurely.
Leaf-curl strain. Mites feeding in leaf buds results in downward curling of emerging leaves, ranging from slight curling to severe curling into a ball. Affected leaves may be associated with stunted shoots, increased growth of lateral shoots, scaring of shoot bark, and necrosis (darkening) of leaves' underside. As with the bud mite strain, erinea are not induced by feeding of the leaf-curl strain.
Solutions
No control is warranted as this mite's feeding generally does not affect fruit yield. If the mite's damage to grapes is intolerable, micronized or wettable sulfur can be applied around bud break to reduce the abundance of grape erineum mite and powdery mildew. Applying sulfur in the fall after harvest but before leaf drop can reduce mite damage the following growing season.
To better time a fall application, wrap several grape twigs near distorted, mite-infested leaves with double-sided, transparent sticky tape as pictured in Grape Erineum Mite: Postharvest Sulfur Use Reduces Subsequent Leaf Blistering (PDF). Remove, examine under magnification, and replace these tape traps at least weekly. Apply sulfur when abundant mites are caught in the traps as they migrate from leaves to overwinter on buds, generally during late September through October. Note that spraying sulfur during hot weather or within 1 month before or after applying horticultural (narrow-range) oil can damage foliage.
Adapted from the publication above and Grape Pest Management, Third Edition from the University of California also available as an eBook. |
Roundish, warty swellings on upper leaf surface caused by feeding of grape erineum mite on the leaf underside.
Roundish swellings from feeding of grape erineum mite shown close-up.
Erineum pockets on the underside of a leaf.
Grape erineum mites magnified.
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