The Willamette mite on grapes

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Introduction

For the past three years the authors have observed extensive damage to grapevines by the Willamette mite, Tetranychus unllamettei McGregor4 throughout the grape-producing areas of Fresno and Tulare counties. Prior to these observations, many growers in these two districts were familiar with the injury, although not aware of its cause. During the past eight years the authors have seen occasional vineyards damaged by this mite also in Sacramento, Santa Clara, Stanislaus, San Joaquin, Yolo, and Sonoma counties. In the spring of 1938 and of 1939 the mite was unusually abundant in Stanislaus and San Joaquin counties, and in 1945 was abundant in Sonoma County.

The species was named and described by (McGregor (1917)), who collected the type material on leaves of white oak, Quercus lobata, at Oregon City, Oregon, August 22, 1915. He stated, “The presence of this species on oaks is revealed from a distance through the rusty appearance.” It has also been reported on elm (Garman, 1940)), and on pears and apples (Gentner, 1937); (Newcomer, 1941); (Overholser, Overley, and Allmendinger, 1944). (Lamiman (1935)) reported the presence of the Willamette mite on grapes, often in association with Tetranychus pacificus McGregor. He states that the Willamette mite “confines its attacks to spring and early summer, disappearing during hot weather.”

Symptoms of Injury

Affected vines in the summer and fall are a characteristic rusty yellowish green, especially the older, basal leaves. On weakened vines with short growth, all leaves may present such an appearance. The mites characteristically feed and produce colonies on the under surface of the leaf. This localized feeding produces injured areas which turn straw colored and then become scarious. This effect is visible in the upper leaf surface as chlorotic-green, straw, or straw-bronze discolorations. The initial feeding is confined to areas enclosed by the larger net veins, and a narrow strip of leaf tissue close to the larger veins remains green. The smaller net veins and often sections of the larger net veins, and the areas they enclose, lose their chlorophyll. During early stages of injury the chlorotic areas may be small and confined within fine net veins. They may consist of a very few spots scattered or grouped, or many spots evenly covering the entire leaf, in appearance sharply differentiated from the strip of normal green tissue banding the veins. This condition results in a faint chlorotic-green mottled appearance on the upper surface, somewhat similar to and easily confused with the mottle of Pierce’s disease of grapevines, especially in late spring.