Pest Damage Symptoms Identifying Mite Presence Infestation Patterns Management More information
Patty Skinkis, Oregon State University
Introduction
Grape rust mite (Calepitrimerus vitis Nalepa), an Eriophyid mite species, is known to be problematic in some grape-growing regions of the Pacific Northwest. Grape rust mites are obligate pests of Vitis vinifera grapes. Because of their economic damage potential, the mite now holds a permanent pest status in many grape-growing regions of the world including Germany, France, Spain, Switzerland, Italy, Brazil, Australia, and the Pacific Northwest of the USA (Bernard et al., 2005; Duso et al., 2010; Walton et al., 2007).
Pest Damage
Grape rust mites are very small (0.15 mm or 1/100 inch long). They are phytophagous (feed on plants), preferring to feed on young grape tissues in early spring, causing damage that leads to deformation of shoots and leaves and results in surface necrosis described as scarring. It is believed that mites prefer newly developed tissues, potentially due to higher nutrient concentrations; however, this preference is not verified. The mite feeds on plant cells by inserting its stylet (mouthparts) into a cell and consuming its contents, taking 10 to 20 minutes to feed. The saliva of some Eriophyid mite species is found to contain plant hormones such as auxin and cytokinin analogs. When deposited onto or into plant cells, these hormones cause deformities in expanding plant tissue (Petanović and Kielkiewicz, 2010). Generally, rust mites are found to feed only on cells in the epidermal (surface) layers of tissues, never penetrating the phloem layer.
It may seem unlikely that rust mites will cause damage by feeding on individual cells. However, large populations in small areas of tissues such as buds and young shoots can cause significant damage such as reduced photosynthetic efficiency, reduced gas exchange, and stunted growth. The wound signal responses of the plant trigger feeding by other Eriophyid mites and activity of predatory insects and mites (Petanović and Kielkiewicz, 2010). Rust mite damage is greatest in cool regions with cool springs when mite populations are active and shoots are small and growing slowly. In warmer grape-growing regions, faster rates of shoot growth in spring likely prevent development of signs and symptoms often associated with rust mite infestations (Walton et al., 2007).
Symptoms of Rust Mite Feeding
When present in high populations on young tissues in spring, rust mites can cause significant damage to tissues. Spring stunting is most common and most visible in the eight to ten leaf stage of development. Symptoms associated with rust mite feeding include the following:
- stunting of young shoots early in spring
- scarring of stem tissues, often on the basal portion of shoots
- deformed, puckered leaves
- damaged inflorescences (which can lead to reduced yields)
- bronzing/discoloration of leaves in late summer
Shoots often outgrow the stunting by mid- to late spring and normal growth ensues. However, vines with particularly high populations of rust mites may have damaged shoot tips and flowers as a result of significant feeding shortly after bud break. In these cases, there can be reduced yields. Vineyards that have high populations of rust mites in the canopy during the summer may have discolored leaves. This discoloration appears to be dependent on climate and pest population density. Discoloration first appears as a blackened area that may change to bronze-red later in the season. This can be an indicator of potentially high populations of mites moving into overwintering sites that may cause damage the following spring. In some cases, high populations of rust mites do not result in discolored leaf tissue.
Identifying Mite Presence
Grape rust mites are shown here on the stem tissue of a young shoot in spring. Photo by R. Paul Schreiner, USDA-ARS.
It is critical to determine if mites are present and causing damage before implementing management practices. Many factors can contribute to stunted growth during spring, including apical dominance, frost, cold damage, thrips, phylloxera, nutrient deficiency, compromised graft union, damaged roots, and low carbohydrates in storage reserves. Verifying mite presence in vineyards will require systematic collection of shoots and scanning the surfaces with a stereomicroscope. These mites are very small (0.15 mm or 0.001 inch long) and difficult to see without at least 40 X magnification (60 X to 80 X magnification is best for verifying the presence of rust mites).
There are a number of time points at which to sample tissues: dormant wood (buds, under bark of canes, trunk head), shoots/leaves in early spring, and leaves in summer. Mites are known to overwinter in outer bud scales and may be found by peeling back those scales on dormant shoots. You may also consider looking under bark of the trunk, cordon, and canes. It is likely higher mite populations are under the bark in comparison to outer bud scales and may suggest better estimates of mite density. Spring is the best time to scout for mites using shoot material. Mites are more easily observed under the microscope on short shoots in early spring than larger shoots later as there is less surface area to scan. Mites have been observed on nearly all areas of the shoot of infested vines, including leaves, stems, shoot tips, and inflorescences. However, there is often higher incidence and density of mites on the basal one to two leaves. Also, mites are found on the underside of leaves. Rust mites tend to hide under thick mats of plant hairs located near veins that merge near the petiole end of the leaf. These plant hairs must be removed with a tweezer to see the mites underneath. During summer, leaves are collected and scanned under magnification. There is much more surface area to scan through, making it difficult to determine population density, but scanning leaves can help detect the presence of mites.
Knowing what to look for
Under magnification, grape rust mites are shaped like wedge-shaped rice grains and have four legs on the anterior portion of their body. The posterior of the body is tapered while the anterior is more broad and angular. Depending on age, grape rust mites vary in size and color. Younger mites look white and become more amber as they age. Mite eggs can be found around adult mites, and they are spherical and clear to translucent white.
Infestation thresholds
While scouting for mites will give you an idea of presence of the pest, it is hard to determine density of the mite population and impact on vine growth. There are few data available to indicate economic threshold levels of the mites from in-season or dormant-season monitoring. However, approximately 400 to 1000 mites per spur were found to cause significant damage to leaves and shoots in Australia, based on sticky tape traps intercepting mite migration from overwintering sites in spring (Bernard et al., 2005).
Infestation Patterns
During the past five years, research has been conducted on grape rust mite in Oregon to better understand pest infestation and damage. Presence of grape rust mite and associated feeding damage has been largely associated with young vineyards (less than three years old) in the coolest growing regions in Oregon. Similarly, presence of the rust mite has been linked to young vineyards (one to two years old) in Europe (Zandigiacomo and Frausin, 1998). These findings suggest that nursery stock may already be infested with Eriophyid mites at planting. However, there are instances where mature vineyards are infested with grape rust mites, and in both young and mature vineyards, within-vineyard patterns of infestation appear random and clustered. This distribution pattern may be due to grape rust mite's mechanism of dispersal. Grape rust mites can move within the vine canopy, by wind, and by human carriers. Mites are most effectively moved by wind, followed by movement from vineyard laborers (Duso et al., 2010; Michalska et al., 2010).
Mite behavioral research indicates mites have the ability to distinguish between host and non-host plants. If grape rust mites are on a host vine, they are able to feed and move more slowly than if they were on a non-host plant (Michalska et al., 2010). However, their ability to “choose” a particular vine or vineyard is not understood. The issue of host choice may be a combination of mite preference and other factors such as population magnification in the proper environment, loss of natural predators, and vine physiological factors, rather than the attraction of large mite populations to a given vineyard site.
Managing Grape Rust Mite Populations
Early-season feeding by grape rust mites has been effectively managed by using appropriately timed sulfur applications at the delayed dormant (woolly bud) stage and another spray applied at bud break (Bernard et al., 2005; Walton et al., 2007b). However, this spray strategy is not effective if cool, rainy weather persists. During conditions of cool weather, growth of tissues is very slow, potentially leading to compounded effects of mite feeding.
Because mite presence later in the growing season may not cause visual symptoms, it is difficult to determine if sprays are warranted unless mites are monitored and damage thresholds are determined. Late-season sprays have not been effective in managing pest populations because by mid- to late summer, mites make their way into overwintering sites (bud scales of canes and bark of vine heads, cordons, and trunks) and, thus are already protected.
During the growing season, miticides are generally avoided to prevent significant reductions in populations of predatory mites, which can help control this pest mite. Sulfur sprays applied to vineyards as fungicides may also help reduce summer grape rust mite numbers; however, populations can increase once these sprays are stopped and alternative fungicides are used.
Literature Cited
Bernard, M. B., P.A. Horne, A.A. Hoffmann. 2005. Eriophyoid mite damage in Vitis vinifera (grapevine) in Australia: Calepitrimerus vitis and Colomerus vitis (Acari: Eriophyidae) as the common cause of widespread ‘Restricted Spring Growth’ syndrome. Experimental and Applied Acarology 35: 83-109.
Duso, C., M. Castanoli, S. Simoni, G. Angeli. 2010. The impact of eriophyoids on crops: recent issues on Aculus schlechtendali, Calepitrimerus vitis and Aculops lycopersici. Experimental and Applied Acarology 51: 151-168.
McNally, P.S., C. Fogg, J. Flynn, J. Horenstein. 1985. Thrips damage in Chenin blanc grapes. California Agriculture. Jan-Feb:23-25.
Michalska, K., A. Skoracka, D. Navia, J.W. Amrine. 2010. Behavioral studies on eriophyid mites: an overview. Experimental and Applied Acarology 51:31-59.
Petanović, R. and M. Kielkiewicz. 2010. Plant-eriophyoid mite interactions: cellular biochemistry and metabolic responses induced in mite-injured plants. Part I. Experimental and Applied Acarology 51:61-80.
Petanović, R. and M. Kielkiewicz. 2010. Plant-eriophyoid mite interactions: specific and unspecific morphological alterations. Part II. Experimental and Applied Acarology 51:81-91.
Walton, V., A.J. Dreves, D.H. Gent, R.R. Martin, U. Chambers, P.A. Skinkis. 2007. Relationship Between Rust Mites, Calepitrimerus vitis (Acari: Eriophyidae), Bud Mites, Colomerus vitis (Acari: Eriophyidae) and Short Shoot Syndrome in Oregon Vineyards. International Journal of Acarology 33:1-12.
Walton, V., A. Dreves, P. Skinkis. 2007b. Short Shoot Syndrome of Grapes in the Pacific Northwest. OSU Extension Publishing. EM-8944-E.
Zandigiacomo, P. and C. Frausin. 1998. Problemi entomologici del vivaismo viticolo in Friuli-Venezia Giulia. Informatore Fitopatologico 48:10–14.
Recommended Resources
Rust Mite Monitoring in Vineyards video, Oregon State University
Grapevine Growth Distortions – A Guide to Identifying Symptoms, Oregon State University Extension
Short Shoot Syndrome of Grapes in the Pacific Northwest. EM-8944-E, Oregon State University Extension
Reviewed by Eric Rebek, Oklahoma State University and Elina Coneva, Auburn University
