Botrytis bunch rot can produce significant yield and quality losses in tight-clustered wine varieties. In addition to desiccation and rot, the disease may provide an entrance for secondary microorganisms that cause additional fungal rots or bacterial sour rots.
Symptoms consist of brownish rotted fruit, usually with tufts of gray fungal growth (hyphae and spores) on the berry surface. The fungal growth usually begins where the grape berry skin has cracks or scars, and will spread over the entire berry, giving a gray moldy appearance. Fungal growth and spread are enhanced by rain, dew, heavy fog and prolonged overhead irrigation.
Cultural Control
Cultural control practices go a long way in managing Botrytis bunch rot. These include:
- Removal of infected cluster mummies (infected dry clusters) from the vines at pruning and discing into the soil.
- Not overhead irrigating vines, especially after bunch closure
- Use of canopy management such as shoot thinning and fruit-zone leaf removal. These practices increase air circulation around clusters and improves spray penetration, and may reduce disease incidence and severity.
Fungicide Programs
While Botrytis bunch rot is not an annual disease in eastern Washington, intervention with fungicides may be necessary. Rather than a season-long control program like what is needed for grapevine powdery mildew, fungicide applications for Botrytis bunch rot can be limited / targeted to these key phenology stages:
- Bloom to early fruit set – Lots of flower parts and debris can be good food sources for the fungus.
- Start of bunch closure – The fungus can get trapped inside clusters, along with other plant debris, and remains protected from pesticide exposure and environmental extremes
- Ripening / 16 Bris through harvest – As the fruit accumulates sugar, it becomes a better food substrate for the fungus. In addition, tight clusters can often squeeze / crack open berries, resulting in leaking juice the fungus can then feed on.
Additional applications around the harvest period may be necessary during prolonged periods of wet weather. Because fungicides can inhibit yeast fermentation, use them with caution within 1 month of harvest on wine grapes.
Some fungicides and fungicide premixes provide control of both powdery mildew and bunch rot when applied according to label instructions. These should be considered at critical times.
Resources
For additional control information see Botrytis Bunch Rot in Commercial Washington Grape Production: Biology and Disease Management (FS046E).
The bacterium infects plants through wounds (such as those that occur during propagation of cuttings) or through root lesions. The bacterium survives systemically in symptomless grapevines and can be carried in dormant grape cuttings used in the propagation of plants. Once introduced to a site, the bacterium can survive on root debris in the soil. Soil fumigation has not been effective in eradicating the crown gall bacterium.
Gall symptoms express after events that increase grapevine callus tissue production, such as after cold damage events or during field grafting. Therefore, after potential winter cold events, it is important to scout vineyards for the development of crown gall symptoms.
The first symptoms are swellings on wood that is more than 2 years old, and typically show up in early summer (June). Young galls are soft, creamy to greenish, with no bark or covering. As the tissue ages, it darkens to brown. The surface can become hard and very rough and black as it dies.
Cultural Practices
- Planting Material (Vineyard Pre-Planting): Planting WSDA, ODA or CDFA certified stock reduces the potential risk of crown gall. This is because certification policies require nurseries to start with plant material that as gone through a Clean Plant Program, such as Foundation Plant Services at UC-Davis or the Clean Plant Center Northwest at Washington State University.
- Vineyard Post-Planting:
- Protect vines from freezing injury. For information on freeze injury and cold damage, visit the Washington State University Grapevine Cold Hardiness Monitoring webpage.
- Avoid wounding plants during cultivation
- Prune grapevines to below visible calls. The vines will remain infected, but new growth will not exhibit symptoms until a damage event occurs.
Treatment Programs
We currently do not recommend any topical treatments of active galls in Washington vineyards. While some treatments may help reduce overall gall size, no topical treatment can rid the vine of the bacteria, and future damage events will trigger additional gall formation.
Resources
See the Washington State University Grape Disease, Pest and Weed Management webpage for more information on managing crown gall.
Disease incidence is especially high in older vine yards where large pruning wounds were made to alter the training system. Most commercial grape cultivars are affected by this disease. Both young and old vines are susceptible. However, Eutypa dieback is generally not found in vines younger than 5 years old.
Symptoms of Eutypa dieback include stunting of spring shoot growth, yellowing and cupping of newly emerged leaves, shedding of blossom clusters, vascular discoloration, and cankers in stems associated with old pruning wounds. In advanced stages, part or all of a vine will die.
Symptoms are best seen in the spring when shoots of healthy grapevines are 10–15 inches long. Later in the growing season affected shoots are stunted, and leaves of infected vines become tattered and scorched. Clusters on infected shoots are poorly developed and often wither and drop. Foliage of infected vines may be covered and masked by the foliage of healthy grapevines.
An important diagnostic symptom of Eutypa dieback on the trunk or arms is a canker associated with a pruning wound. However, the bark must be peeled away to see the canker. It is also common to find one side of a vine dead or with disease symptoms and the other side appearing healthy.
Infection occurs when airborne spores of the fungus come in contact with fresh pruning wounds during or immediately following rainstorms. Spores germinate on the wound, and the fungus grows into the wood and produces a canker. Symptoms may not appear on diseased vines for more than 3 years after infection. Cankers expand lengthwise in both directions from the wound and will eventually girdle and kill arms or trunk of infected vines in 5 to 10 years.
Cultural Pracitces
Losses due to Eutypa dieback can be reduced by identifying and removing portions of diseased vines before the fungus spreads extensively in the infected vine. In the spring, when disease symptoms are most noticeable, scouting and marking diseased vines should occur.
Diseased wood should be removed 4 to 6 inches below the canker and a new, healthy shoot trained into position. Remove the diseased wood after peeling the bark and tracing the canker or by making a series of successive cuts until a final cut is made in healthy tissue with all brown, discolored wood removed. If the canker has grown below ground level, remove and replace the vine. The fungus does not persist in soil, and it is not spread by pruning tools.
In addition, double pruning, or mechanically prepruning early in the dormant period, followed by a hand-pruning later in the dormant period, can also reduce infection risk of the main trunk, by removing segments of cane tissue that may have been recently infected.
Pruning wounds become resistant to infection about 2 to 4 weeks after pruning. The time required for a wound to become resistant depends on when the pruning is done. Pruning cuts made in December remain susceptible for a longer time than cuts made in late winter and early spring. Therefore, waiting to prune until late winter and early spring may be better than pruning earlier, if the weather is not rainy.
Fungicide Programs
Applications of latex paint and other wound dressings on pruning wounds have not been effective in reducing infections. Some fungicides, when sprayed as dormant pruning wound protectants do have some benefit, but the pruning wounds need to be repeatedly protected until they have healed, or until spring sap flow.
This fungus attacks the leaves, shoots, rachis, and berries of grapes in many parts of the country, but has limited importance in Washington wine grape or juice grape production.
It produces small brown to black spots, usually with yellow margins on leaves. Portions on leaves may die if large numbers of spots develop; infections on leaf petioles will cause the leaves to turn yellow and abcise. Spots on shoots are oblong and generally at the basal portion; spots on flower cluster stems are similar to those on leaves and shoots. Wet weather during early shoot growth favors disease development.
Cultural Control
Pruning out infected wood during normal winter pruning can significantly help in the management of Phomopsis. In most cases, when Phomopsis does show up in a Washington vineyard, it’s because that vineyard has undergone minimal pruning for several seasons, resulting in a build-up of diseased wood.
Fungicide Programs
Chemical management in Washington wine and juice grape vineyards is generally not recommended due to the low environmental pressure, and the ability to culturally manage the disease through proper pruning practices.
Grape powdery mildew is one of the most economically important diseases on wine grapes in the state of Washington. It attacks all green tissues of the vine, but is most damaging to leaves and green fruit, covering them with a gray, powdery layer of fungal threads and spores. Affected fruit often cracks, allowing secondary rots to become established. European Vitis vinifera varieties are highly susceptible, and Vitis labruscana varieties (e.g., Concord), have reduced susceptibility. Cloudy, humid, and mild conditions favor powdery mildew development.
The fungus survives winter as chasmothecia (minute fungal fruiting bodies) in bark crevices and leaf litter. Each viable chasmothecium contains numerous spores known as ascospores. The ascospores persist (and the risk of primary infection exists) through bloom. Ascospore release and primary infection require at least 0.1 inch (2.5 mm) of moisture at temperatures of 50°F or greater. Ascospores are dispersed in wind currents and land on leaves, where they germinate, giving rise to microscopic mildew colonies. The first mildew colonies often appear on the undersides of leaves close to the bark. Conidia, the asexual and far more numerous spore type, are produced in the primary mildew colonies. Dispersed by wind to foliage and fruit, conidia infect and produce subsequent secondary mildew colonies. The secondary infection process repeats through the growing season. While already several weeks old, the first visible mildew colonies are typically seen by in mid June.
Cultural Control
Low overnight temperatures (below 45F), and high daytime temperatures (above 95F) will limit powdery mildew growth. Grapevines also display Ontogenic Resistance to powdery mildew. This age-related resistance means that tissue becomes resistant as it ages. Grape berries will become resistant to new infections approximately 4 weeks after full bloom. Foliage will become resistant to new infections once leaves transition from being sinks to sources (i.e., change from glossy to matte). If regulated deficit irrigation is practiced, this means the canopy many not have new young tissue by mid to late summer that is susceptible to infection. Typically, by the time powdery mildew symptoms are seen, the infections are several weeks old.
Practices that open up the grapevine canopy to allow drying of the foliage, reduced canopy humidity, improved spray penetration, and a reduction in late-season canopy growth all help reduce the potential severity of grapevine powdery mildew. Shoot thinning, tucking, and early fruit-zone leaf removal can all help reduce disease pressure on developing grapevine clusters.
Fungicide Programs & Fungicide Resistance Management
The critical management window for grape powdery mildew is from approximately 10 inch shoot growth (EL 13) to 4 weeks after full bloom (EL 31, depending on the growing season). Additional later-season canopy cover sprays may be warranted in cool vintages, or active canopy growth continues through the summer.
From 2016-2019, surveys in Oregon and Washington vineyards that lost powdery mildew disease control early in the season showed that the powdery mildew present in almost all of those vine-yards tested was resistant to FRAC 11 fungicides. Since then, we have also documented resistance in FRAC 3, FRAC 7, and FRAC 13 fungicides, with suspected potential resistance developing in FRAC 50 and FRAC U6 fungicides.
FRAC 11 resistance is all-or-nothing; but regional efforts towards the smart use of this fungicide group can help reduce or eliminate the spread of resistant fungi. These products are important in both the powdery mildew and Botrytis bunch rot management arsenal, so proper management of this developing resistance is critical for our industry. Resistance in FRAC 3 and FRAC 7 fungicides is a gradient; for a time, maximum label rates may perform better than minimum label rates, and/or specific active ingredients within that FRAC group may perform better than others within that same group. Resistance in the remaining FRAC groups is not well understood at this time. In some cases, we have encountered powdery mildew populations that are resistant to multiple classes of fungicides.
The increasing occurrence of fungicide resistance in grape fungal diseases, indicates that we must carefully reconsider our approaches for disease management, with a greater emphasis on product stewardship. General disease management, that adheres to good product stewardship, includes:
- Incorporation of cultural practices that lower disease pressure. Cultural practices such as vigor management, shoot removal and positioning, and leaf removal will lower disease pressure and improve spray penetration.
- Always follow label instructions pertaining to application rates and intervals and always use a properly calibrated sprayer and sufficient spray volume to provide thorough coverage.
- Always use fungicides in a protective, rather than reactive, manner. It is far easier to prevent disease than to cure it.
- Limit the number of applications of individual modes of action (FRAC group) per season and limit sequential applications.
- Tank-mix using multi-site products, such as oil, sulfur, bicarbonates, and many biologicals, when also using medium- or high-risk compounds. However, please check product labels for tank-mixing compatibility.
- Consider starting the season off with a multisite or biological product. That will reduce the initial selection pressure on fungicide-resistant pathogen populations. Use short intervals early-season to ensure proper coverage in a rapidly-developing canopy.
For more information on when certain fungicides should be avoided when faced with fungicide resistance, please visit: https://framenetworks.wsu.edu/grower-information/. Current research is underway on when and how these fungicides can be re-introduced into a spray regime.
Resources
When honeydew (an insect excretion; see also Mealybugs ) covers the vines, leaves, and fruit, a black fungus may develop on the honeydew. This fungus rarely causes any direct damage to most processed grapes, but can be problematic on fresh-eating table grapes.
The primary control of this problem requires controlling the insects that produce honeydew, such as Grape Mealybug or leafhoppers.
Grapevine Fanleaf Disease
Grapevine Fanleaf Disease is caused by Grapevine fanleaf virus (GFLV). All grape species and cultivars are susceptible to GFLV. Infection by GFLV leads to vine decline but not death of the vine. Like other diseases, Grapevine Fanleaf Disease drastically affects vine growth, yield, and fruit quality.
Diseased vines show three distinct types of leaf symptoms: fanleaf deformation, yellow mosaic, and vein banding. All of these symptom patterns are caused by the same virus and reflect varied responses by different cultivars. The disease symptoms are often found in patches in the vineyard.
Grapevine fanleaf virus is spread by soil-borne dagger nematodes. The two known nematode vectors are Xiphinema index and X. italiae. Xiphinema index is by far the more efficient vector and currently not found in Washington State. Thus, vineyards in Washington State that are expressing Grapevine Fanleaf Disease were likely infected at the time of vine propagation. There is no current recommendations for treating impacted vines other than roguing.
Grape Decline
Grapevine Decline is another nematode-transmitted virus disease. At least three distinct viruses—Tomato ring spot virus (TomRSV), Tobacco ring spot virus (TRSV), and Peach rosette mosaic virus—have been implicated in the genesis of the disease. Two distinct strains of TomRSV are present and they induce different symptoms. These viruses are spread by the species of dagger nematode – Xiphinema americanum – that is present in Washington state.
When the nematode vector is present in the state, preplanting vineyard fumigation may be helpful in reducing initial dagger nematode populations. However, this control is not long-term. The best course of action is to plant certified grapevine planting material, or testing plant material prior to propagation.
Grapevine leafroll disease (GLRD) is a complex viral disease of major concern in Washington State Significant reduction in yield is commonly reported due to GLRD, with fewer and smaller bunches. In addition, fruit maturity is delayed substantially depending on severity of symptoms, cultivar and environmental conditions. GLRD affects the quality of grapes by delaying the accumulation of sugars, lowering the accumulation of anthocyanins, and causing up to 50% loss of pigment concentration in red wine varieties. GLRD is, therefore, a particularly serious problem for red wines.
GLRD does not produce symptoms for most of the season—these begin to appear in late summer and/or the early part of the Fall. GLRD symptoms are more dramatic in red- and black-fruited cultivars than white-fruited cultivars. In the former cultivars, the foliar symptoms are characterized initially by red and reddish-purple tints in the inter-veinal areas. These discolorations may coalesce with time, leading to reddish-purple color of interveinal areas and green tissue near the main veins. In the advanced stages, the margins of infected leaves roll downward, expressing the symptom that gives the disease its common name. Thus, the major symptom in late Fall is red leaves with green veins and downward-rolled leaf margins. In the white-fruited cultivars, infected leaves show mild yellowing between the veins and show downward rolling. GLRD symptoms develop during the late summer to fall period in these cultivars and vary depending on the cultivar and time of the year. Usually, the symptoms begin to appear on the mature leaves near the base of the shoots and develop progressively up the canes.
This disease is caused be several distinct viruses termed Grapevine leafroll-associated viruses (GLRaVs), and numbered sequentially GLRaV-1, -2, -3, etc., Recent studies have shown that GLRaV-1, -2, -3, -5 and -9 are present in Washington State vineyards.
Disease Management
The dominant virus in Washington, GLRaV-3, is spread by the grape mealybug (Pseudococcus maritimus). To control the spread of this disease, growers must (in order of top priority and efficacy):
- Identify infected vines, either through visual symptoms when apparent, or through genetic testing.
- Rogue infected vines to remove the source of virus. Disease cannot be contained if roguing is not done.
- Manage grape mealybugs to reduce additional spread of the virus.
Solely focusing on management of the mealybug vector will not result in the successful suppression of disease spread. Roguing of infected vines must occur.
Grapevine red blotch virus (GRBV) is a relatively recent disease in US viticulture. In red-fruited cultivars, mature leaves at the bottom portions of canes show red veins, red blotches and total reddening. No such symptoms are apparent in white-fruited cultivars. In red-fruited cultivars, affected vines show poor growth with significant reduction in fruit yield and berry quality.
This disease is vectored by the Three-cornered Alfalfa Hopper (Spissistilus festinus), which is not currently found in Washington State. Thus far, all know infected vineyards in Washington State are the result of planting already-infected grapevine stock; disease spread between infected blocks has not yet been seen.
Positive diagnosis of this disease can only come from a genetic test. If a vine is infected with GRBV, it should be removed / rogued as soon as possible. If one vine is infected in a block, it is recommended to test other vines, and it is likely the entire block is infected if the planting material came from the same source.
There are many differing grapevine virus diseases reported from different countries and they are of local or minor significance.
With rapid expansion of plantings and changing viticultural practices, new and emerging problems potentially associated with viruses are increasingly apparent in several grape-growing countries, such as virus-induced graft incompatibility disorders, destructive phytoplasma epidemics, and viroid-induced diseases.
Such viruses that have been recently identified in Washington vineyards include:
- Grapevine rupestris vein feathering virus
- Grapevine red globe virus
- Grapevine fleck virus
- Grapevine syrah virus-1
Three viroids have also been documented in the state’s vineyards include:
- Hop stunt viroid
- Grapevine yellow speckle viroid-1
- Grapevine yellow speckle viroid-2
Although other virus and virus-like problems are not yet reported in Washington State vineyards, constant vigilance and careful monitoring of vineyards is important to make sure that no new virus disease becomes established in the state.
It is very important that vines showing any unusual symptoms be brought to the attention of the Grape Virologist (Naidu Rayapati, naidu.rayapati@wsu.edu) at WSU-IAREC, in Prosser.
All the graft-transmitted disorders of the woody trunk are grouped under Rugose Wood (RW) Disease Complex.
RW is characterized by modifications of the woody cylinder, typified by marking with pits and/or grooves. It consists of four different disorders, namely rupestris stem pitting (RSP), Kober stem grooving, LN33 stem grooving, and corky bark. They may occur on the scion, rootstock, or both, depending on the rootstock and scion cultivar. European Vitis vinifera cultivars carry symptomless infections of RW complex until they are grafted onto American rootstocks.
The etiology of RW syndrome has not yet been completely worked out. At least four different viruses—namely, Grapevine Virus A (GVA), B, D to J, L and M, and Grapevine rupestris stem pitting-associated virus (GRSPaV)—have been consistently found in different disorders of RW Complex.
- GVA, GVB and GVD were implicated in disorders of RW complex and the pathogenic role of other viruses remains to be determined. All these viruses are filamentous, phloem-limited, and are graft-transmissible, and thus all four disorders are spread by infected propagation material. GVA and GVB were reported to be transmitted by mealybugs. GVA, GVB and GVE have so far been documented in Washington State vineyards. However, GVA and GVB appear to be more widespread than GVE.
- The natural spread of GRSPaV and other viruses listed above is not yet established. Currently, GRSPaV is known to be present in Washington State and according to some estimations, it affects approximately 5% of the grapevines in the state. When GRSPaV is present alone, grapevines do not produce foliar symptoms and there are no effects on the growth and yield of plants. However, when present as mixed infections with other viruses, RW-type symptoms may occur.
Infections due to RW Complex can significantly reduce the survival rate of grafted vines when compared to grafted non-infected vines. Moreover, the severity of disease may vary according to the genotype of the rootstock that is grafted with infected scion wood. RW affected vines may be dwarfed and less vigorous than normal and may have delayed bud opening in the spring. Some vines decline and may die within a few years after planting, due to graft incompatibility.
Due to increased emphasis of using rootstocks to mitigate nematode- and Phylloxera-induced problems, disorders of the RW Complex might become a concern in future. Therefore, growers should ensure that grafted vines for new plantings are tested free from viruses associated with RW Complex.