Category: Pest & Disease Management

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A bright idea? Ultraviolet light as an integrative pest management tool for grape powdery mildew

Alexander Wong, Graduate Research Assistant, Dept. of Botany and Plant Pathology, OSU
Dr. Walt Mahaffee, Research Plant Pathologist, USDA-ARS and FRAME Networks group (USDA-NIFA-SCRI)

The concerning emergence of fungicide resistant grape powdery mildew (Erysiphe necator) to numerous fungicides (Beresford et al. 2016; Cherrad et al. 2018; Colcol et al. 2012; Colcol and Baudoin 2016; Kunova et al. 2016; Miles et al. 2012; Wong and Wilcox 2002) has led to a need for new integrative pest management strategies in grape production. Ultraviolet spectrum C (UVC) radiation has been used for over a century to kill or disable microorganisms by damaging their DNA. However, prolonged exposure times have been required because the microbes have very efficient DNA damage repair machinery (Beggs 2002). A recent discovery (Janisiewicz et al. 2016; Suthaparan et al. 2016) showed that this machinery is shut off at night to conserve energy, which indicates that UVC light applied at night might be effective with shorter exposure times (e.g., strawberry powdery mildew (Onofre et al. 2019), and wheat powdery mildew spores (Zhu et al. 2019)). In collaboration with David Gadoury at Cornell University and Michelle Moyer at Washington State University, we began testing whether UVC could be used to manage grape powdery mildew and bunch rot this past growing season.

Our research tests were conducted in a 22-year-old block of vertical shoot positioned (VSP) trellised Pinot noir trained at the OSU Botany and Plant Pathology Farm in Corvallis, Oregon using an over-canopy array of UVC light banks (Figure 1). UVC treatments were applied once per week, one hour after sundown at a speed of two or three miles per hour, which relates to a theoretical dose of 120 and 80 joules per square meter (J/m2), respectively. We also applied fungicide programs to subplots within rows of 5 lb/A sulfur on 7- to 14-day intervals, 10 fl oz/A Azoxystrobin on a 14-day interval, an alternation of sulfur and a QoI on a 14-day interval, or untreated control. Powdery mildew incidence ratings were performed every other week starting in mid-May and ending at veraison with leaf and cluster mildew severity ratings completed just before veraison. Grape powdery mildew leaf incidence was significantly (TukeyHSD, p < 0.05) reduced with weekly UV treatments (Figure 2). The UVC treatments did not lead to a significant reduction of mildew severity on clusters, mildew colonies, or rater’s gloves to monitor the amount of mildew (Thiessen et al. 2016) and presence of QoI resistance of each of the plots (Miles et al. 2020).

Botrytis cluster disease severity and incidence was measured at harvest after incubating clusters for 48 hours at 68°F and high humidity. There was no significant difference in Botrytis incidence between treatments. Due to the heavy mildew pressure, many of the clusters with high mildew severity had clusters too desiccated or decayed at harvest to be colonized by Botrytis. Botrytis isolates collected from diseased clusters are being tested for fungicide resistance.

These results suggest that UVC treatments, in conjunction with fungicide programs, has the potential to improve disease management of grape powdery mildew, but the frequency or dose of the application need to be increased. Future field studies at the BPP field site will examine increasing dose and/or UVC application frequency. In collaboration with Willamette Valley Vineyards and Saga Robotics, we will begin exploring the use of an autonomous drive base to apply the treatments on a commercial vineyard scale. Using UVC as part of an integrative pest management tool for powdery mildew will hopefully reduce costs and environmental impacts of disease management by reducing the amount of chemical inputs for disease control.

Figure 1. The tractor mounted UVC array in operation.

Figure 2. Area under disease progress curve for the powdery mildew epidemic based on foliar mildew incidence ratings. Error bars represent the standard error of the mean.

Literature Cited

Beggs CB 2002. A quantitative method for evaluating the photoreactivation of ultraviolet damaged microorganisms. Photochem Photobiol Sci 1:431-437.

Beresford RM, Wright PJ, Wood PN and Agnew RH. 2016. Sensitivity of grapevine powdery mildew (Erysiphe necator) to demethylation inhibitor and quinone outside inhibitor fungicides in New Zealand. N Z Plant Protec 69:1-10.

Cherrad S, Charnay A, Hernandez C, Steva H, Belbahri L and Vacher S. 2018. Emergence of boscalid-resistant strains of Erysiphe necator in French vineyards. Microbiol Res 216:79-84.

Colcol JF and Baudoin AB. 2016. Sensitivity of Erysiphe necator and Plasmopara viticola in Virginia to QoI Fungicides, Boscalid, Quinoxyfen, Thiophanate Methyl, and Mefenoxam. Plant Dis 100(2):337-344.

Colcol JF, Rallos LE and Baudoin AB. 2012. Sensitivity of Erysiphe necator to demethylation inhibitor fungicides in Virginia. Plant Dis 96(1):111-116.

Janisiewicz, WJ, Fumiomi T, Glenn DM, Camp MJ and Jurick WM. 2016. Dark period following UV-C treatment enhances killing of Botrytis cinerea conidia and controls gray mold of strawberries. Phytopathology 106(4):386-394.

Kunova A, Pizzatti C, Bonaldi M and Cortesi P. 2016. Metrafenone resistance in a population of Erysiphe necator in northern Italy. Pest Manag Sci 72(2):398-404.

Miles LA, Miles TD, Kirk WW and Schilder AMC. 2012. Strobilurin (QoI) resistance in populations of Erysiphe necator on grapes in Michigan. Plant Dis 96(11):1621-1628.

Miles TD, Neill T, Colle M, Warneke B, Robinson G, Stergiopoulos I and Mahaffee WF. 2020. Allele-specific detection methods for Qol fungicide resistant Erysiphe necator in vineyards. Plant Dis.

Onofre RB, Ortiz GA, de Mello Neto PP, Gadoury DM, Stensvand A, Rea M, Bierman A and Peres N. 2019. Evaluation of UVC for suppression of powdery mildew and other diseases of strawberry in open field production. In Technical Abstracts for the American Phytopathological Society (APS) Annual Meeting, Plant Health 2019. Cleveland, OH.

Suthaparan A, Solhaug KA, Stensvand A and Gislerød HR. 2016. Determination of UV action spectra affecting the infection process of Oidium neolycopersici, the cause of tomato powdery mildew. J Photoch Photobio B 156:41-49.

Thiessen LD, Keune JA, Neill TM, Turechek WW, Grove GG and Mahaffee WF. 2016. Development of a grower-conducted inoculum detection assay for management of grape powdery mildew. Plant Pathol 65:238-249.

Wong FP and Wilcox WF. 2002. Sensitivity to azoxystrobin among isolates of Uncinula necator: baseline distribution and relationship to myclobutanil sensitivity. Plant Dis 86(4):394-404.

Zhu M, Riederer M and Hildebrandt U. 2019. UV-C irradiation compromises conidial germination, formation of appressoria, and induces transcription of three putative photolyase genes in the barley powdery mildew fungus, Blumeria graminis f. sp. hordei. Fungal Biol 123(3):218-230.

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Avoid mixing biologicals with antimicrobials

Dr. Jay W. Pscheidt and Lisa Jones, Dept. of Botany and Plant Pathology, Oregon State University

Actinovate AG (Streptomyces lydicus WYEC 108) and many other biological products are used in the management of organic grapes. Tank mixing more than one product is both economical and time-saving but tank mix compatibilities with biological control products such as Actinovate have not been thoroughly evaluated. In 2016, we examined the tank mix compatibility of Actinovate AG with commonly used organic products.

Actinovate AG was prepared at a concentration of 0.1g/ml. A 300 ml solution of Actinovate was prepared in a 500 ml beaker then mixed with each material and allowed to stand for 30 minutes. The mixture was then plated onto agar and incubated for 7 days at room temperature. The number of colony-forming units (CFU) of S. lydicus exposed in each mix was assessed daily and compared to an Actinovate plus water only control. The percentage of S. lydicus CFU in each tank mix compared to the CFU in the Actinovate control was calculated.

An average of 3.2×105 S. lydicus CFU developed after 7 days incubation on the various media when Actinovate was just mixed with water. Several products inhibited the growth of S. lydicus when prepared in as a mixture in the laboratory. No growth of S. lydicus was observed on plates when Actinovate was mixed with Horticultural Vinegar, a high rate of Regalia, Rex Lime Sulfur, Serenade Optimum, or Solubor DF. Less than 10% of the S. lydicus CFU grew when Actinovate was mixed with Biomin Calcium, Botector, Neptune’s Harvest 2-4-1 fish fertilizer, or Thuricide. Significantly fewer S. lydicus CFU grew when Double Nickel, the low rate of Regalia, Serenade Max, the high rate of Stimplex or Toggle were mix with Actinovate. There was no significant difference in the number of S. lydicus CFU that grew when Zen-O-Spore was mixed with Actinovate. The number of S. lydicus CFU was greater than double (219%) or quadruple (482%) that of the Actinovate control when mixed with Nitrozyme or the low rate of Stimplex, respectively.

Many of the biological products in this study grew quicker than S. lydicus under laboratory conditions. These fungi or bacteria generally outcompeted S. lydicus for space and resources on the agar plates. The fungus found in Zen-O-Spore was slower to grow and did not outcompete S. lydicus during the 7-day incubation.

This data does not imply a lack of or enhanced disease control in the field. For example, blueberry field trials over a 2-year period where Actinovate was mixed with Simplex did not result in disease control that was different than when either product was used alone. The data does indicate incompatibility between various products used in organic production.

For a complete data set please visit: http://sites.science.oregonstate.edu/bpp/Plant_Clinic/Fungicidebooklet/2016/Blueberry3.pdf

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Pest Alert: Grape Cane Borer

Dr. Patty Skinkis, Professor and Viticulture Extension Specialist, OSU
Dr. Vaughn Walton, Professor and Horticultural Entomologist, OSU

There have been an increasing number of reports of grape cane borer presence and damage in vineyards throughout the Willamette Valley this winter. Typically these reports during the bud break period in April when adults are active and evidence of shoot dieback occurs. However, we have received numerous reports this January and early February as growers begin pruning. This observation may be due to various factors including more suitable weather conditions (winter and summer), higher levels of populations surviving, more suitable host plant materials, increased awareness and improved monitoring. The borers can have a long life cycle within the vine, living as larvae (grubs) within the shoot or cane for nearly one year. Adults lay eggs during early spring and hatch and develop into larvae that feed on the shoot tissues during the growing season. They remain in the wood as pupae during winter and may be found when pruning commences. Both pupae and adults have been reported in southern and mid-Willamette Valley vineyards this winter. This article covers the most salient points for your awareness this winter; please consult additional resources below for further details.

What to look for in the vineyard:
Galleries burrowed by larvae can be observed in cane tissue usually in older or dead wood, canes, spurs, or cordons. These holes are round, drill-like holes of ~0.4 mm diameter, and they are often accompanied with sawdust that was produced by the adult when burrowing into the shoot during late summer or early fall the year prior. Cutting into the wood near these holes during pruning will likely reveal a pupa that is 1-8 mm in length (<0.3 in).

Management:
Insecticide application is often difficult to apply during the dormancy period due to the difficulty for the application to reach the pest and the inability to get into the vineyard with equipment. There are biological controls, such as the Steinernema carpocapsae, an entomopathogenic nematode, that may be used, but care needs to be taken to ensure that the product is handled properly and applied to the entry points of the pest to be effective. In some cases, the best method will be to cut out any canes that have the burrow holes evident. Remove pruning wood, as the wood contains the pupae that will emerge in spring. Removing the pest from the vineyard will ensure that a population does not exist to allow new infestations into tissues.

For more information about the cane borer, please see the following resources:

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Vole Damage in Vineyards

Dr. Patty Skinkis, Associate Professor and Viticulture Extension Specialist, OSU 

I received a number of reports of vole damage in vineyards throughout the Willamette Valley this season. Evidence of their presence became visible in August with feeding damage to trunks (Figure 1) and within the canopy, including damage to shoots and rachises of grape clusters (Figure 2). Voles eat vegetation and typically feed on roots or the base of trunks. Voles do not typically cause issues until a population peak and/or environmental conditions allow for habitation. They may reach epidemic-level populations every ten to 12 years, but these population surges are not predictable and last for one year (Gunn et al. 2011). The Willamette Valley’s last reported vineyard infestation occurred in 2005, and some vineyards lost vines due to the damage.

Preventing and eradicating voles.  Our best suggestions to growers who have been observing vole presence in vineyards has been to encourage eradication. Trapping or baiting voles may not be practical on large acreage or advised with certain farming certifications. For example, zinc phosphide is not allowed in organic production. However, soil tillage or mowing may provide some level of prevention and control. Research in field crops show that tilling the soil is the most effective method of reducing vole populations (Jacob 2003), by disturbing their burrows and causing movement to other vegetated areas. Voles avoid bare ground, so tillage can prevent habitation altogether. In the Jacob (2003) study, they found voles disappeared altogether after disking to a depth of 19 inches. Mowing vegetation was found less effective than tillage, as the mulch from mowing allowed sufficient cover for the voles and did not encourage movement away from the cropped areas. Avoiding mulch layers or vegetation growth under-vine will prevent voles from inhabiting the areas near grapevine trunks and feeding on roots and trunks when food sources are limited.

Scouting for damage. Voles tend to feed on vine roots and at the base of trunks. Look for feeding damage at and just below the soil surface. Since the feeding typically occurs through the phloem and vascular cambium, the cell layers that lie between the phloem to the exterior and xylem to the interior, the vascular system is compromised. As a result, affected vines may turn color abruptly (yellow or red, Figure 3), as they have limited ability to move photosynthates (sugars) and mineral nutrients through the vines to the roots once the phloem and cambium are damaged. Roots are actively acquiring carbohydrates and mineral nutrients from the canopy during late season in preparation for the next year. Having this connection severed is a major issue.

Can anything be done to repair damaged vines? Vines with girdled trunks and root damage may not survive if the damage is done to the circumference of the vine. This is due to the lack of vascular cambium to grow new phloem tissue and “heal” the wound. The best thing to do at this time is flag vines with damage now and check back later in winter during pruning and early spring. If damage was only apparent in the canopy (rachises, berries, and shoots), vines may be able to be pruned to healthy tissue in winter. However, also be sure to flag these vines for follow-up.

Because voles do not hibernate, high populations this winter may pose a threat to vines if they continue feeding in areas where they were observed this season. It will be important to remove vegetation by way of tilling soil or removing mulch layers or vegetation under-vine to avoid any further damage.

Literature Cited

Gunn D, Hirnyck R, Shewmaker G, Takatori S, and Ellis L. 2011. Meadow voles and pocket gophers: Management in lawns, gardens, and cropland. University of Idaho, PNW 627.

Jacob J. 2003. Short-term effects of farming practices on populations of common voles. Ag Ecosyst Environ 95:321-325.

 

Figure 1. Vole damage to the base of a trunk on a mature grapevine. Photo courtesy of Ryan Wilkinson.

 

Figure 2. Feeding damage is apparent on the top of the grape cluster’s rachis (peduncle) and the lower portions of the shoot from which it originates. Photo courtesy of Ryan Wilkinson.

 

Figure 3. Vines with vole damage to the trunk show almost complete reddening of the canopy in Pinot noir vines. Photo courtesy of Ryan Wilkinson.

 

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Resistance is Futile? Strobilurin resistance presence and persistence

Dr. Walt Mahaffee, Research Plant Pathologist, USDA- ARS

In 2015, we found widespread Strobilurin (QoI) resistance in Oregon, and subsequently in California and Washington when we surveyed viticulture regions in those states, it probably seemed like the sky might be falling.  Then when we showed that greater than 70% of the QoI resistant population was tolerant to very high doses of DMI (higher than can be legally applied); it really seemed like the sky would fall.  However, there was a silver lining. We kept all the DNA from all the inoculum monitoring (spore trapping) we had been doing since 2007.

We analyzed all those samples for presence of the genetic mutation responsible for the QoI resistance and found some interesting results. First, we weren’t able to detect QoI resistance before 2013. Second, we detected QoI resistance at least two years prior to growers reporting management problems. This means we had a tool to monitor resistance development which could be useful for warning growers of resistance developing.

Another remarkable finding was that the number and frequency of detecting resistant spores was much lower than the wild-type spores even when QoIs were being used in the vineyard, and we found far more resistant colonies than wild-type on leaves.

These results indicated that there might be a fitness cost to the mutation causing QoI resistance. Given that the mutation alters a protein involved in fungi producing energy, it makes sense that the fungus would not grow as well. This should also mean that moving away from using QoIs should allow the wild-type to out-compete the QoI resistant isolates, and eventually QoIs would become effective management tools again. Sarah Lowder, a PhD student, also made another discovery this past winter – Chasmothecia (the mildew overwintering structure) of QoI resistant populations do not survive as long as wild-type populations. This is more good news.

Now the big question is how to determine how long we need to rotate away from using chemistries with resistance and how to determine when we can use them again. That will be the future work of three graduate students in the lab.

Sarah is going to be working on how to rapidly and efficiently monitor for resistance. She has already made significant advances in this area. Sarah’s work this summer shows that we can swab worker gloves after manipulating the canopy (e.g. shoot thinning, lifting wires, leaf pulling, dropping crop, etc.) and get estimates on the presence of mildew and its resistance. These results are similar to spending hours scouring for mildew colonies. Sarah also developed a simple procedure to test for potential resistance by collecting bark in the winter. Simply grab bark off several vines and stuff it into a mason jar, add ice cold bottled water, shake, then decant through mosquito netting. The material adhering to the net can then be processed using our molecular assays.

Next, Chelsea Newbold (a new MS student) will be examining how the QoI resistance mutation impacts colony formation and sporulation in relation to various environmental conditions?  The big question is can we make predictions about the potential for field failures similar to how we estimate disease risk with the disease forecasting models.

Alex Wong (a new PhD student) will be looking at how fungicide resistance persists and transfers through a population. We need to understand this because resistance to other fungicides will develop, and we will need to know how to manage these resistant populations while they are still in the minority.

Since you might be wondering, here is the results of our 2018 survey for QoI (G143A) resistance. These data are thanks to funding from the Oregon Wine Board, American Vineyard Foundation, and Washington State Wine Commission. It is also a product of numerous folks in each region taking the time to send in samples.  If you would like to send sample, please contact us walt.mahaffee@ars.usda.gov and we will send you kits and instructions.

Figure 1.  Sample frequency categorized as containing only grape powdery mildew with wild-type genotype
(QoI sensitive – green), the G143A mutation for resistance (QoI Resistant – red), sample having both wild-type and
resistant genotypes (yellow) and no GPM detected (purple) in the sample.  Several Oregon vineyards are scouted
on a bi-weekly basis with extensive swab sampling leading to numerous no detection of mildew – that is good news
– since no mildew was found with the early scouting either.

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Managing mycorrhizal fungi and soil health in vineyards

Dr. R. Paul Schreiner, Research Plant Physiologist, USDA-ARS

Renewed interest in vineyard soil health driven in part by advances in microbiome research provides a rationale for reviewing what we know about the foremost component of the root microbiome in grapevines, the arbuscular mycorrhizal fungi (AMF). While other soil bacteria and fungi play important roles in vineyard health and productivity, AMF are unique because of the broad range of benefits they confer. These benefits include improving nutrient uptake from soil (particularly phosphorus (P) and other less mobile ions), increasing soil carbon storage, maintaining soil aggregate stability, and increasing tolerance to drought and pathogens. In the red hill soils of western Oregon, grapevines cannot obtain enough P to grow beyond a few nodes if AMF are absent. They are an integral component of grape and wine production here, and how we treat our soils and vines influences their abundance and the benefits they can provide. There are a few basic issues for viticulturists to consider in managing AMF to get the most from our below-ground fungal partners. These fall under pre-plant and post-plant considerations.

Pre-plant AMF Management.  The key pre-plant issue is whether or not the population of AMF is ample enough to ensure that vine roots are quickly colonized. In most cases the answer to this question is yes. AMF are naturally present in almost all soils worldwide because over 80% of all plant species form this type of mycorrhizal association. However, in modern farming systems certain practices can destroy or greatly reduce AMF in soil. While their use is rare in viticulture, pre-plant soil fumigants (methyl bromide, metam sodium, dichloropropene/chloropicrin, and dimethyl disulfide) typically used to control nematodes and soil-borne fungal diseases can wipe out AMF populations. AMF can also be reduced if host plants are absent for an extended period prior to planting a new crop. This can result from long term fallow periods or from the cultivation of non-host plants. Work in Australia to understand the phenomenon of “long fallow disorder” showed that a fallow period of 1 year or more reduced AMF propagules in soil resulting in poor AMF colonization and P deficiency in subsequently planted crops. Soils from long fallow plots could be rescued by adding AMF back to the system from recently cropped soils. Weeds can also maintain AMF populations in soil and may be important in some cases. For example, my lab showed that soil solarization conducted in the summer reduced AMF populations the following spring in western Oregon because solarization suppressed weeds over the fall and winter that acted as bridge to maintain AMF. Growing cash crops or cover crops that are not hosts for AMF can also reduce AMF propagules in soil. A number of plant species do not form mycorrhizal associations of any type or form other types of mycorrhizas that will not maintain AMF propagules in soil. Common ones used as cash crops or cover crops in agriculture are the mustards (Brassicales) including numerous vegetables, rapeseed, and meadowfoam, as well as spinach, buckwheat, amaranthus, and lupine. A new vineyard planting that follows these crops may benefit from adding AMF at planting or boosting the native AMF population by growing a host plant cover crop before planting. Planting a vineyard after hazelnuts is the most likely scenario where adding AMF will be needed in western Oregon because hazelnuts are ectomycorrhizal and because the orchard floor is kept bare for many years (not allowing host plant weeds or cover crops to maintain AMF).

Exactly when the AMF population is too low for healthy vine establishment is not clear. I conducted numerous AMF inoculation trials when I first began working on grapevines over a decade ago in nurseries and new vineyards. Results from the vineyard trials showed that inoculation with AMF (produced in my lab) enhanced root colonization and improved vine growth in only one of five experiments conducted in the Willamette Valley. By year 2, however, the non-inoculated control vines no longer differed from inoculated ones, and in no case in the nursery or vineyard was vine survival significantly altered by inoculating with AMF. Viable AMF were present at all the sites where we conducted inoculation trials so that the control vines became colonized at every site to at least a small degree.

Post-Plant AMF Management.  Even though grapevines rely heavily on AMF to obtain ample P and often other nutrients, they also can reduce the extent of AMF colonization within their roots when nutrient status (particularly P) is high. Therefore, avoiding fertilizer applications unless a nutrient is demonstrated to be low or deficient is a good practice to reduce negative impacts on AMF. For example, AMF colonization of Pinot noir roots was reduced in vineyards receiving foliar P fertilizer sprays. Root colonization was also negatively correlated to leaf P and leaf nitrogen (N) concentrations across a survey of 31 Chardonnay and Pinot noir vineyards in the Willamette Valley. There is evidence from other farming systems that organic forms of nutrients are less harmful to AMF than synthetic fertilizers, but even organic sources including manure can reduce AMF and potentially reduce other benefits they provide if applied at high rates.

Soil applied fungicides will obviously harm AMF, but what about foliar fungicides? At this time, there is no evidence that the fungicides used in our spray programs to control powdery mildew and grey mold have a negative impact on AMF. However, reducing tillage can benefit AMF because tillage breaks up their hyphal networks in soil. Indeed, we showed that in-row cultivation reduced AMF colonization in Oregon vineyards as compared to herbicides (mainly glyphosate) used to suppress in-row weeds. Finally, in separate studies both east and west of the Cascades, AMF colonization in grapevine roots was lower in vines at wetter sites (west) or in vines that received more irrigation water (east). Therefore, applying less water will also enhance AMF in vineyards. Since AMF provide other benefits beyond their key role helping grapevines obtain P, choosing management options that enhance their abundance (or at least do the least harm) also improves other aspects of soil health.

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Bunch Rots in the Pacific Northwest

Dr. Jay W. Pscheidt, Professor and Extension Plant Pathology Specialist, OSU Dept. of Botany and Plant Pathology
Dr. Patty Skinkis, Associate Professor, Viticulture Extension Specialist, OSU Dept. of Horticulture

As we get into fall with a little rain, we wanted to highlight the potential for various bunch rots. These bunch rots are weather-, disease- and insect-related. Botrytis bunch rot and sour rot are the two most frequently encountered in this region, but others that are important around the world are not common here.

Botrytis Bunch Rot
We in Extension have written about the ubiquitous Botrytis bunch rot off and on over the years. Water in the form of rain or irrigation drives this disease, especially at bloom and near harvest. The fungus can infect (gain entrance to) ovaries and colonize floral tissue at bloom. It then becomes inactive (quiescent) and does not reactivate until berries begin to ripen in the fall. Open training systems and cluster zone leaf removal help create an environment that does not favor the disease. Fungicides are less effective than canopy management but are useful in wet years. Fungicide use can be challenging since sprays need to go on well before you know whether it will be a wet season, and fungicide resistance is common and complicated by fungicides used in your powdery mildew program. Read more about Botrytis bunch rot here:

Powdery Mildew
Powdery mildew is not really a bunch rot. Depending on how early infection occurs, the result may be poor fruit set or small and split berries. By the time véraison rolls around there is not much of a cluster to rot.  Small or light infections of the berry, however, can also allow Botrytis to get a foothold. Good powdery mildew control will aid Botrytis bunch rot control. 

Sour Rot
New research out of New York has defined sour rot and given us clues as to how to manage it in the vineyard. Very specifically, sour rot occurs when the berry becomes brown AND has both ethanol and acetic acid accumulation, which gives it the characteristic sour vinegar smell. The ethanol is no surprise as it comes from yeasts, but the acetic acid comes from bacteria. There is a sequence of events that is required for sour rot to occur, and it starts with wounding.

Somehow the berry skin breaks, allowing entry of these organisms. This can happen through berry growth, rainy weather during ripening (as we had a few years ago) and/or insect or bird damage. The yeasts produce ethanol that is then converted to acetic acid by the bacteria. This is still not enough to get sour rot symptoms. In New York, fruit flies were critical for sour rot symptom development. They do not need to introduce the microorganisms but are a factor all in themselves, and that factor is unknown at this time. It is unknown whether other insects, such as yellow jackets, can also induce symptoms. Targeting fruit flies with insecticides in the vineyard did result in less sour rot development. Interestingly, targeting the microbes with anti-microbial sprays alone was not effective. You can learn more by reading:

Other Grape Rots
A few other grape rots have been reported or observed in the PNW. Several more have been described in other viticultural regions of the world, including the following list. (We mention these various rots because it is always possible for new exotic organisms to be introduced into our region. They may just be a temporary “flash in the pan” problem or could establish as an annual concern over time)

  • Phomopsis: I have seen Phomopsis fruit rot only once in my 30 years here in Oregon and that was in an unmanaged vineyard used for nursery stock. A disease with similar symptoms from the southeastern USA is called bitter rot. The only way to tell the difference is by taste, which I had enough of during my postdoctoral research in New York!
  • Black rot has been reported from eastern Washington on Concord grapes but is not a common problem.
  • Anthracnose (or better named “bird’s eye rot”) and ripe rot are also fungal fruit rots more commonly found in the southeast USA.
  • White rot is a real fungal disease of grape and not someone just joking around about bird doodoo on a leaf!
  • Downy mildew: This is not a problem here but is common in many other regions of the world.

In the Winery
Grapes affected by fruit rot diseases can cause problems in the cellar as well.  Dr. James Osborne wrote this article titled, Dealing with Compromised Fruit in the Winery, for Wines & Vines magazine in August, 2014.

Bottom Line
It is most important to manage powdery mildew and Botrytis bunch rot, and to scout for fruit flies around harvest. Also, keep an eye out for unusual problems or rots. If you find some suspect diseases or unusual rots, contact your local Extension team member. We hope that the harvest will go smoothly with few problems.

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2017 Pest Management Guide for Wine Grapes in Oregon

Now available through OSU Extension is the 2017 Pest Management Guide for Wine Grapes in Oregon. This guide is co-authored by viticulture, horticulture and pathology extension faculty at Oregon State University and updated annually. It provides chemical and cultural control information for insects, weeds, and diseases based on grapevine phenology (growth stages throughout the year). Updated information from fungicide efficacy trials is included as well as other resources and an air blast sprayer calibration worksheet.

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Treehopper Observations in Oregon Vineyards

Growers in Southern Oregon have observed treehopper damage in vineyards during the 2016 growing season. Researchers at OSU would like to know if any grapevines in other regions are displaying similar symptoms. These observations will assist them in determining the current distribution of treehoppers in Oregon vineyards. Symptoms of treehopper feeding include leaf petiole or cane girdling (see photo below). Approximately one week after the feeding and girdling, the leaf may change color and stand out clearly against the remainder of the canopy. If you observe such symptoms, please contact Rick Hilton or Vaughn Walton. This research is a component of a collaborative Red Blotch grant funded by the Oregon Wine Board.
 
To report symptoms or for more information, contact:
 
Rick Hilton
Senior Faculty Research Assistant/SOREC

Vaughn Walton
OSU Horticultural Entomologist

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Rust Mites Can Cause Damage Shortly After Budbreak

Rust Mites Can Cause Damage Shortly After Budbreak

Dr. Patty Skinkis, Viticulture Extension Specialist & Associate Professor

Grape rust mites have been a nuisance pest in vineyards of western Oregon for years. They can be found living on grape tissues from early spring through summer. Grape rust mite has been known to cause shoot deformity early in the growing season with most notable damage in years when vines have delayed growth under cool conditions.

Being aware of the first signs and symptoms of rust mite infestation in early spring is important to determine if there is a problem. However, visual symptoms are not enough for action. It is critical to determine presence of grape rust mites before considering application of miticide sprays. The presence of high numbers of rust mites have been found to cause severe stunting of emerging buds and  young shoots. For examples of these symptoms, see the grape rust mite section of the PNW Insect Management Handbook. There can be numerous other causes of stunted shoots, but with the hype of rust mite concerns, many growers blamed rust mites as the cause of all stunted shoots. As a result, there have been potentially unnecessary applications of miticides (sulfur, lime sulfur, stylet oil, or other miticide products) early in the season.

Grape rust mites are impossible to see with the naked eye, so tissue collection and viewing under magnification is required. A user-friendly method was recently developed by a team at the OWRI to monitor grape rust mites on vine tissues. This method has since been employed by growers in Oregon to determine presence of rust mites. The protocol is available for use and links provided below:

Using this method, we were able to determine a strong correlation of rust mite presence on stunted shoots early in the season. Damaged shoots often had hundreds of mites; there were over 100 mites found on shoots <10 cm in length using the rinse in bag protocol and up to 500 mites when evaluated upon subsequent extractions (Schreiner et al. 2014). Since there can be great variability in mite numbers and rapid growth of tissues early season, it is difficult to determine clear action thresholds. However, action is warranted if there is significant shoot stunting, deformity and confirmed high populations of rust mites. In-season sulfur sprays that are applied as a means to prevent powdery mildew has been found to keep rust mite populations in check (Schreiner et al. 2014). Current recommendations exist for early season rust mite control, and those can be found in the 2015 Pest Management Guide for Wine Grapes in Oregon.

For more information about monitoring for rust mites and management, see the following publications and resources:

Schreiner, R.P., P.A. Skinkis, and A.J. Dreves. 2014. A rapid method to assess grape rust mites on leaves and observations from case studies in Western Oregon vineyards. HortTechnology. 24: 38-47.

Skinkis, P.A., J.W. Pscheidt, E. Peachey, A.J. Dreves, V.M. Walton, I. Zasada, R. Martin, D. Sanchez, and C. Kaiser. 2015. 2015 Pest Management Guide for Wine Grapes in Oregon. OSU Extension Publishing.  https://catalog.extension.oregonstate.edu/sites/catalog.extension.oregonstate.edu/files/project/pdf/em8413_0.pdf

Skinkis, P. 2014. Grape Rust Mites, eXtension/eViticulture.org. http://www.extension.org/pages/33107/grape-rust-mite#.U_yZCHcXOVo

Skinkis, P., J. DeFrancesco, and V. Walton. 2015. Grape Rust Mite, PNW Insect Management Handbook. http://insect.pnwhandbooks.org/small-fruit/grape/grape-grape-rust-mite