Category: Vineyard Info

By Judit Monis, Ph.D.

Last January I attended the virus management session organized by the Unified Symposium in Sacramento, CA. Maher Al Rwahnih, James Stamp, Rick Hamman, and Eric Pooler were invited speakers.  While it is common sense that it is important to plant healthy vines to avoid the perpetration of viral diseases, we are learning that it is not that simple.  A couple of years ago we heard that even the most tested and best maintained foundation block in California is susceptible to becoming infected by viruses.  I will present my take on the different presentations and add my ideas on solving such important issues.

The Russell Ranch Foundation Block Virus Status

Maher Al Rwahnih from The University of California Foundation Plant Services (FPS) opened the session describing the discovery in 2017 of five vines infected with Grapevine red blotch virus (GRBV) in the Russell Ranch mother block.  The block was planted in 2010 and is the regulated by for the California Food and Agriculture (CDFA) Registration and Certification (R&C).  After completing a two-mile radius survey, researchers at FPS were able to trace the origin of the positive findings in the mother block to vines planted in a home backyard, a school yard, and a home garden center.  The vines from the outside source as well as the mother block selections were removed and destroyed as soon as the infections were confirmed.  Due to the seriousness of finding virus in their mother block, FPS decided to test annually all the plants in the CDFA R&C mother blocks for Grapevine leafroll virus associated -3 (GLRaV-3) and GRBV.  After completing the tests in 2018, FPS found 24 vines infected with GRBV and none infected with GLRaV-3.

To reduce the inoculum levels in the foundation block, all infected vines have been removed.  To control the spread of virus the FPS has devised a strategy that consists in monitored the blocks routinely for vectors (GRBV is transmitted by the Three-cornered alfalfa tree hopper and GLRaV-3 is transmitted by mealybugs and soft scale insects).  Additionally, preventative treatments are carried out with feeding deterrents and systemic and contact insecticides.  A survey of the block’s surrounding areas has been increased to a five-mile radius to find the original source of infection.  Moving forward all ordered selections will be tested individually for GRBV (Test to Order program) prior to distribution to assure that material has no detectable levels of virus.

Preventing the Introduction of Disease in the Vineyard

James Stamp, from James Stamp and Associates, voiced his concern on the difficulty of securing clean planting material and presented standards he applies prior to ordering vines for planting.  The mother plants must be tested and show no evidence of infection of GLRaV-3 and GRBV.  In addition, grafted vines need to display good physical characteristics.  In his opinion, a vine with a solid graft union is likely to have a lower incidence of fungal pathogens.

Similar to my advice, he recommends to work cooperatively with nursery personnel to obtain testing results and always perform independent sampling and testing.  The vines that are sampled from a nursery increase block should be marked so that if the results show no detectable levels of virus, cuttings can be collected for grafting.  He prefers “protocol 2010” vines (these are the vines that are planted in the Russell Ranch mother block described above).  However, he has had success with nurseries that propagate older vines from the “classic” U.C. Davis mother blocks.

It is best to choose a nursery that has adopted cleanliness standards (i.e., no dust, paved roads, etc.). It is recommended to visit the nursery increase blocks in the fall to observe typical symptoms (red leaves in red grape varieties) and be present when cuttings are being collected (those that were submitted to testing) to assure that these are used to fill the order.  It is advisable to keep in touch with the nursery personnel to make sure that the delivery of the order will be on time.  Generally, the nursery will graft more material than needed to make sure that the correct number of grafted vines are delivered – but it is best to be in touch to make sure this happens.  If vines are to be finished in the field, it is recommended to take samples and test them for GLRaV-3 and GRBV as these viruses can be transmitted in the field.

Stamp’s presentation concludes suggesting that consumers need to be educated and engaged to accept genetic engineered resistance.  He urges for the availability of more funding for research to apply CRISPR (a gene editing technology) for disease resistance development.  In his opinion, this will be the only way to fight diseases as he has given up that the certification programs will be able to keep viruses out of planting material.

The Economics of leafroll Disease and the Need to Prevent Virus Spread

Rick Hamman, a vineyard manager in Washington State focused on the economic impact viruses have on grape production (particularly leafroll viruses).  He stated that he has learned the lesson the hard way and refers to the loses due to virus infection as a “virus tax.”

In 2001 leafroll symptoms were noticed in a three-acre block and after testing it was found to be 100% infected with GLRaV-3. The vineyard was removed and replaced with “clean” planting stock.  However, in a short period of time the block became 85%. Infected.  This was due to the presence of mealybugs in the residual roots that were not carefully removed.  The speaker was able to calculate losses due to GLRaV 3 infection and these are significant. In 2010, the crop did not meet the required winery Brix value and represented a $500/ton loss.  This vineyard manager has experimented with rogueing material as symptoms appear in the vineyard block.  In Washington state, growers are able to burn the removed material but he admits that this may not be possible in other wine growing areas.  In one case, the virus presence and vine removal in the block was monitored by Naidu Rayapati’s research at Washington State University.  The study showed that by replacing infected vines with healthy stock the inoculum levels decreased each year and allowed to keep the block productive.  However, in other cases the removal of symptomatic vines was not as successful as the infection rate in the block continued to increase (perhaps due to spread of virus from infected vines not showing symptoms).

Eric Pooler from the Silverado Investment Management Co. is very familiar with viral infected block as the company manages vineyard blocks throughout California.  The company has worked very hard to monitor and control the life cycle of mealybug vectors that transmit leafroll viruses.  The speaker noted that it is Important to continue with insecticidal treatments after harvest and during dormancy as vector population will increase if they are not treated.  Just as important is to make sure to get full coverage of plants during insecticidal treatment.  The company has designed a modified sprayer that has many spray nozzles to assure the complete coverage of the vines with the insecticides. The speaker suggested that their personnel manages disease by increasing irrigation and /or the application of cytokines (plant growth hormones).  Their observation has been that a healthy canopy has correlated with better fruit set regardless of infection.  When it comes to planting a new vineyard, the company has developed a checklist of requirements prior to ordering vines from a nursery.

Conclusions

As a plant pathologist I recommend always planting healthy (pathogen tested) vines.  Since viruses, their vectors, and other grapevine pathogens can be present in the vineyard, growers need to be vigilant of pathogen spread and potential new infections.  For viruses, unfortunately the industry is focusing only in GLRaV-3 and GRBV exclusion.

However, I recommend to test for other leafroll viruses (GLRaV-1 through -4), Vitiviruses (Grapevine virus A = GVA, GVB, GVD, etc.), and Grapevine fanleaf virus (GFLV).  Presently, the crown gall causal agent (certain pathogenic strains of Agrobacterium vitis) and fungal pathogens are not regulated or excluded from certification programs.  However, these pathogens should also be kept in check as they cause important diseases.  The good news is that disease diagnostic assays have evolved to be able to be more sensitive and specific.  The application of next generation sequencing also known as high throughput sequencing will help reduce inoculum levels and consequently reduce infections in the vineyard.  Vineyard managers and growers must continue to be attentive of the infection status in their vineyards and their neighbor’s vineyards.  Coordinating insecticidal treatments and communicating openly on the presence of disease and vectors in vineyards is imperative.  We all must be reminded that it is possible to fight viral spread in the vineyards cooperatively.  Work performed in South Africa has shown a drastic area-wide reduction of leafroll virus with a simple but dedicated management strategy.  The heavy use of insecticidal treatment of mealybugs combined with complete removal of infected vines and replacement with virus tested vines was a success in reducing disease incidence In South African vineyards.

One question brought up by the audience was if it was necessary to have a fallow period after removing an entire vineyard.  In my opinion, the fallow period is needed to control leafroll viruses transmitted by mealybugs.  However, we are just learning about red blotch disease epidemiology and expect that in the coming years better disease management recommendations will be available to help keep foundation blocks and vineyards free of disease.  In the future I plan to contribute another article to expand with my recommendations for virus control in the vineyard to help growers manage disease.

Judit Monis, Ph.D. provides specialized services to help growers, vineyard managers, and nursery personnel avoid the propagation and transmission of disease caused by bacteria, fungi, and viruses in their vineyard blocks.   Judit (based in California) is fluent in Spanish and is available to consult in all wine grape growing regions of the word.  Please visit juditmonis.com for information or contact juditmonis@yahoo.com to request a consulting session at your vineyard.

I am writing again about Grapevine Trunk Diseases (GTD) in the Midwest, following the article with Mike White (ISU extension viticulturist) published in this magazine in September-October 2018. In that article we raised the question as to whether the commonly seen “winter kill” symptoms of dead cordons and spurs, and poor budbreak, may be mis-diagnosed in the region, as they also correspond to common GTD fungi symptoms. I raised this with some “old hands” in the industry, and they laughed at the suggestion. I hope this article might cause them to reconsider (but probably not).

We have two developments to report. Firstly, there has been quite some activity in testing of samples, much of it by Mike. Initially this was by local diagnostic laboratories, but there was concern with apparently inconsistent and negative results which caused us to send samples further afield. We have been concerned about test results for some samples which may lead growers to believe that there is no problem. However, as we have discovered some testing procedures are more reliable than others. This problem is not unique to samples from the Midwest, as Richard has also had problems with some samples from California tested in different laboratories.

GTD Fungi Isolated in Iowa

The following is a list of fungi and one bacterium which have been diagnosed in samples from Iowa, provided by Mike:

1. Botryosphaeria dothidea (“Bot”)
2. Crown Gall (Agrobacterium vitis, a bacterium)
3. Cytospora viticola (associated with trunk disease)
4. Diatrylpella species. (associated with Eutypa, a major trunk disease)
5. Eutypa species including Eutypa lata
6. Phaeoacremonium species. (associated with Young Esca (Petri Disease)

Phaeoacremonium minimum

1. Phomopsis species (associated with foliar, cane, fruit and trunk disease)
2. Seimatosporium species. (associated with Dead Arm Disease)
3. Pestalatiopsis (foliage, fruit and trunk disease)
4. Phaeomoniella species. (associated with Young Esca – Petri Disease)

Phaeomoniella chlamydospora

1. Diaporthe species (Phomopsis)
2. Stereum species (typically saprophytic feeding on dead wood)

In fact, as more samples are properly tested, it is likely there will be found other fungi. It would appear from this limited experience that Polymerase Chain Reaction (PCR) testing as is carried out by some Californian laboratories is the more reliable method. Many samples will have multiple infections.

The attached figures show the vine appearance of three samples analysed in California. Concord samples which showed internal streaking and necrosis (dead tissue), centered on the pith, and necrosis associated with cankers. The sample contained four fungi, Eutypa spp., Phomopsis spp., Phaeoachremonium minimum and Phaeomoniella chlamydospore, see Figure 1.

Figure 1

Similarly Marechal Foch yielded Eutypa spp.,Diatrypella spp. and Phaeomoniella minimum, see Figure 2.

The last sample to be discussed was especially interesting, being Vitis riparia from the wild, harvested at Indianola, Iowa, from a creek bottom. Mike and I on our travels together had found suspicious staining in several wild vines, here was some confirmation. The sample showed necrosis and streaking, and some dark tar-like deposits, and white rot in the center. The diagnosis was for Phaeomoniella minima, Phaeoachremonium chlamydospora and Botryosphaeria dothidea, see Figure 3. There may be some implications for new varieties bred from Riparia parents re GTD susceptibility.

The Riparia was to be the first confirmation of Botryosphaeria in Iowa. In my tour with Mike last summer, I often spoke of “Bot” in a general sense as being the likely cause of some symptoms we saw. I should have been more careful. Botryosphaeria symptoms are like those of other GTD, but in particular those of Phomopsis. This finding in Riparia was interesting from other points of view. Was Botryosphaeria present in wild vines before present grapevine plantings were introduced? Whatever the answer to that question, wild vines may constitute a reservoir of spores for infection of adjacent vineyards. Of course, other native plant species may be infected is as well.

Outstanding Webinar on GTD by World Expert Dr. José Ramón Úrbez Torres of British Columbia 

This webinar was held Tuesday, December 11, and organized by Dr. Tim Martinson of Cornell University. Around 300 persons registered for the event, indicating a heightened awareness and interest, especially in cold winter climate regions. The webinar lasting a little over an hour is now posted on the Northern Grapes Project website for viewing. If you have unexplained dead or unhealthy vines in your vineyard, we suggest that you watch it here: https://northerngrapesproject.org/

The seminar was very comprehensive. It began by describing the many species of fungi now known to be involved in grapevine trunk disease, and described how they are spread, both between and in vineyards. The seminar concluded by presenting a range of methods of protecting vineyards from GTD.

GTDs are caused by a group of fungi of different genera and species, and they often occur as mixed infections. The fungi have in common a typically insidious nature when pathogenic on grapevines. For some diseases, there are no conspicuous foliar symptoms, and the first a grower may know of the problem is spur then cordon then vine death. Unfortunately, by the time the first vine dies, many others can be already infected and they will gradually die unless the problem is treated. But treatment must commence early if the vines are to be saved. So, it is very important to recognise early disease symptoms in just a few vines where they exist.

Grapevine trunk diseases can affect and kill vineyards of many ages. Some deaths may even be recorded in the year of planting, and, typically as the vineyard ages, more and more vines die. For growers concerned with vineyard profitability this is an issue; some vineyards in some parts of the world are being replanted at 20 years of age due to trunk disease. Unfortunately, vines delivered to growers from nurseries may be already infected with grapevine trunk diseases. This is a worldwide problem, and research shows that it can be involved with the bench grafting process, but even own rooted vines may be infected, as can occur in the Midwest.

Vigilant growers should inspect nursery stock for staining within the stem. It is normally particularly evident at the base of the rootstock cutting and in the graft union. The webinar was excellent for showing a full range of symptoms in both young and old vines, including many sections across the trunk. Sometimes fungi will be resident within young vines without causing problems; however, it is known that when the vines experience any stress that these fungi will become pathogenic and will affect the growth of the vine and eventually kill it. Severe winter temperatures are known as an important stress. In fact, as was emphasized in the article referred to above, the common death of cordon and spurs which is typically called “winter injury” may in fact sometimes be due to trunk disease.

There has been much more concern globally about GTD since 2000 even though there are reports that the diseases had been present for a long time, and some of the early research has been forgotten. For example, the roles of Phomopsis in trunk disease was shown by early research at Geneva New York in 1909 and again more recently with Eutypa in several Eastern states from the 1970s onwards. In France it has been declared that “wood diseases are a national crisis responsible for 12% of French vineyards being non-productive”. So, while some might think trunk diseases are new, they are not, but there is clear evidence that they are becoming more of a problem worldwide.

 What to do about trunk disease? They can be controlled using a range of management techniques. In regions of cold winters there is evidence that early pruning may be more effective, whereas the opposite is the case in California for example where late pruning in the dry part of the spring helps to prevent spread.

It is imperative to protect pruning wounds, and fortunately a range of fungicides are available in the USA although not in Canada. There is certainly a need for local research in places with very cold winters as to how these may be applied in freezing temperatures. For larger vineyards in other regions using a modified vineyard sprayer is becoming popular. Protection of pruning and other wounds is the first line of defence against GTD, and likely in the majority of Midwest and Eastern vineyards this is not now practiced, which may change if GTDs are found to be a widespread problem.

The second line of defence against trunk disease is trunk renewal. This is now practiced of course to overcome “winter injury”, and thankfully at the same time can be protective against trunk disease. Provided the trunk is renewed from below the level of staining in the trunk, then the vine may be rejuvenated and is free of disease.

Conclusion

Growers in the Midwest and Eastern states have not been concerned as they might about trunk diseases, probably because some symptoms have been thought due to winter injury. Yet as was declared in the webinar, GTD occur everywhere in the world where grapes are grown! At the earliest signs of disease/poor health we encourage growers to take sections of the trunk and look for staining. If any is found, a lab confirmation should be sought.

I urge all growers, advisers and nurserymen to watch this webinar. It is an hour very well spent and may give you a new perspective on your vineyard and its health and profitability.