Grapevine Leafroll Associated Virus and Mealybug in the Gimblett Gravels region of Hawke's Bay By Daniel P. Brennan VIT7.09 Vine Health
In recent years the Gimblett Gravels region has developed into one of the premium growing regions in New Zealand and has gained international acclaim for wines grown in this region. Its unique rocky soils, terroir and weather provide the unique scenario to allow certain wines in high demand at premium quality such as Cabernet Sauvignon, Syrah and Merlot. The popularity of these varietals and quality of that the region is producing has made the Gimblett Gravels some of the most highly valued land in New Zealand Gimblett Gravels Autumn, 2010 But like most things in viticulture, there are benefits and problems with the unique terroir.
It had been discovered in recent years that the Gimblett Gravels was suffering from serious disease infection from Grapevine Leafroll Associated Virus Type 3. Because GLRaV-3 has been recognized as the largest problem facing NZ viticulture today, this was a major concern for the vineyard owners producing these premium wines. Much research and efforts have been put into controlling the virus and its primary vector. But many times it is overlooked as to 'why' this happened in the first place and 'why' this is a unique problem for the Gimblett Gravels region.
To the right is a picture of a Gimblett Gravels vineyard in late spring/early summer. Observe the vineyard floor. Only patches of green exist. The free draining soils that allow for interesting fruit development in the vines also allow for minimal vineyard floor. Some consider this a blessing as it does help with weed control. And often a few days of hot weather and all of the vineyard floor will turn to yellow because the grass cannot cope with the free draining, dry, gravelly soils. Notice the bottom picture background taken after a few days of hot weather.
First lets look at the problem from the point of view of the virus' main vector, the mealybug. Mealybug will overwinter in the bark and underground. But once summer hits and it begins its growth and reproductive stages, where can it go in an environment like this. Research currently being conducted by David Reid on the mealybug, its relationship with ants and controlling mealybug as a vector for the virus, shows how many different grasses in which the mealybug and the ants can normally live. So in the case of the Gimblett Gravels soil, where can the mealybug and ants live since the vineyard soil becomes so scarce? Naturally they would gravitate to the vines and the luscious saps. David Reid has even shown that different ants can facilitate this movement from underground, to the vineyard floor, and into the vines. Mealybug is the major vector of GLRaV-3 and it quickly overtook the Gimblett Gravels in recent years. Photo courtesy of David Reid showing canopy ant food: small insects and honeydew
After doing some research this year on another subject at Villa Maria's Joseph Soler block I began to inquire about Mealybug and incidence of virus. The Soler vineyard has both conventional and organic blocks and it lies only partially on the G-4 soils of the Gimblett Gravels. So this provided some interesting contrasts to observe all under the eye of one vineyard manager, Jonathan Hamlet. Jonathan is in favor of managing the blocks organically and even biodynamically in the future. Having access to many vineyards throughout the region and country with Villa Maria provides Jonathan with observations from all types of scenarios. His opinions are stated here pretty clearly: “ From my experience, there are much less mealy in organic vineyards, we had a few in the Savy block, but in our reds we virtually have no mealy, Food and Plant Research took 75 leaf samples out of my organic merlot block this season and found none (he even examined them under a microscope) Two gates vineyard has the same experiences. The experience was the same in the HB apple industry, when orchards stopped spraying organophosphate insecticides (broad spectrum) they achieved much better control. I feel a large part of the control is not disrupting the natural predators, and also having some ground cover under the vines means the mealy bug stay in the soil rather than migrating up into the vines.” Jonathan Hamlet – May 2010
Here is a satellite image of the Soler vineyard as it was still partially planted, but as you move out of the gravely soils (top left), and down along Portsmouth Road you can observe how the soils are richer and therefore can hold more growth on the vineyard floor.
So as we begin to look at the 'why' we can begin to attack the problem at hand and see what exactly we are dealing with here. But it may not be as simple as planting cover crops and grasses to maintain control of mealybug. For the soils may not be able to support white clover, hawke's beard or other grasses that provide environments for the ants and mealybugs to inhabit. But, as Jonathan pointed out, Two Gates Vineyard which lies in the heart of the Gimblett Gravels has seen similar success with vineyard floor management. Organic vineyards and moreover biodynamic vineyards are carefully thought out systems that consider the entire blocks as ecological niches and many growers and vineyard owners don't have the knowledge, time or desire to change the conventional vineyards. So other techniques have been used.
Before you jump into biodynamics or even organic methods one must consider that a responding polyculutre is required to give homes and nourishment to the predators and parasites. Some would argue that this is not attainable on the Gimblett Gravels where the free draining soils can be so extreme that many other issues in the vineyard can enhance the symptoms of Grapevine Leafroll Virus. The soils complicate the issue in a few ways, not only with not providing an alternate habitat for the mealybug. First because the roots have to struggle to get all the elements and nutrients necessary to maintain a healthy vine. This makes combating the virus and its symptoms very difficult. Many times a disease is able to express itself and takeover an unhealthy, struggling vine. Problems can loom over potentially unhealthy vines
That is not to say it is too hard. Here is a picture of the Murdoch Vineyard just off of Gimblett Road in the Gravels. This is a certified organic vineyard. Notice the limited weed control and growth in the interrows even as the picture was taken well into late autumn. They experience very low levels of mealybug incidence at this vineyard according to viticulturist Chris Henry.
Detection Issues Without proper knowledge of the symptoms of GLRaV one could confuse the vine as having the virus. But first of all, this photo was taken in the spring and discoloration of GLRaV infected vines show in early autumn. This is a young Syrah vine with an elemental deffiency. Because they all struggled the previous season, we laid some mulch down (notice at base of all vines) to allow retention of nutrients and water for the young vines.
Here is another young Syrah vine that is not as far along in its deficiency as in the previous slide. Notice how on some of the leaves the red blotches have started from the outside and then eventually become more prominent in others. Also there is not significant downward rolling as there is in GLRaV. And again, after a fairly wet spring, notice the vineyard floor interow as fairly yellow and baron. Detection Issues
Wind The Gimblett Gravels lie on a former river bed between some prominent foothills. The area experiences regular high winds. This allows for easier passage of the vectors like mealybug to travel from vine to adjacent vine, across rows to other vines and even on to vineyard equipment to be passed on to previously unaffected rows and neighboring blocks.
Identification and control However you manage infestation in the Gimblett Gravels, identification has been led by Caine Thompson's technique of Normalised Distribution Vegetation Index and GPS technology to track and monitor the Virus. Great strides have been made in development and use of this technique. Using the technology along with rouging and pulling out vines has greatly reduced the spread of the virus and loss of blocks that would have been taken over by the virus in the coming years. To the right observe a block in which individual vines have been marked as infected. The color shading allows to better identify areas of the vineyard that need the most attention. This also allows to consider outside influences, soil structure and other factors as to the pattern of the virus spreading. For instance, those areas more heavily affected could be getting it from a neighboring block or from populations of infected mealybug previously in the soil. Photo provided by Caine Thompson
Rouging vines After identification of infected vines they must be ripped out. Then those areas must be treated with an herbicide and/or pesticide. Then a fallow period of 1-2 years is implemented. This can be very expensive in both labor costs and loss of production to a grower, so careful identification is essential. But it does work as Thompson states here: “ If leaf roll virus exists in low numbers in a vineyard (under 20%) It can be effectively rouged out of the vineyard and we have many examples of this already from the mapping work we have done. When rouging is not done then we see an increase in virus spread especially if mealy bug numbers are high. There are numerous studies that have shown the economic benefit of ‘rouging’ where virus levels are low.” Cain Thompson May 2010
“ When a block is over 20% it is very difficult to ‘rouge out’ the virused vines as at this stage it is a large number of replants to manage through the block and there is potential for other vines not yet expressing virus that may also be infected but will not express until the following year. Therefore the only option once a block gets to this stage is to manage mealy bug numbers and try to slow virus spread as much as possible. Eventually blocks like this will need to be removed as they will not reach quality/quantity targets in the future. When these blocks are left in the ground they provide a source of infection for neighboring blocks that may be clean so a thorough mealy bug control program is necessary.” - Caine Thompson May 2010 Rouging Vines But this statement is complex for a grower who sees no definite answer to solving the virus issue by pulling out the entire block. No research or product on the market has proven that the virus will definitely be gone after pulling out the vines, treating the area and then leaving a fallow period. The cost of doing this is near impossible for a grower to take on at this point. Most struggle to get through each season and most are enjoying good to great vintages in recent years, especially those with older vines even if they know eventually the virus will decline the quality.
Owner of Unison Vineyard, Philip Horn is very hesitant to go around pulling out vines until there is a definite method for eradicating the soil and old root systems from the virus. Until this happens it is impossible to take on the financial burden. And now that mealybug infestations have been brought to tolerable levels the vines are able to temporarily overcome the effects of the virus. But it will surely overtake those vines which have the virus. And though the Gimblett Gravels is able to ripen the big reds such as Syrah and Cabernet, it is still a cool climate and eventually ripening will become and issue. And this is not just an issue for the small grower in the Gimblett Gravels. Heavily diseased blocks from Delegats are just next door. They have been known to be diseased for a few years and they have not yet ripped out the vineyard. But working together is something that is being implemented now by Hawke's Bay Winegrowers.
Thompson, in some ways, agrees with this: It’s important to recognize that in vineyards we can tolerate mealy bug if the virus is eliminated from vineyards. It is where the two coexist is when we come into problems. We have seen this extensively on the gravels where this scenario exists virus spread is rapid moving through blocks at a very quick pace. Leaf roll virus is permanent and once a vine is infected it will remain so for the duration of its life and the virus ‘level’ in the vine will increase which puts more pressure on the vine effecting crop load, lowering brix, and increasing acid. As vines become older fruit quality really suffers even though the vines are older they cannot ‘overcome’ or ‘tolerate’ the virus. Fruit quality is always poorer especially in red varieties.'' Caine Thompson May 2010.
Back to soil issues.... Complicating the issue for the Gimblett Gravels once again is the soil. Not only does the rocky soils make it very difficult to dig up and remove vines. But because the root systems have to grow so deep in the rocky soils to get the proper nutrients, many times large sections of the infected root systems will remain in the soil after rouging. So not only could infected mealybug remain in the soil but infected plant materials as well. From www.gimblettgravels.com
Still, rouging and GPS tracking is so very important because it shows what is working and what is not working. Maps from this years (2010) tracking of the virus have not been made available yet. But once all of the data is sorted, the Gimblett Gravels will be able to see if this year's spraying protocol established by Hawke's Bay wine growers has helped slow the spread of the virus by the mealybug. These vineyard management programs and virus/mealybug fact sheets are the first of its kind in attacking the virus at a sub-regional level, coordinating between all the vineyards to help solve the problem together. Early reports from vineyards in the Gimblett Gravels have been a large decrease in mealybug populations. Working Together
Riversun Nurseries in Gisborne are setting the standard for Virus free, traceable grafting and planting materials. Though it is now obvious that much disease was spread because of tainted grafting materials and rootstocks, it can't be stressed enough. Traceability and tested materials are becoming a very important part of the battle against the virus. And with all of the rouging needed on the Gimblett Gravels, it is essential to the process. The sins of our fathers...
The protocol... The idea was to set the conventional vineyards on the Gimblett Gravels on the same chemical control pattern of mealybug. It called for a late spring/early season application of Tukuthion and oil followed by 2 applications of Applaud. What was interesting this year was to see the communication between vineyards on this protocol. It not only set up protocol but created a system on which neighboring vineyards could share information on slight augmentations and best application techniques. For instance, upon advice from Phil Holden who manages the Gimblett Gravels blocks for Villa Maria, it was deemed best to wait as late as possible before capfall to apply the second treatment of the Applaud etc. This was very important as it was the last chance of getting the mealybug in the very important larvae stage when the spray can be very effective in eradicating at high population times.
What next?.... The data will be sorted soon for the effect of all the measures being taken in the Gimblett Gravels to control mealybug and GLRaV. So keep an eye out for that. But the state of managing the virus as a whole will still be a matter of rouging while New Zealand waits for new products to be approved in cleansing the soil and old root systems of the virus. In the mean time, all will be closely watching how this unique terroir takes to dealing with this huge issue. New Zealand as a whole is in that tough position where it is warm enough for the major vector of the virus in mealybug, but not warm enough to compensate for the loss in ripening and possibly not warm enough to provide adequate predators for the mealybug. This is compounded in the Gimblett Gravels where interows and vineyard floors many times lack the vegetation to provide a better environment for the mealybug than the vines themselves. This issue will come more to the forefront as Sustainable Winegrowing New Zealand deals with the effects of chemical sprays and their effects on soils and the environment over time.
In summary... The Gimblett Gravels is getting there because of the sub-regional effort and though its problems are unique, so are its solutions. With the help of all the skilled viticulturists, Food and Plant Research and Hawke's Bay Winegrowers, over time they will get on top of virus and this program is a great step towards understating virus/mealy bug relations.
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