Grapevine Pruning to Reduce Canker Diseases – Benefits of Procrastination

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Dormant grapevine pruning is by far my favorite activity in the vineyard. Pruning takes years of experience to master and even the most skillful pruner will still occasionally be stumped by a vine. Pruning is a time where the art of compromise is often employed in the vineyard, where some vines conform nicely and others march to the beat of a different drummer. No other vineyard task has such a wide window for completion. This can be very good news for those growers out there that like to go out on family ski vacations in the winter. However, if you have a winery in addition to the vineyard, then sorry, you will need to work about 365 days a year. But do not be discouraged, there is still a window for skiing after the barrels are topped and malolactic fermentation is complete!

The long window for vineyard pruning may not be long enough for larger vineyards. On the Central Coast of California, for example, the larger growers will begin pruning in early December. With hundreds (if not thousands) of acres to complete before bud break, there is no time for procrastination. However, the smaller vineyards may benefit by waiting just a bit longer to sharpen those pruning shears. Research by Plant Pathologists at UC Davis has shown that delaying pruning can result in significantly less pruning wound diseases.  That’s right, diseases are trying to infect your vines even while they are sleeping. I will describe a few of these diseases below, and follow up with a summary of actions that growers can take to reduce infections.

Fungal Diseases of Pruning Wounds

A multitude of fungal diseases have been identified with canker diseases of trunks and cordons in the vineyard and the associated decline. A healthy vineyard should be productive well into its thirties if cordons and trunks remain healthy. However, what we see in the field is that vineyards can begin to decline due to canker diseases as early as 10 years into development in extreme cases. Although these diseases do not always kill vines outright, they cause the need for costly redevelopment of cordons and trunks. The major canker diseases you will hear mentioned in the literature are Eutypa, Botryosphaeria (Bot Canker), and a host of others that have been associated with vine decline, including Phaeomoniella, Phaeoacremonium, and Phomopsis.

Stunted growth of shoots can be seen here growing from a spur position that was infected by Eutypa.

Stunted growth of shoots can be seen here growing from a spur position that was infected by Eutypa.

By far, the two canker diseases most commonly mentioned by growers are Eutypa and Bot Canker. These fungal diseases commonly enter the wood of vines through fresh pruning wounds. Pruning wounds offer a point of access where spores can land and directly contact the vascular tissue of the vine. It used to be thought that these spores would primarily infect large pruning wounds (nickel size or greater) however we now know that they can infect even small pruning wounds of one-year-old canes. These fungal diseases can be found anywhere where grapes are growing and have numerous wild hosts, making them ubiquitous, and therefore a concern for all grape growers across the country (not just California). These diseases have been identified in GA, NC, NY, PA TX, VA, and other areas of the east.

Closing the Gap on Wound Infection

With the number of diseases we are constantly battling in the vineyard, it can be discouraging to hear that we now have more to think about while we are peacefully pruning out in the vineyard. The good news is that there are some passive methods to help reduce infection. Let’s begin with those before moving on to spray products and chemical methods. I recently invited UC Davis Plant Pathologist Dr. Doug Gubler to speak about his research on canker diseases at the Sustainable Ag Expo in San Luis Obispo (hosted by the Vineyard Team).

Below are a few interesting facts we learned from Dr. Gulber’s talk:

  • Spores of canker diseases need only up to 5 hours of wetness (rain) to infect wounds
  • Pruning wounds are susceptible to Eutypa infection up to 2 weeks after pruning
  • Pruning wounds are susceptible to Bot Canker for up to 12 weeks after pruning
  • Pruning wounds are susceptible to Phaeomoniella and Phaoacremonium for up to 16 weeks
  • Pruning wounds heal faster in warmer weather (closer to budbreak)
  • After infection, Eutypa can move up to 2 inches per year (primarily toward the trunk)
  • After infection, Bot Canker can move up to 5-7 inches per year
  • Bot Canker is more likely to kill whole spurs (dead spur positions are a key symptom)
  • Eutypa shows up in spring foliage (symptoms include stunted growth, short zig-zag internodes)
  • Canker disease spores are released and spread with rainfall events
  • Spores are less active later in the winter (…in California, however dry winters may result in later spore release)

From the above information we can see two important messages:

  1. Pruning during or just before the rain will increase the chance of canker disease infection
  2. Delaying pruning as late as possible will help to reduce infection

Why does delaying pruning help? As listed above, we see that there is more spore activity earlier in the winter and the pruning wounds take longer to heal (longer window for infection to occur). The top drawback to delaying pruning is that it takes the previously long window of opportunity to prune and crunches it into a last minute ordeal. This is where double pruning can come into play. Normally, spur pruned vines are pruned back to one or two count buds (for a review of dormant pruning you can view these three web tutorials). Double pruning is the action of partially pruning canes to retain a spur containing about 6 to 10 count buds. These long spurs are then cut back to the final count of 1 or 2 buds just before or at the onset of bud break.

These vines were partially pruned mechanically and will be final pruned just before bud break.

These vines were partially pruned mechanically and will be final pruned just before bud break.

The process of double pruning requires an extra, hasty pass through the vineyard but usually it can be done with unskilled labor or by machine. The final pruning can then be completed by skilled labor, but at a much faster rate than if whole canes needed to be pruned and removed in one pass. Double pruning also has the potential to induce delayed bud break – an important side effect for those in sites prone to late spring frost events. Finally, all dead wood should be removed at dormant pruning, as it is a harbor for canker disease spores.

Preventing Infection at the Wound

By simply delaying pruning until later in the winter and removing all dead wood from the vineyard, UC Davis researchers predict that up to 90 to 95% control of canker diseases can be achieved (yes that sounds high to me too). Yet there are still other practices to prevent fungal infection. The number one recommendation for prevention of wound infection of final pruning wounds is the application of a physical or fungicidal barrier on the wound. The idea here is that a physical or chemical barrier on the pruning wound will allow protection for long enough for the wound to heal naturally and thus not succumbed to infection.

Depending on which state your vineyard is in, there are a few products labeled for application on pruning wounds. These products need to be applied within 24 hours of pruning in order to be effective. A second application may also be needed if rainfall occurs soon after the first application (as always, read the label). Two fungicide products used in California to prevent Eutypa and Bot Canker are Rally 40WSP and Topsin M. If you prefer to avoid fungicides, there are a few “softer” products that act as physical barriers on freshly cut pruning wounds. The two that seem to be most tested in California are B-lock and VitiSeal. These products are sprayed directly onto pruning wounds, and once dry are supposed to form an impenetrable barrier to disease spores. The above products are listed on the UC Davis chart for fungicide efficacy and treatment timing.

Large pruning wounds made to retrain arms for a lyre training system are shown here painted with a pruning wound sealant (B-Lock).

Large pruning wounds made to retrain arms for a lyre training system are shown here painted with a pruning wound sealant (B-Lock).

There seems to be much enthusiasm around these “softer” products, however there is not a great deal of research based information to draw from, and the economics are not yet well-defined. Several growers I have worked with in the past have simply applied latex paint on larger pruning wounds in hopes to obtain a wound protection barrier. I would have to agree that this is better than doing nothing, although labor intensive to hand apply. I am interested to see the new results from ongoing trials with these products.

Putting it all Together

Clearly there is much research in major grape production areas that show the economic importance of canker diseases and the effectiveness of several methods to prevent infection. Growers with small vineyards may have the luxury of delaying pruning until the last minute, whereas larger vineyards will need to develop strategies such as double pruning to reduce infection by canker diseases. Regardless of vineyard size, there are an increasing number of products that are now available for application to pruning wounds. These products also have a place for preventing infection of pruning wounds when rain is predicted after pruning.

As we learn more about the efficacy and economics of preventing canker diseases, I predict these practices will become as routine as a pre-bloom spray for powdery mildew. I am personally exploring the use of low volume electrostatic sprayers and recycling sprayers for dormant application of wound protectants, to determine the potential savings these technologies might afford. For each new action recommended in the vineyard, there needs to be another innovation or return to offset the cost. I look forward to sharing some of these experiences with you in the future.

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