MSU Extension Grapes

As the season begins: Pruning, observation and early signs of vine health

Each year in early spring, the Michigan State University (MSU) viticulture team travels to vineyards across Michigan to engage with growers, observe the vineyards in the early phases of vine growth and focus on the effects of one of the most critical aspects of vineyard management: winter pruning. These visits are not only an opportunity to evaluate how different pruning strategies are influencing early-season vine growth, but also a vital time to identify the first signs of grapevine decline, a growing concern in many Michigan vineyards.

This article explores the current understanding of grapevine decline in Michigan, with a focus on practical, research-informed strategies that can help growers extend vine longevity, reduce disease pressure, and make informed decisions in the face of evolving challenges.

Trunk diseases – a threat to vineyard longevity in Michigan

Grapevine trunk diseases are a complex of chronic wood-infecting diseases that can drastically shorten a vineyard’s productive life. These diseases are caused by many different fungi, often with a single vine hosting multiple pathogens at once. In young vineyards (under 7 years old), Petri disease (young vine decline) and black foot are the most common trunk diseases. In mature vineyards (older than 8 years), Eutypa dieback, Botryosphaeria dieback (often called bot canker), Phomopsis dieback (related to Phomopsis cane and leaf spot) and the Esca complex are widespread.

These pathogens spread insidiously via spores carried by air and rain, entering the vine through pruning cuts and other wounds. Once established, they cause internal wood decay (cankers) that leads to chronic vine decline, dead arms, reduced yields and eventual vine death. In severely infected vineyards, yield losses of 30-50% are common, and in extreme cases up to 90% yield loss has been reported.

Michigan’s cool-cold climate adds an extra challenge to trunk disease management. Cold winter injury can wound vines and mask underlying trunk disease symptoms or even predispose vines to infection. For example, following the polar vortex winters of 2014 and 2015, many Michigan growers initially blamed extensive vine losses entirely on freeze damage. Only later MSU did lab analyses, revealing that opportunistic trunk disease fungi had invaded through the cold-cracked wood, accelerating vine decline.

The take-home message is clear: Trunk diseases are an ever-present threat, especially in Michigan’s climate, and proactive prevention is crucial to maintain vineyard longevity.

Pruning wounds: The infection gateway

Every pruning cut is a double-edged sword: it’s necessary to train the vine, but it creates an open door for pathogens. Most grapevine trunk disease infections begin when fungal spores land on fresh pruning wounds and colonize the exposed wood. 

How long is a pruning wound susceptible? Research shows grape pruning wounds can remain susceptible for a few weeks up to several weeks after cutting, in some cases a month or more under cool conditions. Eutypa, for example, can infect a wound even four to six weeks after it’s made, though risk declines over time. The highest risk is immediately after the cut and the first one to two weeks following, especially if weather conditions are wet.

Crucially, the pathogens require moisture to infect. Spores are typically released during rain events (or snowmelt) in the dormant season when temperatures rise above freezing. In Michigan, late winter and early spring often bring frequent precipitation. If pruning cuts are made during a thaw in February or March and it rains shortly after, spores of Eutypa, Botryosphaeria and other fungi can be splashed or blown onto the fresh wounds and germinate there. This is why timing and weather conditions for pruning are so important for disease prevention.

Avoid pruning in wet weather

A fundamental rule is to never prune during rainy or wet conditions. If a rain or snow event is forecast within the next day or two, it’s wise to postpone pruning, because fresh cuts made just before rain are almost certain to get doused with spores. Pruning during cold, dry, sunning weather allows cuts to dry out quickly and gives the vine’s natural defenses a chance to start sealing the wound before any spores find it. Michigan often has intermittent thaws in late winter, taking advantage of a dry window in late winter or early spring to prune can greatly reduce infection risk. Conversely, if extended wet weather has settled in and pruning can’t be delayed further, be prepared to use wound protectants, as discussed in the next section.

Prune late (delayed pruning)

Rather than pruning at the earliest opportunity in winter, Michigan growers are advised to delay pruning to as late in the dormant season as practical. In California viticulture regions, pruning as late as February or later was shown to dramatically reduce trunk disease infections because it avoids the peak spore dispersal period of December-January and exploits the natural decrease in wound susceptibility over time.

In Michigan, our pruning window is shifted later (budbreak is later than California), so the equivalent advice is to prune in March or early April rather than in mid-winter. The MSU viticulture team also suggests delaying pruning as long as possible not only for disease reasons, but to allow assessment of winter bud injury before final pruning decisions.

The disease control benefit comes from the fact that wounds made in late March or early April (when temperatures are warmer and vines will soon start bleeding sap) tend to heal faster; research indicates pruning wounds made in warmer spring conditions heal more rapidly and have a shorter infection period than mid-winter cuts. By late March in Michigan, the frequency of intense rain events is often lower than mid-winter, and any spores that do land may face a vine that is starting to become biologically active, further limiting infection. For instance, it is important to delay large pruning cuts until spring (no earlier than late March) for exactly these reasons.

Double pruning technique

For large vineyards or where early pre-pruning is needed, “double pruning” is an effective technique to limit infection on final cuts. This involves making an initial pruning pass in winter, leaving long spurs or canes well beyond the intended final cut position (e.g., leave eight to 10 bud spurs, or leave canes a foot longer than needed). Then, just before budbreak, a second pruning pass trims the spur down to its final two-bud length (or removes the extra length of cane). The idea is that any spores that landed on the first cut (the stub you left) will only travel a short distance into the wood. When you come back and make the final cut in spring, you’re removing that potentially infected stub entirely, leaving a fresh wound at a time of year when inoculum levels are lower and wounds heal faster.

Many Michigan growers with spur-pruned vineyards (e.g., Concord or some hybrid blocks) practice mechanical pre-pruning in winter followed by hand follow-up pruning; this is essentially double pruning. Even in cane-pruned systems (Guyot), one could prune canes long in winter and shorten them at bud swell. Double pruning does involve an extra pass, but it’s a proven method to reduce trunk disease risk in high-inoculum regions. If only a single pruning pass is feasible, try to do it late (Figure 1).

Prune healthy blocks first

One practical tip is to prune your healthiest or youngest vineyards first and save any known infected or older blocks for last. By doing so, you minimize the chance of spreading spores or infected sawdust on tools from a diseased block to a clean block. Also, crews are less likely to inadvertently carry fungus on their clothes or equipment from sick vines to healthy vines if the sequence goes from healthy to diseased, rather than vice versa. While the main mode of spread is via airborne spores, this “clean-to-dirty” order of work is a standard sanitation practice that can help, especially when tool disinfection between vines is limited.

Minimize large cuts and old wood wounds

When pruning, try to cut into older wood as little as possible. Grapevines don’t “compartmentalize” (seal off) large wounds very well, so big cuts through old trunk or cordon wood remain vulnerable and can become permanent infection courts. Whenever possible, make pruning cuts on 1- or 2-year-old wood rather than cutting deeply into a trunk or the base of an old cordon.

If you need to remove an old arm or trunk, consider doing so by cutting it in stages (double pruning concept) or waiting until dry weather in late spring. Also, angle your cuts on large diameter wood to shed water; a slightly angled cut (not flush horizontal) will prevent water from sitting on the wound surface. If possible, orienting the cut so that the cut face points downward also helps keep rain out. These little details in pruning technique can help keep wounds drier and less hospitable to spores.

Leave a desiccation stub

A technique related to minimizing damage to the permanent structure is to leave a short “stub” of wood when cutting off large arms. Rather than cutting flush, cut a few centimeters out (about twice the diameter of the wood in length) so that as the wood dies back a bit from the cut, the dieback doesn’t immediately reach into the main trunk. This area of expected dieback is called the desiccation cone. By respecting it (i.e., leaving a stub that can die back) you protect the main trunk from being hit by that dieback.

The stub can be removed entirely the next year once it’s clear where healthy tissue ended. This practice is recommended in pruning guidelines to minimize internal spread of wood diseases.

By carefully choosing when and how you prune, you can greatly reduce the opportunities for trunk disease pathogens to infect your vines. Of course, even the best timing and technique can’t guarantee every wound stays clean. That’s where wound protection treatments come in, which we will cover next.

Protecting pruning wounds with fungicides and sealants

Because we can’t always prune under ideal dry conditions, and because inoculum is often pervasive in the environment, applying protectants to pruning wounds is a recommended additional layer of defense.

Wound protectants fall into two categories: chemical fungicides (Topsin-M and Rally 40W) or biological sealants. These can be used alone or in combination, and many Michigan growers are beginning to incorporate them, especially when pruning must occur during higher-risk periods.

The typical use pattern for these fungicides is to treat the vines within 24 hours of pruning (the sooner the better). For instance, a common practice is to prune a block and have a second person follow behind with a backpack sprayer to mist the pruning wound sites with the solution. 

Lime sulfur (Sulforix) and other dormant sprays

An alternative chemical approach is using a dormant spray of lime sulfur immediately after pruning. Products like Sulforix (calcium polysulfide) applied to the vines can act as a surface sterilant, killing fungal spores on contact and penetrating superficial wound tissues. Michigan State University Extension specifically mentions that a dormant application of Sulforix after pruning will help protect wounds.

Lime sulfur has known efficacy against Phomopsis and some other fungi and is used in some regions to reduce inoculum. It should be applied soon after pruning is completed (before any rain) to essentially “disinfect” the fresh cuts. Do note that lime sulfur has a strong odor and can be caustic. Also, it must be applied during full dormancy (before any green tissue emerges) to avoid phytotoxicity.

Another benefit is that lime sulfur applications can clean up overwintering inoculum of powdery mildew and anthracnose on the vine, so it can serve multiple purposes in the early season spray program. 

Copper-based dormant sprays have also been suggested by some as a general wound antimicrobial, but copper is less specifically effective on trunk pathogens than lime sulfur.

Avoid straight paint or wax alone

It might be tempting to just slap some paint or wax on pruning cuts to seal them. However, be cautious. Plain latex paint (with no fungicide) has been abandoned all over the world because it is not proven very effective and can even trap moisture, possibly worsening conditions under the paint. Likewise, wax pruning sealants used in tree care are generally not recommended for grapes. Grape cuts are numerous, and not all sealants adhere well or persist through weather.

If you do use a paint, make sure it’s combined with a fungicide as described above.

In practice, each grower can tailor a strategy based on vineyard size and infection risk. The key is to treat the wounds as soon as possible after pruning, the same day, ideally within hours. For Michigan conditions, consider using protectants, especially if you prune earlier than ideal (e.g., pruning in January or February due to labor timing) or if you know your vineyard has a history of trunk disease.

Vineyard sanitation: Tool disinfection and wood disposal

While timing and protectants address the “front line” (the pruning wounds), sanitation measures address the background level of inoculum and the risk of spreading infection mechanically. Good sanitation in the vineyard can significantly lower the pressure of trunk disease over the years.

Remove infected wood from the vineyard. Any wood that is known or suspected to be infected (old cordons with cankers, pruned-off diseased trunks, dead arms) should be physically removed from the vineyard and destroyed (burned or buried). Do not toss infected wood into the row middle to rot; many trunk disease fungi will continue to sporulate on dead wood for months or even years, shooting spores around each time it rains. By burning or deeply burying that material, you eliminate a local source of millions of spores. Even pruning debris from healthy vines can harbor fungi like Phomopsis.

A common practice is to rake or blow prunings to the row centers and then chop them with a brush chopping. Chopping can help small canes decompose faster, potentially reducing pathogen survival. However, if you removed a large piece of diseased wood like an old trunk or cordon, do not just leave it in the aisle. Those pieces should be picked up and removed entirely. Some growers will burn piles of pulled wood during the pruning season (taking care of course to follow local fire safety rules), which is an excellent way to eradicate the spores on that wood. Others may bury or compost the wood far away from vineyards.

The bottom line: Eliminate infected wood from in and around the vineyard. This includes old grape stumps or neglected vines at the edge of the field, which can also harbor trunk pathogens.

Tool disinfection

Pruning tools (hand shears, loppers, saws) can potentially carry fungal spores or even bits of infected tissue from vine to vine. In a worst-case scenario, cutting through an Eutypa-infected spur and then immediately using the same blade on a healthy vine could introduce the fungus directly into the fresh wound.

In practice, the risk of tool transmission is considered lower than airborne spread, but it’s not zero, especially when making cuts through obviously diseased wood (e.g., a cankered trunk). Therefore, it is a recommended best practice to disinfect pruning tools regularly.

Admittedly, sanitizing pruners between every vine is often not practical (it would slow the work). But you can target your sanitation. For instance, after cutting out a clearly infected vine or after finishing a row/block where some vines were diseased, take a moment to dip or spray your tools. Use 70% alcohol (isopropyl) in a spray bottle to spritz the blades or use a 10% bleach solution in a bucket to dip the tools. Bleach (sodium hypochlorite) at 10% is a strong disinfectant but is corrosive to metal, so if you use it, be sure to rinse and oil your tools later to prevent rust. Alcohol is handy because it doesn’t require rinsing and can be sprayed on in the field quickly; a downside is it evaporates fast, so ensure a thorough wetting.

Another effective option some use is Lysol concentrate (a quaternary ammonium disinfectant) mixed as directed. It is less corrosive than bleach and can be quite effective on surfaces.

Whatever the agent, the important thing is contact time. For bleach, dip for about 1-2 minutes to really kill spores. With alcohol, a thorough wetting for 30 seconds is usually sufficient as it kills on contact. Also, consider having two sets of pruners: one can be in use while the other soaks, allowing you to rotate and not slow down too much.

For large cuts made with chainsaws (for example, if cutting out an entire trunk), disinfecting the saw bar/chain between vines is trickier. If a chainsaw was used on obviously infected vines, you might mist the blade with alcohol or bleach afterward (with the tool off and cool) and let it sit. At the very least, knock off sawdust and avoid dragging the saw from a sick vine straight into a healthy vine’s cut.

Vineyard floor management

Keep the vineyard floor clean of large debris. Old cut vine pieces left on the ground, or pieces of posts or tree branches, can also host fungi that might not be trunk pathogens but could complicate disease management. For trunk diseases specifically, the main concern is the grape wood. Some growers also practice weed control around vine trunks to reduce humidity and debris that could harbor fungi.

By rigorously removing diseased wood and sanitizing tools, you drastically cut down the reservoir of spores in your vineyard. Remember, many trunk disease fungi can also live as saprobes on dead wood, so a dead spur left hanging on the trellis is not just an eyesore, it’s a factory for spores. Good sanitation is labor-intensive upfront, but it pays off by slowing the spread of these diseases.

Trunk renewal and rehabilitation of infected vines

Despite our best preventive measures, trunk disease symptoms will eventually appear in some vines. When you see the signs (e.g., a cordon that’s died back, a section of trunk with a canker, or stunted shoots on one side of a vine) it is time to act with trunk renewal or surgical removal of the infected tissue. The goal is to eliminate or bypass the disease-infected wood and regenerate a healthy vine structure.

“Surgery” – cutting out cankers

If a vine has a localized canker, you can attempt a surgical fix: prune out the infected wood, cutting at least 4 inches below the visible infection into healthy wood. Some experts recommend an even larger margin, up to 12 inches below the cankered area, to be sure no residual fungus is left. In practical terms, this might mean removing an entire cordon or cutting the trunk off a foot below where a diseased cordon was attached. It may feel drastic, but incremental cutting usually fails to stop the disease. You need to get well into clean wood.

After cutting, inspect the cross-section. If you still see brown wedge-shaped discoloration, you should cut lower. Continue until the wood is uniformly light and healthy-looking. Once removed, destroy that infected wood as we discussed (don’t leave it lying by the vine). If the surgery involved a large trunk cut, consider applying a wound protectant on that big cut surface to guard it while the vine heals. This specialized form of surgery used in Europe for Esca is called “curettage” (Figure 2), where vines are literally carved open and the rotted wood gouged out, then the trunk is left to callus. It requires a specialized work force, and it is not commonly practiced in Michigan until now. In the future, this technique will be pivotal for Michigan growers and carefully considered before doing the classical amputation of the diseased part.

Trunk renewal

Thankfully, grapevines can sprout new shoots from their base (latent buds on the rootstock or lower trunk), often called suckers. In a diseased vine, these can be utilized. MSU Extension recommends training new trunks as a method to “rehabilitate” diseased vines. Here’s how it works: If one trunk or cordon is diseased but the vine is not completely gone, select a healthy sucker (or two) arising from near the crown or base of the vine. Cut out the old diseased trunk/cordon (as described above) and nurture the sucker through the growing season to become a new trunk or cordon.

This trunk renewal essentially replaces the old, infected wood with new, disease-free wood. It may take a couple of seasons to get full production back since the new trunk needs to grow and be trained, but it can prolong the life of the vine significantly. An infected vine that might have died in one to two years can instead keep producing on its new trunk for many years more.

A common strategy in cold climates like Michigan is to maintain multiple trunks per vine, often two trunks, not only for cold injury backup but also as a hedge against trunk disease. If one trunk becomes diseased or winter-killed, the second trunk can carry the crop while you regenerate a new one. 

Training vines with two trunks and periodically replacing the older trunk is a highly recommended practice. For example, on a mature vine, one trunk may look unhealthy while a couple of suckers are coming from the base. Rather than stripping off those suckers, keep the most vigorous one and cut the diseased trunk off above it either immediately or at next pruning. The sucker is then trained up onto the trellis to form a new trunk and tied as needed. Within a year or two, it can replace the old trunk completely.

When to remove the whole vine

There are times when trunk renewal isn’t sufficient. If a vine is extensively infected, say both trunks in a dual-trunk vine have cankers, or the infection has clearly reached the graft union or rootstock, the best course may be to pull the vine entirely (roots and all) and replant. Use your judgment: if over half the vine is compromised and suckers are weak or absent, starting over may be wiser.

Keep in mind trunk diseases are not like a foliar disease that can be sprayed away. Once the heart of the vine is rotted, it’s hard to reclaim it. That said, many vines can be rescued if action is taken early.

Putting it all together for Michigan growers

Michigan’s vineyards, with their mix of cold winters, humid summers, and a range of vinifera and hybrid varieties, face unique challenges from trunk diseases. By following an integrated approach of timely pruning, wound protection, thorough sanitation and strategic vine training, growers can significantly mitigate those challenges.

The best practices outlined here align with recommendations from Michigan State University Extension and viticulture experts, tailored to our cool-climate conditions:

• Prune as late as possible in the dormant period (late winter/early spring) and avoid pruning during wet weather. Consider double pruning to further reduce risk.
• Protect pruning cuts with labeled fungicides or wound sealants especially on large cuts or when conditions are conducive to infection. These treatments can greatly reduce spore establishment in the wound.
• Keep it clean, sanitize tools periodically, prune healthy vines before diseased ones, and remove all infected wood from the vineyard for destruction. Good sanitation lowers the overall inoculum load that your vines are exposed to.
• Renew and replace. Don’t hesitate to retrain new trunks or cordons when you spot disease. Maintain two-trunk systems so you can surgically remove one trunk when needed and not lose the vine. Cutting out a canker 4-12 inches below the infection can save a vine if done early.
• Stay vigilant. Scout in spring for tell-tale symptoms like stunted, cupped shoots, dead spurs or weak early growth, which can flag an infection. Mark those vines for summer or next winter’s surgery. Early intervention is everything.

By implementing these practices, Michigan growers can expect to significantly prolong the productive life of their vineyards. Trunk diseases may never be 100% avoidable, but they can be managed to minimize economic loss. As one more point of encouragement: A recent study in Italy found that by using such preventative strategies, vineyards reduced trunk disease incidence enough to extend vineyard lifespan by several years, effectively delaying expensive replant decisions. Our Michigan wine industry’s continued growth means many vineyards are now entering ages (15-30 years) where trunk diseases could escalate, making now the time to act on prevention.

In conclusion, trunk disease prevention and pruning wound management should be a routine part of vine care in Michigan, just like spraying for mildews or netting for birds. With careful pruning, protective treatments, sanitation and vine renewal, growers can safeguard vine health.