Case study.

In mid-July of this year, I received a call from a grape grower about symptoms of decline in several Marquette vines. The vines were planted five years ago, which means they survived the record rainfall in 2018 and polar vortex in 2019 and appeared to be pushing through the very hot and dry summer of 2021. However, with little rain during the spring and summer, vines were starting to feel the stress. When I arrived at this vineyard in late July, several vines had completely collapsed with berries still on the vine. Entire vines (Figure 1) or single cordons (Figure 2) turned completely brown. It was not clear what could’ve caused this mid-summer decline, but more vines were starting to show symptoms. Looking for a trend, it became clear that many vines experiencing these symptoms were in a low part of the vineyard. Lower parts of the vineyard are often prone to increased cold damage and these areas retain more water which can lead to ‘wet feet’ of the vines. Furthermore, symptoms were only observed on the ‘Marquette’ variety, which was planted next to ‘Petite Pearl’ vines that were not showing symptoms of this decline.

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We decided to further investigate the reason for the decline. Due to the lack of consistency of symptoms throughout the vineyard, we knew drought alone could not be blamed for this collapse. A cut was made through the trunk of the vine displaying symptoms on one cordon (Figure 2), and a small canker was observed (Figure 3). This brown canker extended up the length of the vine but did not affect the roots.

In the lab, vines were tested for the presence of fungal canker pathogens. Isolations from the cankered tissues revealed the presence of the fungus, Diaporthe sp. (Figure 4). Diaporthe cankers are widely prevalent in grapevine production regions worldwide, in fact, Diaporthe sp. was identified as one of the top 3 pathogens associated with grapevine trunk diseases during a recent and ongoing survey in Minnesota vineyards. Symptoms associated with Diaporthe cankers may include a reduction in spring growth, shoot dieback, shortened internodes, and fruit shrivel during the mid- to late summer. These infections are also associated with cankers or regions of dead tissue in the vascular system of the trunks or cordons.

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Canker diseases.

Cankers are typically caused by fungal pathogens that enter through pruning wounds or wounds in the bark. Canker pathogens infect vines that have been predisposed to some type of external stress like freeze damage. They are considered opportunistic pathogens; they do not attack healthy, vigorous vines, they target weak vines. Once vines are infected, the pathogen(s) may exist as latent (symptomless) infections. Symptoms like dieback often don’t appear until another stressor such as drought comes along, and the pathogen exploits the weakened state of the vine. This is why we often see grapevine trunk diseases causing a gradual decline in vines. It may take several seasons for a vine to succumb to a canker disease, but predisposing factors such as water stress (drought) can speed up that process.

Once the main trunk of a vine is infected, canker diseases cannot be cured. However, if only one side (cordon) of the vine is infected then it can be removed without disturbing the other cordon. Current recommendations for canker disease management in the upper Midwest include:

• Remove pruning’s from the fields. This helps reduce the fungal inoculum present in the vineyard and minimizes the risk of infection.
• Keep vines healthy and Plant on good sites only. As mentioned earlier, canker pathogens are opportunistic fungi and target vines that are stressed.
• Keep some suckers on your vine. These suckers can be used to renew grapevines following severe winter injury. This practice allows you to remove damaged or infected parts of the vines while offering the opportunity to retrain a new, healthy vine.
• Keep an eye on vines in low spots in your vineyards as these will be more susceptible to environmental stressors and therefore be more susceptible to infections.

If you suspect your vines have been infected by a pathogen, submit samples to the UW Plant Disease Diagnostic Clinic (https://pddc.wisc.edu/) for an accurate diagnosis.

Plant stress and predisposition.

Plant stress can be defined as a sudden change in the environment that results in non-ideal growth conditions which increase the demand on the plant. These changes in the environment can lead to irreversible changes that can cause eventual plant death or changes that are reversible. However, stresses that were once considered reversible can become irreversible in prolonged stress situations. Plants have many mechanisms to maintain their optimum, but an adaptation in one direction (ex. water stress) may not provide adequate adaptation in another direction (ex. pathogen attack). Plant stress can be caused by several environmental factors including heat, chilling, salinity, desiccation, freezing, water stress (drought or flooding), nutrient imbalances, etc. However, not all stress is bad! When certain stressors are imposed gradually (not rapidly) plants can slowly make adjustments to tolerate these stressors.

Predisposition is when there is an environmental stress that occurs prior to infection that can affect the susceptibility of plants to diseases, making them more susceptible. A major factor in Wisconsin and Minnesota that likely predisposes our vineyards is winter injury, which can reduce yields, kill vines, or reduce the longevity of the vine by making it more susceptible to attack by pathogens and pests. While we grow many of the cold-climate varieties, there is always an upper limit to their hardiness. Winters are also inconsistent from year to year, making it difficult for plants to adjust to constantly changing conditions. So how do we mitigate these issues:

• Select disease resistant and cold-tolerant varieties
• Help vines acclimate for winter: Keep the shoots long in the summer, avoid fall-applied nitrogen, and practice balanced cropping (don’t overcrop)
• Manage water and nutrients
• Remove dead tissues from orchard
• Prune and manage the canopy to allow good airflow and sunlight penetration (pathogens need humidity and/or moisture to infect and spread)

Not all vine decline is caused by grapevine trunk diseases.

This case study represents one situation where a gradual decline of vines was associated with a canker pathogen. However, vines can decline for other reasons, and the impact of environmental conditions on vine health should always be considered. For example, a vine can die back from “wet feet” or winter injury even without the presence of trunk disease. Therefore, we encourage growers to consider all possible stressors such as the environment, canopy management, and overcropping before drawing conclusions about the cause of vine decline. Secondly, remember that trunk disease and environmental stress can act together to kill a vine, because stressed vines are more vulnerable to dieback from trunk disease.

This article was posted in Disease, Grapes and tagged Annie Klodd, case study, disease, Grapes, Leslie Holland, vine disease, vine stress.