By Kathryn Carter, Fruit Specialist, OMAFRA
This year many vineyards in Ontario have been affected by winter injury, while others have survived the winter well. The extent of winter injury varies between blocks, varieties and geographical locations.
Why are we seeing winter injury this year?
Although traditionally we associate winter injury in grapes with extreme cold events such as the polar vortex in 2014/2015, this past winter wasn’t extremely cold. Most weather stations did not record temperatures low enough (<-23 ° C) to result in the extent of the winter injury we are seeing.
Grape vines acclimate very slowly in the late fall, so rapid drops in temperature in November can result in significant winter injury. The conditions we experienced last fall including wet weather, delayed ripening and high yields may have impacted the cold hardiness of some grape varieties. Research by Dr. Jim Willwerth (CCOVI) suggests that delays in fruit maturity (due to high cropping level or delayed harvest) can also result in delays or reduction in cold tolerance of vines under Ontario’s climatic conditions. The impact of cropping level or harvest date on cold hardiness varies between grape varieties. Chardonnay, Pinot noir and Merlot are more likely to have reduced winter hardiness after high crop load and late harvests. Crop level and/or harvest date has less impact on the vines’ ability to reach maximum hardiness levels in Cabernet franc, Sauvignon blanc and Riesling. The wet weather last fall, also made disease management challenging. Vines that experienced pre-mature leaf drop (due to foliar diseases) in the fall of 2021, may have been more vulnerable to cold injury if the vines were not able to lignify (harden off) before cold temperatures hit.
Unseasonably warm periods in January and February can also cause vines to de-acclimate, making them susceptible to winter injury, at temperatures they could have withstood before the warm weather. In January 2022, there were a few days of temperatures above 2° C followed by a rapid drop in temperature on January 16 to -20° C (Environment Canada, Vineland Station). This may have resulted in some winter injury.
Freeze injury can be a result of a single event, or multiple cold events, that accumulate the cold damage throughout the season.
Early identification of winter injury helps growers to make informed decisions about vineyard management practices and evaluate the need to re-plant or re-train the vineyard.
What are the symptoms of winter injury?
Bud Injury: Winter injury can be observed when buds begin to leaf out and it is apparent whether the surviving buds are primary (highly fruitful), secondary (less fruitful, with fewer and small clusters) or tertiary (fruitless). The number of buds and the type of buds (primary, secondary or tertiary) that survive have an impact on crop yields. Vines with high amounts of bud damage may respond with a flush of suckers from the trunk or ground. In some cases, uneven development or weak/stunted buds (Figure 1) may be an indication that the vascular tissue of the vine (cordon or trunk) has been badly damaged, and it can not support a canopy through the high transpiration rates of mid-summer.
Trunk and cane injury: Severe winter injury can cause trunk splitting. In some cases orange slimy growth may be visible on pruning cuts of winter injured grape vines. Trunk injury can be difficult to assess, and internal injury to the vascular tissue in the trunks may not be visible. Vines may start the spring with poor or uneven growth and collapse later in the season due to vascular injury that has gone unnoticed. If there is little or no shoot growth along the cordon, cut the dead cordons back to the head of the vine (where the trunk and cordon meet) and look to see if the phloem and xylem are green/ cream (live) or brown (injured). If the xylem and phloem are dead, then the trunk is severely injured. If there is little or no sucker growth at the base of the trunk on the trunk, then the trunk is damaged (Figure 2)
Figure 1. Cold injured buds on grape vine
(arrows indicate dead buds)
Figure 2. Grape vine with severe trunk injury
How to deal with injured vines
Ensure that crop insurance adjusters visit your vineyard to document the damage before making decisions about removing or cutting back vines.
In the spring, if bud damage is close to 100%, leave approximately five healthy suckers (by removing excess suckers) at the base of the trunk for potential trunk replacement. If there are fewer then five suckers remaining, leave them all, as they will help to distribute the excess energy in the vine’s root system and help manage excess vigour (bull wood). Protect these suckers from herbicide injury, by using grow tubes. Be sure to remove the grow tubes in late summer or early fall to allow enough time for the shoots to acclimate for the winter. If there is no sucker growth even at the base of the vine, then the vine is dead.
For sites with uneven or poor bud break, keep in mind that the injury may have been more severe then observed initially. Internal vascular damage to vines can result in vine collapse in the summer when temperatures increase and vines are unable to access the nutrients they require. If shoot development is uneven, or weak (especially in the renewal zone), it is a good idea to maintain suckers, in case it is necessary to renew the trunk. Failure to retain suckers, could result in few options available for pruning and managing vines next year. In cases where the vines are older and do not produce suckers, it will be necessary to replace the vines after this year.
During winter pruning select a healthy sucker for trunk replacement. Avoid selecting suckers that are more than pencil sized in diameter (bull canes) as they will have limited fruitfulness and be more vulnerable to cold injury the following winter. Secure the new trunk shoot vertically upward on the old trunk, or onto a pole. Consider keeping one extra trunk as a back up.
What to expect
For vines with uneven bud development or dead buds expect more vegetative growth this season, which can lead to more shading and less fruitful buds next year. With winter injured vines it is important to leave enough shoots to distribute the growth potential and minimize excess vigour. Continue to monitor these vines for vine collapse throughout the season.
Cold injury can also activate latent crown gall infections which will start to become evident around July. Often galls appear near the graft union, trunks or cordons, and can appear the year the winter injury occurred, or in subsequent years. Symptoms of crown gall are similar to trunk injury: stunted shoot growth, wilting and eventual collapse during the heat of summer.
Prior to replanting winter injured sites, take the time to assess the site, variety and training systems as well as cold mitigation strategies.
With a reduced crop this year, there will be reduced requirements for Nitrogen (N), so N fertilizer applications can be reduced or eliminated. Managing N will help minimize an overabundance of growth that can result in excess shading.
Even without a crop, it is important to manage disease pressure in the vineyards and ensure vines are healthy. Maintain appropriate shoot density and shoot positioning to maintain airflow through the canopy and manage disease pressure.