Erratic Bud Break and Irregular Shoot Growth in Spring: Potential Causes and What Can Be Done to Fix Them

Tian Tian, Viticulture Farm Advisor Kern County

Budbreak is the first observable phenological stage at the start of the new growing season. Variability in budbreak can be caused by a variety of factors including adverse weather conditions, disease problems, and pest issues, and can even be a characteristic of certain varieties. Uneven budbreak has a domino effect across the growing season by increasing variability during bloom, which will also increase the number of harvest passes for a vineyard. This unevenness will complicate the timing of bloom sprays and make harvest events impossible to set up evenly across a vineyard. This variability is not always noticeable directly after budbreak but will become more evident once shoots are around one-foot tall. This article will provide the main causes and mitigation solutions for erratic bud break and uneven shoot growth, as well as insights on monitoring for problems.

Winter weather.

The uniformity of bud break can be affected by winter weather, even though vines are dormant at that time. Buds need exposure to low temperatures (32 to 45 °F) for a period of time in the winter to break dormancy. Despite debates over the number of chilling hours required for optimal bud break, it is clear that insufficient chilling contributes to variable shoot growth in the spring. To overcome the issue involving a lack of chilling, Dormex (H2CN2) is an effective product used to increase the uniformity of bud break and possibly advance its occurrence. The use of Dormex is popular in table grape vineyards. However, it is worthwhile noting that some growers observed a decrease in vine longevity upon regular Dormex use. However, this effect has not been described in the literature. Dormex is highly toxic, so caution should be used when applying this product. We are currently conducting bud stimulant trials in 2022 and 2023 to evaluate newer products that may serve as alternatives to Dormex. Check back in with future editions of VitTips for the results of those trials.

Low precipitation in the winter is another factor contributing to variable bud break and delayed shoot growth in the spring. Under normal moist soil conditions, roots supply water through xylem tissue for the swelling bud. However, when the soil is too dry, the roots are unable to supply enough water. This leads to the formation of air bubbles, known as embolisms, in the xylem vessels. In severe cases, buds are not able to rehydrate properly. This can lead to a disconnection between the bud and the rest of the vine’s vascular system, leading to issues with the transportation of nutrients and water to the young shoots in spring. As a result, shoot growth is stunted and cluster development can suffer from poor canopy development.   We encountered the issue of delayed spring growth in 2021. If these conditions return in a future year, irrigation prior to budbreak to refill the soil moisture profile will help prevent the poor canopy development seen in the spring of 2021. Luckily, the cold rainy weather we had this winter would protect us from a such problem in 2023.

Variety characteristics.

Some grape varieties have strong apical dominance. This leads to buds on the distal end of the cordon breaking bud first and slowing down budbreak on the rest of the cordon. In old days, growers would assign crews to remove the distal young shoots manually during bud break, to improve bud uniformity. However, this practice has become cost inhibitive with current labor rates. Bud stimulants, such as Dormex, are applied to mitigate the variability in bud break in those varieties. Moreover, varieties that lag on dormancy appear to have more issues in variable bud break. Delaying pruning may help with such situations.

Vine diseases.

Erratic bud break can be an indicator of vine decline due to disease issues, such as trunk disease and crown gall. Trunk disease is a chronic disease caused by fungal pathogens colonized in arms, cordons, and trunks. The pathogens that cause trunk disease enter the vine through wounds, they can cause necrosis of the perennial wood structures (arms, cordons, and trunks). As the fungi slowly spread in the permanent wood structures, they reduce the vine’s ability to supply water to portions of the vine past the infection. As the fungal pathogens move relatively slowly, you will find tiny shoots adjacent to normal young shoots in one vine, during the early to mid-stage of infection. Usually, trunk disease is found in older vineyards (> 10 years old) than in young vineyards.

Some trunk disease pathogens are more aggressive than others, but prevention is a powerful tool, nevertheless. Spores of trunk disease pathogens release under cold and wet conditions. Delaying pruning and avoiding pruning right before rain reduces wounds presenting at the time when spores prevail. Applying wound protectants is also effective to protect vines. For product selection, please see the reports on pruning wound protection trials from Dr. Akif Eskalen’s lab at UC Davis (https://ucanr.edu/sites/eskalenlab/Fruit_Crop_Fungicide_Trials/).

In addition, crown gall is a disease caused by a bacteria called Agrobacterium vitis. Galls can be found in cordons, trunks, and roots. This disease occurs more often in cool production regions where cold injury presents.

Figure 1. Photo of bud break status at the end of March in a table grape vineyard

Pest issues.

Pests like cutworm feed on buds and young shoots. If you suspect that uneven bud break may result from pest issues, please use the checklist developed by UC IPM for table grape (https://ipm.ucanr.edu/agriculture/grape/table-grapes-delayed-dormancy/) and raisin/wine grapes (https://ipm.ucanr.edu/agriculture/grape/wine-and-raisin-grapes-delayed-dormancy/) for monitoring guide.

Also, in own-rooted vineyards, parasitic nematodes, and phylloxera can lead to the gradual decline of vines. The slow shoot growth in the spring can be related to the loss of root mass due to their activities.

Monitoring bud break and early shoot growth

I found keeping records using pictures is a quick and easy way to monitor bud break and shoot growth in the spring. To do so, I start with labeling a few vines in one vineyard block with color flagging wrapped around the trunk and writing the block number and row number on the flagging. Then take pictures of flagged trunks and canopy periodically. The pictures need to be clear enough to see the young shoots without inferences of other vines in the background (example as shown in Figure 1). Saving pictures in designated albums is recommended. As most phones would record the time and GPS location of pictures, one could sort photos by location or by date for future reference. Please note that the GPS of pictures may not be accurate enough to separate vineyard blocks in areas with poor phone service. Satellite images can also be helpful to compare vine growth between years at a similar time. Please see the article written by Mark Battany on using satellite NDVI for vineyard diagnostics (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=46551).