Retrospective Analysis of Vineyard Problem in 2024

All growing years are never the same and 2024 was another year to learn yet another lesson in viticulture. The summer was warm with a rain event almost every week, which in any other year is a welcomed event, saving us from having to irrigate the vineyard. However, the warmth and the rain caused our vines to grow uncontrollably and probably lead to what we began to observe in the vineyard. Here are some photos of the grapes as they matured through veraison.

There may be more than one problem going on here but a few of the symptoms suggest that our grape bunches suffered from something called late bunch stem necrosis(LBSN).¹
LBSN is a complex physiological disorder of grapevines, where the bunch stems (rachises) shrivel during ripening, followed closely by berry shrivel. The cause of this shriveling may be due to environmental stresses that causes the vine's xylem that transports water and nutrients, to shut down. Without a working xylem network, water and nutrients do not get to the grape bunches leading to the shriveling.
In reading this article on LBSN, one thing stood out that might have relevance to our situation: Aerial roots (roots forming on the cordons) can happen in response to winter injury, late frost damage, or excessively wet, humid conditions. While they do not negatively affect the plant, they do serve as a useful clue that the vine is experiencing these conditions.
We did observe aerial roots in many of our vines. In 2023, we had a late frost event on May 18th that decimated the south half of our vineyard. In 2024, we noticed that the north half of the vineyard fared better than the south half, being able to ripen the grape bunches. So it could be that our vineyard was still trying to recover from the damage done in 2023. The aerial roots and the berry shriveling were the visual cues that indicated a stressed vine.
We wait with anticipation as to what lessons Mother Nature has in store for us in the vineyard this year.
References:
1. Amaya Atucha, Berry Shriveling in Grapes: Late Bunch Stem Necrosis, UW-Madison Department Horticulture, September 13, 2019.