2024 Chenin Blanc and SAD: Part Two

Recently, I blogged about 2024 Chenin Blanc and SAD: Part One. This post is a continuation with the goal of trying to figure out what causes impaired sugar import into the grape berries. Sugar accumulation disorder (SAD) and suppression of uniform ripening (SOUR) are both classified as grape ripening disorders.
In order to understand why SAD and SOUR can occur in grapes, we have to go back to how grapes ripen since these disorders occur shortly after the onset of ripening. After fruitset and the first growth stage, the grapes enter into a lag phase before véraison. This progression of first growth, lag phase and véraison in grapes is characterized by a double-sigmoid curve, which is best summarized by this graphic:

For additional information about grape berry ripening, please check out this post: Grape Berry Development - Heading into Véraison.
Ripening involves higher levels of abscisic acid (ABA), brassinosteroids, and ethylene. Grapes affected with SAD or SOUR exhibit the following symptoms:

• Grape berries look deflated
• Reduced berry weight, symptoms appear shortly after onset of ripening
• Low (10–13 °Brix) due to arrested phloem influx
• Titratable acids often high due to berry dehydration
• Tartaric and malic acid content per berry not changed; oxalic and citric acid reduced
• Most amino acids reduced; higher hydroxyproline, arginine, and alanine
• Low yeast-assimilable N, K+, and other nutrients; low K+ in rachis and pedicels
• Low anthocyanins, genes for biosynthesis delayed; elevated skin tannins
• Rachis and pedicels are green and show no symptoms

Here are pictures of unripe Chenin blanc that we dropped prior to our harvest. Many of the grapes did have low °Brix that we checked for using our hand held refractometer or by taste test. The rachis and the pedicels did appear green so that did not help us in the culling but in the process of dropping nearly one ton of grapes, we were able to hone in on which bunches were likely to be affected with SAD and SOUR.

Next blogpost will be on the possible causes of SAD and SOUR mentioned in the review.
References:
1. Griesser M, Savoi S, Bondada B, Forneck A, Keller M. Berry shrivel in grapevine: a review considering multiple approaches. J Exp Bot. 2024 Apr 15;75(8):2196-2213. doi: 10.1093/jxb/erae001. PMID: 38174592; PMCID: PMC11016843.