Sugar Accumulation Disorder (SAD)
What is SAD?
Sugar accumulation disorder (SAD) is a post-veraison ripening disorder characterized by reduced sugar accumulation and anthocyanin content, as well as a lower pH and reduced weight and volume (Krasnow et al. 2010). The cause of SAD is under investigation, but has yet to be determined.
Comparing SAD to other berry shrivel conditions
Shriveled berries on vines with bunch stem necrosis, sunburn and late season dehydration may appear similar to SAD. Other characteristic symptoms are used to distinguish between these disorders.
Bunch Stem Necrosis (BSN)
BSN clusters have necrotic (dead) rachis tissue whereas SAD vines have green and healthy rachis.
Sunburn usually affects berries that are most exposed to the sun, compared to whole cluster shrivel observed with SAD.
Late Season Dehydration
Late season dehydration is difficult to distinguish from SAD and BSN. Unlike BSN, the rachis remains healthy. Dehydrated berries taste sweet, whereas SAD berries lack sugar.
To learn more about these disorders see Krasnow et al. 2010
UCCE Viticulture Team Research
The UCCE Napa Viticulture team has a long-term study with one site containing a block symptomatic for SAD. The study aims to explore how SAD affects different components of infected vines.
From 2020-2021 we have monitored Brix levels in one affected (symptomatic) block and another unaffected (asymptomatic) block. Vines were chosen from both blocks to have their Brix tested on one date during early ripening stages and another date towards the end of ripening.
Local Research on SAD
The UCCE Napa Viticulture team established a multi-year SAD study in Napa County. We are measuring sugar accumulation (°Brix) in one affected (symptomatic) and one unaffected (asymptomatic) block. Both blocks are V. vinifera cv. Cabernet Sauvignon on 110R. Sugar accumulation is measured for 50 randomly selected vines in each block, at two time points corresponding to early and late ripening stages.
Results from both years show a clear distinction between the two blocks. In the SAD block, sugar accumulation was reduced and there was greater variability in °Brix measurements across sampled vines.