Inception, Diagnosis, and Consequences of the Berry Shrivel Disorder
Berry shrivel (BS) is a disorder of unknown cause and sporadic appearance that has been increasingly observed in vineyards in Napa and Sonoma Counties. BS is commonly misdiagnosed as bunch stem necrosis (BSN), but we have found that the disorders can be clearly distinguished based on the presence of a healthy, green rachis in BS compared to a necrotic rachis in BSN-affected clusters, and the fact that BSN berries have normal to high Brix whereas BS berries have much lower Brix than normal berries. This was the second year of our study of BS development, and the first in which BS was prevalent enough to obtain a representative sample of BS berries over time. In order to determine whether BS is a berry, cluster, or vine level phenomenon, a substantial amount of over-sampling was required, and hence not all the 2005 samples of berry composition have been analyzed to date.
In both 2004 and 2005, around the time of veraison, and prior to any appearance of BS symptoms, BS berries were firmer than control berries, but the decline in firmness at veraison was faster in BS than in control berries, so that for most of the post-veraison period, BS berries were softer than controls. In 2004, BS fruit skin had a slightly increased mass of cell wall material compared to the skin of control berries, and a slightly reduced amount of cell wall material in the mesocarp, indicating that the disorder may be related to metabolic aberrations in cell wall metabolism in the mesocarp. In 2005, with a larger sample size, this was not the case. It is also unclear to what extent BS is associated with water stress. In Oakville, vines which historically have exhibited BS have been consistently less stressed than controls, but the opposite trend was observed in Sonoma County, and in nether location was the degree of BS associated with the level of water stress, as measured by leaf water potential.
In both 2004 and 2005, visual symptoms of shriveling, whether caused by BS, cluster girdling, or cluster excision, were always associated with a loss in mesocarp cell viability (as indicated by a fluorescent vital stain), and hence we can attribute the shriveling to cell death, rather than simply to berry desiccation. In both years there has been no apparent difference between BS and control berries in the xylem connection between the berry and the pedicle. For the 2005 berry composition data that is available, BS berries were similar to berries on girdled clusters, in that they both accumulated less soluble solids and had a lower pH than control berries. One key result from 2005 is that there is also evidence that BS symptoms, as measured by essentially all of the major indicators of berry development (Brix, pH, dry weight), are progressive over the season and are also expressed at the whole-vine level. Berries that show early symptoms of BS are the most affected, but lesser degrees of symptoms also occur in berries that are affected late, and still lesser, but measurable, symptoms occur on apparently healthy berries from affected vines. These results strongly suggest that, for a vineyard that is affected by the BS disorder, the effects of the disorder may not be limited only to clusters exhibiting shriveled berries.
Our current hypothesis is that BS is not a disorder related to xylem function and vine water relations, as thought previously, but rather a disorder either of phloem function and/or of vine photosynthesis. Wines were made from commercial Cabernet fruit (BV vineyard in Rutherford) that was either free from BS, or had varying levels of BS (5, 10, or 15%by weight), or BSN (40%by weight) fruit. In all cases, fermentation proceeded normally, and these wines will be subject to sensory and chemical analysis.
