Ouch, I’m Burning!
Leaf-stripping is a technique that many viticulturists use to increase berry exposure, hence color and sugar development, mainly in red wine grape varieties. It also can be used to increase the air circulation in and around the cluster, to prevent bunch rot and other types of diseases that thrive in moist conditions.
This year we have had a number of rain events that may cause some concern for increased incidence of diseases, and many are thinking about or have started to leaf-strip around the cluster zone. Remember though, that many of the sugars that accumulate come from leaves surrounding the cluster, so be careful to not remove too many leaves, otherwise sugar accumulation may be delayed.
To the right (top) is a picture of a couple of vines that have been leaf-stripped to increase cluster exposure, but a number of leaves have also been left to continue photosynthates that will be translocated to the cluster later in the season. The second picture (middle) indicates what may happen if clusters are exposed on the west side of the vine all season long, namely sunburn. This may also be possible in clusters that are exposed suddenly, around veraison as in the third picture.
Think of it like this…you are out in the first warm, sunny day of the year, and you forgot your sunscreen because you haven’t needed it, thanks to the cloudy February and March. What happens? You get a sunburn! The same thing can happen in a number of different fruit crops.
So what’s so bad about that? Well… sunburn can lead to a number of off-flavors that winemakers may not be enthusiastic about. It can also lead to a reduction in yield, due to the dehydration of numerous berries. There are a number of studies that are in the works to explore exactly what these compounds are from various research programs. However, sunburn is most prevalent in areas of the country/world that have high solar radiation on a continuous basis (all of WA state!).
The short of the story is… be careful how much leaf-stripping you do. It can go too far, which can result in sunburn. However, if you do it early in the season, to allow for gradual exposure, then the berry skin will thicken and build up tolerance to the increased sunlight. This can result in good development of anthocyanins and flavonols, both important in making a great wine to last for all years.
– Dr. Mercy Olmstead, Former WSU Extension Viticulture Specialist