Fungal pathogen: Botrytis cinerea
Botrytis bunch rot is the most common rot for wine grape vineyards in Oregon and the Pacific Northwest. Infection can occur as early as bloom but may not be visible until post-veraison.
Preventing Botrytis in the vineyard
To prevent Botrytis bunch rot, it is critical to have good canopy management practices that allow air flow and fungicide spray penetration/coverage of the inflorescence or fruit cluster. These canopy practices must be appropriately timed during vine growth and should be used in vineyards with moderate to high vegetative vigor. One of the most effective canopy management practices is cluster zone leaf removal on the morning-sun side of the canopy, and it should be conducted on vines where you cannot see the clusters and should be done any time from just before bloom to just after fruit set. Do not leaf pull after berries have grown bigger than about pea-size and should be done well before veraison is anticipated. Late season leaf removal can lead to sun/heat burn of fruit if high temperatures persist. Recently winemakers have been asking vineyard managers to reduce the amount of leaf removal or to conduct later in the season out of fear of over-exposure in recent warm seasons. However, research in Oregon shows that typical leaf removal (~50% leaves removed) and up to 100% of cluster zone leaf removal conducted from bloom to pea-size does not result in sun/heat burn of fruit, even in warm seasons. This research has also shown that early leaf removal can increase color (anthocyanins) and aroma compounds of Pinot noir. Note: If you have weak or small vines without much canopy density, clusters are typically visible and leaf removal is not necessary.
The second step in prevention is the use of well-timed fungicide applications, targeting applications to the cluster zone at bloom and bunch closure. As needed, fungicides may need to be applied before rain events post-veraison to prevent conditions for infection. Details on the timing of fungicide applications and products to use are available in the Pest Management Guide for Wine Grapes in Oregon. See page 34 of this guide to determine conditions for Botrytis infection based on air temperature and hours of berry wetness.
What to do once a vineyard has infected fruit
Once you have active infections of bunch rot late season, you will have to assess the level of infection and determine your next step. Some wineries have very low thresholds for Botrytis, and you may need to remove that fruit from the vine before harvest. It is possible to remove infected clusters at the winery if they have a sorting line. There are methods that can be used in the winery to deal with Botrytis infected fruit. See Dealing with compromised fruit in the winery for more details.
For further information on Botrytis bunch rot, see the following resources: