Fall 2013: Botrytis Bunch Rot of Grapes: To spray or not to Spray?
Jay Pscheidt, Ph.D., Extension Plant Pathology Specialist, OSU
Walt Mahaffee, Ph.D., Research Plant Pathologist, USDA-ARS
Patty Skinkis, Ph.D., Viticulture Extension Specialist, OSU
The 2013 growing season has progressed smoothly, but these last few weeks of ripening may prove challenging with the appearance of Botrytis bunch rot in some vineyards. The rainfall since véraison has encouraged the development of this problem, and more wet weather is predicted in the coming weeks. The question arises as to the value of bunch rot sprays at this time of the year.
Fungicides will not get rid of Botrytis once it has started in a cluster. That gray fuzzy stuff you see in the cluster now will continue to be there. Also, healthy appearing clusters may have microscopic infections within the cluster on berries, caps, and other trash that you cannot see. These infections will continue to grow even if you apply a fungicide spray. At this point, fungicides can only act to protect healthy tissue from new fungal infections. The key is making applications before rainfall.
There were wet conditions during bloom in some parts of the state, and these conditions were suitable to Botrytis, as it can colonize many of the flower parts that remain in the cluster. The recent rains and those predicted in the forecast will activate the Botrytis in these tissues if the cluster remains wet. To help reduce disease development, canopy management (leaf removal or hedging) can help to increase airflow and allow clusters to dry.
This is an uneasy time for many with clusters ripening and harvest just around the corner, but you may be wondering whether to spray or to just use canopy management. Sprays applied before this rain may be helpful if rainfall keeps clusters wet for an extended period of time.
The bottom line–do not spray, just wait for now! If you have been practicing leaf removal and making appropriately timed fungicide applications at bloom, bunch close, and véraison, then your grapes should be looking fine. If not, spraying now would not help control active Botrytis infections and may encourage fungicide resistance development. The forecasted heat this week should arrest bunch rot development that is currently underway.
For more information, see the 2013 Pest Management Guide for Wine Grapes in Oregon for temperature and berry wetness models for Botrytis infection (page 25).