Grape Harvest and Rain

2018 is looking a bit like 2016 with all this late summer rain. We’ve had statewide rainfall each of the past 5 weeks. Most unwelcome! Sour rot and Ripe rot are becoming common in our research trials and vineyards around the state. Early ripening thin-skinned varieties are especially prone to skin splitting and cracking due to rain. Once opened, the berries are quickly invaded by yeasts, bacterial and fruit flies. Sour rot with its characteristic vinegar odor is the end result. There is nothing a grower can do to stop the rain, but there are some approaches to reducing sour rot. Canopy management is an important aspect for many varieties. Sprays of insecticide to control fruit flies plus an antimicrobial such as Oxidate or Fracture have shown some success. Best results occurred when sprays were started at 15 Brix. Sour rot is mainly a concern on the thin-skinned, tight cluster varieties such as Vignoles, Chardonel, Seyval, etc. We’ve also seen it this year on La Crescent, Itasca, Prairie Star and others. Tough skinned varieties like Vidal, Chambourcin, Noiret, etc. seldom experience skin splitting and thus are not prone to sour rot.

Ripe rot is a relatively new disease in our region and is most likely to occur on Marquette or Frontenac. Verona and Crimson Pearl are also showing heavy infection. I have a trial underway this year on Marquette and Frontenac. Preliminary data suggest that Abound (a.i. azoxystrobin, a FRAC 11 strobilurin) applied at veraison and again 14 days later provided excellent control. Rally and Captan applied on the same schedule have provided about 50% control, which is not commercially acceptable.  Untreated control plots of Marquette have essentially 100% infected clusters so this is a real concern for growers of that variety. We’ll be collecting data on Frontenac later this week and will have an update at the Fall Wine Grape Workshop in September.