I think only Grateful Dead fans will understand the unworthy attempt at being funny in the title… sorry.
But it has been a long, strange pre-bloom / bloom period in Georgia vineyards, and perhaps throughout other grape growing regions in the southeastern US. Most bunch grape vineyards in Georgia are well into bloom or early fruit set – perhaps excepting later developing cultivars planted at higher altitudes. Muscadines in south Georgia are smack dab in the middle of bloom; both self fertile and female muscadines may thus experience reduced fruit set in those vineyards. Muscadines have yet to bloom in the Georgia piedmont – from Atlanta latitudes and north (see picture below); northern-positioned muscadine vineyards may experience good fruit set if bloom is accompanied by drier weather.
Fruit set and yield. Many growers have asked about how the weather patterns will affect fruit set and crop yield potential. Sunny, dry weather hastens the bloom-to-fruit set-transition and improves fruit set percent. Extended bloom periods are typically unwelcomed as they are caused by cool, cloudy, rainy weather. Such weather conditions are unfavorable for pollen viability and germination rates (Koblet 1966) and thus fruit set (and crop yield potential). The lower threshold for optimal for grape pollen germination is 77 deg. F (Keller 2010). Georgia vineyards have experienced adequate temperatures, but poor meteorological conditions, for fruit set. Grape clusters will likely have fewer berries this year at sites experiencing extended cloudy and rainy periods throughout bloom. Rainfall and weather patterns throughout the rest of the season will determine how crop yield will ultimately be affected. Sustained rainfall could increase the average berry weight in clusters containing fewer berries; the net effect could be similar weights relative to clusters composed of relatively greater berry numbers and lower average berry weights. Practically, this information does not mean much because we cannot control the weather. If we could, we would not have requested our recent weather. This is simply some food for thought since some growers have asked about it.
Disease control. Bloom represents the first critical period of seasonal disease protection on grape clusters. Many fungal diseases can infect clusters beginning at bloom. Our recently-experienced weather patterns will exacerbate fungal disease infection due to three primary factors – (1) fungal diseases thrive in warm, humid conditions; (2) many have not been able to get out to spray due to persistent rain and/or wet, unstable vineyard floors; and (3) we anticipate a large amount of flower debris (which may serve as a source of infection) within clusters due to reduced fruit set. Please consider tightening spray intervals and using effective products for controlling powdery, downy, and black rot, amongst other diseases that can infect clusters. Spraying effective materials throughout fruit set and before bunch closure is paramount for optimal disease control every year in the humid southeastern US; this year is no different. Please consider the recent comments from blog posts by our regional grape pathology experts – Dr. Phil Brannen (https://site.extension.uga.edu/viticulture/2018/05/downy-mildew-and-other-grape-diseases-management-during-bloom/) and Dr. Mizuho Nita (https://grapepathology.blogspot.com/2018/05/bloom-time-fungicide-application.html). As I have said in previous posts, I would encourage all to sign up and visit Mizuho’s blog site at https://grapepathology.blogspot.com/.
The silver lining to the recent wet weather may be that newly-planted vineyards that were not equipped with drip irrigation have received ample water supply… perhaps more than ample in some cases. This is a reminder that young vines do require pest management so that foliage can stay healthy to optimize photosynthesis and carbon gain. Consider implementing an insect and fungal disease management plan for your young vines. Reach out to your county extension agent or statewide fruit entomology and plant pathology specialists for pest management recommendations.
Best of luck for drier weather and getting your sprays on in a timely fashion. Here’s to hoping for better weather in June.
Thanks y’all, Cain
Koblet, W. 1966. Berry set of grape vines related to shoot treatments and climatic factors. Wein Wiss 21: 293-379.
Keller, M. 2010. The Science of Grapevines: Anatomy and Physiology, 1st Ed. Academic Press.