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By Maria Smith, HCS-OSU

Veraison, or the onset of ripening, marks the home stretch of the growing season (Fig. 1). At this stage, berries change colors, soften, and swell with water and sugars. During ripening, vines are heavily taxed by the fruit’s carbohydrate, water, and nutrient demands. In the case that one or more of these 3 vine resources are limited, both fruit maturity and vine vegetation can visibly suffer.

Fig. 1 Early veraison indicated by color change from green to red in Marechal Foch. July 27, 2020.

While this growing season hasn’t been as stressful for disease management as 2018 and 2019, we have had notably dry weather over more than a 2 month stretch. This has brought about “abnormally dry” to “moderate drought” conditions that now cover nearly 85% of the state (Fig. 2). For young vines, this has driven a need for irrigation throughout this growing season to aid in establishing root systems and canopy growth, and for both young and mature vines, a need to keep a closer eye on changes to vine nutrient status (Fig. 3).

 
Fig. 2: Drought conditions for Ohio, as of 7/30/2020 (source: https://droughtmonitor.unl.edu/CurrentMap/StateDroughtMonitor.aspx?OH)

Fig. 3: Leaf chlorosis example in 2-year-old vines. July 27, 2020.

Chlorosis in basic terms means a yellowing of leaves due to the loss of chlorophyll (the molecules responsible for light absorption during photosynthesis). Several reasons may explain chlorosis, including nutrient deficiency and drought stress. Visual symptoms aid in detecting acute issues with the vine, but ultimately for remediating nutrient deficiencies, vine nutrient status should be assessed to determine whether and which deficiencies are the exact cause.

Why vine nutrient status? Soil testing is great for identifying availability and, in the case of pH, accessibility of nutrients (it should still be performed regularly every 3-5 years), but it does not tell you what is in the actual vine. That’s where petiole tissue testing enters. Long-term nutrient management is best when the two are used together to develop an annual fertilization management plan.

As a reminder, veraison is the preferred time to sample and test for vine nutrient status (regardless of symptoms) due to the relative stability of nutrient concentrations. For details on how to sample, recommended values and testing facilities, please see the following resources:

• OSU factsheet on Grapevine Nutrient Management: https://ohioline.osu.edu/factsheet/hyg-1438
• Taking foliar samples in vineyards and orchards (UMN Extension): https://www.youtube.com/watch?v=eyPs1rTFoNc&t=2s

Fixing nutrient deficiencies now. Is it too late?

Veraison is about the last point of the growing season that fertilization should be done to avoid delaying or harming vine acclimation in the fall. For acute deficiencies, foliar fertilization with soluble formulations is ideal for quickest uptake. Additionally, late-season foliar applications of urea (46-0-0), especially under drought conditions, may increase yeast assimilable nitrogen (YAN) in fruit at harvest. Stuck/sluggish fermentations and hydrogen sulfide by-products that generate “rotten egg” aromas can often be attributed to low YAN. However, baseline YAN values for a given cultivar and site should be understood before using foliar urea as a means of adjusting YAN values in fruit.

Foliar applications are temporary band-aids and do not adequately address chronic deficiencies. It is best to avoid deficiencies altogether through routine monitoring of soil and vine nutrition and annual soil fertility maintenance.

The list below addresses commonly observed deficiencies and remediations via foliar fertilizing. These recommendations are only intended as a short-term solution to situations where deficiencies have been clearly identified through tissue testing and/or visual symptoms. In general, a healthy vineyard with routine soil fertility maintenance does not benefit from additional foliar fertilization, and its use may create unnecessary management costs. We additionally suggest avoiding the use of commercial mixed-nutrient products for foliar fertilizer, as they may either provide insufficient quantities of a target nutrient or unnecessary additions of others. When applying any fertilizer, applicators must be careful in calculating rates to avoid excess fertilization and, with foliar application, burning vines. Tank mixing foliar fertilizers and pesticides may cause phytotoxicity. Consult all labels and/or spot-test before applying to all vines.

Nitrogen (N):  

• No. applications: 3x
• Rate: 5 lbs urea (2.3 lbs N) per 100 gallons water, applied at 200 gallons per acre

Potassium (K): 

• No. applications: 1x
• Rate: 6-10 lbs potassium sulfate (0-0-53) or chloride (0-0-63) per 100 gallons water, apply at 200 gallons per acre

*Note: excess K additions can result in Mg deficiencies and lead to high pH in fruit.

Phosphorous (P): Foliar application is not recommended

Magnesium (Mg):

• No. applications: 2x, 1 then again after 2 weeks (bloom time preferred for first application)
• Rate: 16 lbs Epsom salts per 100 gallons water, applied at 200 gallons per acre

Boron (B): 

• No. applications: 2x (pre-bloom to bloom time preferred for application)
• Rate: 1-2 lbs Solubor (20% B) per acre