Many growers have observed late season foliar nutrient deficiencies in Concord vineyards while harvesting and there have been questions concerning diagnosis and management for next season. Knowing your vineyard system combined with visual observations can give clues to the mineral nutrient in question but only a tissue nutrient analysis can tell you for sure the nutrient status of your grapevines.
Currently, we have standard target tissue values for petioles collected at bloom and/or veraison and these standards are published in the Winegrape Production Guide for Eastern North America by Tony Wolf. One of the outcome goals of the Hi-Res Vineyard Nutrition project is to expand standard target tissue values for leaf blade, as well. Obviously, we are well past veraison in Concord vineyards so the published standard values will not be very much help in diagnosis if you collect tissue at this time. However, if you have healthy areas and nutrient deficient areas in your vineyard, you can always collect tissue from these separate regions, keep the tissue samples separate, and do a comparative analysis. Once you have your tissue results, we can assist with the interpretation.
Many vineyards are showing signs of either Magnesium (Mg) deficiency or Potassium (K) deficiency. Sometimes it can be difficult, especially in early stages, to tell the difference between Mg or K or even other nutrient deficiencies, like N, because many nutrient deficiencies cause some type of interveinal chlorosis (yellowing between leaf veins). Again, this speaks to the importance of a tissue test for a true diagnosis. Magnesium and Potassium have an antagonistic relationship in grapevine nutrition; therefore, it is important to know which one is deficient to take the right corrective action. For example, if you have Mg deficiency and you think it is K deficiency so you add more potassium fertilizer (potash), you will most likely make the Mg deficiency worse, not better.
This is what typical Magnesium Deficiency looks like in Concord (Figure by Dr. Terry Bates).
Magnesium deficiency causes interveinal chlorosis and then eventually leaf scorching starting around the leaf margins. Symptoms tend to start on mid-shoot leaves but can be somewhat random and give the canopy a tiger stripe appearance. Look for Magnesium deficiency where the vineyard has low soil pH, cool/wet conditions, or excessive potassium fertilizer applications. Nine times out of ten in Lake Erie Concord vineyards, Mg deficiency is solved by fixing the soil pH issue with lime applications. See: https://www.efficientvineyard.com/blog/blog-post-title-one-bfj39
This is what typical Potassium Deficiency looks like in Concord (Figure by Dr. Terry Bates).
Late season potassium deficiency is commonly associated with high crop load in Concord. Remember, crop load is a function of both yield and vine size (the Y:PW ratio…not just yield alone). Concord fruit has a high potassium demand, especially from veraison to harvest. If the crop size is too large for the given vine size (i.e. root size and ability to get K from the soil), then the fruit will steal the K from the leaves and cause leaf deficiency symptoms. K deficiency will also start as interveinal chlorosis (and can look like Mg deficiency) but usually turns more black than yellow.
Also, K is considered a mobile element in the plant and will move to the fruit and growing shoot tip when in short supply. Therefore, K deficiency is almost always observed first on the older basal leaves of the shoot. To correct potassium deficiency for next year, add potassium fertilizer (potassium chloride, KCl, aka potash) and, more importantly, measure, understand, and manage for proper crop load balance through any combination of pruning level, early-season shoot thinning, and mid-season fruit thinning. — By Terry Bates, Director, Cornell Lake Erie Research & Extension