Did you notice more shatter at bloom than normal in your table grape vineyards this year? What happened with coloration at veraison? Anything unusual? If so, your vines may be getting too much nitrogen. Another sign of excess nitrogen is an unusually high number of flat canes at the end of the season, says Ashraf El-kereamy, University of California Cooperative Extension viticulture advisor for Kern County.

Although nitrogen is one of the essential elements required for vine growth and development, excessive levels can decrease vine productivity and reduce quality of the grapes, he says in his article, Effects of Excessive Nitrogen in Table Grapes, in the Spring, 2017, issue of VitTips, the UCCE viticulture newsletter for San Joaquin Valley growers.

Vine roots take up nitrogen in the form of nitrate or ammonium which are incorporated into amino acids in the roots and shoots. These amino acids are the building blocks of proteins and enzymes involved in vine growth and development.

The amount of nitrogen required by a particular grape variety depends on how well it can take up and use the nitrogen available in the soil. However, taking up more nitrogen that needed can stimulate the vines to produce excessive vegetative growth, resulting in a shaded canopy. Too much shade can inhibit initiation of flower buds, resulting in lower bud fruitfulness the following season. Also, a shaded canopy may have a high percentage of necrotic buds that won’t grow the following spring.

That’s not all.

“A vigorous canopy may compete with fruit for sugars, negatively affecting fruit sugar accumulation and red color development,” El-kereamy says. “Blocking too much sunlight from the berries also inhibits anthocyanin and phenolic accumulation in red color varieties. Under these undesirable conditions, normal leaf removal around clusters may increase the risk of sunburn due to high nitrogen and low phenol level in the berries.”

Shading due to excess nitrogen levels can increase the risk of fungal diseases, since the microclimate of shaded canopies is typically more humid than those with less shade, he notes.  Also, excessively leafy canopies reduce spray coverage and efficiency.

The impact on the vine of too much nitrogen varies with the phenological stage, El-kereamy adds

“Excessive nitrogen at bloom could result in early bunch stem necrosis, especially in cool, wet weather, or excessive shattering resulting in inadequate berry set and almost empty loose clusters,” he says. “Similar effects are associated with shaded clusters.”

You can determine nitrogen levels in the vine using a bloom petiole analysis and soil and water nitrate readings.

The vineyard, itself, can provide clues of excess nitrogen. They include a higher percentage of flat canes than usual by end of the season; lower bud break percentages and fruitfulness early in the spring; and, during the season, excessive shatter at bloom, lower anthocyanin accumulation and less coloration at ripening.