Some grape growers throughout California – particularly on the Central Coast – have noticed strange leaf symptoms this spring, including yellowing or bronzing of blades along with necrotic margins or interveinal areas, a University of California Cooperative Extension advisor notes.

The affected leaves are always the most basal leaves, with newer growth not showing symptoms, says Mark Battany, a water management and biometeorology advisor based in San Luis Obispo.

The culprit, Battany explains, is the cool and wet weather in May, which led to reduced vine transpiration and less uptake of needed magnesium and potassium from the soil.

“At sites with limited availability of these nutrients in the active root zone, this resulted in deficiency symptoms in the basal leaves,” Battany writes in his Grape Notes blog. “The symptoms appear in the basal leaves because both nutrients are readily mobile in the vine and will move from the older basal leaves to the newly expanding tissues as needed.”

The number of leaves showing these symptoms isn’t large, but these basal leaves are important for shading grapes later in the season, Battany writes.

“This alteration of the fruit zone shading condition may be the most important consequence this season for mature vineyards that observed strong symptoms this spring,” he adds.

Though the problem may be minor in most instances, it puts Central Coast vineyards on the long list of other crops that suffered because of unusually late rain, cold winds and even snow in the Golden State.

Cherry growers in the northern San Joaquin Valley say rain split some of the ripening fruit on their trees, while mid-May storms interrupted berry harvest in Central California, delayed rice planting and brought concern for growers of tree nuts and other crops.

Two vineyard types

Battany notes that leaf symptoms are showing up in two types of vineyards the most: ones of any age on calcareous soils, and very young vines. Calcareous soils have low native contents of magnesium and potassium, unless they’re added in treatments, he says. And very young vines have small root systems that limit their ability to take up nutrients anyway.

Mature vines with symptoms were likely in vineyards where fertilization with magnesium and potassium has been limited, perhaps because it was withheld, he writes.

“Symptoms on young vines have been very dramatic, even in some areas on non-calcareous soils,” Battany observes. “This may indicate insufficient levels of magnesium and potassium in these soils, or that these nutrients are found deeper in the soil beyond the shallow roots of the young vines.”

Warmer weather should bring about newer growth that is normal, he says. With young vines suffering more extensive loss of foliage, soil testing might be useful to determine if these sites would benefit from more fertilization, Battany advises.

To read his full blog post, visit https://bit.ly/2FqGPLe.

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