Does Nitrogen Cause Pinot Leaf Curl?
Researchers analyze mystery ailment at Sonoma County Grape Day
The ailment also can affect Pinot Blanc vines and has been observed less frequently in Pinot Meunier. Pinot Leaf Curl, or PLC, appears during spring; symptoms can range from stunted or distorted leaf growth to severe instances in which an entire shoot and node can die.
Rhonda Smith, University of California extension advisor for Sonoma County, made the ailment a focus of Sonoma County Grape Day on Feb. 16. She said she wanted to draw attention to the sickness and explore a possible relationship between the disease and nitrogen. She said the disease, which has been found in every Pinot Noir region of California, is known to be more common during cold, wet springs.
Dr. Doug Adams, a professor of viticulture with UC Davis, joined Smith on the panel. Adams said that when vines enter dormancy, they store nitrogen to support growth in the coming spring and summer. When the growing season arrives, the plants convert the stored nitrogen into a form that can be metabolized by the plant. The nitrogen compounds used during this process can also yield putrescine. Elevated levels of putrescine can be toxics to vines, he said.
Is putrescine the culprit?
The next step, Adams said, is to find a 100% asymptomatic Pinot vineyard and determine “normal” levels of nitrogen compounds. Then researchers can evaluate if symptomatic tissues contain elevated levels of putrescine. “You can find putrescine in most plants, but only in trace amounts,” Adams said.
Various terms have been used to describe PLC. Smith said the condition was known, but it has not been a major problem until recent years, when Sonoma County growers began to become concerned by the increased incidence and severity of symptoms.
Smith said the condition can reduce crop loads, but it was hard to discern how much of an effect it may have had on the 2011 harvest totals because of losses from botrytis and other, more common pests.
Smith also wanted to draw growers’ attention to the fact that while PLC can sometimes look like botrytis, it is a wholly separate condition. She said PLC first weakens tissue, and then opportunistic botrytis finds a way to attack the vine. Therefore, Smith stressed that fungicide applications do not work on PLC.
There appears to be some variation in clonal susceptibility for PLC: When present in a vineyard, the condition tends to be found in mature blocks, and mild symptoms can occur in up to 80% of vines, Smith said.
Symptoms to watch
According to a report written by Smith, symptoms of PLC can range from mild to severe. Onset occurs early in the growing season, most commonly when three or more leaves separate from the shoot tip. The leaves then appear to “bend or fold downward across the middle of the blade, perpendicular to the main vein”—hence the descriptive “leaf curl” name. The vines will appear to have “broom-like growth” with multiple lateral shoots pushing at all nodes, with abnormal leaves until a single lateral becomes dominant.
Mild symptoms are often limited to darkened or necrotic regions on the underside of leaves. In a severe case of PLC, the necrotic region will extend to the petiole: Both the blade and petiole abscise from the shoot. If that is a cluster-bearing node, the disease can reduce crop load.
Sonoma Grape Day is an annual event for grapegrowers, organized by UC Cooperative Extension and sponsored by the Sonoma County Winegrape Commission.