During the last 10 days of March, Serr Tulare and Chandler walnuts varieties in Madera and Merced counties were shedding catkins and approaching pistillate bloom, reports Chris Morgner, PCA, Agri-Valley Consulting, Merced, Calif. Leaves on about half of the Tulares he observed were in the prayer stage.
In the meantime, predictions of unsettled weather the week of March 29 prompted him to recommend that his growers treat Tulares and possibly Chandlers with a copper spray to protect against walnut blight. Although the fungus disease normally doesn’t pose a big threat in his area, he likes to treat orchards with at least one spray each season. He likes to time the spray within the 10-day so window between 50 percent prayer stage and early pistillate bloom. He holds off on the spray as long as possible before rain falls to maximize the amount of green tissue – leaves, catkins and nutlets – that will be protected by the copper.
“Once we reach the stage where we have to pay attention to the possibility of walnut blight, we base the treatment on the weather,” Morgner says. “Some growers buy copper for the season and store it in the barn. Then, if it doesn’t rain, they send it back.”
Last year, because there were no prolonged periods of wetness during walnut bloom, none of his growers treated their trees for the disease. This year, Morgner expects they’ll spray twice, possibly three times.
His growers will probably treat their blocks of Serr with ReTain plant growth regulator in early April to reduce flower drop caused by pistillate flower abortion (PFA).