Walnut growers hold spray
Updated: Saturday, May 08, 2010 9:28 AM
Residue-threshold ruling on tap; blight conditions ripe
By TIM HEARDEN
RED BLUFF, Calif. -- Though the spring's wet start is making conditions ripe for walnut blight, some growers are waiting to spray because of a change in available fungicides.
For nearly two decades, growers have used a mixture of copper and an ethylene bisdithiocarbamate product called Manex to battle blight, which can cause significant crop damage if left unabated.
But the unavailability of the product's technical ingredient, maneb, forced manufacturer DuPont Crop Protection to develop a similar spray called Manzate, which required approval from the U.S. Environmental Protection Agency.
The EPA granted the product's registration in February, but without setting a threshold for how much residue would be allowed on harvested nuts. Without such a threshold, referred to as a time-limited tolerance, it would be illegal to sell nuts with any residue on the shell, said Rick Buchner, a University of California Cooperative Extension farm advisor in Red Bluff, Calif.
"The intention from what we can gather ... is to have that issued well in advance of harvest," said Dennis Balint, chief executive officer of the California Walnut Commission. "Farmers tend to be very conservative people, and if all the steps aren't clearly lined out there are questions. I think that's where we are right now."
Sprays are applied in the spring when there's not yet a nut on the tree. With Manex, the allowable level of residue at harvest was 0.015 parts per million, but most nuts had no residue whatsoever or a minute level, Balint said.
Growers hope that a tolerance level will be approved by the EPA by mid-summer, Buchner said.
"Some are going to go ahead and apply anyway because the odds are good that it'll get a tolerance," he said. Some growers of early varieties have already made one or two spray applications, he said.
But others for the time being are simply using copper, which was the sole material used for managing the devastating fruit and leaf disease until pathogens started developing resistances to it in the early 1990s.
If the disease-causing bacterium Xanthomonas arboricola is on the tree, wet and warm conditions can increase the tree's risk for blight, Balint said. Several storms have brought rain to California's Central Valley in the past two weeks, but temperatures so far have been low enough to reduce the risk, he said.
The California Walnut Commission requested both a one-year and a permanent registration for Manzate, and the EPA granted use for one year while it considers permanent approval, Balint said.
The EPA has indicated it will issue the residue threshold before nuts are harvested in the fall, said John Chrosniak, DuPont's regional business director for North America in Wilmington, Del.