DES MOINES, Iowa (AP) — Nearly 1,000 Iowa residents have contacted a state agency about sickly oak tree leaves, which officials say were likely caused by farm chemicals and made worse by weather fluctuations.

Iowa Department of Natural Resources district forester Mark Vitosh told The Des Moines Register (http://dmreg.co/2rEGmyw ) that little can be done to prevent the deterioration of the oak leaves besides stopping the use of herbicides.

The condition, called leaf tatters, causes leaves to appear as if they've been eaten down to the veins.

Research by the University of Illinois in 2004 found a "strong correlation" between leaf tatters and exposure to chloroacetanilide herbicides.

"If that chemical was not there, this wouldn't happen, if you believe the research," Vitosh said.

There have been more oak tatters this year because the weather caused the leaves to emerge at the same time chemicals were most prevalent in the atmosphere, said Tivon Feeley, the department's forest health program leader. The number of citizen complaints this year is higher than previous years, but it's not at a record level, he said.

Iowa residents make more reports about sickly oak trees than other Midwest states, according to U.S. Department of Agriculture Forest Service officials.

The USDA Forest Service recommends reducing other stresses to affected trees and keeping them in good health.

Iowa has seen a significant decline in white oak trees over the last five years, and the tatters may be contributing to that decrease, Vitosh said.

But funding for research on the declining oak population and leaf tatters is difficult to secure.

"It is considered regional — a few Midwest states — and no one has proven (tree) mortality with it," Feeley said. "Join that with a shrinking federal budget, and there isn't enough funding to really look further into this."

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Information from: The Des Moines Register, http://www.desmoinesregister.com