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Posted 20 November 2010. PMN Crop News.

Late Fall Cleanup Limits Iris Pests

Source: Kansas State University Press Release.

Manhattan, Kansas (November 8, 2010)--Taking action after the first hard freeze can be an effective way to limit two common iris pests. Just getting rid of the plants’ dead leaves, however, may not be enough, according to Ward Upham, horticulturist with Kansas State University Research and Extension.


Old iris foliage can and does serve as an over-winter haven for the fungus that causes iris leaf spot disease and the eggs of a destructive insect called the iris borer. As a result, removing irises’ post-season debris can significantly reduce the numbers on hand to attack the following spring, Upham said.

“But, some of the earliest borer research turned up additional facts that we sometimes forget,” he added. “In the 1930s, Cornell University discovered iris borer moths weren’t as selective as first thought. When the females were ready to lay eggs each fall, any roughened surface would do -- dead leaves, twigs, rusty nails, cloth, wood chips, even wire screen.”

Iris leaf spot weakens plants and makes them look ugly, Upham said, but iris borers can cause deadly damage. Besides, just one female borer moth can lay hundreds of eggs every night, totaling more than 1,000 eggs before she’s done. And, North Americans’ control efforts since the 1930s haven’t kept the borer population in check.

Iris borers are a multiple threat, he said. When the tiny, new larvae hatch in spring, they chew their way up iris leaf margins. Then they make a pinhead-size hole, enter a leaf and eat their way back down on the inside.

Not every borer makes it to soil level. One theory is that iris borers eat each other, too, Upham said. But, those that do can be more than an inch long. They’re able to bore into and start mining their iris’ rhizome. Often, they’re also carrying the bacterium that causes soft rot, which makes rhizomes soft, slimy and foul-smelling.

“To limit their activity, as well as iris leaf spot’s spread, we need to be doing a better job of sanitation after freezing weather arrives,” the horticulturist said. “Some commercial growers burn their iris fields every winter. But, I just remove everything near or on my iris beds. I put down some new mulch after the ground freezes.”

His removal includes any green iris leaves that may remain. Contrary to popular opinion, they won’t be making food any more. With the first freeze, iris rhizomes shut down for winter, Upham said.

Ward Upham