Released February 25, 2010
MANHATTAN, Kan. – An easy, “green” way to promote iris health is to remove last year’s leaves and other debris before the plant sends up new leaves this spring.
“Irises have two major pests -- both of which overwinter in or on old iris foliage and flower stalks,” explained Ward Upham, horticulturist with Kansas State University Research and Extension.
The fungus that causes iris leaf spot lives in iris beds year-round. It spreads by producing spores that travel from last year’s to this year’s foliage -- typically via wind or splashing rain during spring, he said.
The first symptoms reflect the disease’s name: small leaf spots, one-eighth to one-fourth inch wide. Often, the rim of each spot is reddish. The surrounding leaf tissue often looks water-soaked, but then yellows. The spots enlarge after the plant flowers. They may grow together, killing part or all of infected leaves.
“Sometimes the disease will affect the flower stalks and buds, too. But, even then, an outbreak won’t kill the plant. Iris leaf spot is deadly only in that repeated attacks can reduce plant vigor to the point that the iris dies anyway,” Upham said. “Unfortunately, irises in some parts of Kansas may be reaching that point, because we’ve had such a string of wet springs.”
Iris borers are the newly hatched young of a moth that lays its eggs on dead iris debris during fall. In April, the tiny larvae emerge to chew on the surface of the year’s new iris leaves, causing scars. As they grow, however, the larvae disappear. They bore into the leaves – each one generally leaving behind a pinhole, small slit or ragged leaf edge.
“Then they mine down toward the rhizome, eating and growing as they go. By July, though, watchful gardeners in this part of the country can kill the larvae by hand. All they have to do is squeeze infested leaves in the vicinity of the plant injury,” the horticulturist said. “That stops the borers before they can enter the rhizome and hollow it out, maturing at 2 inches long and often setting the stage for bacterial rot.”
Upham said other “green” management steps that promote iris health are 1) keep iris beds uncrowded, 2) don’t water irises from overhead, 3) remove and dispose of spotty leaves and/or borer-infested rhizome parts, and 4) initially clean up iris beds in late fall.
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http://www.ksre.ksu.edu/news/story/briefs022510.aspx
Editor: Elaine Edwards, elainee@ksu.edu
