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Nursery & Forest
Volume 60 Number 12 Date 07/09/2015 HETEROSPORIUM LEAF SPOT - This common leaf spot disease of iris was observed in a St. Croix County nursery this week. Symptoms appear early in the season as brown spots with water-soaked margins near the leaf tips. Once irises bloom, the spots enlarge rapidly, coalesce, and cause leaf dieback starting at the tip. Later the spots turn yellow to reddish-brown with characteristic gray centers. Although the heterosporium fungus does not attack the bulbs or rhizomes, it weakens and may eventually kill plants. Since the fungus overwinters on dead iris leaves, removing and disposing of all diseased leaves in fall will reduce the amount of inoculum available for infection the following spring. A fungicide spray program is recommended for severe cases involving many plants. PHYLLOSTICTA LEAF SPOT - The distinctive light brown lesions with purple-red borders appearing on maples in Columbia and Sawyer counties are symptoms of phyllosticta leaf spot, a primarily aesthetic disease of maples also known as purple-bordered leaf spot or eye spot. Severe infection induces premature yellowing and shedding of leaves, but damage from the disease is usually minimal. Controls consist of removing infected leaves, increasing air flow and avoiding overhead watering. Fungicide treatment is not advised. HOLLYHOCK SAWFLY - The green and black larvae of this insect were determined to be the cause of severe defoliation of hollyhocks at a Vernon County residence. The gregarious caterpillar-like worms feed in groups on the underside of hollyhock leaves, causing a lacy or skeletonized pattern of damage similar to that of the Japanese beetle. Control must be initiated as soon as the first holes appear on the lower leaves. -- Ellen Natzke, DATCP Nursery Inspector EUROPEAN ELM SCALE - Nymphs are emerging from beneath the gray, waxy female scale coverings and have been observed on 'Valley Forge' American elms in Sawyer County. The tiny yellow crawlers will spend the summer feeding on the undersides of leaves before migrating back to branches and twigs in fall. This scale pest of elm and hackberry can cause yellowing of foliage prior to natural autumnal color change, stunting, premature leaf drop, and branch dieback?symptoms similar to Dutch elm disease. In addition, branches on heavily infested trees are often blackened by sooty mold growth on the honeydew produced by the crawlers. For severe infestations, soil applications of the systemic insecticide imidacloprid at the base of trees have been very effective. -- Tim Allen, DATCP Nursery Inspector 
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