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Nursery & Forest
Volume 61 Number 11 Date 07/14/2016 HETEROSPORIUM LEAF SPOT - This common leaf spot disease of iris was observed in a Chippewa County garden center 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 affected 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. VERTICILLIUM WILT - Catalpa trees at a nursery in Ozaukee County were exhibiting symptoms of this fatal vascular disease, frequently misidentified as decline caused by environmental factors. Symptoms are highly variable and can be acute or chronic. Acute symptoms include premature fall coloration, wilting, defoliation, branch dieback and death, whereas chronic symptoms typically reflect damage from earlier infections and may include slow growth, sparse foliage, stunted leaves and twigs, leaf scorch and abnormally heavy seed crops. Trees with severe wilt and dieback cannot be saved and should be replaced with a species not susceptible to Verticillium such as aspen, beech, sycamore, poplar, willow or any conifer. BLACK KNOT OF CHERRY - A light infection on cherry trees was noted at a nursery in Ozaukee County. This common fungal disease is characterized by irregular, black swollen galls or 'knots' which form on branches and can range in size from ½ inch to one foot long. Shoots and branches bearing knots should be pruned in winter or early spring, before fungal spores are released. Multiple infections of black knot reduce tree vigor and cause eventual decline. Nursery trees with knots on the trunk must be removed from sale. GUIGNARDIA LEAF BLOTCH - This leaf spot disease is developing on horse chestnut trees in Ozaukee County. Symptoms include irregular, reddish-brown leaf lesions with yellow margins that distort affected foliage as they increase in size and severity. Disease development can be suppressed by disposing of fallen leaves in autumn to reduce inoculum levels. VIBURNUM LEAF BEETLE - Nursery inspectors recently found this invasive European beetle on arrowwood viburnums at a nursery in Washington County, making Washington the fourth Wisconsin County where VLB has been detected since 2009. Viburnum leaf beetle is particularly damaging because both the adult and immature forms rapidly defoliate viburnums. Successive feeding by larvae and adults prevents shrubs from refoliating and can kill otherwise healthy plants after 2-3 years of heavy infestation. Gardeners, landscapers, nursery stock growers and retailers in southeastern Wisconsin should be alert to the characteristic, unique skeletonization of viburnum leaves caused by this insect and implement an aggressive treatment program to prevent this pest from spreading, if detected. -- Ellen Natzke, DATCP Nursery Inspector PHYTOPHTHORA CACTORUM - This root rot pathogen was confirmed by laboratory diagnosis on rhododendron plants from Price and Oneida counties. The infected rhododendrons were showing thickened leaves with distinct dark-red margins between healthy green leaf tissue and necrotic tissue, although symptom expression can vary. Most Phytophthora root rot infections occur during periods of wet, warm weather when spores enter plants through wounds. Because saturated soils provide ideal conditions for P. cactorum development and spread, cultural practices that improve drainage should help reduce problems. Lowering soil pH is another protective measure, as low pH inhibits spore activity. Symptomatic plants should be removed and destroyed to prevent spreading spores to other host plants of P. cactorum, including hundreds of ornamentals, trees and fruit crops as well as valuable agricultural crops like apples, ginseng and strawberries. -- Timothy Allen, DATCP Nursery Inspector 
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