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Nursery & Forest
Volume 59 Number 6 Date 06/05/2014 IMPATIENS DOWNY MILDEW - This destructive disease of impatiens was diagnosed by the Plant Industry Bureau Laboratory on samples from a Wood County greenhouse. Impatiens downy mildew (IDM) has been widespread in U.S. greenhouses and landscape settings in the last two years, with Wisconsin and more than 30 other states reporting cases. In 2013, the disease was found by DATCP and the UW-Madison in Dane, Clark, Jackson, Kewaunee, Monroe and Oneida counties, as well as in Waukesha County where it devastated a garden planting in September. To reduce IDM problems this season, commercial growers of impatiens are advised to dispose of plants with light green stippled leaves, curled leaves, or the characteristic white, downy mycelia growth on the undersides of foliage. Home gardeners should also carefully examine impatiens for symptoms before purchasing flowers, and consider planting the mildew-resistant New Guinea impatiens or a New Guinea hybrid. Infected plants should be immediately removed and destroyed. -- Anette Phibbs, DATCP Plant Pathologist PSEUDOMONAS BLIGHT - Potted lilacs in Kenosha, Sheboygan and Walworth counties were showing symptoms of this disease. The principal diagnostic characteristics are yellowish-brown leaf lesions, black streaks along the leaf veins and midribs, and withered, black shoots which bend to form a distinctive shepherd's crook. Pseudomonas blight develops on lilacs during cool, wet weather and spreads by rain and splashing water. Control consists of pruning out blighted twigs as soon as they occur, thinning plants to increase air circulation, and growing resistant varieties. Pruning shears should be disinfected with bleach or 70% alcohol between cuts to prevent spread of the disease. BOTRYTIS BLIGHT - Nursery inspectors observed this gray mold disease of greenhouse floral crops on geranium, impatiens and peony in Kenosha, Milwaukee, Racine, Sheboygan and Walworth counties. Symptoms of botrytis appear as brown spots on flower petals and irregularly-shaped necrotic areas on the leaves. The leaf spots develop a grayish mass of fungal spores that disperse on splashing water or wind. Botrytis can develop at any stage and may affect any plant part. Measures that reduce humidity levels below 85% and increase air circulation can help minimize its occurrence. Treatment with an appropriate fungicide or removal from the greenhouse is recommended for symptomatic plants. AZALEA SAWFLY - This insect has been feeding on azaleas in the past week, and if uncontrolled, will defoliate entire plants, leaving only the leaf midrib. Larvae observed in Dane County varied in color from green to dark brown depending on whether they had fed on the leaves or flowers. Insecticidal soaps or manual removal of the larvae will usually give reasonable control. Severe infestations may require Neem oil or an insecticide spray. RHIZOSPHAERA NEEDLE CAST - Discoloration of Colorado blue spruce trees in Sheboygan County has been attributed to this fungal disease, a common and widespread problem in Wisconsin this season. The UW-Madison Plant Disease Diagnostic Clinic has also reported 13 cases on spruce in at least 11 counties. Rhizosphaera is characterized by browning and early needle loss starting on the lower branches. Needles are infected in spring, turn yellow in July, and then become purplish-brown by late summer or fall. The presence of rows of small black dots or fruiting bodies on the surface of shed needles is diagnostic. To prevent infection, spruce trees may be treated with a fungicide in spring when the new growth reaches ½-2 inches long, and again 4-6 weeks later. POWDERY MILDEW - This common fungal disease of ornamental plants was found on various roses at garden centers in Marinette and Milwaukee counties. Powdery mildew is characterized on most plants by its grayish white powdery dusting on the upper leaves, which later causes the foliage to turn yellow and senesce prematurely. This disease is favored by high humidity and wet weather. Reducing humidity levels and increasing air circulation will help to alleviate the problem. Fungicidal control is usually not necessary as this disease is usually only a cosmetic concern. -- Ellen Hermanson, DATCP Nursery Inspector 
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