Diseases and Pests

Pest of the Month: Box Tree Moth Disease of the Month: Powdery Mildew

by Heather Prince

Pest of the Month: Box Tree Moth

As of May 26, 2021, The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) has confirmed the presence of box tree moth (Cydalima perspectalis) in the continental United States. APHIS has issued a federal order to halt the importation of host plants from Canada including boxwood, euonymus, and holly species. Between August 2020 and April 2021, an Ontario nursery shipped boxwood that may have been infested to 25 retail facilities in Connecticut, Massachusetts, Michigan, New York, Ohio, and South Carolina, and a distribution center in Tennessee. As of late May, it has been found in three facilities in Michigan, one in Connecticut, and one in South Carolina. Box tree moth is native to East Asia and has become a serious invasive pest in Europe. In 2018, it was found in the Toronto area and was controlled.

The caterpillars of box tree moth feed mostly on boxwood and can defoliate plants quickly. Once leaves are consumed, the caterpillars attack the bark, girdling twigs, and killing plants. Box tree moths overwinter as second to fifth stage instar larvae and can survive temperatures to at least -22°F (-30°C). Overwintering caterpillars are green and yellow with white, yellow, and black stripes and black spots. They pupate for about 14 days among webbing and damaged leaves. Adult moths are white with a brown border. They lay pale yellow eggs in groups of 5 to 20 overlapping like shingles. Once hatched, they feed on the undersides of leaves and older caterpillars build silken retreats.

Treatment:

Horticultural oil and insecticidal soaps will likely be able to control young caterpillars and biopesticides that work on other caterpillars, such as Bt and spinosad, have been used to kill box tree moths in Europe. Broad spectrum pesticides that are labeled for caterpillar control on ornamental plants will also likely provide acceptable control.

Disease of the Month: Powdery Mildew

Powdery mildew is a common fungal disease on annuals, perennials, shrubs, and trees. The six common genera thrive in hot, humid weather. Powdery mildew resembles white or dusty gray patches on the top surface of leaves, shoots, buds, flowers, or stems. In some cases, it can look like snow. Powdery mildew fungi overwinter on plant tissues and dormant buds. Spores are released with spring rains and infect tissue through rain or wind. Sometimes it will cause leaves to become dwarfed or distorted and new growth tends to be more sensitive to infection.

Treatment: Powdery Mildew

Managing the air flow through the plant is one sustainable way to manage powdery mildew. Prune or thin plants for better air movement. Choose resistant varieties of plants for best performance. You can also treat it with a fungicide, although rotate through products to minimize disease resistance. Treat as soon as the fungus is spotted for best results. Look for the active ingredients myclobutanil, propiconazole, and thiophanatemethyl that have systemic properties and can be applied less often than chlorothalonil, sulfur, or copper-based fungicides.

Additional resources:

University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php 217-333-0519 The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972