Diseases & Pests Boxwood leafminer and Anthracnose

by Heather Prince

Pest of the Month: Boxwood Leafminer This spring we’re seeing more boxwood leafminer (Monarthropalpus buxi) as summer begins. Overwintering under the leaf surface as partially grown larvae, this insect is set up to start feeding and metamorphosizing into adults as temperatures warm. Affected Boxwood leafminer leaves will have tan blister-like spots that may shatter if probed. Larvae finish out their cycle and pupate in spring, adults emerging usually when weigela blooms. The adult flies are very tiny yellowish-orange flies about the size of the head of a pin. Female flies land on the leaf upperside and insert eggs into Anthracnose on maple the leaf tissue, causing stippling. These eggs hatch into yellowish-white larvae that feed on the soft tissue in the interior of the boxwood leaf. Up to twenty larvae can be present per leaf. They munch away slowly through the summer, initially causing raised, green blisters on the leaf upperside and small, tan blisters on the underside. After overwintering, the mature larvae cause the lower epidermis of the leaf to separate and appear whitish. If many larvae are present, this will happen to the entire leaf underside. Treatment: Treatment is most effective during egg-laying and early larval feeding. These include carbaryl and labeled pyrethroids for adult activity. For larvae, a systemic insecticide will penetrate leaf tissue and kill the insect as it eats. These include abamectin, acephate, and imidacloprid. Since it takes time for a systemic to move into all plant tissues, apply as soon as leafminer presence is confirmed. Disease of the Month: Anthracnose on Shade Trees A cool, very wet spring has created the perfect conditions for anthracnose disease to infect a range of different types of shade trees including sycamore, ash, and maple. Anthracnose is a group of closely related pathogens that cause a variety of symptoms including leaf blight and deformation, as well as cankers on stems and branches, depending on the species of tree. On ash, symptoms are dark blotches along the margins of leaves, which cause distortion, and small purplish-brown spots located near the leaf centers. Clients will often notice the abundant leaf drop, when new green leaves fall from the tree. A single year of infection will not cause significant stress, but three or more consecutive years of heavy defoliation of ash trees can leave them more susceptible to other diseases. On maple, symptoms are similar, including purplish brown blotching along leaf veins that can span out to the leaf margin. However, it is usually not a severe disease on maples, being more cosmetic.

This disease is very dramatic on sycamores, as you may experience delayed leafing on the tree and leaves and growing tips dying as they emerge. Upon closer examination of twigs, look for cankers which are sunken, dead-looking areas. Cankers on twigs can restrict water and nutrient movement to leaves, causing leaf and twig dieback, while cankers on larger branches can girdle and kill entire branches. The small black fruiting bodies of anthracnose have a pimple-like appearance. You may also see witches brooms around terminal shoots that were killed by the infection.

Treatment: In most cases, cultural controls such as raking up infected leaves and pruning out twigs with cankers is recommended. Keep trees watered and healthy so they can push out new growth. However, if anthracnose fungi have repeatedly hit a tree or a very young tree is involved, fungicides may be applied. Spray in the early spring when buds begin to swell and reapply every 7 to 14 days when it is a rainy, wet spring. Fungicides listed for anthracnose include chlorothalonil, thiophante-methyl, mancozeb, propiconazole, and copper fungicides.

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

1/11/19 8:11 AM Sales and marketing statistics show that the single best way to reach buyers is through highly-targeted specialty magazines

This award-winning magazine is frequently hailed as the best magazine of its kind. Put your message in this flattering environment.

The Landscape Contractor has an affordable advertising program for every budget.