Garden calendar: For the week of Jan. 6
Wreaths, boughs, swags, potted evergreen arrangements and garland: The state Department of Agriculture, Trade and Consumer Protection (DATCP) has reported finding boxwood blight and elongate hemlock scale, an invasive insect, on holiday evergreen decorations this winter. When you are ready to dispose of the decorations, burn or bag and discard the material rather than putting it out for collection or composting it — even if no symptoms are visible.
This is especially important if you or your neighbors grow boxwood, pachysandra, spruce, hemlock, fir and other conifers. Boxwood blight was found at a Wisconsin nursery for the first time in July 2018. A fungus called Calonectria pseudonaviculata causes the disease. It affects all cultivars of boxwood, but some are more susceptible. A fact sheet from Purdue Extension lists resistant and susceptible cultivars and has photos at https://www.extension.purdue.edu/extmedia/BP/BP-203-W.pdf.
Boxwood blight also affects pachysandra species (Japanese spurge and Allegheny spurge). There are no treatments to cure it, but preventive treatments are available for non-symptomatic plants. Destroying affected plants is necessary to control the disease. Dead plants and fallen leaves should be carefully and immediately removed from the garden and bagged for disposal so the disease does not continue to spread.
The blight begins with leaves developing brown leaf spots, often with dark borders, then the spots enlarge and coalesce. Leaves drop, often soon after leaf spots appear. Stems of infected boxwood may also develop dark brown to black lesions, often in a diamond-shaped pattern. Don’t confuse boxwood blight with winter kill. In Wisconsin, winter kill is not uncommon in boxwood, but dead leaves usually remain on the plant rather than dropping. For pachysandra, watch for brown spots on leaves and lesions on stems as well.
If you think you may have the disease, you can send a sample to the Plant Disease Diagnostic Clinic for a free test — be sure to say you want it tested for boxwood blight. Visit https://pddc.wisc.edu/ for instructions on submitting a sample.
Inspectors found an insect called elongate hemlock scale, or EHS this winter. EHS removes nutrients as it feeds on the underside of conifer needles, weakening the plants. EHS is a threat to Wisconsin Christmas tree farms, native hemlock and balsam fir forests, and ornamental conifers in yards and parks. Unfortunately, cold winter weather will not kill it. Elongate hemlock scale is native to Asia and has been introduced into Michigan and many Eastern states. EHS has several growth stages. After hatching from eggs, tiny “crawlers” feed on the underside of needles and build a hard shell around themselves as they grow, creating the “scale” that is visible. Crawlers can start new infestations; wind and birds may also disperse scales to new trees. EHS is very hard to control with pesticides, due to the overlapping generations with crawlers present all year long, and because adult scales are protected under their hard, waxy shells. EHS feeds on more than 40 conifer species, with hemlock, spruces and firs being among the most susceptible.