Watch for boxwood blight on shrubs, pachysandra
By Nick Schultz
For a moment, set aside the joy of seeing your crocuses and daffodils bloom. Overlook all the pruning and yard chores a new season brings. Instead, as you survey your landscape, look for early signs of plant disease and winter injury.
A relatively new fungal disease is Boxwood blight (also known as box blight). This disease hasn’t been discovered in Wisconsin yet, but is in several Eastern and southern states and as near as Illinois.
Once introduced to a landscape, boxwood blight is difficult and costly to control.
Boxwood is a common shrub for landscape hedges where winters are a bit milder than in central Wisconsin. It has a USDA hardiness of zone 5, suitable for southern Wisconsin and counties along Lake Michigan. A few varieties of the evergreen shrub are hardy in central Wisconsin, especially in sheltered urban areas.
The blight causes brown spots on boxwood’s small leaves, or bleaches plant tissue in addition to browning, said Brian Hudelson, of University of Wisconsin-Extension (UW-Extension) Diagnostic Services in Madison. Plant dieback will result. “It will eventually kill these.”
It also affects pachysandra, a glossy evergreen ground cover common in central and northern Wisconsin. Also known as Japanese spurge, leaves develops brown spots when diseased. Both are part of the same family of plants, Buxaceae.
Boxwood blight is caused by the fungus Calonectria pseudonaviculata or Cylindrocladium buxicola. A serious fungal disease, it results in defoliation and decline of susceptible boxwood. Boxwood blight was first identified in the United States in 2011 in Virginia and Connecticut. It was spread to states as close as Illinois.
Symptoms may include tan spots with dark edges. Boxwood stems may also have black streaking.
Cool, wet weather creates a favorable environment for boxwood light. Try to keep leaves from staying wet. Never overhead water, Hudelson said. Use drip irrigation or soaker hoses.
To reduce the chance of boxwood blight, buy plants produced in Wisconsin, he said. Ask the nursery if you’re unsure. Avoid boxwood plants grown in Eastern states.
Inspect plants carefully before purchasing them, and avoid any that show symptoms of blight. Keep new plants isolated, monitoring them for signs of blight.
If you buy plants from different sources, keep them physically separate, Hudelson said. Spacing plants, rather than creating a close hedge, improves air circulation and helps plants dry.
Two varieties have some resistance to boxwood blight, he said: “Green Mound” and “Glencoe” or Chicagoland Green.
If you notice diseased branches, prune them out and notify Hudelson’s diagnostic clinic. Send a sample branch to the UW Plant Disease Diagnostic Clinic so Hudelson’s team can test it for boxwood blight. Diagnosis will be free, he said, because Extension specialists want to identify and control this disease as much as possible.
Remove and destroy infected plants. Burn them, if this is allowed in your community. Or, deep bury the infected material. Do not compost.
To prevent the spread of this or any disease, be sure to disinfect pruning tools and other garden supplies. Disinfect with 70 percent alcohol solution, a commercial disinfectant, such as dairy farmers use, and a 10 percent bleach solution if no other options exist. The latter hastens rust, so look at other options first.
To prevent boxwood blight, several fungicides are available. Note, they must be used before the blight is diagnosed, not to treat the disease.
This preventive treatment is only for those willing to use strong fungicides at weekly intervals. Fungicides with alternate active ingredients must be used in combination, Hudelson said. At least one breaks down to a known carcinogen.
Some bleaching on boxwood is actually caused by winter injury. Winter burn to boxwoods often occurs above a “snowline.” Foliage protected by snow from wind and severe cold remains green, while exposed parts of the plant turn brown. A plant infected with boxwood blight may have a more random pattern of browning or brown from the bottom up.
When warm spells occur in winter, such as the February thaw earlier this year, some plants break dormancy and begin to photosynthesize. They use up moisture retained in their plant structure over winter and become dehydrated. Water stress, extreme winter temperatures, insufficient snow cover or alternating thawing and freezing can cause browning and winter burn.
In addition to boxwood, yews and Albert spruce are among evergreens affected by winter injury. Fruit trees and redbud are among deciduous trees most likely to be affected.
The best way to prevent winter injury if to ensure trees and shrubs, especially those less than three years old, are adequately watered through the growing season. They need one inch of water weekly until the ground is frozen or snow covered.
Other possible diseases Hudelson and others will watch for in 2017 are:
* Thousand cankers disease – Twig beetle infects black walnuts, other walnuts.
* Tar spot – Dark, oil spots on maples are unsightly by won’t kill the tree.
* Fungal leaf blights – affect tomato, potato plants. Grow resistant varieties, crop rotation.
* Black rot – affects cole vegetables. Buy pathogen-free seed, rotate crops.
* Impatiens downy mildew – Water mold turns impatiens yellow, then slimy. Rotate and do not crowd plants.
Prevent these diseases with good quality plants or certified pathogen-free seed, Hudelson said. Remove and dispose of contaminated plants and decontaminate tools. Avoid overhead watering.