Excellent talk last night at the Chappaqua Library by plant pathologist Margery Daughtrey, on “Global Gardening: Protecting Plants in the 21st Century.”
She focused on the new impatiens downy mildew that is wiping out the main garden variety of impatiens and the boxwood blight that has now been seen in 10 states, including New York and Connecticut. The blight has also been seen in Oregon and North Carolina and Va., where many of our boxwoods are grown by large commercial nurseries.
But the good news for boxwood owners is that the blight doesn’t seem to be as devastating and fast moving as the impatience mildew. (Here’s a link to my post from yesterday on the latest on the boxwood blight.)
The blight spores are more lightly to move yard to yard rather than county to county, Daughtrey says.
After being seen in the UK in 1994, the disease was first reported in this country in Oct. 2011, simultaneously in NC, Va and Conn., she says. It has now spread across most of Europe and has also been seen in Eurasia (Georgia) and New Zealand.
Early signs of the blight include little black spots and cankers on leaves and twigs.
(Photos from Margery Daughtrey; the one of the cluster of boxwoods was shot locally by arborist Tom Marino, of SavATree in Bedford Hills.)
A third factor is environmental conditions, and Daughtrey points out that we had some big storms in 2011, including hurricane Irene and tropical storm Lee, with lots of wind and rain that “probably contributed to the drama of the disease.”
The American boxwood appears to be one of the most susceptible varieties, and the classic English box is the most susceptible of all, she says. “We might have to part with English boxwoods.”
Less aggressive pruning (into tight, usually round shapes) would also help. Allowing the branches and leaves to get more air flow and circulation seems to help fight the disease. Give each shrub plenty of space if you can.
Giving them full sun also seems to help, she says. And it’s very important to vacuum up all diseased leaves.
If you see signs of the disease on your boxwoods, aggressively prune out all diseased branches. Avoid composting them (bag them up for the trash).
If you can, keep old, valuable specimens of boxwood away from new boxwoods.
Also, avoid overhead watering.
It’s not just boxwoods that are affected by this blight. Also, Sarcococca (sweet box) and pachysandra, Daughtrey says. Diseased pachysandra has been seen on 20 properties in Conn., and these specimens will drop some leaves.
As far as fungicides that may fight the boxwood blight, Daughtrey says it’s too soon to know what will be effective. It will take years of testing to figure out which fungicides will be effective. “We need time to figure out what to do.”