Margaret Lauterbach

Cool, rainy spring prompts alert for fire blight on Valley trees, plants

Fire blight is a bacteria that can turn colorful plants such as apple trees a deep, ugly brown.
Fire blight is a bacteria that can turn colorful plants such as apple trees a deep, ugly brown.

We're being warned by the Pacific Northwest Pest Alert Network to watch out for fire blight this spring. This bacterial disease afflicts only apple, pear, and hawthorn trees, and pyracantha and cotoneaster shrubs. Branch ends and especially the tops of these trees and shrubs look like they've been burned in fire and left charcoal remains. It first infects the blossoms, then succulent new growth, twigs and leaves, leaving everything brown on apple trees and black on pear trees.

The disease may start in cool, rainy weather and not be obvious until the weather warms, and then it really gets going. Prune off the infested twig or branch at least 8 inches below the infection (toward the trunk). Disinfect pruning equipment before using clippers or saws on other woody plants, using at least a 10 percent bleach solution. Some folks use Lysol spray for disinfecting, but I don't know how effective it is. After pruning, use a copper spray such as Bordeaux mixture, carefully following the label directions. You may be directed to use that after fruit has been harvested this season.

The alert network, from a consortium including the University of Idaho, also is advising that we watch for signs of coryneum blight on peach, apricot, nectarine, plum, almond and cherry trees. This fungus starts as tiny red or purplish spots on leaves that expand in size, developing a brown center that will drop out, leaving the leaf looking as if it's been shot by a shotgun. The common name for this disease is shothole fungus.

To prevent the disease, irrigate or water with low-volume sprinklers or drip irrigation to keep from splashing the foliage with water. Control efforts include spraying with zinc sulfate in October to cause early leaf fall, and prevent the disease from increasing and then emerging more widespread than ever in spring. Bordeaux mixture is used also, during the tree's dormancy.

We've had rainy springs in the past, but I don't recall being warned by the alert network about insect appearances such as pear psylla, peach twig borer, white apple leaf hopper and fruit orchard leaf rollers, as they're advising this spring. Damage will be most serious on the youngest trees, and is of most concern to commercial growers because of cosmetic damage to fruit.

Watch for peach twig borer during bloom, checking for wilted leaf shoots and feeding damage at the base of flowers. Recommended control is a treatment using spinosad (Entrust or Success), or two applications of bacillus thuringiensis. Follow the label directions carefully. Another way to spot any twig or shoot invasion is to watch for birds feeding in those areas.

The fruit tree leaf roller is usually controlled by prebloom sprays for other insects on fruit trees.

The white apple leafhopper closely resembles white flies. It's been very damaging in orchards in Utah, and is a matter of concern in Washington and Oregon as well. It has developed resistance to some insecticides, so I'd suggest home orchardists be wary of using any on this insect and just ignore tiny white dots on fruit or leaves.

Pear psylla is a more threatening pest, even in home orchards. These insect parents resemble tiny cicadas, with transparent wings held over their bodies as mini-roofs. They rapidly develop resistance to insecticides, and worse yet, some carry the pathogen that carries pear decline. Pear decline can destroy trees.

They tend to attack European pears more readily than Asian pears, and the devastation they wreak varies according to pear variety, the health of the tree and the number of these insects. There are five instars (that is, stages of development resulting in skin shedding) of the nymphs. If you see honeydew with black sooty mold, leaves yellowing or parts turning black, you may have pear psylla. I'd hope you have encouraged beneficial insects to take care of this problem for you.

Don't confuse this with pear leaf blister mite damage; those latter insects leave black "skid marks" on leaves, and should be controlled by sulfur and oil sprays after harvest.

To encourage beneficial insects — predators or parasites — grow plants that bloom with shallow nectar reservoirs. Dandelions, lilacs, purple giant hyssop, chaste tree, mints, chervil or parsley, dill, etc., sport tiny flowers that contain nectar these insects can reach.

Send garden questions to melauter@earthlink.net or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.
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