By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture/plant pathology
It's hard to even think it, but Spring sometimes can be a mixed blessing.
Just ask the homeowner who's trying to cope with a laundry list of plant diseases -- fire blight, for example.
Fire blight thrives in warmer, wet weather when the disease-causing bacterium becomes active. Apple, pear, quince, crabapple and mountain ash, but NOT GREEN ASH, commonly are affected. The bacterium oozes out of cracks and crevices in the bark, and bees or other pollinating insects pick it up on their bodies. The disease spreads as these insects pollinate other flowers.
If your trees are affected by fire blight, they will begin to show symptoms just before their flower petals fall. This is the blossom blight stage when flowers begin to turn brown and mushy and wilt. Eventually, the bacteria will move down into the branches and leaves of the tree. Leaves darken and wilt but remain attached to the tree (see above), giving it a scorched or burned look. Branch tips blacken and curl, causing a "shepherd's crook" symptom.
Fruit also can be affected. Bacteria often oozes out of the infected fruit and, as symptoms progress, fruits remain attached to the tree as shriveled "mummies" (see above). Cankers, which are sunken areas darker in color than the surrounding bark, form as the disease progresses. If present on the main trunk, cankers often are fatal, as they eventually will girdle the tree.
Canker on trunk of mountain ash
Cankers also serve as the overwintering source of the bacterium. The following spring, bacteria will ooze from the cankers or cracks in bark. Insects may come in contact with the ooze and spread the disease to other trees.
Fire blight control can be a challenge. A variety of controls are available and success usually is greatest when we integrate all of them.
The use of resistant varieties is the first line of defense. Listed on the table below are apple and crabapple trees with certain degrees of resistance to the blight. Resistance doesn't necessarily mean immunity. A tree with resistance can become infected, but the problem won't be nearly as severe as if there were no degree of resistance.
Avoid overfertilization with nitrogen. Nitrogen stimulates new growth and the new growth is highly susceptible to fire blight infection.
Practice sanitation when pruning trees. Prune out and destroy all infected and dead plant tissue. Make pruning cuts 6 to 12 inches into healthy tissue. Always dip tools in a disinfectant solution, such as 10 percent household bleach, between each cut to minimize disease spread. Prune newly infected twigs as soon as possible in the spring. Do all other pruning during winter.
You may want to spray next spring to prevent the infection. Chemicals will have little or no effect on existing infections. Copper compounds, such as Bordeaux mixture, Kocide or lime sulfur, or streptomycin (Agristrep) are effective chemicals. Timing, however, is critical. If you use copper, you must apply during dormancy and bud break. Copper may burn leaves and fruit if it is applied later in the season. If you use streptomycin, apply when bloom is 50 percent and repeat twice at 4-to-5 day intervals.
Tree varieties with some degree of fire blight resistance*
* Resistance does not mean immunity.
Photographs courtesy of Judy Sedbrook..
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010