Last week I got the first call I have been anticipating for several weeks now. The caller began to describe how the leaves on her ornamental or Bradford pear tree were turning black and the tree looked bad. After a few questions, I speculated it was a common disease called fire blight.
A tour around town confirmed my initial diagnosis. Quite a few of the ornamental pear trees I observed have the tell-tale signs of fire blight. These signs include new leaves turning black or brown, new shoots suddenly dying and curling, and the remains of blooms still attached to the shoots. A closer look will reveal blackened stems, often near the node where the blooms and leaves are attached.
Since tender new growth dies so quickly, the ends of young, tender shoots bend over, giving the appearance often referred to as a “shepherd’s crook”.
Fire blight is caused by a bacterium called Erwinia amylovora, and attacks certain members of the Rosaceae family, including pears, apples, crabapples, hawthorns, mayhaws, and flowering quince. On susceptible varieties, fire blight can be very destructive, causing extensive dieback and even death if re-infected regularly over the years.
Infected pear leaves turn black while apple leaves typically are brown.
The bacterium first gets into a plant through natural openings in blooms, or wounds caused by insects, hail, broken or freshly pruned branches. Once a tree has fire blight, the bacteria can then infect older wood.
The ‘Bradford’ variety of ornamental pear is considered to be quite resistant to fire blight, but under favorable conditions, it can also become moderately infected, but fortunately this doesn’t happen too often. ‘Aristocrat’, another ornamental pear variety, while having other favorable characteristics, is much more susceptible to fire blight. Many of the pear varieties we grow for the edible fruit are also prone to fire blight. The delicious ‘Bartlett’ pear is highly susceptible, and should never be planted in east Texas. Fruiting pear varieties with better fire blight resistance include ‘Warren’, ‘Ayres’, ‘Magness’, ‘Kieffer’ and ‘Orient’.
So, just what are those favorable conditions that favor disease development? The conditions for a “perfect storm” include warm temperatures in February and March just prior to and during bloom, along with high humidity and/or rainfall during the blooming period. The warm weather and moist conditions promotes the bacteria to actively grow, and splashing rain can spread the disease within a tree. Honey bees and other insects transmit it both within a tree and to other trees in the area as they visit flowers for their nectar.
Another factor that favors disease development is soft, succulent growth during spring and summer. The tender shoots and leaves are more easily damaged allowing entry of the disease. Avoid applications of nitrogen fertilizer around pear trees to reduce their vigor and amount of succulent growth.
So, what can you do about it? Prevention with fungicides or bactericides is only effective during bloom. Copper-based fungicides and agricultural streptomycin are recommended as treatments during bloom. These products are not effective at other times of the year. Once the tree is infected, the only option is to prune out infected branches, which can be a daunting task if you’re dealing with a large tree and lots of dieback.
This coming winter, prune out infected branches at least 8 inches below the damage. If practical, remove shoots at their point of attachment rather than leaving stubs. Inspect the shoots and look for cankers (darkened, shriveled, sunken areas of the stems). Bacteria will likely occur below the cankers which is why you need to prune several inches below obvious cankers.
Avoid pruning when the plants are wet. Dip pruning tools in 70 percent isopropyl alcohol (rubbing alcohol) or 10 percent bleach solution (1 part bleach to 9 parts water solution) between each cut. (see Linda Chalker-Scott’s article on why she does not recommend using bleach – her disinfectant of choice is Lysol). Wash and oil tools when you are finished – bleach is extremely corrosive and can ruin metal (not to mention your clothes). If you don’t disinfect your pruning tools between cuts, you will likely spread the bacteria even more through the fresh wounds created by contaminated pruning equipment.