Fire Blight in Trees
Dr. Mario A. Villarino, county extension agent for agriculture and natural resources
In early April, many people began noticing dead twigs in their flowering pear trees. The twigs look like they have been scorched with a blow torch. The ends of tender new growth curls, looking like the crook of a shepherd's staff. The blackened leaves cling to the blighted twigs. These are the classic symptoms of fire blight .
Fire blight is a bacterial disease, and is one of the worst diseases of pears and apples, K. C. Hansen, Extension Horticulturist indicates, since it is capable of killing trees of susceptible varieties. Many edible pears and apples are very susceptible to fire blight and easily contract the disease, and is thus one of the factors limiting their production in our area. Many ornamental plants are also attacked, including ornamental or flowering pear, crabapple, quince, pyracantha, cotoneaster and hawthorn. The symptoms on apples and crabapples are slightly different than on pears. Instead of blackened leaves and shoots, apple leaves turn brown. Most ornamental pears are resistant, but they are not immune to fire blight. Certain varieties, like Aristocrat, are more susceptible than Bradford, one of the more common varieties of ornamental pear. Most years, fire blight is not a problem on ornamental pear.
Requirements for Disease. Fire blight, like all diseases, requires a certain set of circumstances to occur in order for the disease to take place. This includes a warm period prior to and during bloom, and rain during bloom. When conditions are favorable for bacterial growth, populations can develop rapidly. At 70 degrees, bacterial numbers double every 20 minutes, becoming billions in less than a day. Each bacterium is capable of causing an infection! The bacterium overwinters in cankers, or dead areas, on twigs and limbs. In the spring, the bacteria in the cankers become active and an ooze containing bacteria will emerge which attracts various types of insects. These insects carry the bacterium to blooms, and then honeybees visiting the flowers pick up the bacterium and pass it to other blooms, continuing the spread of the disease.
The bacterium can also be spread by hard, driving rain, and it will infect plants through wounds caused by hail, insects, and fresh pruning cuts.
Besides rainfall, other environmental conditions and cultural practices that can promote fire blight development include excess moisture, high rates of nitrogen, and excessive pruning. Anything that promotes rapid and soft, succulent growth will make that tissue more susceptible to infection.
Gardener's Response. So, what can you do if your trees are showing symptoms? Pruning is one method of reducing the source of future infections. Whether to prune now or later partly depends on the age and size of the tree. If you have a very small, young tree showing symptoms, further spread of the disease could seriously damage the tree. So, removal of infected growth should be done now. For older, larger trees, or plants with only a few infections, it may be better to wait until later in summer when all the new growth has hardened, and stimulating new growth through pruning is not as likely.
It is very important when pruning to remove the affected portions several inches below any signs of canker on the shoots. Look carefully on the affected shoots for cankers which will appear sunken, shriveled, and are often elongated, progressing downward from the dead shoot tissue. Pruning equipment should be dipped in a 10 percent bleach solution or alcohol for 2 seconds to prevent spreading the bacterium from one cut to another. To prevent the bleach from corroding your tools, clean and wipe them with a light oil at the end of the day.
Affected trees can be treated with a copper fungicide or antibiotics, but ONLY DURING THE BLOOMING PERIOD. More than one treatment might be needed, especially if blooming is prolonged. For more information on this or other related agricultural topics, please contact the Hopkins County Extension Office at 903-885-3443.