Another disease we are likely to see in warm, wet conditions is fire blight. It is caused by a bacterium, Erwinia amylovora. Susceptible hosts include apple, crabapple, pear, ornamental pear, cotoneaster, hawthorn, firethorn, and mountainash. All hosts are in the Rosaceae family, but not all Rosaceae are affected. Resistant varieties are available.
When weather is warm and wet in the spring, flowers serve as the site of infection. The bacterium can infect natural openings or wounds and is spread by wind, water, equipment, and even animals. As long as warm, wet conditions continue, the bacterium can continue to infect new sites and continue to spread.
Symptoms rapidly progress in the plant. Look for water-soaked or wilted new growth that quickly turns brown to black and remains attached to the stem. Stem tips often curl over in a characteristic "shepherd's crook." Dark cankers develop in the wood as the bacterium moves down the shoots or flowers.
Most of the primary inoculum in the spring comes from bacteria that have overwintered in cankers on stems. Removing the cankers significantly reduces the bacteria in the area. Preferably, such removal should be done in the dormant season, as was suggested in the fall issues of this newsletter. Spring removal of dead or infected wood often stimulates more succulent growth on the tree. That might be good in most cases, but the fire blight bacterium infects succulent new growth. If you have tissue to remove now, consider waiting until the weather turns hot and dry. Avoid heavy nitrogen fertilization because this practice also promotes succulent new growth susceptible to the fire blight bacterium. If pruning is done in the growing season, tools should be disinfected before each cut. It is necessary to make pruning cuts 6 to 8 inches down the stem from the dead tissue and into the green, healthy growth. The stem does not show symptoms that far from the cankered area, but the bacterium may still be present in low populations.
If fire blight has been a problem in past years, some chemicals may be used as protectants. You can find the recommendations in the pest management handbooks listed in the scab article. Copper formulations and antibiotics are generally used in commercial fruit production aimed at protecting the flowers from infection. Dormant copper sprays are recommended for control of fire blight, particularly following severe fire blight years. We are past the dormant spray time, so if fire blight is a perennial problem for you, mark your calendars to remind yourself to spray in the dormant season. If establishing new plantings, look for resistant varieties. The U of I publication Recommended Crabapples for Illinois Landscapes (see the scab article) lists fire blight resistance.
Consult Report on Plant Disease, no. 801, for details about fire blight on fruit trees. This publication is available in Illinois Extension offices or on the Web at the VISTA site http://www.ag.uiuc.edu/~vista/horticul.htm.. Commercial producers or those interested in details about diseases that can affect fruit (and often their ornamental counterparts such as crabapple) can refer to the Midwest Commercial Small Fruit and Grape Guide 2002, also available through Extension offices. The Illinois Fruit and Vegetable News is available free of charge on the Internet at http://www.aces.uiuc.edu/~ipm/news/fvnews.html.