In Illinois, we are very fortunate to have the ability to grow multiple tree fruit crops in our climate. However, we do have quite a few insect and disease challenges to address when reaching for that perfect apple or pear.
On a recent trip to the Pacific Northwest region, I was in awe of the virtually disease-free trees I observed in parks and home gardens. Compared to the typical apple tree here in Illinois in July, these trees had perfectly unblemished leaves and fruits, which is an utter rarity in our region without extensive disease and insect treatments starting at or near budbreak in the spring.
Nonetheless, the state of Washington is perhaps the best place in the U.S. to grow apples, supporting nearly 60 percent of apple production in the U.S. each year. Most of this growing craze is due to a near ideal climate across most of the state, which features dry, sunny days with cooler nights. It’s ideal for apple growing and much less than ideal for disease spread. In contrast, Illinois’ hot and humid climate is quite favorable for the development and spread of a plethora of tree diseases.
Fire blight is one of the worst fruit tree pathogens, and it is a constant pressure on both landscape plants and orchards in Illinois. This disease is caused by the bacterium Erwina amylovora that infects a wide range of hosts in the rose family, including many of our most common fruit trees, such as peach, plum, cherry, apple and pear.
Fire blight is a significant problem for fruit-tree growers, causing drastic damage to most tree canopies and reducing production significantly over time. Once infected, there is typically little that can be done other than slowing the disease progression, and infected trees remain a source of spread to health plants around them.
Fire blight is somewhat unique in that it can infect multiple plant parts from flowers and fruits to twigs or large stems. The bacteria are most active in our wetter spring time weather. Under wet conditions (over 65 degrees), it multiples rapidly, producing a sticky ooze from infection sites. Insects are attracted to this ooze and spread it quite effectively to other hosts.
Flowers are most often infected and are commonly the place this pathogen starts before spreading to twigs and eventually producing stem-gridling cankers. Infected flowers appear wet or water-soaked and begin to wilt and die. This infection nearly always spreads to stems, creating cankers that also produce a wet oozing substance. Cankers appear as dark sunken areas as they expand, creating larger and larger zones of dead tissue and ultimately circling and killing stems.
Infected shoots are probably one of the most telling symptoms, as the twigs and leaves turn black or dark brown. This characteristic has earned the pathogen its common name of “fire” blight, given the burnt appearance of infected twigs.
In addition, dead twigs commonly bend into a curled shape as they die, resembling a shepherd’s crook. This happens as the young tender growth in new twigs rapidly die from the disease and curl up. It’s known as the most common and identifiable symptom.
While it’s most active in spring, risk of fire blight spread occurs throughout the year. It most commonly infects pears and apples but has a wide host range, including many common landscape plants in the rose family, such as crabapple, Callery pear, hawthorn, serviceberry, quince and spirea.
Since infections typically start from flowers or in small twigs, the disease presents itself as black or brown dead branch tips in its early stages, which may or may not have the telltale shepherd’s hook shape.
Pruning is the most effective treatment for fire blight. Infected twigs and stems can be pruned out (cut 6 to 12 inches below the infected area) to limit further spread. Pruning is best done during winter dormancy but may need to be performed during the growing season on rapidly spreading infections. Remember, fire blight spreads during wet weather, so plan pruning during dry periods, and sanitize pruning tools between every cut.
Some pesticides are available to limit or prevent spread of this disease but likely require professional application to be effective across an entire tree canopy. Contact a certified arborist (searchable by county at treesaregood.org) for more information about treatment options.
Resistant cultivars exist for most fruit tree species. Therefore, the best prevention is to simply plant a resistant variety.
Ryan Pankau is horticulture extension educator with University of Illinois Extension serving Champaign, Ford, Iroquois and Vermilion counties.