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 US to grow apples, supporting nearly 60% of apple production in the US each year. Most of this growing craze is due to the ideal climate across most of the state, which features a dry, sunny climate with cooler nights. Ideal for apple growing and less than ideal for disease spread. In contrast, Illinois’ hot and humid climate is simply perfect for the development and spread of a plethora of tree diseases.
What is fire blight?
Fire blight is perhaps the worst of all 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 and infects a wide range of hosts in the Rose family (Rosaceae), which includes many of our most common fruit trees such as peach, plum, cherry, apple and pear,
While fire blight may not be the most widespread pathogen infecting fruit trees, it is the most significant, causing drastic damage to most hosts. 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.
How does it infect plants?
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 springtime weather. Under wet conditions (over 65°F), 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 stems and producing 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 areas of dead tissue and ultimately circling and killing stems.
How can you identify fire blight?
Infected shoots are probably one of the most telling symptoms as the stems 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 branch tips. In addition, they 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 is the most commonly talked about symptom of fire blight, although its not always present on infected plants. In addressing this disease, I’ve found it most useful to identify and track stem canker formation on larger stems since they cause the most damage in the long run.
While its most active in spring, risk of fire blight spread occurs thought out 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, crabapple, quince and spirea. One key to identification is confirming that the presumed host is, in fact a member of the rose family because this pathogen typically only infects Rosaceae.
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 a shepherd's hook shape.
Managing infected plants
Pruning is the most effective treatment for fire blight, since infected twigs and stems can be removed (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.
Regardless of when you prune, be sure to sanitize after every cut. This can be accomplished using a variety of sanitizing products either sprayed or rubbed onto the cutting surface of your pruning tools. My favorite method involves rubbing alcohol, which is commonly available and easy to get. I like to have a small spray bottle of rubbing alcohol on hand, along with some clean, absorbent rags.
Sanitation can easily be accomplished with a spray on cutting surfaces followed by a quick wipe with the rag. Alternatively, the spray can be applied to the rag and then rubbed across the cutting blade. Be sure to remove all debris, as small chunks of wood and bark can often become wedged in between cutting teeth or smashed onto smooth blades. For this reason, its important to actually rub or wipe to surface of the tool to ensure the alcohol covers it sufficiently and all debris is removed.
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 www.treesaregood.org) for more information about treatment options.
Resistant varieties
Nowadays, much work has been done in the development of fire blight resistant varieties for many genera of Rosaceae. While no variety is absolutely immune, most resistant plants that are kept healthy offer sufficient resilience to fire blight for their expected lifetime. If you plan to plant any new fruits trees or other members of the rose family, take the time to seek out resistant varieties. Fire blight is a widespread pathogen in Illinois with active infections everywhere, making resistance about the only way to avoid this devastating disease.
Ryan Pankau is Horticulture Extension Educator with University of Illinois Extension serving Champaign, Ford, Iroquois, and Vermilion Counties. This column also appears on his “Garden Scoop” blog at go.illinois.edu/gardenscoop.