This bacterial disease can spread rapidly and be devastating to pears (both edible and ornamental), as well as apples and crabapple trees. We see it on cotoneaster, hawthorn, quince, firethorn, and mountainash, as well as a few other species in the Rosacea family. If you have not seen this disease, you’ll know it by this description. The newest leaves and flowers wilt, turn brown or black, and remain attached to the infected stems. The stem tips curl over in what is called a “shepherd’s crook” symptom. It appears that the branch tips have been scorched with fire, so appropriately the disease is called fire blight. Bacteria quickly move down the shoot to the wood, causing it to turn brown to black as well. The causal bacterium is Erwinia amylovora. Symptoms could be confused with frost injury or wind burn, but wood is affected as well with fire blight. Peel back the bark to reveal dead wood underneath. Most labs can confirm the presence of bacteria fairly quickly. Most infection occurs via flowers, but wounds can also serve as infection sites.
If you have had problems with this disease in the past, consider using a resistant variety (or more resistant variety) as a replacement. Another option is to use protective sprays. Sprays are usually timed to protect flowers from infection, as that is the major infection site for this disease. If the disease has been a problem in the past, you can use a copper spray before green tip. Green tip means leaves are just beginning to emerge and you can see green tips on the buds. Most of Illinois is beyond this stage now, but fruit pathologist Dr. Babadoost says it is not too late for some benefit from copper compounds. Copper sprays will help because they are applied to the tree where the bacterium might have overwintered. They kill the bac terium as it begins to ooze out of these areas. In addition, if storms occur, tissue will be wounded, and the copper sprays may provide additional protection of those sites. Antiobiotic sprays are applied to protect the tree from infection during bloom. These sprays are applied when the tree is 0% to 10% bloom or when no more than 10% of the blooms are open on the tree. They provide a system protection rather than the protective/contact action of the copper compounds. Antibiotics are used in commercial fruit production. There are a few products available for use in landscape settings as well, but we do not recommend their use due to resistance concerns. Refer to the Commercial Landscape and Turfgrass Pest Management Handbook or Home, Yard, and Garden Pest Guide for registered chemicals for your specific host plant. If you wish to use an antibiotic, consult your local garden center or chemical supplier.
It is also helpful to remove cankered wood that may harbor the fire blight bacterium, but do this in the dormant season. Doing so now will stimulate new growth, which is more susceptible to fire blight infection because it is easily wounded. High nitrogen rates should be avoided for the same reason.
Ornamental pears have been particularly hard hit by fire blight in the last few years. Information on fire blight–resistant species can be found on multiple Web sites, but I find conflicting reports on cultivar resistance. Try to use a resistant variety and then keep dead wood cleaned out in the dormant season and use a low level of fertilization. Also refer to the University of Illinois Report on Plant Disease on fire blight at http://www.ag.uiuc.edu/~vista/.