Fire blight can affect several parts of the host plant. Bees can introduce the bacterium into the flowers during the pollination process. Raindrops can also spread the bacterium into the flowers. About 14 days after the flowers have opened, the petals become water-soaked, turn brown, and then black. This condition, which affects single flowers or entire clusters, is called blossom blight. New leaf growth can also be affected. The leaves wilt suddenly and turn black or brown, giving the plant an appearance of having been scorched by fire. Often, the ends of infected twigs curl over, forming a shepherd’s crook. Fruit may become blighted, turning brown to black and often oozing bacteria. Bark lesions, called cankers, can develop on the bark of larger branches that have become infected through flowers or twigs. Cankers are localized dead areas in the bark of twigs, branches, or trunks. They appear as unusually shrunken, depressed, or discolored areas of dying tissue, which may split open and expose the wood beneath. These depressed and discolored canker areas produce an opaque grayish or brown liquid, which is the source of infection, the bacterium. Cankered branches will die and a tree can decline rapidly. Trees may die or be so damaged that they are no longer viable specimens.

Fire blight overwinters as bacteria at the margins of cankers. In spring, warm, wet weather, above 65 degree F, initiates bacterial activity, resulting in oozing from the canker. This ooze is transmitted to flowers and twigs by water, birds, pollinating bees, and humans (often through pruning). One to three weeks later, fire blight symptoms appear. As new branches become infected, new cankers form and serve as sites for future spread of the bacteria.

Cultural Management:

Fire blight can be difficult to manage. Remove all infected branches in spring as symptoms appear. Prune at least 6 inches below the visibly infected area. Do not prune when trees are wet. Pruning tools should be sterilized after each cut. Avoid excess use of nitrogen fertilizer which may lead to over-stimulation of plant growth and make tissues more susceptible to infection.

Resistant cultivars:

The best way to prevent fire blight is to plant resistant cultivars when available. Many species and newer cultivars of apple and crabapple are resistant to fire blight. A partial list is available on The Morton Arboretum website. Check with local nurseries regarding the availability of these and additional disease resistant cultivars. Other hosts have limited or no disease resistant cultivars.

Chemical management: 

Chemical management for fire blight is limited, and timing is very important for the products that are available. Fungicides do not affect fire blight, since the disease is caused by a bacterium. Streptomycin, an antibiotic, can be used during flowering to prevent infection in the flowers, but timing is critical. Follow label directions carefully. Copper-based products can be used in the dormant season to prevent a buildup of bacteria on the surface of tree parts. Once the disease is in the wood, these products offer no protection. Certified arborists do have newer antibiotic products available, but again, timing of the use of these products is very important. To find a certified arborist, go to  https://www.treesaregood.org/

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).