Suppress Fire Blight with Pruning

Fire blight is a very destructive disease in apples, pears and many other rosaceous ornamental plants. The bacterial pathogen, Erwinia amylovora, infects blossoms, fruits, shoots and rootstock crowns. In apple and pear, the primary infection usually affects blossoms and young shoots when moisture is present and the average daily temperature is above 60 degrees F. The pathogen then moves from the initial site of infection further into the branches. The blight stops spreading when the terminal shoot bud is set later in the growing season. It is not uncommon for fire blight to kill young trees of highly susceptible cultivars or entire scaffold branches of older trees.

The first symptom that can be observed on apple trees, is a drooping of the first or second fully expanded leaves on the terminal portions of the shoots. Next, black necrotic areas appear along the mid-vein at the base of leaf blades. Then, the typical shepherd’s crook symptom, or curving under of the terminal shoots is apparent. There are several management strategies that can be used to prevent fire light including the selection of resistant or moderately resistant cultivars, and avoiding heavy fertilization, excessive vegetative growth, and wet foliage by overhead irrigation. For apple, moderately resistant cultivars include Arkansas Black, Empire, Liberty and Red Delicious. Golden Delicious, Fuji, Staymen, CandyCrisp and Redfree are considered fire blight susceptible. Some of the very blight susceptible cultivars are Gala, Jonathan, Idared, Rome, Bradbury, York, Ginger Gold, Jonagold, Paulared, Mutsu and Lodi. Susceptible rootstocks to avoid are B.9, M.9, M.26, Ottawa 3 and all of the Polish series. MM.11 is considered moderately resistant. M.7 and all of the Geneva rootstocks (recently released from Cornell University) are blight resistant. While the use of fire blight resistant plant material is recommended, homeowners usually do not get to choose the rootstock as the nursery simply lists the tree in the catalog as dwarf or semi-dwarf tree.

As noted previously, many ornamental plants in the Rosaceae, such as crabapples and ornamental pears, are susceptible to fire blight and it is important to choose resistant cultivars for landscape planting. Information on the relative susceptibilities of crabapple and ornamental pear cultivars can be found at http://agebb.missouri.edu/pdc/diagnosticreports/resistant01.htm.

For apple or pear, hand thinning or removal of the fruit is done to increase the size of the remaining fruit on the tree and to decrease disease and insect infection sites. Bud pinching is another cultural practice occasionally done to train the central leader of the trees. Similarly, pinching succulent terminal shoots for tree training is also a common practice. However, if these cultural practices are done during warm temperatures when moisture is present, this is an excellent way to spread the infection! Delayed hand thinning will result in some loss of fruit size but the risks of further fire blight infection outweigh the benefits of early hand thinning.

Fire blight damage on a tulip. Photo by Clemson University, courtesy of www.forestryimages.com

Protective chemical fire blight management strategies also can be used for apple and pear trees. Copper hydroxide can be applied on apple and pear trees before growth begins in the spring and when the temperature is above 45 degrees before bud break. Streptomycin can be applied only during bloom. The growth regulator, Apogee, can also be applied at full bloom and causes early cessation of terminal growth. Thus, fire blight infection is limited by promoting early formation of the terminal bud. However, these products are used for commercial orchards and without complete tree coverage, they have limited usefulness on large trees in the home landscape.

The final strategy used to suppress fire blight is pruning. Some advocate leaving the fire blight-infected branches untouched until winter pruning. This recommendation to leave the infected tree unpruned may be appropriate for hot, arid, low humidity growing regions. Because of the high humidity, high dew and rainfall conditions during bloom, the removal of infected wood is always recommended in Missouri. When blight infections occur during severe hail storms or “wind whipping” episodes, whole trees can be killed. To prevent the spread of fire blight, the pruning cut should be made 8 to 12 inches below the last visible point of healthy and infected bark when infections occur on the central leader or the terminals of scaffold branches. Pruning cuts should be made in between the nodes (stem area between the leaves). On scaffold branches, do not make flush cuts near the main trunk. To remove the blight, leave stubs of branches at least a few inches away from the main trunk. . If the fire blight spreads from the infected portion a flush cut into the main trunk, then the trunk would then require removal, destroying the natural form of the tree for several seasons. While most pruning literature advocates making a flush cut outside the branch collar, pruning for fire blight suppression requires leaving branch stubs in the tree. In very large trees, it may not be possible to prune every infected branch. However, by removing as many as the sources of inoculum as possible, the risk of reinfection or spread of the disease is lower.

Fire blight damage on apple. Photo by Clemson University - USDA Cooperative Extension Slide Series www.forestryimages.com

Another topic of controversy is the need to disinfect pruning tools between cuts when removing infected wood. In arid climates, such as California, it is not necessary to disinfect tools if pruning in dry weather. However, tools should be disinfected between cuts in a bleach solution (1 part bleach; 10 parts water) when pruning fire blight infected trees in Missouri to avoid spreading the disease. After, pruning is completed, wash the corrosive bleach solution off the blades of the pruning tools with soapy water.

After pruning infected trees, the final decision is whether to remove the prunings from the orchard or landscape. Unfortunately, no research has been conducted to resolve this issue. "Some believe that moving infected wood out of the orchard area may spread more blight than when prunings are left to dry in the row middles. Others advocate pushing prunings into row middles and letting them dry before chopping them with a mower. Because the great potential for fire blight infection occurs during the period of heaviest rainfall in Missouri, it is recommended that the prunings be removed from the orchard as soon as it is feasible.

In summary, fire blight infected wood in young trees should be pruned immediately after the dew dries. In older trees, prune out as much infected wood as practical, leaving stub-cut branches. Disinfect pruning shears between each cut and remove prunes from the landscape as soon as possible.

Michele Warmund
Ag Ext.-Plant Sciences
573-882-9632