Fire Blight Management Tips From Michigan: When to Prune or Not to Prune

The question of pruning out fire blight strikes during the growing season, leaving them until the winter or using the "ugly Stub method" has long been debated. The answer to this question may depend upon the number of strikes and the weather conditions suitable for infection.

During the 2000 season, a warm, humid spring and a long bloom period resulted in a severe outbreak of fire blight in Michigan apples. To complicate the situation further, a severe storm brought 2 ½ inches of rain and hail to the region in mid May resulting in severe trauma blight several weeks later. After two years, fire blight is still a concern for many Michigan apple growers.

What did Michigan researchers, extension agents and farmers learn about managing fire blight in 2000? Proper timing and application of Streptomycin is very important for the management of this disease. During the 2000 fire blight epidemic in Michigan, weather conditions at bloom allowed bacteria populations to rise to very high levels. When rains came late in bloom, growers did not have time for streptomycin applications in all their orchards and only sprayed the most susceptible varieties. Consequently, blossom blight developed on the unsprayed rows in many orchards and a trauma event in May spread the disease throughout the orchards in the region. Many Michigan apple growers immediately pruned out fire blight infections. While other growers did not prune fire blight strikes until winter. Regardless of when they pruned, growers found some fire blight in their orchards the following season. Dr. Mark Longstroth, an extension agent with Michigan State University, suggested that under dry conditions when only a few strikes occur, immediately pruning down to non-infected 3-year-old wood would reduce the potential of the disease spreading. Prunings that have dried out on the orchard floor do not present a danger to spreading the disease. Longstroth stresses that if warm, wet weather persists, the risk of spreading the disease during pruning would be great and it may be better to leave the fire blight strikes in the orchard until a period of dry weather occurs. Unfortunately, leaving the disease in the orchard may serve as a source of inoculum should a severe storm roll through that could result in a "trauma blight" event. There is some uncertainty of whether the "ugly stub" method works better than leaving the diseased strikes in the orchard until winter. A comparison of the two methods under Michigan climate and situation is required.

"We had a lot of orchards where the grower walked away from older trees and the blight killed all the young wood less than 3 or 4 years old" says Dr. Longstroth, "by late July the trees had grown new shoots and you could hardly tell they were infected from the road." Some Michigan growers pruned out all the fire blight affected wood the following winter and others waited a year and did their pruning after the tree's growth had settled down. "Both strategies seem to work well and I think waiting a year was cheaper," says Longstroth, "but you do have a much-increased risk the following spring if you leave it in for a year."

Dr. Longstroth suggested trees that show symptoms of fire blight for two years in row should be removed and burned. This continuos infection is an indication that the fire blight bacteria is systemic in that tree and it will continue to express symptoms each year and serve as an infection source inoculating the orchard each year. Continuous pruning of the same infected trees year after year is not sustainable, since the trees would probably become unproductive due to the infection and pruning.

Also the infected trees will continue to act as a source of inoculum for the rest of the orchard.

Longstroth suggests that pruning out strikes during years when fire blight is not a major problem allows growers to reduce the amount of bacteria that will be present in the orchard in the years when bloom conditions are right for the spread of the disease. "I tell my growers to scout and prune out strikes when the weather is dry.