During the summer months, managing shoot blight infections is a top priority. Many growers are faced with the question of pruning or not. Pruning can reduce fire blight inoculum and therefore reduce the spread of the pathogen; however, it can also increase shoot growth and make trees more susceptible to fire blight. So, the answer depends on the severity of infection and the environmental conditions. This season, we are experiencing drought conditions and fire blight has not been reported widely in Wisconsin suggesting that this season may be lower disease pressure compared to previous years. However, until terminal buds are set, it is critical to keep an eye on weather conditions as heavy dew or rain could support fire blight growth and spread.

In trees that are older, take care to minimize pruning too early in the season. Aggressive pruning early in the season can stimulate the production of young succulent shoots. In younger trees, pruning should take place as soon as infections appear to reduce the risk of spreading.

Recently, a multistate (NY, PA, WA, and OR) study was conducted to assess certain fire blight management practices to provide growers with research-based recommendations. Many of the current recommendations for fire blight seem to vary based on who you ask or which resources you consult, but this study aimed to demystify some of the uncertainties associated with fire blight management. The major outcomes of the study (DuPont et al. 2023) are:

• Fire blight infected tissue should be removed in a timely manner to reduce tree mortality.
• In a comparison of several different summer pruning practices, the best management practice of cutting 12-18 inches from the edge of the symptoms using sanitized pruning shears provided the best management, specifically by reducing new symptom development and reducing the systemic movement of the bacteria in the tree.
• Aggressive pruning, defined as cutting 2.5 feet from the edge of the symptoms with sanitized shears or loppers, did not improve upon the best management practice of cutting 12-18 inches from the edge of the symptoms using sanitized pruning shears.
• The long stub method (5-inch stub from structural wood) helped reduce the number of cankers on structural wood. This method was least helpful in less susceptible wood or following terminal bud set.
• Breaking infected branches by hand (instead of pruning using shears or loppers) is sometimes used to speed up the fire blight sanitation process, however this study demonstrated that this breaking method can result in more cankers (reformed cankers and new symptoms).
• Sanitizing pruning shears has long been recommended to prevent the spread of fire blight infections. New findings from this study suggest that whether you sanitize shears or not when using the best management practice of pruning 12-18 inches past the edge of symptoms, there is no significant difference. There was also no significant difference in the number of cankers formed in the tree with sanitized vs. no sanitized shears. If pruning through ooze, then sanitation may provide some benefits, however, depending on the severity of the infection in the orchard, timely removal of fire blight strikes is the top priority.

Further details about the study can be found here (Tianna DuPont, Aina Baro, Washington State University; Ken Johnson, Oregon State University; Kerik Cox, Cornell University; Kari Peter, Penn State University. Pruning Out Fire Blight Infections. May 1, 2023).

When pruning fire blight strikes, make cuts into at least 2-year-old wood. The fire blight bacterium is less likely to multiply in this older wood. Pruning should take place during dry weather as this minimizes the risk of spreading the pathogen. In high density orchards, if the central leader of a tree is infected with fire blight that tree should be removed. Preventative applications of prohexadione calcium (ex. Apogee) should be considered to reduce shoot growth and help mitigate the impacts of fire blight. Under drought conditions shoot growth may not be as vigorous and prohexadione calcium applications should be adjusted accordingly for fire blight management. Apogee is most critical for vigorous, susceptible varieties.

This article was posted in Apples, Disease and tagged Apples, disease, Fire Blight, Leslie Holland, Pruning, pruning management.