After a challenging season last year with increased reports of fire blight, many growers are concerned looking at the season ahead. Wisconsin experienced varied weather conditions last season that likely contributed to the uptick in fire blight incidence. From freezing temperatures after bloom, to warmer than normal temperatures in June. Many were surprised to see such severe shoot blight infections with the dry and warm weather, but these conditions were favorable for systemic shoot blight infections.  Looking forward to the 2022 season, many are concerned about fire blight disease pressure in the orchard. Season long management is your best approach to minimize the spread of the pathogen.

Does fire blight last season mean fire blight this season?

No, not necessarily, but if there was significant overwintering inoculum (holdover cankers) left in the orchard this can increase disease pressure. Holdover cankers should be removed during winter pruning.

Consider using NEWA for assessing blossom blight risk

The Network for Environment and Weather Applications (NEWA) provides two models for assessing blossom blight risk, Cougar blight and Epiphytic Infection Potential (EIP). Keep track of your first blossom open dates for all varieties and use these dates in the fire blight model for the most accurate predictions. The model also allows you to add your recent streptomycin application date to re-evaluate risks and provides a trauma blight tool that helps track the development of shoot blight.

• Prediction models DO: Identify risks of infection based on weather conditions. Models tell us when and how favorable environmental conditions are for blossom infection. The models are less effective for shoot blight as this is dependent on the internal movement of bacteria to developing shoot tips or external injury.
• Prediction models DO NOT: Replace scouting or in-field observations of phenology. Predict control failures. Predict future disease.

Weather conditions (and resulting predictions) can change quickly. Utilize the 1- and 2-day forecast for fire blight predictions as these are the most reliable. Remember, it is more cost-effective to spray for blossom blight when there are conducive conditions for fire blight. The risk of fire blight may be low if 1) the apple variety is not highly susceptible, 2) there is no prior history of fire blight, and 3) nitrogen levels aren’t pushing trees into high vigor. Do not make unnecessary applications. The model interpretations should be adapted to your orchard’s features, specifically – age, variety, and rootstock.

Managing fire blight

Pre-season: During winter pruning, all fire blight strikes should be removed. If you observed trees where the central leader (main trunk) was infected, these trees should be removed; do not try to graft a scion onto these trees, remove the entire tree. You can replant in the spot where the tree was removed. Infected pruning’s should be removed from the orchard. If pruning’s cannot be burned, then leave them in a place where they can dry out quickly.

Early season: If applying copper, this application should occur at green tip. Check the label for any restrictions.

Bloom: Follow a blossom blight forecasting system such as NEWA. Time applications during high-risk periods. This is especially important if you’ve had a history of the disease; if you have little or no history of fire blight the applications may not be required during every high-risk event predicted by the models. If using streptomycin, apply at bloom. In the first streptomycin application you can include the surfactant Regulaid to enhance effectiveness; discontinue use after the first application to minimize leaf yellowing that can be associated with streptomycin applications.

Post-bloom: Monitor fire blight strikes and prune out, ensuring you remove at least 12 inches past the canker margin. Strikes should be left to dry out or burned to minimize the risk of further spread of the pathogen.

Summer: Summer pruning can invigorate shoot growth resulting in the production of highly susceptible tissues. Preventative applications of prohexadione calcium (ex. Apogee) should be considered to reduce shoot growth and help mitigate the impacts of fire blight. This is especially critical for vigorous, susceptible varieties. Application should be made during shoot elongation at late bloom. One application should be enough to limit fire blight development, but an additional application may be beneficial on vigorous trees of highly susceptible varieties. Check out the Midwest Fruit Pest Management Guide for detailed information on timing, rates, and precautions for prohexadione calcium.

• Preventative applications of copper may be used post-bloom and, in the summer, to protect shoots from fire blight infections. Copper should be applied before infection occurs because it only acts to reduce bacteria on the surface. Copper should not be used to cure existing shoot blight infections as it will have no effect. Copper may cause fruit russet. Make applications in quick drying conditions; hydrated lime can help reduce phytotoxicity associated with copper.

New plantings? If you have or soon plan to have new apple trees at your orchard fire blight is not something you should overlook in these plantings, especially in high density orchards. If you are in the process of selecting varieties, be sure to plant varieties that are grafted onto fire blight-resistant rootstocks. Carefully inspect trees for symptoms and signs of fire blight infection before planting. If you suspect any trees are infected submit a sample to the UW Plant Disease Diagnostic Clinic for diagnosis and dispose of the tree. After planting new trees apply copper using the lower copper rates labeled for use after green tip. A second application of copper can be applied 14 days later. To minimize the risks for phytotoxicity to the roots be sure the soil has settled. Trees should be scouted weekly for fire blight strikes until the end of July. If infected trees are observed remove them. If conducive conditions or wounding events (i.e., storms or hail) occur, trees should be scouted frequently for fire blight infections. Scout new plantings at the end of the season. Often new plantings have a significant number of flowers, if feasible, remove flowers before they open in dry weather and before the accumulation of too many heat units. A similar program as mentioned above may be used for new plantings. Note that copper should be applied at a stage equivalent to bloom (check REI before flower removal). Prohexadione calcium applications do offer shoot blight protection and according to studies in New York, applications on young plantings do not compromise establishment by late fall.

For more information on fire blight biology and control, read my recap from the 2021 Apple Field Day here: https://fruit.wisc.edu/2021/07/22/fire-blight-management-in-high-density-orchards/

The 2022 season is off to a cooler start with several glimpses of spring. Stay alert and utilize fire blight forecasting systems at the onset of bloom.

**Thank you to Dr. Kerik Cox (Cornell University) for providing information shared in this article.

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