The 2014 growing season was surprisingly difficult for many apple and pear growers in Southern New England. The warm, wet spring resulted in widespread fire blight, a bacterial disease whose symptoms include blossom blight, shoot blight, cankers, fruit lesions and in extreme cases tree death. To help farmers combat this emerging disease, the Rhode Island Fruit Growers’ Association invited Jon Clements from University of Massachusetts Extension to address fire blight at their annual meeting held in March at the RI Farm Bureau offices in West Greenwich, RI.
Why Fire Blight is a Concern
Fire blight is a particularly challenging disease for many reasons. First of all, any orchard that has had it in the past or neighbors an orchard that has had it is susceptible. The inoculum can be, and most likely is, harboring in infected trees (many of which occur in the landscape), leaf or blossom litter, or pruning debris. This makes fire blight a particular concern this year. The inoculum is present; the only question is how favorable the environmental conditions will be.
A second difficulty in controlling fire blight is that nearly every variety of apple and pear is susceptible: given the right conditions every variety can be affected. At particular risk are young trees (aged 3-8 years), plantings on M9 dwarfing rootstocks, and trees high in nitrogen. The M9s are so susceptible that the fire blight can enter the rootstock and kill the entire tree, rather than isolated branches on more resistant rootstocks. The same issue arises with young trees due to their smaller mass and fewer branches. The difficulty with heavily fed trees is their tendency to flush longer and harder, extending the potential infection period.
A third concern with fire blight is climate change. As we seem to be moving into a period of warmer, wetter conditions, fire blight has the potential to become an increasing threat. The bacteria that cause fire blight thrive in those conditions and will have more opportunities to reproduce.
Fire Blight Disease Cycle
All is not lost, however. While managing fire blight presents challenges, it is not impossible. Understanding the disease is the first step in controlling it. The bacteria can only enter soft plant tissue such as blossoms, new growth, and areas of injury such as hail or wind damage. The bacteria cannot infect healthy, intact bark. Therefore the greatest risk of infection is during the spring flush through blossom.
In order to infect trees, however, the bacteria also requires specific external conditions. Warm, wet weather activates the bacteria. (“Wet” does not merely mean rain: heavy dews, frost control, and possibly even spray moisture can qualify.) Existing cankers will begin to ooze, enabling the bacteria to spread via wind and/or insect activity. If the inoculum lands on sensitive plant tissue, infection occurs easily.
To determine whether the conditions are favorable for fire blight and require preventative treatment, it is advisable to subscribe to one of the many disease-forecasting programs. Local Extension and/or tree fruit organizations should have additional information about forecasting models, their use and availability.
Managing Fire Blight
Prevention is key with fire blight, and growers can start from the ground up. When planting new trees, people in areas prone to fire blight should consider purchasing trees on Geneva rootstocks as they are bred to be resistant to fire blight. Clements also recommends removing blossoms in the first year or two of new plantings, as blossoms are the primary means for infection.
Sanitation is also a key factor. Dormant pruning of infected branches can help remove inoculum-filled cankers. One must be careful to avoid cutting into active cankers, however, as that will actually spread the disease. Some experts recommend pruning 12″-18″ below a visible canker. Since that may not always be practical or possible, Clements reminds growers to use “common sense” and prune according to their instincts. If infections are either so widespread that control is nearly impossible or so small that they are not worth the effort, he says the best course of action may be to “just go fishing”! Additional control measures are available.
There are some chemical measures for fire blight control, but timing is crucial. A copper application at green tip — up to 1/2″ green —will provide initial protection. The copper can be mixed with oil for those who routinely apply it, but good coverage is important. Any uncovered flush will be susceptible.
When a forecasting model indicates a fire blight infection period, an orchard-wide application of streptomycin is called for. There is a 24-hour efficacy period for strep, so in some seasons, repeated applications may be necessary. Strep treatments should not be used in the summer or fall unless favorable weather conditions occur within 24 hours of trees sustaining hail or heavy wind damage.
Some states have developed strep-resistant strains of fire blight. This should not be the case in most areas of Southern New England. If you have been using strep frequently for many years and suspect resistance, your local fruit extension specialist can advise on alternative treatments.
Fire blight can be devastating, leaving trees marked by blighted branches, cankers and in worst cases tree death. It is, however, a disease that can be managed with good sanitation and timely chemical applications. For further information on fire blight and its control, UMASS has a fact sheet entitled An Annual Fire Blight Management Program for Apples available at https://extension.umass.edu/fruitadvisor/sites/fruitadvisor/files/fact-sheets/pdf/F-133.pdf.