Control of Shoot and blossom blight in the presence of streptomycin Resistance

Sherman Thomson

USU Extension Plant Pathologist

Dept of Biology

Utah State University

Streptomycin resistance of the fire blight pathogen is currently localized in orchards in the Santaquin and Payson areas. Surveys in 2000 and 2001 did not reveal significant spread or detection of streptomycin resistant Erwinia amylovora in areas outside of the original detection sites.

Careful use of bactericides to prevent the selection of resistant strains is necessary for all pome fruit growers in Utah. Constant vigilance and regular surveys of orchards for the presence of streptomycin resistant bacteria is essential for effective control with bactericides.

Control of fire blight in the presence or threat of streptomycin resistance requires a change in the strategies to control fire blight. It is not wise to use only a single bactericide, especially streptomycin, for the entire season. Incorporate compounds into the spray program that have different modes of action. Copper used at times when the chance of phytotoxicity is low will help reduce the selection for resistance. Use Mycoshield (tetracycline) on pears or apples if Utah has a Section 18 for its use. Other choices include Blightban A506 and Messenger although they have not been proven effective in some tests.

Evaluations of other bactericides for control of fire blight are a regular part of our research program. We test potential compounds at the USU Kaysville research orchard where we can inoculate with the fire blight pathogen. Tests in 2001 revealed that the combination of Streptomycin plus Actigard provided the best control of fire blight in both pear and apples. Unfortunately this combination is not very practical since streptomycin resistance is present in some orchards and Actigard is not yet registered. However this trial verifies that using an antibiotic plus a compound that stimulates the natural resistance in trees provides better control than anything else we have available. Streptomycin alone was also effective but Messenger, Starner, Vacciplant, and Serenade did not control fire blight.

Shoot blight is very difficult to control since there are no bactericides registered for its use. Even streptomycin has not been effective for shoot blight in many tests worldwide. The reason for lack of control is probably because it is difficult to know when to apply a bactericide to coincide with infection periods. We evaluated Apogee, a plant growth regulator, for control of shoot blight in pears and apples at the USU Kaysville farm. Apogee was applied when shoots were about 1 inch long and applications repeated every 2 weeks for a total of 3 applications. Apogee significantly reduced shoot growth on apples but did not have an effect on pear shoot growth. We inoculated shoots on trees treated with Apogee by injuring with a hole punch loaded with E. amylovora. Apogee had no effect on growth of pear shoots or on shoot blight of pears.

Apogee provided excellent control of shoot blight in apples even when shoots were injured and inoculated. 100% of the check shoots became infected whereas none of the Apogee treated shoots became infected. However a mid vein on 30% of the inoculated leaves on the Apogee treated shoots turned black. The vein infection did not progress into the shoot and did not cause shoot blight. Apogee significantly reduced the growth of new shoots. The average shoot length of Apogee-treated trees on May 14 was 12 cm while the shoots on the check trees averaged 17 cm.