SURVEY RESULTS OF ERWINIA AMYLOVORA IN UTAH FOR RESISTANCE TO STREPTOMYCIN AND INVESTIGATIONS COMPARING KASUGAMYCIN (KASUMIN) TO STREPTOMYCIN AND OXYTETRACYCLINE FOR CONTROL OF FIRE BLIGHT

Isolates of Erwinia amylovora (Ea), surveyed from apple (Malus domestica) orchards in Utah in 2006, were screened for frequency and level of resistance to streptomycin, oxytetracycline and kasugamycin in laboratory tests. Twenty-six % of Ea isolates surveyed were resistant to streptomycin at 100 ppm. Fifty-six % of strep-resistant Ea isolates at 100 ppm were also resistant at 200 ppm and 44% of strep-resistant isolates were also resistant at 400 ppm streptomycin. No Ea isolates were found resistant to oxytetracycline at 30 or 100 ppm. One Ea isolate was found to be resistant to kasugamycin at 30 ppm but was susceptible at 100 ppm kasugamycin and was not resistant to either streptomycin or oxytetracycline at 30 ppm or higher. None of the strep-resistant Ea isolates were resistant to Kasumin at 30 or 100 ppm. In field experiments, conducted in 2006 and 2007, for fire blight shoot strike control, kasugamycin (Kasumin, Arysta LifeScience) was compared to streptomycin (Agrimycin) and oxytetracycline (Mycoshield). Kasumin was as effective as or better than Agrimycin for controlling shoot strikes in 2006. In 2007 Kasumin was equal to Agrimycin in 2 of 4 apple cultivars that were evaluated for fire blight shoot strikes. A floral strike experiment was conducted in 2007 and significant differences (P=0.05) among antibiotic treatments were observed when averaged over four apple cultivars. Floral fire blight strike percentages for water-treated blossoms was 26.3%; for Mycoshield at 100 ppm floral strike percentage was 14.5 %; for Kasumin at 100 ppm floral strike percentage was 8.6%; for Kasumin at 80 ppm floral strike percentage was 7.4%; and for Agrimycin at 100 ppm, floral strike percentage was 5.0% (lsd=2.0). Kasumin is useful for control of fire blight in Utah orchards. Ea isolates identified as resistant to streptomycin in 2006 have not shown resistance to kasugamycin or oxytetracycline in laboratory tests to date.