Investigating the distribution of strains of Erwinia amylovora and streptomycin resistance in apple orchards in New York using CRISPR profiles: a six-year follow-up
Fire blight, caused by the bacterium Erwinia amylovora, is one of the most important diseases of apple. The antibiotic streptomycin is routinely used in the commercial apple industries of New York and New England to manage the disease. In 2002, and again from 2011 to 2014, outbreaks of streptomycin resistance (SmR) were reported and investigated in NY. Motivated by new grower reports of control failures, we conducted a follow-up investigation of the distribution of SmR and E. amylovora strains for major apple production regions of NY over the last six years (2015-2020). Characterization of clustered regularly interspaced short palindromic repeat (CRISPR) profiles revealed that a few ‘cosmopolitan’ strains were widely prevalent across regions, while many other ‘resident’ strains were confined to one location. In addition, we uncovered novel CRISPR profile diversity in all investigated regions. SmR E. amylovora was detected only in a small area spanning two counties from 2017 to 2020, and always associated with one CRISPR profile (41:23:38), which matched the profile of SmR E. amylovora discovered in 2002. This suggests the original SmR E. amylovora was never fully eradicated and went undetected due to several seasons of low disease pressure in this region. Investigation of several representative isolates under controlled greenhouse conditions indicated significant differences in aggressiveness on ‘Gala’ apples. Potential implications of strain differences include the propensity of strains to become distributed across wide geographic regions and associated resistance management practices. Results from this work will directly influence sustainable fire blight management recommendations for commercial apple industries in NY State and other regions.