Applications of Germicidal Ultraviolet Light as a Tool for Fire Blight Management (Erwinia amylovora) in Apple Plantings.

Nighttime applications of germicidal UV light (UV-C) have been used to suppress several fungal diseases of plants, but less is known of UV-C's potential to suppress bacterial plant pathogens. Fire blight of apple and pear, caused by the bacterium Erwinia amylovora, is difficult to suppress using cultural practices, antibiotics, and host resistance. We therefore investigated the potential of UV-C as an additional means to manage the disease. Laboratory assays confirmed that in vitro exposure of cultures E. amylovora to UV-C at doses ranging from 0 to 400 J/m2 in the absence of visible light was more than 200% as effective as cultures exposed to visible light after the same UV-C treatments. In a 2-year orchard study, we demonstrated that with only two nighttime applications of UV-C at 200 J/m2 made at bloom resulted in an incidence of blossom blight and shoot blight equivalent to the results viewed when antibiotic and biopesticide commercial standards were applied. In vitro dose-response studies indicated consistency in pathogen response to suppressive UV-C doses, including pathogen isolates that were resistant to streptomycin. Based on these results, UV-C may be useful in managing bacterial populations with antibiotic resistance. Concurrent measurements of host growth after UV-C applications indicated that the dose required to suppress E. amylovora had no significant (P > 0.05) effects on foliar growth, shoot extension, internode length, or fruit finish but substantially reduced epiphytic populations of E. amylovora on host tissues.

Investigating the Distribution of Strains of Erwinia amylovora and Streptomycin Resistance in Apple Orchards in New York Using Clustered Regularly Interspaced Short Palindromic Repeat Profiles: A 6-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 (NY) 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 6 years (2015 to 2020). Characterization of clustered regularly interspaced short palindromic repeat (CRISPR) profiles revealed that a few "cosmopolitan" strains were widely prevalent across regions, whereas 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 was 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 because of 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.

Comparison of Bioassays Used to Determine Onion Thrips (Thysanoptera: Thripidae) Susceptibility to Spinetoram.

Onion thrips (Thrips tabaci Lindeman) is one of onion's most damaging insect pests and has a history of developing resistance across insecticide classes. The susceptibility of T. tabaci populations to insecticides can be determined using laboratory bioassays. Three types of bioassays have been documented in the literature specifically for use with T. tabaci: vial assay (contact only), feeding assay (ingestion only), and leaf-dip assay (contact + ingestion). The objectives of this study were to 1) compare insecticide susceptibility levels of a T. tabaci population using these three assays and 2) determine which bioassay's results were most similar to those generated from exposing thrips to whole plants treated with insecticide. All experiments were conducted using a colony of T. tabaci known to be susceptible to insecticides and all were evaluated for their susceptibility to spinetoram (Radiant SC). Results indicated that 1) each bioassay generated a unique concentration-mortality relationship and LC50 value (0.01, 0.03 and 1.6 ppm for leaf-dip, vial, and feeding assays, respectively), and 2) all bioassays overestimated the susceptibility of the population relative to the whole-plant assay (LC50 = 5.3 ppm). Attributes of these bioassays are discussed relative to their future use in insecticide resistance monitoring programs for T. tabaci.