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Etiology and management of Septoria leaf spot on stevia

Alyssa Koehler: North Carolina State University


<div>Stevia (<em>Stevia rebaudiana)</em> is a new perennial crop in the US, produced for leaves that contain numerous glycosides extracted for use as nonnutritive sweeteners. In 2015, leaf lesions caused by a <em>Septoria </em>sp. were observed in multiple locations in North Carolina. Lesions were present throughout the season but spread rapidly and coalesced to cause significant defoliation during favorable environmental conditions in September, prior to harvest. Type culture isolates of <em>Septoria steviae, </em>reported in Japan in 1982, were obtained for comparison to NC isolates. All isolates were sequenced for six loci: actin, β-tubulin, calmodulin, internal transcribed spacer, RNA polymerase II second largest subunit, and translation elongation factor-1alpha. Isolates from NC were identified as <em>S. steviae.</em> In 2017, efficacy trials were conducted at two locations in NC, testing seven fungicides and one biological control. Products were applied twice late in the growing season and plants were rated for percent of leaf area with lesions. Trials were combined for analysis and there were significant treatment effects (<em>P<0.001</em>). All products tested reduced lesion area compared to the control (57% leaf area covered at first harvest). In vitro fungicide trials were conducted to document the initial sensitivity profiles of NC <em>S. steviae</em> isolates to a range of fungicide chemistries. Successful management of Septoria leaf spot will be critical to the successful establishment of stevia in the US.</div>