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Aggressiveness evaluation of Diaporthe species causing soybean stem canker in the United States

Kristina Petrovic: Institute of Field and Vegetable Crops

<div>In 2014, the estimated yield loss of soybean (<em>Glycine max</em> L.) caused by stem canker was 350,000 tons (175 million dollars). A total of 135 isolates were recovered from symptomatic soybean stems collected from six U.S. states (IA, IN, IL, KY, MI, and SD) between 2002 and 2016. Using morphology and DNA sequence analysis, isolates were identified as <em>D. longicolla</em> (66%), <em>D. aspalathi</em> (25%) and<em> D. caulivora</em> (9%). In a greenhouse experiment, aggressiveness of five isolates each of <em>D. caulivora</em> and <em>D. aspalathi</em> was compared in two independent experiments using four inoculation methods (toothpick, stem-wound, mycelium-contact, and spore-injection). Isolates of <em>D. caulivora</em> were compared on cv. Hawkeye, whereas <em>D. aspalathi</em> isolates were compared on cv. Bragg. The experiment was established in a completely randomized design in a factorial arrangement with inoculation methods and isolates as factors. At 21 days after inoculation, lesion length and stem length were measured; both were affected by a two-way interaction (<em>P </em>< 0.001) between isolates and inoculation methods. For <em>D. caulivora</em>, isolate SD-29 produced significantly (<em>P </em>≤ 0.05) longer lesions and shorter stems by the stem-wound method compared to other inoculation methods. For <em>D. aspalathi</em>, isolate KY-41 produced significantly (<em>P </em>≤ 0.05) longer lesions compared to the other isolates. The toothpick method resulted in significantly (<em>P </em>≤ 0.05) shorter stems across all <em>D. aspalathi </em>isolates compared to the other methods. Our study determined that the toothpick and stem-wound methods can be used effectively for screening soybean germplasm for resistance to <em>D. aspalathi </em>and<em> D. caulivora</em>.</div>