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Role of cation concentration in pepper tissue in suppressing bacterial leaf spot severity

Bhabesh Dutta: University of Georgia

<div>The phytopathogenic bacterium <em>Xanthomonas euvesicatoria</em> causes bacterial leaf spot (BLS) of pepper and has a worldwide distribution. Several models were developed from 2012-2014 that were based on how macronutrients, micronutrients and micronutrient ratios affect BLS severity. Factors used to select a model for validation included highly significant <em>P</em> values, high adjusted <em>R<sup>2</sup></em> values, low variance inflation factor (VIF) values (<5), root mean square error, and Mallow’s Cp, and high Akaike’s information criterion correction values. In addition, salicylic acid concentrations and relative expression of non-expressor pathogenesis-related gene1 (<em>NPR1</em>) and pathogenesis-related protein 1 (<em>PR1</em>) in pepper tissues were also considered in model selection. A model (ECGA1) consisting of concentrations of copper, manganese and potassium and the iron to zinc ratio as independent variables was used for validation in three different commercial pepper fields in Georgia [Colquitt, Worth and Tift County]. When AUDPC values for two field sites (Colquitt and Worth Counties) in 2015 were pooled and plotted against ECGA1, predicted values for both sites resulted in a a highly significant relationship (<em>P</em>= 0.0001) with an R-square value of 0.92. A significant relationship between observed AUDPC vs. predicted values was also observed in Tift County (2016) (<em>P</em><0.001; adj. <em>R<sup>2</sup></em>=0.98). Relative gene expression of both <em>NPR1</em> and <em>PR1</em> genes were significantly (<em>P</em><0.01) higher in peppers grown in predicted low risk sites compared to peppers from high risk sites in Colquitt, Worth and Tift counties. Although BLS severity fluctuated depending on environmental conditions, the data suggested that the level of risk at a particular location may be influenced by how macronutrient and micronutrient concentrations affect plant disease resistance genes in the SAR pathway.</div>

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