Phytopathology 2017 07 07() doi 10.1094/PHYTO-05-17-0187-R
The phytopathogenic bacterium Xanthomonas euvesicatoria causes bacterial leaf spot (BLS) of pepper and has a worldwide distribution. BLS is difficult to control and an integrated management strategy that incorporates crop rotation, use of clean seeds and clean plants, weed control, resistant varieties, and applications of bactericides, biocontrol agents, and systemic acquired resistance (SAR) inducers is generally recommended. However, even with that arsenal of weapons, BLS can still be responsible for severe losses under favorable environmental conditions. Thus additional tools need to be added to an overall integrated management strategy to combat BLS. In this paper, we developed several models from 2012-2014 that were based on how macronutrients, micronutrients and micronutrient ratios affect disease severity. Factors used to select a model for validation included highly significant P values, high adjusted R2 values, low variance inflation factor (VIF) values (<5), root mean square error, and Mallow's Cp, and high Akaike's information criterion correction (AICc) values. In addition, salicylic acid (SA) concentrations and relative expression of non-expresser pathogenesis-related gene1 (NPR1) and pathogenesis-related protein 1 (PR1) in pepper tissues were also considered in model selection. A model (ECGA1) consisting of concentrations of copper (Cu), manganese (Mn) and potassium (K) and the iron to zinc ratio (Fe:Zn) as independent variables was used for validation in three different commercial pepper fields in Georgia [Colquitt County and Worth County (2015) and Tift County (2016)]. When AUDPC values for two field sites (Colquitt and Worth Counties) in 2015 were pulled together and plotted against ECGA1 predicted values for both sites resulting relationship was highly significant (P= 0.0001) with R-square value of 0.92. Significant relationship between observed AUDPC vs. predicted values was also observed in Tift County (2016) (P<0.001; adj. R2=0.98). Relative gene expression of both NPR1 and PR1 genes were significantly (P< 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 will fluctuate depending on environmental conditions, the data indicate 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.