Microsatellite Markers from Peronospora tabacina, the Cause of Blue Mold of Tobacco, Reveal Species Origin, Population Structure, and High Gene Flow

Peronospora tabacina is an obligate parasite that causes blue mold of tobacco. The pathogen reproduces primarily by sporangia, whereas the sexual oospores are rarely observed. A collection of 122 isolates of P. tabacina was genotyped using nine microsatellites to assess the population structure of individuals from subpopulations collected from central, southern, and western Europe; the Middle East; Central America; North America; and Australia. Genetic variations among the six subpopulations accounted for ∼8% of the total variation, including moderate levels of genetic differentiation, high gene flow among these subpopulations, and a positive correlation between geographic and genetic distance (r = 0.225; P < 0.001). Evidence of linkage disequilibrium (P < 0.001) showed that populations contained partially clonal subpopulations but that subpopulations from Australia and Mediterranean Europe did not. High genetic variation and population structure among samples could be explained by continuous gene flow across continents via infected transplant exchange and/or long-distance dispersal of sporangia via wind currents. This study analyzed the most numerous P. tabacina collection and allowed conclusions regarding the migration, mutation, and evolutionary history of this obligate biotrophic oomycete. The evidence pointed to the species origin in Australia and identified intracontinental and intercontinental migration patterns of this important pathogen.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Multiyear Evaluation of Fungicide Efficacy and Application Timing for Control of Southern Rust in Hybrid Corn in Arkansas

The efficacy and timing of eight foliar fungicides to manage southern rust of corn (caused by Puccinia polysora Underwood) was investigated over 4 years in three field experiments. Each experiment consisted of one-, two-, or three-fungicide application timings at tassel, milk, or dent growth stages with quinone outside inhibitor (QoI), demethylation inhibitor (DMI), or QoI + DMI fungicides. Each year trace amounts of southern rust were observed in the field at tassel, except in 2018, when rust was not observed until physiological maturity. Southern rust severity on ear leaf and two leaves above the ear leaf was approximately 50, 35, 75, and 0% at dent in 2015, 2016, 2017, and 2018, respectively. Applications that contained a QoI or QoI + DMI fungicide provided greater southern rust control than DMI fungicides, with little variation within fungicide classes. Applications of QoI or QoI + DMI fungicides applied at tassel provided greater disease control (52.5%) than those applied at milk (5.8%) or dent (1.4%), and greater yield protection (40.4%) than those applied at milk (23.7%) or dent (2.6%) when final rust development was severe (>40%). When rust development increased later in the season, after milk growth stage, a trend of better disease control was observed with fungicides applied at milk (57.8%) compared with tassel (35.2%), but grain yield protection was similar, with an average yield protection of 7.4%. There was no yield benefit with fungicides applied in the absence of disease or at the dent growth stage. Southern rust was most effectively managed with QoI or QoI + DMI fungicides applied at tassel when southern rust was present and environmental conditions favored rust development.