Managing Diplodia ear rot in corn: Short and long-term solutions

Diplodia ear rot caused by Stenocarpella maydis is considered the most common ear rot pathogen in hybrid corn, and seed quality is compromised in infected ears. The absence of high levels of resistance makes it necessary to explore alternative options for management. The objectives of this study were i) determine the efficacy of fungicide applications for Diplodia ear rot and ii) identify and map resistance to Diplodia ear rot by screening a population of recombinant inbred lines (RIL). Field experiments were conducted during 2011 and 2012 at two locations in Indiana to determine efficacy of the fungicides Quilt Xcel® (azoxystrobin + propiconazole; Syngenta Crop Protection) and Proline® (prothioconazole; Bayer Crop Science) on disease severity. The experimental design was a split-plot with inoculation treatment as the whole plot, and fungicide application timing as the subplot. For plants receiving an inoculation treatment, S. maydis colonized sorghum was placed in the whorl at the vegetative growth stage V7. Fungicide treatments were applied at the label rate of each fungicide when plants were at the vegetative growth stage V6, tasseling/silking (R1), or kernel milk stage (R3). Results indicated that fungicide treatment did not significantly reduce disease severity for any application time, in either non-inoculated or inoculated plots in both 2011 and 2012. To identify potential quantitative trait loci (QTL) and molecular markers associated with resistance to Diplodia ear rot, a study was conducted near Lafayette, Indiana on the corn intermated B73 x Mo17 (IBM) population of recombinant inbred lines (RIL). This population was planted in 2010, 2011 and 2012. Each line was inoculated at the growth stage V7 with sterilized sorghum seed colonized with S. maydis. Analysis of the data detected a single QTL each year for 2010 and 2011. No QTL was detected in the 2012 due to adverse environment conditions that reduced ear production and population size. This study provides new information for corn producers about the efficacy of applying fungicides to control Diplodia ear rot. Although a QTL consistent over the three growing seasons was not identified, one line was identified as highly resistant and needs further evaluation. Also, enhancements in progeny type and population size need to be considered before establishing further QTL mapping studies with this IBM population