Emerging issues in the U.S. Corn Belt: infection of corn by Clavibacter michiganensis subsp. nebraskensis and use of foliar fungicides to protect yield

Obura, Salome
Major Professor
Alison E. Robertson
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Plant Pathology and Microbiology

Currently two major issues pertaining to disease management in corn production in the US Corn Belt are the spread of Goss's wilt and leaf blight caused by Clavibacter michiganensis subsp. nebraskensis (Cmn), and the use of foliar fungicides on corn for `plant health' benefits. This dissertation presents data regarding infection of corn by Cmn, and fungicide effects on disease management and yield.

Greenhouse and laboratory research was undertaken to determine if infection of corn by Cmn may occur without severe wounding, such as that caused by hail, sand blasting or wind damage. Corn plants with no obvious wounding were spray inoculated with a suspension of Cmn (~108 cfu/ml) and compared to plants that were wounded and spray inoculated. The incidence of Goss's wilt and leaf blight on non-wounded inoculated plants was 37.0% and 60.0% under ambient and increased humidity conditions, respectively compared to 100% incidence on plants that were wounded and inoculated with the pathogen. Leaf wash assays showed populations of epiphytic Cmn survived and increased over time on corn leaves. Fluorescence and scanning electron microscopy (SEM) revealed that Cmn colonized leaves in localized sites including the junctions between epidermal cells, cuticle depressions, in and around stomata, and at the bases of glandular trichomes. Furthermore, single cells and aggregates of Cmn were observed within substomatal chambers using SEM. We propose that sites of epiphytic Cmn localization such as stomata and trichome bases may serve as infection courts for Cmn in the absence of severe wounding.

Field research was conducted from2010 to 2012 at multiple locations in the Corn Belt to assess the effects of fungicide application timing on disease development and yield of hybrids that varied in susceptibility to Cercospora zeae-maydis (Czm) and Setosphaera turcica (St), cause gray leaf spot (GLS) and northern corn leaf blight (NCLB), respectively. A standard experimental design and set of treatments (hybrid, multiple pathogen inoculation types and timing and fungicide applications timing) were used in all four states (Illinois, Iowa, Ohio and Wisconsin). Generally foliar disease severity was low, final severities in the non-sprayed control were less than 5% at all site years except for Illinois in 2010 and 2011 (disease severity ranged from16 - 22%). Fungicide application (VT/ R1) significantly reduced disease severity relative to the control in five of eight site years (P <0.05). Disease was reduced by 30% at Wisconsin in 2011, by approximately 20% at Illinois in 2010, 29% at Iowa in 2010, and 32% and 30% at Ohio in 2010 and 2012, respectively. The untreated control had significantly lower yield (P <0.05) than the fungicide-treated in three site-years. Fungicide application increased the yield by approximately 6% at Ohio in 2010, 5% at Wisconsin in 2010 and 6% in 2011. Yield differences ranged from 30- 550 kg /ha and could not consistently be attributed to benefits related to physiological processes enhanced by the use of fungicides. Results suggest susceptibility to disease and prevailing environment are important drivers of observed differences among treatments and sites.

A case study teaching resource that communicates a grower's decision-making process when considering the application of foliar fungicides in corn was developed and presented to students at Iowa State University in an introductory plant pathology course. The case study presentation is designed for a 50 minute class session, using both a small group and whole-class review and discussion style. Students were also given the opportunity to write a 140 character "tweet-like" statement, comment or question for an expert. Tweeting enhanced discussion and enabled the instructors to moderate discussion, connect student ideas to the lesson and inform the entire class on issues raised related to corn production in the US Corn Belt such as scouting, fungicide application, break even prices and physiological processes enhanced by fungicides. More than 80% of the students appreciated the "real life scenario" approach and felt it enhanced their appreciation of the issue.