Evaluation of corn germplasm accessions for resistance to Clavibacter nebraskensis, causal agent of Goss's bacterial wilt and leaf blight.
Goss's bacterial wilt and leaf blight of corn (Zea mays), caused by Clavibacter nebraskensis, is a reemerging disease in the Midwestern United States. From 2011 to 2013, field studies and a greenhouse study were conducted to assess the University of Illinois maize inbred collection for putative sources of resistance to Goss's bacterial wilt and leaf blight. This inbred collection consisted of over 2,000 diverse inbred corn lines that have been collected from all over the world. An initial field screen of over 1,000 inbred lines from the collection was conducted in Urbana, IL in 2011. These lines were inoculated with a C. nebraskensis cell suspension and rated for Goss's bacterial wilt and leaf blight severity using a 1-to-9 scale, with a score of 1 being most resistant. Means for Goss's bacterial wilt and leaf blight ratings ranged from 1 to 8.5. The initial screen identified over 150 lines that had high levels of resistance (severity score of ≤2.5). In total, 177 lines were used in the second stage of field screening. In the second stage, average Goss's bacterial wilt and leaf blight severity ranged from 1.1 to 7.4. Nine lines with high levels of resistance in 2011 and 2012 were advanced to the third stage of field screening. The mean Goss's bacterial wilt and leaf blight severity rating of the resistant lines in the last stage was 1.9, while the susceptible check had a mean score of 6.4. These nine lines were also used in the greenhouse to assess whether resistance varied based on inoculating roots, stems, or leaves. Disease severity was significantly (P ≤ 0.05) less when roots were inoculated compared with both leaf and stem inoculations, which were not significantly different from each other. Lines having high levels of field resistance were also found to be resistant in greenhouse screening regardless of inoculation method. Clustering of pedigree distance of the 34 resistant lines (severity score of ≤2.5) with known pedigree information found that 21 clustered with the Lancaster heterotic family, 4 were related to the Iowa Stiff Stalk Synthetic family, and 9 did not cluster with an identifiable heterotic family. These results show that the Lancaster family is an excellent source of Goss's wilt resistance, and that fewer sources of resistance were found in other families. The most resistant lines identified from this research are potential sources of resistance to Goss's bacterial wilt and leaf blight, and their lineage can be used in corn breeding programs to develop resistant hybrids.