Background

Southern leaf blight (SLB), caused by Cochliobolus heterostrophus (Drechs.) Drechs. [anamorph = Bipolaris maydis (Nisikado) Shoemaker], is a widespread maize disease which causes significant yield losses in hot, humid tropical and sub-tropical regions, such as the southeastern USA, parts of India, Africa, Latin America and Southern Europe. In 1970-71 an SLB epidemic caused by C. heterostrophus race T infecting hybrids carrying Texas male-sterile cytoplasm (cms-T) caused an estimated 15% loss in total maize production in the US (Ullstrup, 1972). After the 1970 epidemic, cms-T maize was replaced by race T-resistant, normal cytoplasm maize.

Currently, race O is the predominant cause of SLB in the US and worldwide (Wang et al., 2017). SLB resistance to C. heterostrophus race O is quantitatively inherited with primarily additive or partially dominant gene action (Holley and Goodman, 1989). Under experimental conditions, yield losses due to infection with C. heterostrophus race O as high as 46% have been observed (Fisher et al., 1976; Byrnes and Pataky, 1989). However, losses in commercial production are generally much less severe (Mueller et al., 2016).

This approach to inoculation and rating is based on methodology developed by Carson et al. (2004), though similar methods had been used in numerous previous studies (e.g., Fisher et al., 1976). We have used it in a number of studies to screen germplasm for SLB resistance and to elucidate its genetic basis (Balint-Kurti et al., 2006; 2007 and 2008b; Zwonitzer et al., 2009 and 2010; Kump et al., 2011; Negeri et al., 2011; Belcher et al., 2012; Santa-Cruz et al., 2014; Yang et al., 2017). We have also used an essentially identical method to assess resistance to two other foliar fungal diseases; Gray leaf spot caused by Cercospora zeae-maydis (e.g., Balint-Kurti et al., 2008a) and northern leaf blight (NLB) caused by Exserohilum turcicum (e.g., Balint-Kurti et al., 2010; Chung et al., 2010; Zwonitzer et al., 2010). This method has provided reliable data with high correlations between replications and environments.