Background

Northern corn leaf blight (NCLB) is a ubiquitous foliar wilt disease that threatens maize production worldwide (Welz and Geiger, 2000). The disease is caused by the hemibiotrophic fungus Exserohilum turcicum (anamorph of Setosphaeria turcica), which favors a high-humidity and cool temperature environment. Under favorable conditions, fungal infection manifests itself as large and irregularly emerging lesions that destroy the entire foliage. Therefore, this disease decreases the active leaf area and the accumulation of photosynthesized products. Up to 50% grain yield loss was reported but the reduction largely depended on environmental parameters (e.g., temperature, humidity), phases of maize development and hybrid susceptibility (Ullstrup, 1970; Pataky et al., 1998).

Precision phenotyping for NCLB disease resistance is critical for the determination of host resistance against E. turcicum. Testing for disease resistance in the field is well established, e.g., by placing or distributing inoculums in the leaf whorl at the 4 to 6 leaf stage (or even older) plants (Dingerdissen et al., 1996; Lipps et al., 1997; Brown et al., 2001; Asea et al., 2009; Chung et al., 2010; Chung et al., 2011). Scoring for resistance can be conducted by determining the levels of susceptibility (1 to 9; 1, complete resistance, no symptoms; 9, 90-100% of leaf area infected), the primary diseased leaf area (PrimDLA) that was defined as the percentage of infected leaf area of the inoculated leaf, the diseased leaf area of the entire plant (DLA), the incubation period (IP) rated as the number of days post inoculation until first observing the wilting/lesion, the lesion number (LN) at 14 to 21 days post inoculation and finally the area under the disease progress curve (AUDPC). However, tests for resistance in the field are environmentally-dependent and time-consuming. Here we describe a simple greenhouse seedling assay by testing only the second leaf, thus being suitable for quantifying thousands of seedlings in a single experiment within 37 days.