Maize Genomics and Genetics 2025, Vol.16, No.2, 98-107 http://cropscipublisher.com/index.php/mgg 100 Figure 1 The relationship between the logit of the final Northern Corn Leaf Blight (NCLB) incidence and Julian date when NCLB incidence reaches 1.0% (estimated using a logistic model) (Adopted from Liu et al., 2022) Image caption: These 14 data points come from 12 commercial maize fields (300 plants per field) located in Yanqing District, Miyun District, Daxing District, and Haidian District of Beijing in 2017, as well as experimental fields at Shangzhuang Experimental Station in Beijing in 2016 (2,768 plants) and 2017 (3,385 plants) (Adopted from Liu et al., 2022) In Malaysia, experiments using different culture media to compare pathogens found that corn flour agar grew fastest, which indirectly confirmed the role of warm and humid environments in promoting its reproduction (Kutawa et al., 2017). Similar findings have been made in South Africa - some corn hybrids perform better in specific climates, which means that even if the genotype is the same, environmental conditions can greatly change the severity of the disease (Mtyobile and Miya, 2023). Therefore, choosing suitable varieties in different regions is actually negotiating with natural conditions. 3 Overview of Genetic Diversity of Corn Germplasm 3.1 Corn germplasm resources and their value in disease resistance research The germplasm resources of corn include a variety of genetic materials such as varieties, inbred lines and hybrids. These resources are very rich and very important. Although they seem to be just different types of corn, they are actually a genetic treasure trove that is particularly critical for studying disease resistance. Take the Northern Corn Leaf Spot (NCLB) as an example. Most of the alleles used to enhance disease resistance in breeding come from these germplasm resources. For example, the Htn1 gene that was introduced into modern corn breeding lines in the 1970s was originally discovered in local varieties in Mexico (Hurni et al., 2015). Of course, it is not just Htn1. In various corn populations, scientists have found many qualitative and quantitative disease resistance genes, which also shows the necessity of protecting and utilizing these resources (Welz and Geiger, 2000). However, traditional breeding alone is not enough. With the development of technology, genome-wide association studies (GWAS) and quantitative trait loci (QTL) mapping allow us to gain a deeper understanding of the genetic basis of disease resistance. For example, in tropical maize, GWAS found 22 SNP loci associated with NCLB resistance, indicating that these germplasm resources can indeed help us discover new disease resistance genes (Rashid et al., 2020). In addition, through QTL analysis of recombinant inbred lines, multiple QTLs for NCLB resistance were also identified (Chen et al., 2015), which makes the research and application of these resources more reliable. 3.2 Differences in genetic background of maize varieties around the world Maize is widely distributed, and the genetic backgrounds of varieties in different regions vary significantly, which also leads to differences in their disease resistance. For example, when studying nearly 1,500 maize inbred lines in Europe, multiple SNP markers associated with NCLB resistance were found, indicating that European maize has its own unique genetic background (Van Inghelandt et al., 2012). In contrast, tropical maize lines adapted to different agricultural environments in Asia show another set of genetic variation, and GWAS analysis has found haplotypes specifically associated with NCLB resistance (Rashid et al., 2020; Zhou and Liang, 2024).
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