Maize Genomics and Genetics 2025, Vol.16, No.2, 98-107 http://cropscipublisher.com/index.php/mgg 99 resistance genes, and also discussed the application of marker-assisted selection (MAS) technology in breeding, aiming to provide theoretical support for the development of a durable and effective disease resistance breeding strategy. 2 Pathogen and Epidemiology of Northern Corn Leaf Spot 2.1 Basic characteristics and infection mode of pathogenExserohilum turcicum During corn cultivation, long brown spots appear on leaves from time to time, which often reminds people of northern corn leaf spot (NCLB). The culprit is a fungus called Exserohilum turcicum, which belongs to the heterothallic gametosporic fungi. Although the spores of this fungus are slender and shaped like elongated spindles, and multiple septa (ranging from 2 to 12) can be seen on the surface, what really makes it difficult is its highly adaptable genetic "equipment". For example, a study pointed out that E. turcicum shows obvious polymorphism at a gene locus called ZmWAK-RLK1, which is related to the structure of corn cell walls and is particularly active during pathogen infection (Yang et al., 2021). However, just one key gene cannot explain everything - its entire genome has been sequenced, encoding about 13 000 proteins, which provides clues to understanding how it "uses its brain" to invade crops (Cao et al., 2020). It is worth mentioning that this fungus does not launch an attack immediately after landing on the leaves. It must first germinate on the surface of the leaves through spores, and then slowly penetrate into the tissue. After the infection is successful, it can generate spores again to form a secondary transmission. In other words, it can not only fire the "first shot", but also continue to create "recoil" (Navarro et al., 2023). However, corn is not completely passive. Resistance genes such as Ht1, Ht2, Ht3 and Htn1 can quickly induce reactions when facing invasion, such as accumulating reactive oxygen and adjusting photosynthesis efficiency (Yang et al., 2021; Navarro et al., 2023), although the intensity of the reaction varies from variety to variety. 2.2 Epidemic patterns and data differences in different production areas The spread speed and severity of NCLB actually depend to a large extent on "time, place and people". Environmental differences and variety differences in different corn-producing areas will cause the disease to show different rhythms. In China, field data over the years have shown that the development process of this disease can be traced. Generally speaking, the earliest incidence often determines the final severity. The earlier the problem occurs, the more difficult it is to control in the later stage (Liu et al., 2022). Moreover, it is not randomly distributed in the field, and often a few plants in one area are concentrated, suggesting that the local release of pathogenic spores is the key link. But looking overseas, things become more complicated. In Bihar, India and Malaysia, the strains of E. turcicum are different from those in China in appearance, growth rate and pathogenicity (Kutawa et al., 2017; Anwer et al., 2022). For example, there are obvious differences in colony color. Some strains grow fast, while others grow slowly. This may be due to genetic drift caused by different environmental selection pressures. South Africa has also conducted some hybrid disease resistance assessments. The results are not surprising: different genotypes show obvious differences, and some hybrids can maintain moderate resistance in areas with high incidence of NCLB (Mtyobile and Miya, 2023). This type of variety is particularly important in areas with severe diseases. After all, it is easy to change the environment and the cost of changing seeds is lower. 2.3 The impact of environmental factors such as humidity and temperature on the progression of the disease NCLB does not break out every year, and not every field will have problems. Environmental conditions are often "catalysts". Especially humidity and temperature - once the two are properly matched, E. turcicumis as active as a cheat. Research data from Beijing show that under high humidity and medium temperature conditions, the disease development speed is significantly accelerated, and large-scale expansion can be seen in a short period of time (Figure 1) (Liu et al., 2022).
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