About 5-6 weeks after pollination is a good time to evaluate corn hybrids for resistance to leaf disease. This Corn Journal blog from 2017 discusses some of the dynamics in resistance to leaf diseases.
Leaf epidermal cell’s walls and the waxy leaf surface provide the first line of defense against microbes. Pathogens adapted to overcoming this defense set off the next defense system after penetrating the leaf. This is initiated by the plant detecting the presence of the intruder. Plant cells nearby detect the presence of a protein exuded by the pathogen. Such proteins are called effectors, as they are detected chemically by host cells near the invader. Upon detection, these adjacent host cells produce potential microbe-inhibiting compounds such as reactive oxygen, nitric oxide, specific enzymes, salicylic acid and other hormones to effectively thwart the pathogen growth. Much initial reaction is limited to host cells adjacent to the infection site.
Resistance to corn leaf pathogens such as Exserohilum turcicum, cause of northern leaf blight, Cercospora zeae-maydis (gray leaf spot) and Bipolaris maydis (southern corn leaf blight)
Involve detection of that specific pathogen and production of more general antimicrobial products in the immediate area of the pathogen. These two steps are inherited independently. Perhaps the pathogen detection system is more specific to the pathogen, accounting for a corn variety being more resistant to one pathogen than another. On the other hand, I am suspicious that if two pathogens arrive in the same area of the plant, only one will survive, as if the plant reacts to the first one by producing general resistance compound that inhibit the infection by the second one to arrive in the same area.
The system described above is referred to as general or horizontal resistance. It is controlled by 3-5 genes for products to detect and reduce spread of the pathogen. Horizontal resistance is expressed in corn plants by fewer leaf disease lesions. Evaluation of varieties for this type of lesion has some ambiguity however, because the number of lesions or amount of leaf damage is also affected by the intensity of disease pressure. Heavily diseased leaves from the previous season in fields of low tillage, with frequent early season rain can result in more leaf lesions in a variety of good general resistance to a pathogen than will occur in one of poor resistance with little disease pressure.
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The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.