Corn, like most plants, have two basic types of disease resistance systems. Quantitative, also called horizontal, resistance limits the spread of the pathogen by limiting the number and size of lesions. Qualitative resistance, also called vertical resistance, drastically limits the pathogen, often with sacrifice of the infected cells.
Quantitative resistance is initiated by detection of a general feature of the invading pathogen, such as the chitin component of a fungus like Exserohilum turcicum (cause of northern corn leaf blight). Chemical detection of the pathogen triggers production somewhat general antimicrobial enzymes in the area close to the invasion, limiting the pathogen. Quick detection and response results in fewer and smaller lesions. Relative differences for this resistance can be evaluated when varieties are compared when placed under the same pathogen pressure. This evaluation is usually done on a numerical scale ranging from very susceptible to very resistant. These ratings are always dependent upon equal exposure to the pathogen. Because reactions by the plant are metabolic processes that tend to decrease as the corn plant approaches completion of grain fill, relative maturity can be a factor in evaluating accurately comparing genotypes. Quantitative resistance ratings are not absolute but carry some relativity to disease pressure.
Quantitative resistance is usually affected by several genes in the host plant. This allows stability against races of the pathogen but also becomes somewhat awkward for the corn breeder to use in the breeding process because of the necessity of constant disease pressure.
Qualitative breeding is more attractive because it usually involves a single gene and it is more absolute- present or not. This gene produces a protein that responds to a specific protein in the pathogen, triggering a quick host cell that is different than that of quantitative resistance. For example, the Ht1 gene in corn, initiate stops the fungus (E. turcicum) from producing spores and further damaging the leaf. This single gene was attractive to breeders because of the ease of selection and strong control of the pathogen. Unfortunately, like in most cases of qualitative resistance, the single gene for resistance is matched with a variant of the fungus that no longer produces that specific protein detected by the host gene. Ht1 gene was used in much of the commercial corn hybrids during the 70’s but is of limited value now because of the increased frequency of the ‘race’ or pathotype not detected by Ht1 product. This is a common experience with other diseases as well and represents the plant breeder’s conundrum of balancing the expediency of breeding for qualitative resistance versus the probable short life of effective resistance.
The battle of corn genetic variability and pathogen genetic variability continues.
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About Corn Journal
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.