Fungi, bacteria and viruses have constant genetic mutations and evolution favor those that adapt to potential food sources. This pathogen diversity must be matched with diversity in corn to avoid crop losses. The list of new, unexpected occurrences in USA and internationally seem to occur every few years. Probably we should not be surprised. As we attempt to improve the hybrids for yield, standability, performance under changing environments we can also inadvertently and unknowingly include genes for susceptibility. Especially vulnerable are genes that allow the recognition of the invader and therefore quick defense response. Furthermore, a corn hybrid that may work well, and not express susceptibility to local pathogens, but when moved to another environment, a pathogen reacts differently.
Microbes also have genes that vary from mutations and sexual recombination. Rapid production of huge numbers of spores, ability to infect multiple hosts often near corn field, and broad, widespread populations of these pathogens is their strength. New variants of the pathogen, adapted to at least a few current hybrids with higher level of pathogenicity, must initially show only in isolated spots in corn fields and easily overlooked. History nearly all ‘new’ races in the USA were not noted until they were widespread. My guess is the genetic variant allowing for the new race (pathotype) was infrequent but present in the pathogen population for some time but was not recognized until damage was common. The more recent occurrence of the bacterial streak of corn, caused by Xanthomonas varicola pv. vasculorum, previously known only in South Africa but within only a few years it was been identified in several US states and Argentina. Was it here for a long time, or distributed by seed or even grain debris? Perhaps it has been a long-time pathogen of other grasses. Or perhaps is a mutant of a related bacterium and we inadvertently selected for susceptibility in corn.
It is doubtful that the battle between new pathogen variants and corn will end. Our best protection must come from careful observations in corn fields and submitting suspicious samples to appropriate specialists for identification. Corn genetic diversity has always allowed selection of resistance, but it does take a few years to implement the hybrid seed production before serious damage to the crop.
The fun never stops!
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.