Last few posts have described only a few of the disease surprises in the 1970s and 1980s. Several others have occurred, some showing up for a year or two and then becoming less noticeable. The trend to new occurrences has continued. Bacterial leaf streak, caused by Xanthomonas vasicola, pv vasculorumwas initially found in Nebraska and then in several other midwestern states in USA in 2016. It was only known to occur in South Africa previously. It also was found in Argentina in 2017 but perhaps was there since 2010.
Physodermabrown spot showed up in the scattered areas of the US corn belt in 2017. Although it was known to occur sporadically in southern USA, it showed up in our small nursery in northern Illinois on a few plants.
Tar spot of corn, caused by an obligate parasite (Phyllochora maydis) and usually accompanied by another fungus, Monographella maydis, showed up in Northern Illinois and Southern Wisconsin in 2015/2016 and with more intensity in 2018. It had been known previously in highland areas of South America. There is more to learn about these pathogens, including how they live through the winter.
Were these pathogens in an area long before being identified? Was it only a matter of time before the disease was noticed? Did they spread by wind or seed? We know that spores of Puccinia sorghi, cause of common rust, spread the disease from South Texas of Mexico by wind annually. Corn kernels can easily carry fungal spores whether used as seed or as grain.
Perhaps these corn pathogens were infecting another grass species but a mutation in the pathogen allowed infection of corn. The Physodermain in my nursery was in an outside row near other grasses. Xanthomonas vasicolahas related variants that infect sugar cane and perhaps other grasses. Tar spot spread to northern Illinois is very hard to explain, except the similarity with the environments of highlands in South America. Or were these present here but insignificant and thus unnoticed until susceptible corn genotypes became widespread. Or perhaps susceptible hybrids, lack of crop rotation and minimal tilling allowed increase of the pathogens. It may be a combination of multiple factors.
The Compendium of Corn Diseases, 4th edition, published in 2016 lists more than 70 corn diseases. Many are minor causes of significant damage, at least currently. Unexpected environmental changes, including those related to climate change or inadvertent mutations in corn breeding may result in changes in significance of a corn disease.
We should expect seeing ‘new’ occurrences of diseases to occur probably everywhere corn is grown. Inspecting and reporting to corn disease specialists observation of unusual symptoms in corn fields each season is important to avoiding significant damage by ‘new’ corn diseases.
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.