Two corn diseases were reported in 2015 as basically new to USA Midwest. Tar spot, reported in Illinois and Indiana is caused by a fungus, Phyllachora maydis and is known from its occurrence in Mexico. The other one called bacterial leaf stripe, caused by Burkholdia andropogonis, was noted in the 70’s in Nebraska. Neither disease appeared to be damaging but were noticeable enough to be sent to State Extension Pathologists for identity. Physoderma brown spot also was more easily found last summer in the Midwest than the past many years.
When new diseases are first found there is a good chance that they were present previously but not noticed at least by the right people to realize it was significant. Large fields, many acres per farm operation, high plant densities make it obvious that only a few infected plants will go unseen. But this is a reminder of the importance of getting diagnosis done by experts that have the knowledge and methods to give an accurate identification.
One of the most important roles for plant pathologists working for a company or government institution is to access the significance of a new occurrence of a disease. Was it because of a change in weather, crop culture practice, host genetics or pathogen genetics? Is it a future threat that needs to be addressed or simply a rare, non-damaging occurrence?
Within my 45 years of working with corn diseases we have seen several ‘new’ diseases. I initially thought gray leaf spot would remain mostly in the valleys of North Carolina and perhaps in humid areas along rivers. Missed that one as it turned out that the fungus enjoyed some of the newer genetics of the late 70’s and general humidity of the Midwest including those irrigated fields in western corn belt. When Corn Lethal Necrosis was first seen in Nebraska and realized that one of the viruses was transmitted by corn root worm beetles and the other virus was either wheat streak virus vectored by the wheat curl mite or sugar cane virus that virtually gets spread throughout the corn belt later in the season by aphids, I thought the disease would spread. I won’t confess to all of my errant predictions but only state that it is really difficult to understand all the variables involved when an unexpected occurrence of a disease first is present.
It is very important that everyone is aware when seeing unusual symptoms that they get reported to experts, including the seed producer and corn pathologists. A corn disease working group composed of state, federal and private company corn pathologists meet annually to discuss diseases noted the previous year as they attempt to predict significance.
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.