New corn diseases

​Every year there seems to be a newly found corn disease, or at least one that has been found for the first time in some corn-growing region.  Given the wide range of environments, corn genetics, pathogen genetics and changing seasons this should not surprise us.  Yet, when it occurs, everyone concerned with corn needs to try to understand the significance of the ‘new’ disease.
 
The 4th edition of Compendium of corn Diseases published by The American Phytopathological  Society in 2016, covers at least 100 corn diseases plus lists multiple pathogens for some of the diseases.  Many of these were prominent for a few years but changes in host genetics made them less significant only to later become notable again as susceptible genetics were used, or environments changed or the pathogen genetics changed. 
 
Goss’ wilt surprised us in the early 70’s, then became much less significant for most dent corn as resistant hybrids became common. But, during the last few years this disease has become damaging again.  Did the pathogen change or is this totally because of use of susceptible hybrids?  Gray leaf spot was known as insignificant until about 40 yrs. ago, and then only in the humid, foggy eastern USA valleys. A few years later, it became notable in the river valleys of the Midwest and then we discovered it likes the irrigated fields of the western Midwest.  On the other hand, sorghum downy mildew had a spurt of attention in the 70’s in which It overwintered on Johnson grass, increased on sorghum and then went to corn in areas of Missouri and Kansas and south.  Better herbicide control of the Johnson grass broke the cycle, greatly limiting the destruction from this disease.
 
It is probable that new corn disease dynamics are occurring somewhere each year but undiscovered until the disease increases to an extent that it is noticeable and spread over some area.  Recently there were announcements concerning bacterial leaf streak, caused by Xanthomonas vasicola pv. vasculorum, occurring in Midwestern fields. Early symptoms could be confused with gray leaf spot and thus far it has not been identified with damage to performance to the crop.  We do not know if it will be damaging, how it spreads (it is not believed to be seed borne), does it have other corn belt hosts (it is also found on sugar cane in Africa), is it attacking only some genetics or is it favored by certain environments.  It was first reported in 2015 in Nebraska.  Public and private researchers are trying to learn more about this disease and it is probable that as the dynamics are understood, this disease will join the many others that occur but are insignificant to corn production. The first role for a corn pathologist involved with a new corn disease is to identify the pathogen but the most important, and most difficult, is to predict the significance of the disease.