New corn disease distribution

​One of the more recent ‘new’ corn diseases was first noted in USA in 2016 is bacterial leaf streak caused by Xanthomonas vascicolapv vasculorum.  This disease was known in South Africa since 1949 but in its first year of identifying in the USA, it was seen in several counties of Nebraska and Iowa and Illinois.  There are indications that it was also seen in Argentina in that same year.  If it was distributed by seed from South Africa, why did it suddenly appear in so many places in one year?  This bacterium is a variant of a more common plant pathogen, Xanthomonas campestris.  A recent publication by U. Nebraska plant pathology extension
(https://cropwatch.unl.edu/2019/bacterial-leaf-streak-corn-nebraska) listed 16 common grasses in the areas that also host the bacterial leaf streak pathogen.    Did the outbreak develop as a mutant of another variant of this species, infecting grasses and building up sufficient intensity until brought to the attention of plant pathologists?  Perhaps continuous corn cropping, limited tillage and hybrid susceptibilities also contributed to the ‘sudden’ appearance of this corn disease.
 
It is frequent that we are surprised by a new distribution of a corn disease and analysis of the cause of the change is often difficult.  Genetic variation of pathogen, adaptation of new corn varieties, susceptibility of other hosts, distribution of insect vectors, change in environments including weather and farm culture practices are all potential contributors to distribution changes.  Movements of the pathogen can occur with wind, infected seed, grain shipments, equipment movements and even people clothing can be factors.  All of these potential contributors probably could account for the initial introduction of a new pathogen and generally goes without knowledge for a few seasons before intensity is sufficient to get attention.  After initial identification and understanding significance, breeders can select sufficient resistance to overcome the worst effects of the ‘new’ disease.
 
I think every corn disease that was identified in the last 47 years of my experience was not first found in only a single location but in multiple locations the same season of initial identification.