Bacterial Leaf Streak of corn

​A corn disease only known to be in Africa since first discovered in 1949 was found in Nebraska in 2016 but by 2017 scattered fields in several USA Midwestern states. It was found in Argentina in 2017. The bacterium, currently named Xanthomonas vasicola pv. vasculorum (previously Xanthomonas campestris pv. zeae) can exist in dead infected leaves not only of corn but perhaps several other grasses.  It is believed to spread to corn leaves in infected debris at least as young as V4 stage, probably during storms.  From there, bacteria apparently enter to leaf through stomata.  The bacteria move between cells reaching the parenchyma cells surrounding xylem cells of the smaller leaf veins. Receiving nutrition from the parenchyma cells and then multiplication in the xylem tubes, plugging these tissues results in dead, elongated streaks in the leaf tissues. These lesions tend to be limited laterally by large leaf veins and the plant resistance system eventually limits the length of the streak as well.  It is probable that varieties differ in how quickly and completely the resistance system inhibits the expansion of the pathogen but it appears to not be significantly affecting grain yields on most hybrids.  More study will better identify differences in resistance.
 
This disease seems to be only the most recent example of unpredictable occurrence of a corn pathogen.  Did it arrive from South Africa or was it a mutated form of a bacterium that was already present on native grasses in the USA, adapting to vulnerable host genetics inadvertently bred into some corn hybrids? The wide and seemingly sudden wide distribution could possibly be explained by carrying with seed or even with ‘dust’ within grain. Wind, vehicles, ships, and people could move infected leaf debris. Proving that seed is the primary source is difficult because of the problem of adequately sampling.
 
A few other bacterial pathogens of corn such as cause of chocolate spot of corn (Pseudomonas syringae pv coronafaciens) and Goss Wilt appeared unexpectedly and fungal races such as race t of Biploaris maydis, race 1 of Bipolaris carbonum as well as several races of Exserohilum turcicum suddenly were found.  We should expect this phenomenon to continue.