Weather and corn rusts

Distribution of the two main corn rust diseases, common rust, caused by Puccinia sorghi and southern rust, caused by Puccinia polysora, is closely related to origin of storms.  Both of these fungi are obligate parasites, producing spores only on living plants.  They have complex life cycles involving other spore types but the asexually produced urediniospores are the primary means of the spreading of the disease.
 
Puccinia sorghi, despite the Latin name, only infects corn (not sorghum), and thus temperate zone corn must receive urediniospores from where corn is grown in subtropical areas.  In the USA that is usually in Mexico.  Urediniospores are small single cells with a thick wall that delays desiccation when carried by high altitude winds.  Storms during month of April-June often originate in the Southwestern regions of the country, moving across Central corn belt states.  Urediniospores require about 6 hours of moisture to germinate and penetrate the corn leaf.  This is the environment of the corn leaf whorl during these months, allowing early establishment of the disease in a field.  More spores are formed from this initial infection within 7 days, allowing more local spread.  Timing of infection is significant because of continual moisture present in the whorl.  If storms carrying the spores arrived during the early corn growth period, further spread within the field is likely.  Late planted corn frequently has more vulnerability to the disease, but timing and origin of storms also is significant.  
 
Urediniospores are orange in color, but spores produced as the corn plant matures are rust pustules producing black teliospores.  These will not infect corn but only the alternate host, a tropical Oxalisspecies.  That process includes sexual reproduction allowing for meiosis and generation of new genetic combinations of rust.  That becomes the source of common rust races that overcome the single gene resistance used in corn.  The new races originate in the tropical areas where the rust fungus completes its life cycle, including recombination of its genetics.  The new races spread with storms, increasing as they overcome common single gene resistance.
 
Puccinia polysora has a higher temperature requirement for infection than Puccinia sorghi, but the spread to temperate zone is also affected by storms.  Although the alternate host for completion of its life cycle is unknown, genetic variability in the fungus suggests that it does exist.
 
Whereas many corn disease pathogens are simply dependent on moisture and humidity for disease development, the rusts are also depending on direction of early season storms to spread from tropical to temperate areas.