New corn disease outbreaks

​Gray leaf spot, caused by the fungus Cercospora zeae-maydisin USA, was identified on corn in 1925, but was notable in the 1970’s.  The fungus is favored by humid environments and susceptible hosts.  Backgrounds that featured B73 was commonly associated with susceptibility that was intensified if the other parent of a hybrid was also susceptible.  A few very successful hybrids in terms of other desirable features like grain yield and stalk quality were driven from the commercial market by this disease as it spread through much of the central corn belt.  Emergence, spread and significance of this disease was a surprise to most in the corn industry.
 
Maize chlorotic mottle virus (MCMV) was first identified as a corn pathogen in Peru in 1974.  In 1976 it was associated with severe damage to corn in Kansas and Nebraska when plants were also infected with another virus such as MDMV or wheat streak mosaic virus (WSMV).  Since then it has also been identified in corn South America, Asia and Africa.  Disease caused by MCMV and either of these other virus is called maize lethal necrosis or corn lethal necrosis.  Most commonly used genotypes are susceptible, but resistance can be found with effort.  MCMV is transmitted by insects such as beetles and thrips.  MDMV is spread by aphids and WSMV by the wheat leaf curl mite.  MCMV can remain in beetle larvae overwinter and transmitted to young corn seedlings by rootworm feeding.  Most damaging affect on corn happens when the other viruses are also infecting corn at very young development stage.
 
Head smut of corn, caused by the fungus Sphacelotheca reiliana, has caused damage to corn erratically in North, Central and South America, Australia, China, Europe and South Africa.  The fungus teliospores commonly are spread to the soil, where they germinate and infect seedlings.  The mycelium grows within the plant towards the floral tissue, ultimately replacing the ovules and pollen with fungal tissue including more teliospores.  It is commonly associated with susceptible genotypes, continuous corn, light and dry soils.
 
We get surprised continually with outbreaks of corn diseases.  There is no reason to think that this pattern will change.