A recent epidemic to hit the U.S. took place in 1970 and caused over $1 billion loss in corn. The fungus called Bipolaris maydis caused mass destruction in corn. It would produce dark, multicellular conidia that were easily spread to neighboring plants, allowing the epidemic to spread quickly.
In 1969, the disease southern corn leaf blight along with a few other foliar diseases of corn only attributed to 2.3% loss. But something happened in 1970. To fully understand what happened, it is necessary to look at the seed production of corn. Early in the 20th century breeders discovered that inbreeding corn did not increase yield, it actually decreased it. So how do they produce the corn seed? Each year they must carefully cross two inbred parental lines that are genetically different for many genes. To actually produce hybrid corn in this manner, the tassels that produce pollen at the top must be prevented from fertilizing the ears of corn on the same plant. This is done by detasseling or removing the tassels. Seed is harvested only from the rows with detasseled corn. This type of detasseling involved expensive hand labor and correct timing so as not to detassel after pollen was shed. A genetic factor was found that caused plants to become male-sterile. Plants from the female parental line no longer had to be detasseled. Since this factor was found associated with the cytoplasm it was termed Texas male-sterile cytoplasm (TMS). By 1970, nearly 80% of the hybrid field corn produced in the United States contained TMS cytoplasm. At the same time the pathogen Bipolaris maydis was undergoing a genetic change. A new race of the fungus was discovered and named Race T to differentiate it from the previously existing Race O. This new race was found to be particulary virulent on TMS corn. In many areas entire fields were lost and losses of 80-100% were common in others. What was different about this new race of B. maydis? The new race produced a toxin specific for the mitochondria of TMS-cytoplasm corn.
