work in cross breeding. In 1908, George Harrison Shull described heterosis, also known as hybrid vigor. Heterosis describes the tendency of the progeny of a specific cross to outperform both parents. In 1917, a process was developed that would make this hybridization commercially viable. In 1933, less than 1% of the corn produced in the United States was produced from hybrid seed; by 1944, over 83% was. This hybrid seed is produced by crossing two inbred lines by planting a row of one inbred variety followed by several rows of a second variety. The tassels of the second variety were removed by hand so that the second variety could be pollinated by the first. Late 20th century Hybrid corn was detasseled manually until the mid-1950s, when a cytoplasm was discovered that would cause one of the inbred lines to be male sterile while the hybridized seed corn it produced would regain male fertility. This gene allowed seed corn companies to greatly reduce their labor costs by producing seed corn without the need for manual detasseling. By the mid-1960s, nearly all seed corn was produced with this gene. This situation changed in 1971 with an outbreak of the fungus southern corn leaf blight. The cytoplasm used to produce male sterility was highly susceptible to this fungus. At the time, approximately 90% of hybrid corn used in the United States contained this gene. About 15% of the corn crop was lost to infection, and for the next few years, male sterility was abandoned and nearly all seed corn was again detasseled manually. In the mid-1970s, machines were developed to help reduce the large labor costs associated with manual detasseling and as a response to a shrinking rural teen labor force. In the 1980s, male sterile varieties that were not susceptible to southern corn leaf blight were reintroduced; however, the reliance on a single sterile variety seen in the 1960s has not been repeated. Today, corn hybridization is accomplished by a combination of machine and manual detasseling as well as male-sterile genes. Seed corn fields Seed corn fields are planted in a repetitive pattern known as a "panel", "block" or "set", depending on the area of the country. There are two main planting patterns for these panels. A panel may be planted in a 6:2 pattern, where six "female" rows, the rows to be detasseled, are followed by two "male" or "bull" rows,