Improvement of Sweet Corn

L HE only genetic difference between the composition of a sweet corn kernel and that of field corn is due to a single recessive gene out of the hundreds or thousands of genes in corn. This gene makes the sugary condition of the kernel persist in sweet corn; or, in other words, prevents the conversion of some of the sugar into starch. The first recorded observation of sweet corn by w^hite men appeared in 1779, when Lt. Richard Bagnall, of Sullivan's Indian Expedition, returned from an Indian campaign to the west of the Susquehanna with several sugary kerneled ears procured from the natives. We know today that the Iroquois Indians cultivated at least two sweet varieties, one white and one black—similar to Black Mexican—and that the Indians of the upper Missouri included four sweet corns among the 104 corn varieties they cultivated (4S)^ In 1828 Thorburn's seed catalog listed a single sweet corn variety, and by 1881 the number of advertised varieties in all seed catalogs had increased to 16. Today, by reason of an increased interest in the production of hybrid sweet corn, it would be difficult to estimate with any accuracy the number of sweet corn varieties and stocks on sale, but it must run into many hundreds. Considering its popularity, it may be surprising to some that the region of sweet corn cultivation is confined practically to southern Canada and the northern half of the United States, There are three reasons for this restricted distribution. (1) The period during which sweet corn kernels remain sweet after picking is of very short duration, and at the higher temperatures farther south the sugars are rapidly converted to starch. (2) The corn earworm, Heliothis obsoleta Fabr., is less injurious or is absent only in latitudes with winter temperatures low enough to prevent winter pupation of the larvae. Earworm injury in the Southern States is so great that the main dependence for roasting ears is the naturally more resistant field corn, including such varieties as Trucker's Favorite, Mexican June, and Tuxpan. These varieties, together with practically all the field corns of the South, have become relatively resistant to the corn earworm through natural selection extending over a long period. This fact is being used to good advantage (S8) in converting some of the leading southern dents into highly resistant sweet corns by appropriate crosses. The third reason is much like the second, but in this case the condition is bacterial wilt instead of an insect. This disease is especially severe in the South,