Plant breeders have noted that corn inbreds and hybrids shovv marked differences in their ability to withstand even mild periods of drouth and high temperatures. Sweet corn inbreds and hybrids as a group do not have as high resistance as field corn in breds and hybrids. The most resistant sweet corn lines are not equal to the most resistant field corn lines.

Since 1929, in central Iowa, it has been comparatively easy to classify sweet corn inbreds in the field according to their relative resistance. Less than normal rainfall and higher than normal temperatures during a part of the growing season each year have made possible the classification of the lines into one or more of the following four groups: Those which are subject to (a) tassel firing, (b) firing of the top two or three leaves, (c) firing of the lowest three or four leaves and (d) excessive rolling and wilting of the Ieaves. Plants of lines which are classed in group (d) roll their leaves during periods of high temperature and drouth to the extent that the leaves may fail to unroll entirely when conditions are more favorable. Stunting or death may result.