As we reach late August, we can begin to judge the severity of gray leaf spot and its impact on the corn crop. In general, Iowa seems to be fortunate this year because gray leaf spot severity is low in most areas and I expect that the overall impact on yields will be small. Unless disease severity is near 10 percent of the ear leaf at the dough stage, there is little chance of yield loss. I have seen only a few fields with severity of up to 25 percent of the ear leaf in the late-milk stage, and these fields will suffer yield loss. But most fields appear to have considerably less than 5 percent of the ear leaf diseased at this time, and are probably safe from measurable yield loss.
||Gray leaf spot symptoms.
Gray leaf spot tends to increase rapidly as the plants near maturity, and we have had some weather over the last month that was favorable for infection to occur. So we should expect to see significantly more symptoms develop during late August and September. But the later the symptoms appear, the less impact they have on yield.
||Stewart's disease symptoms.
Another leaf disease, Stewart's wilt, is making a very noticeable appearance in southern Iowa. In some fields, this disease will have greater impact than gray leaf spot. This disease is caused by a bacterium that is transmitted by the corn flea beetle. Flea beetle activity occurring throughout the season sometimes results in a significant amount of leaf blighting. This problem is not widespread, but some fields in southern Iowa will suffer yield losses due to Stewart's wilt this year.
This article originally appeared on page 169 of the IC-478(21) -- September 2, 1997 issue.