The cool weather has been ideal for soybean aphids, and we’ve seen populations explode from nothing in July to well over the economic threshold of 250 aphids per plant in many area soybean fields.

The earliest planted soybeans have now reached the R5.5 stage where we can stop spraying, but most fields have a few days to a week plus to go before they reach that stage. One early planted Asian soybean rust sentinel plot that I’ve been scouting in Iowa County had a population of 9 aphids per plant on August 5, 69 on August 11, 384 on August 18, when the beans reached R5.5, and 998 aphids/plant on August 25. Many other fields have also shown this rapid increase, with populations doubling every few days.

Populations have been especially high in late planted fields. As long as aphids are found on most plants and numbers are 250/plant or more and increasing, an insecticide may pay until after soybeans reach the R5.5 stage. This is a few days before seeds are full size in the pods on the upper four nodes. Remember to check pre harvest intervals on the insecticide labels. Sudden death syndrome is fairly common again this year in the area, and some white mold is showing up as well.

Most corn fields are in the milk to dough stage. At a nitrogen management plot tour conducted last week in Linn County it was evident that corn yields will likely suffer more where all of the nitrogen was fall applied. Corn in the plots and in area fields are showing nitrogen deficiency symptoms, with symptoms worse where the N was fall applied.

Although some corn leaf diseases, including gray leaf spot and common rust, are showing up in the area, there have not been widespread problems yet. There have been scattered reports of southern corn rust in the area, which is a much more serious disease than common rust. For pictures of common and southern rust, see this ICM News article.