Crops Put In a 4-Day Work Week

July 21, 2009
ICM News

By Rich Pope, Department of Plant Pathology

For organisms, heat drives development.  If you can regulate your own heat like we humans do every day is a balmy 98.6 or so, so development every day is the same and can be measured by the calendar.  But for our field crops and most of the pests they face, development is based on the heat they get from the environment.  And, less heat means slower growth.  For the week of July 12--19 alone, Iowa normally accumulates 200 base-50 degree days  as a statewide average.  Last week that accumulation was 133,  or about four and a half "normal" days for the week. So, for the season since May 1, we are now 143 to 205 degree days behind normal. 

Iowa degree day accumulations through July 19, 2009

So what does that mean? Here are just a few fine points for discussion at field edges and coffeeshops:

  • Will soybeans be damaged by the cold during flowering?

Pod set is determined in part by biomass accumulation, which is slowed by the cold. But soybean has a great ability to flower over long periods, and warmth in the next 2--3 weeks should allow for ample pod set. We will start to have more concern if it gets to August and we are still unusually cool, but even 80-degree highs and 60-degree lows will help. 

  • What diseases might become issues?

Some diseases are more prevalent in the heat, some in cooler weather. Conditions may favor downy mildew on soybean in lowlying areas, and eyespot on corn is a concern in some areas, while southwest and south central Iowa have cornfields with significant gray leaf spot infections.

  • What about all the weeds?

Soybeans are growing slowly, and late to canopy. That allows light penetration and encourages weed growth. Add that to fields where weed control programs were delayed and weed pressures have built up, means that 2009 may be remembered as a year of the weeds.

  • It is an odd year, so does that mean soybean aphids are a huge concern?

So far, soybean aphids have been reported in some fields in the northern half of Iowa, but not genearlly at threshold levels. Soybean aphids bear watching because populations can grow rapidly with favorable conditions, which includes cool temperatures.

Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at or by calling (515) 294-5899.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 21, 2009. The information contained within may not be the most current and accurate depending on when it is accessed.