Welcome rains benefit crop development

By David Nicolai, University of Minnesota Extension

Most of the drier areas received a welcome rain. This is important as we go into the critical seed-set and pod-fill periods. Some areas with heavy rain, wind and hail have root-lodged and above ground-lodged corn. Poor root systems can be caused by several reasons: rootworm feeding, genetics, disease and poor seedbed conditions are equally likely. Rootworm Bt hybrids only protect against one of these. Corn has an easier time righting itself when lodging occurs before tasseling.

The USDA National Ag Statistics Service and the Minnesota Department of Agriculture reported the field crops summary as of Sunday, July 20th:

The average corn height was estimated at 59 inches compared to 78 inches last year and 69 inches for the 5-year average. Seven percent of corn was silking compared to 82 percent last year and 52-percent average.

The average height of soybeans was 17 inches compared to 22 inches last year and 20 inches for the 5-year average. Fifty percent of soybeans were blooming compared to 77 percent last year and 68 percent average.

Spring wheat was 23 percent ripening compared to 66 percent last year and 45-percent average. Barley was 18 percent ripening compared to 80 percent last year and 54-percent average. Oats were 36 percent ripening compared to 87 percent last year and 67-percent average. One percent of oats were harvested compared to 20 percent last year and 8 percent average.

As of Friday, July 18, 69 percent of corn and soybeans were rated in good or excellent condition. Eighty-four percent of spring wheat, 77 percent of barley, and 80 percent of oats were rated in good or excellent condition.

Soybean aphid numbers continued to increase last week. Bruce Potter, integrated pest management specialist from the Southwest Research and Outreach Center at Lamberton reports that localized problem fields appear to be widely distributed through western Minnesota. Several consultants have reported threshold numbers in fields that typically have early aphid populations (e. g. fields near rivers, creeks, lakes and other areas with buckthorn concentration) in south central, southwestern and west central Minnesota. Most have reported that larger fields, away from buckthorn, typically have low populations. Again, this is not unusual.

At this time, economic levels of soybean aphid are not widespread. Fortunately, Bruce reports that there are not any large geographic areas in the upper Midwest with a high incidence of heavily infested fields to supply aphids downwind. Unfortunately, the wide geographic area with scattered fields near threshold may make this year’s problem less predictable than others.

While the fields examined have a less clear edge effect than previous years, it is still there. Most aphids are concentrated on the upper foliage of these early reproductive stage soybeans. This will change as new leaf development ceases and pod fill begins (R4-5). Extreme heat, heavy rainfall, and wind/hail can lead to aphids using the lower portion of early reproductive soybean canopy to a greater extent.

Bruce expects that aphid populations will increase more quickly in areas with less rainfall or in fields that hold less moisture. On a calendar basis, southwest Minnesota soybeans are behind in development but aphid populations are tracking similar to 2006. Hopefully, the recent rains and some moderate temperatures can help generate some fungal controls in more heavily infested fields.

Remember that the economic threshold for treatment of 250 aphids per plant, when 80 percent of plants in the field have aphids and populations are growing, is still valid, so field scouting is an important management practice.

(David Nicolai is a crops educator with University of Minnesota Regional Extension in Hutchinson.)