Cropping Issues in Northwest Minnesota

A survey trip last week of soybean fields in Red Lake, Pennington, Marshall, Roseau and Kittson Counties found the soybean tentiform leafminer (STL), a pest that until recently was only known to feed on two native plants in the same plant family as soybean, American hog peanut and slickseed fuzzybean. The survey trip focused on examining soybeans growing near trees as it is thought that this may be where STL survives the winter.  Figure 1. The bottom of a soybean leaf with soybean tentiform leafminer mines on one leaflet.   While STL was found in Crookston at the Northwest Research & Outreach Center several weeks ago, during last week's survey, mines of STL ( Figure 1 ) were found not in soybeans growing near trees in field windbreaks or shelter belts, but rather only in two soybean fields that were immediately (within 5 feet or less) adjacent to larger wooded areas. Figure 2 shows what the top side of the leaf mines in Figure 1 look like; this did have the 'tented' o

Figure 1. Linear stem lesion and girdling of the stem, characteristic of Phytophthora stem rot. Photo courtesy of A. E. Dorrance; Reproduced, by permission, from Hartman, G. L., et al., eds. 2015. Compendium of Soybean Diseases and Pests. 5th ed. American Phytopathological Society, St. Paul, MN. Phytophthora root and stem rot In the last couple of weeks, I have visited a couple of soybean fields in northwest Minnesota in the Red River Valley that were suffering from Phytophthora root and stem rot (PRR). This particular disease is caused by a soil-borne, fungus-like organism ( Phytophthora sojae, Ps ) called an oomycete, that produces spores that swim in the soil toward soybean roots. The disease is favored by compacted, poorly drained and saturated soils, rain and warm temperatures.  PRR can occur as soon as the crop is planted, resulting in rotted seeds or seedlings. Other plants that are infected by Ps don't exhibit severe symptoms until later in the growing season. Symptoms char