OK, since you survived the blog on glyphosate yellowing, you may recall that we talked about discussing some of the other issues that may be causing yellow areas in our bean fields.

The entry would be way too long (and potentially boring…) if we got real in-depth on each one, so you’re going to get the cliff notes version. We like to refer folks to the ICM News site, our ISUE Field Agronomists, and their Local Dealers for more info, using this blog as an appetizer.

Sudden Death Syndrome (SDS) has is showing up in a few spots our area fields. Early planted fields are at highest risk for infection (I know, we didn’t have a lot of those), and the disease tends to like highly productive soils and/or soils that tend to be on the wet side. X.B. Yang (ISU Plant Pathologist) describes symptoms as scattered yellow spots between leaf veins, that often eventually coalesce (my big word of the week) to form brown streaks between the veins (interveinal necrosis)(ok, two more big words…). Only the mid vein and major lateral veins remain green. Leaflets drop eventually, but petioles often remain on the bean plant. Diseased plants have deteriorated taproots and lateral roots, with the root cortex is light gray to brown, and the discoloration may extend up into the stem a couple inches above ground. Sometimes bluish fungal colonies can be seen on the root if soil moisture is high.

Dr. Yang hits on SDS and some other diseases in this ICM News article.

Soybean Cyst Nematode (SCN) infestations are responsible for many stunted and yellowing areas as well. Dr. Greg Tylka tells us that SCN can often be found on plant roots from flowering on throughout the season, and digging plants to check for cysts on roots is a good technique to determine whether cyst nematodes are present (they can get harder to find as we get close to soybean maturity). Examination of plant roots for presence of cysts can help determine presence or absence of SCN, but we still recommend soil sampling post harvest to be sure and help quantify the level. If you have some of these yellow/early maturing/ stunted areas, you may want to have your dealer sample them now since they are easy to find, or set out a couple of flags in these areas (use 3 or 4, as a couple of them will lose the “flag” part to the cutter deck), or set some GPS waypoints to help your dealer find them later.

Read more from Dr. Tylka here

Iron Chlorosis has shown up in some fields as well. Iron chlorosis usually first shows up in low and wet areas of soybean fields. These fields often have high pH (greater than 7) and poor drainage in these affected areas. Symptoms of iron chlorosis are yellowing of the interveinal area of young leaves. Brown and necrotic spots may occur in leaf margins, and plants can be stressed or killed if the disease is severe. The symptoms are more pronounced when soil temperature is low and soil moisture is high. I am told that manganese deficiency can cause similar symptoms, but I don’t have much experience with this in soybean fields. More here.

Spider Mites can cause some stunting/yellowing (it looks more like a sandblasting or bronzing effect) in our bean fields. We’ve heard very little about them in SW IA this year, but with it being warm and dry down here, we want to keep an eye on our fields, you can learn more here.

On another pest management note, some of the fields treated for soybean aphids around 3 weeks ago are starting to show a rise in aphid numbers again. While relatively few of these fields have been treated a second time, we want to give a “heads up” to you watch them. While not a very common occurrence down here in SW IA, it isn’t unheard of in other parts of the Midwest that soybean aphids sometimes need a second application of insecticide to control them. With that in mind, looking at the calendar, the growth stage of our fields, and the aphid pops, we will want to keep an eye on our beans over the next few weeks. However, after we hit R 5.5 (seeds are filling on one or more of the 4 uppermost main stem nodes), the yield benefits of insecticide apps drops off substantially and we typically are done worrying about soybean aphids. Just a rough guess based on what I saw the last few days, we are maybe still a couple weeks from mid R5.