Soybean Murder Mystery

 

Solving murder mysteries is a popular game but when it affects your crops, it is serious business! Soybean yields have been stagnating for years. Some farmers get great yields (70-90 bushel/acre), but many farmers average 50-65 bushel/acre. Soybean yields may be 20-30 bushel below optimum, even with good genetics. With depressed crop prices, good soybean yields are essential to making a profit and staying in business.

The Allen County SWCD (Decatur, Indiana) held a field day two weeks ago. A local farmer (Nate) approached me after I gave a presentation on how a lack of plant available nutrients increases crop disease. Nate, a trained agronomist, says he noticed a “soybean mystery disease” starting in 2014. It occurs around August 1-10th almost every year but is worse some years.

Symptoms: Usually, low areas or compacted areas show up first but also hill tops. It starts on lower leaves which turn pale, yellow, then brown and fall off. Usually, bean pods start aborting from the bottom third of the plant. Often, Manganese (Mn) deficiency signs characterized by dark green veins with yellow leaves occurs. Some leaves have yellow and brown halos that look like septoria leaf spot on steroids. In severe cases, the top soybean leaves thin out. It looks like a Charlie Brown Xmas tree. Small top leaves, aborted blossoms, and aborted pods signal Calcium (Ca) deficiency. Another odd characteristic: On the top trifoliate leaves, the middle leaf stands almost perpendicular to the sun. (Almost looks like a middle finger sticking up.) Severely impacted plants are at least 6 to 8 inches, even a foot shorter than healthier plants nearby. Plants start maturing 2-3 weeks early.

Nate sent several samples to Purdue University in 2018 and got variable explanations. Symptoms mirrored charcoal stem rot, sudden death, soybean cyst nematode, and septoria. Purdue could not ID the exact disease but widespread yield losses could be documented, they might get a grant ($250-$500K) to study it.

We toured several soybean fields. In some fields, the damage was relatively minor. A few yellow leaves on the bottom but a tell-tale sign was upright leaf on the soybean trifoliate. I noticed many soybean plants with stems with no leaves and no pods. Almost like deer nipping the top leaves off, except it was too wide spread for deer damage (Ca deficiency, aborted blossoms, no leaves). On healthy soybean plants, leaves can sometimes be almost the size of your palm of your hand. On infected plants, leaf size is very small.

Another observation: Nate is seeing similar symptoms in trees and bushes in the fence row. Fence row trees, shrubs, and weeds had pale, yellow leaves, and have somewhat similar characteristics: small falling leaves, septoria looking spots, Mn and Ca deficiency.

Here is my educated guess but I could be wrong. Glyphosate (Roundup) is the most widely used chemical in the world. Almost a 1# of glyphosate/acre is applied on almost all our crops. Glyphosate is a major chelator, tying up Ca, Mn, and many other nutrients. Yellow and pale leaves (chlorosis) is a sign of Iron (Fe) and Magnesium (Mg) deficiency. Iron (Fe) activates the enzyme that makes chlorophyll, Mg is the central element in chlorophyl. Glyphosate also ties up Cobalt (Co) and Nickel (Ni). Both Co and Ni are needed by soybean plants to make nodules to fix nitrogen for the plant to make proteins. When nutrients are lacking, insect and diseases attack stressed plants and yields decline.

In GMO or roundup ready (glyphosate tolerant) crops, glyphosate is a major chelator, tying up key nutrients. Glyphosate increases drought stress and plant stress. If weather conditions are great, the symptoms are relatively mild and yield may not be affected much. However, when plants are stressed due to compaction, heat, dry weather; the plant becomes severely deficient in many micro-nutrients. Stressed plants become targets for increased insect and a slew of diseases.

Tips: Try to reduce glyphosate applications especially after soybeans bloom (R1). Do not tank mix Mn with glyphosate because it gets tied up. It should be applied separately, if possible, perhaps with a drone. If you plan to plant soybeans back to soybeans, try planting a cover crop to increase microbial activity and reduce diseases. Scout fields now to look for the damage described above before the crop totally matures.

With the widespread use of Enlist soybeans, fields are almost completely weed free, but is it possible we are paying a price by using too much glyphosate??? Glyphosate is persistent and slowly broken down in soil and in plants. If someone has a better explanation, give me call or consult your agronomist, Extension specialist, for more information.