Matt Morris, Extension Educator, Frederick County
As we enter the middle of August, you’ve likely made some key management decisions already to protect your soybean crop’s yield potential. Unfortunately your job is not over: late season stress during pod set and pod fill has the most impact on yield. This means walking your fields, no matter how entangled, weekly up until pod fill has just about finished.
One of the things to monitor for is nutrient deficiencies, specifically micronutrients. Many of these problems should have been corrected earlier in the season, but can be rescued now with a foliar application. Boron, for example, is a key micronutrient that influences the number of pods per plant, seeds per pod, and the overall number of branches on the plant. Our soils here in MD are naturally boron deficient. Manganese is another micronutrient that can be deficient in our area. Manganese is key in chlorophyll production and can become deficient following applications of glyphosate or cool, wet weather like we’ve had this month. Foliar applications of manganese can be made as a corrective action if deficiency occurs. With these two micronutrients and all other nutrient issues, in-season soil and tissue sampling can help diagnose a problem. I’d recommend routine tissue sampling, especially if your yield goal is high. Many nutrient deficiencies will start to reduce yield long before the plant will show visual symptoms. A tissue sample may even tell you that your plants are falling short on nitrogen requirements once you hit the 70 bushels/acre threshold.
Another stressor to monitor for is disease. Diseases stress the plant’s systems and reduce the amount of leaf surface that can absorb sunlight. When scouting your fields look at the leaf surface both upper and lower, the stem, and the roots. These are all areas where disease can occur and begin to reduce yield. Discolored or brown spots on the leaves, white molds on the leaves or stems, lesions, and red/black spores can all be signs of disease. Our recent weather pattern of cool, wet weather can amplify disease pressure, especially under the canopy where it stays moist with little airflow. Fungicide applications can be used to prevent, treat, or suppress diseases, but most chemicals only work on specific diseases so proper identification is important. If you cannot visually ID the disease, the University of Maryland has a plant diagnostic lab on campus that can make an ID for you. If you plan on submitting a sample, give me a call and I can help you with that process. The most effective method is to make a fungicide application prior to disease occurrence and to select varieties with low disease susceptibility.
Insects are one more factor that can impact soybeans later in the season and lower yield. Insects can feed on vegetative structures like leaves and stems, but also on reproductive structures like pods or flowers. There are a number of insect pests that can occur late in the season. Many are brought on by hot, dry weather like spider mites while others are less weather dependent. One insect pest that could be of concern for soybeans is corn earworm, known as podworm in soybeans. They cause their damage during the R1 to R5 growth stages by feeding on the growing pods. Recent trap counts in Burkittsville indicate that there is an active adult population in the area so be mindful of their presence. Insecticides should be used only when a population reaches the economic threshold, which varies based upon the insect species.
Lastly, weeds are one factor that may not directly impact yield, but can cause harvest issues and produce seed for next year. Most weeds should have been controlled earlier in the season before the canopy closed, but if some did escape you have few options. Most postemergence products are only effective on smaller weeds or have harvest interval restrictions. I would say that the biggest late season weed problem in Frederick County right now is marestail. If you have marestail visible in your soybean field and they are any bigger than 4 inches you do not have many choices. Assume all of our marestail population is resistant to glyphosate and ALS herbicides. We also have issues with palmer amaranth and waterhemp in Frederick and these two are serious issues. If you have either of them in your soybean crop I advise you get anyone you can find to hand pull and destroy them as soon as you can. These plants can produce hundreds of thousands of seeds that will haunt you next spring if left uncontrolled. Seriously evaluate your 2018 weed control program with the thought in mind that these same weeds will be back with a vengeance next year.
Monitoring for these important issues can be the key to preserving your soybean’s yield potential. Hopefully our weather pattern continues so water won’t be a limiting factor.
