Tuesday, August 03, 2010
Is There a Nasty Surprise in Your Soybeans?
If it is August, it must be the scouting season for soybeans. While nationally 86% are blooming and 53% are setting pods it is the time of year that diseases and insects can create damage of exponential proportion. Foliar diseases can shut down photosynthesis. Insects can draw the life-giving moisture from soybean plants, and also spread diseases. You want to protect your crop, so here’s what to look for as you wade through soybeans fields.Weather has been variable throughout the Cornbelt, with both dryness and flooding reported, which will result in a variety of disease and insect issues for soybeans. For those soybeans that are relatively stress free, and in the R3-R6 stage, their yield is being determined now, says University of Illinois soybean specialist Vince Davis. While Davis says there are no widespread reports regarding disease or insects in Illinois, he says it is an important time to be in your bean field scouting for weed control failures, insects, and diseases and getting to know the progress of pod setting. And he adds, “While I don't have the answer for how to improve pod numbers on every acre, learning more about your field conditions during these reproductive stages is a good place to start.”
Insects:
Entomologists have long advised soybean producers to beware of stink bugs, which pierce soybean pods to remove moisture and then spread various viruses that discolor soybeans. 2010 has brought a pair of new species of stink bugs, the red banded stink bug and the red shouldered stink bug. Both have migrated from the Southeastern US into the Cornbelt. IL Extension entomologist Mike Gray says it does not take as many of these new species to do the same damage as the ones which have long been present. “The suggested economic threshold for the red banded stink bug is 24 insects per 100 sweeps. This compares with a threshold of 36 insects per 100 sweeps for the brown stink bug.”
One of the non-starters this year has been the soybean aphid, and most authorities believe that a pathogen that decimated populations last year prior to their over wintering, had a hand in reducing outbreaks this year. Entomologist Ron Hammond at OH St. says, “As we get into August and the later part of the summer, the susceptibility of soybeans to aphids goes down. However, as in other years with low numbers aphid, we do expect the numbers of aphids to begin to rise prior to the end of the summer. These aphids then will move to buckthorn in the fall and lay eggs, and will overwinter, possibly resulting in problems in 2011.”
On the issue of soybean aphids, Purdue entomologists say, “Reports from “aphid central” (i.e., Minnesota and Wisconsin) are much the same, much lower than normal numbers. David Ragsdale, University of Minnesota Entomologist, reports that abundance of heavy rains has decreased aphid numbers, some mortality measured over 60% with single rain events. As we venture into the critical pod–forming and filling stages of soybean, low aphid numbers is very good news for producers!
One nemesis of the soybean, the two spotted spider mite is usually ignored in wet weather, but is troublesome in dry weather. The OH St. specialists say, “While mite populations might build up in dry areas, remember that the weather has also been very humid, which encourages a natural-occurring pathogen that helps to keep populations down. Before taking action against this mite in soybeans, make sure that it is extensive enough and also an expanding problem.”
Weeds:
Some soybean fields may still be more weedy than you care to harvest, but it is getting late in the season to consider many late season herbicide applications. University of Illinois weed specialist Aaron Hager says the issue is the length of time required between the application and the date of harvest. Among 27 of the more popular herbicides, the shortest pre-harvest interval is 30 days for an application of Basagran. Others require 45 to 90 days for the minimum time before harvest. Most of those would push harvest into October or November and most soybeans will be ready for harvest long before then.
Diseases:
Iowa State University plant pathologists Alison Robertson and Daren Mueller report that excessive rains have brought on several diseases which like wet and humid weather. Those include Sudden Death Syndrome, Phytophthora root rot, White mold, and Stem canker. Robertson and Mueller say foliar fungicides are either not effective to manage these diseases, and application is not recommended at this time. But they say there is value in scouting because it will help you create a management plan to reduce the problems in coming year. In some cases tillage and crop rotation will help, but in other cases just marking the field as a hotspot for a specific pathogen will help you select seed that will be resistant to that disease. They say it is all about future management. That policy gained the support of another Iowa State plant pathologist X. B. Yang, who says the hot and wet season leads to different diseases, testing the disease management skills of the producer. Regarding SDS, he says, “With high pressure, some tolerant varieties may not hold up in some fields. The disease, if found, will be an indication of variety tolerance. Good recordkeeping of the disease location will help variety selection strategy for next soybean planting.” If you would like a photo example of SDS on soybean leaves, use this picture from Purdue.
At the University of Wisconsin, plant pathologist Paul Esker describes several risks that lead to SDS development. “The pathogen overwinters in soybean debris as chlamydospores, which as resistant fungal structures. Disease is favored by high soil moisture during vegetative growth and wet and cooler conditions around flowering. What if I have SDS...what should I do? First of all, make sure to get a proper diagnosis. If you see evidence of SDS in the field, take a sample (including roots) and send it to the Plant Disease Diagnostic Clinic. After proper identification and if yield was impacted by SDS, consider the use of cultivars with increased resistance to SDS. Also, monitor conditions at planting to avoid cool soil temperatures that are favorable for infection by the pathogen and consider tillage to help increase soil temperature and drainage.”
Another good reference is the Soybean Station, produced by Purdue soybean specialist Shawn Casteel.
Having a handy photo guide of soybean diseases can be helpful in scouting. The University of Missouri offers such a guide .
Summary:
Scouting soybeans is quite important to the success of the crop. There are many insects, pathogens and other yield robbers that may be controllable if found in a timely manner by good crop scouting. However, if diseases are too far along or weeds cannot be controlled with herbicides, it is always helpful to note the problems in a given field, and use good management to avoid problems in coming years.
Posted by Stu Ellis on 08/03 at 12:00 PM | Permalink