July 20, 2010
Japanese beetles are showing up again in the usual locations, especially in the Iowa City and Cedar Rapids area. There are a couple of other insects that are also defoliating soybeans in some fields – green cloverworm, and celery leaf tiers. It’s been about 30 years since we have had a major problem with green cloverworms, and that is the last time much research was done on the pest. Green cloverworms are a pale green caterpillar with one or two white stripes on their sides. They are now in the second generation and go through 3 generations per year, but usually the third generation is not as destructive. Economic thresholds were based on number of green cloverworms per foot of row using a drop cloth (about 10 or so depending on insecticide cost and soybean value). It may be easier to use the general defoliation guideline for making a treatment decision, especially if more than one pest is involved. For soybeans in or past beginning bloom, the threshold for treatment is 20% loss of total leaf area. Most people tend to overestimate defoliation. One thing to keep in mind is that soybean aphids may become a problem later this summer, and spraying too early can actually increase aphid problems by killing the beneficials, so it’s best not to pull the trigger too soon on these defoliating pests. The following graphic may help you in determining the level of defoliation. 20% defoliation would be all of the leaves showing the degree of feeding shown with the 20% picture.
For more information on Japanese beetles and their management, see http://www.extension.iastate.edu/CropNews/2009/0630hodgson.htm and http://www.extension.iastate.edu/CropNews/2009/0727hodgson.htm.
For more information on green cloverworms and their management, see http://www.extension.iastate.edu/CropNews/2010/0712hodgson.htm.
Celery leaf tiers are not known to be a problem in soybean, so information related to soybean is scarce. The mature larva is slender and pale green with a narrow, darker green band along the back and with a broader, whitish band along each side, and it tapers toward each end. The underside of the caterpillar is yellowish and faintly mottled with brownish yellow. The full grown larva is two-thirds to three-fourths of an inch long. The larvae usually damage the underside of the leaves by skelonizing them. The plants have a silvery appearance when heavily infested. Later, the damaged areas become pitted. A slight, silken web is spun in a leaf, or between two adjacent leaves and the leaf is folded or woven together to form a shelter area in which the larvae feed; thus, the name leaf tier. They seem to be concentrating in the lower part of the soybean canopy. A picture can be found here http://extension.entm.purdue.edu/veg/insect_ID_pics/CH10_Celery/leaf_tier.jpg.
Many corn fields are now showing symptoms of nitrogen deficiency with the lower leaves “fired” or yellowing along the midrib and upper leaves also showing a pale color. Some of the yellowing may just be due to wet soils, although a lot of N has likely been lost again this year. For most fields it is likely getting too late for emergency N applications.
Gray leaf spot is showing up in some fields. Now is the time to be making decisions on making fungicide applications. Fields with hybrids that are more susceptible to foliar diseases that are showing fungal disease lesions within three leaves of the ear leaf at this time are the fields that are most likely to respond to a fungicide application.
High numbers of potato leafhoppers can be found in some hay fields. Be sure to also use a sweep net to monitor potato leafhopper numbers and treat if numbers exceed the threshold. For more information on managing potato leafhopper, see pages 107 - 110 of the June 21, 1999 Iowa State University Integrated Crop Management Newsletter or http://www.extension.iastate.edu/Pages/eccrops/potatoleafhopper.html. Remember, waiting to see hopperburn is waiting too long as substantial losses have already occurred by that time.
Lepto Leafspot and Common Leafspot
Lepto Leafspot and Common Leafspot are showing up in many alfalfa fields. In general, there is nothing that can be done except to harvest early to salvage as much leaf material as possible. For more information on Lepto Leafspot, see the following sites at The Ohio State University: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/alfalfa/lepto.htm, http://agcrops.osu.edu/cropdoc/b827_128.html. For more information on Common Leafspot, see: http://nu-distance.unl.edu/homer/disease/agron/alfalfa/AlfCmnLfSpt.html & http://www.plantpath.wisc.edu/PDDCEducation/ScoutSchool/General/Slide22.htm
FOR YOUR CALENDAR
CROP TECHNOLOGY TOUR
Jeff Bermal Farm on NW Corner of Keota (Corner of W15 & G32)
Co-Sponsored by ISU Extension and Vision Ag
Corn and soybean disease identification and management will be the focus of the tour, with scouting techniques and fungicide trial results presented by Mark Carlton, ISU Extension Field Agronomist. Free Meal Sponsored by BASF.
Northeast Iowa Research and Demonstration Farm, Nashua
Demonstration Garden Field Day on August 7, 2010 at 4:00 p.m.
Fall Field Day on August 26, 2010, 1:30 p.m.
Details are and will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetnerf.html.
Southeast Iowa Research and Demonstration Farm, Crawfordsville
Fall Field Day on September 15, 2010
Tentatively there will be a manure application field day in the morning followed by a “more traditional” field day in the afternoon. More details will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetserc.html.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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