July 30, 2010
Green cloverworm populations have crashed in the last few days in many fields, partly from a fungus that is killing the larva, but also because the larger larva have pupated and turned into the moth. Moths are very easy to find in many fields. They are fairly small and tan with a snout (proboscis). The celery leaf tier moths look similar and are also common in fields where the leaf tier larva were present. Most of the leaf tier damage was done in the lower part of the soybean canopy. It skeletonizes the leaves and makes webs.
The green cloverworm moths will be laying eggs for the 3rd (last) generation of the worms, but in the past the cloverworms that hatch in August have not been very destructive because they are killed very soon by the fungus. One thing to keep in mind when debating about whether to spray soybeans with a fungicide is that the fungicide may actually help protect the cloverworms.
Japanese beetle populations do not seem to be as high as in some past years. Very few fields have reached the 20% defoliation threshold. 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.
Soybean aphids are still at low levels in the area. The warm weather we have had recently should help to slow down their reproduction and allow beneficial insects to keep on top of them. Insecticides sprayed when aphid numbers are very low can actually create an aphid problem by killing all of the beneficials, allowing the aphid populations to explode. So far it is looking like this may be an “off year” for aphids, but the next 2-3 weeks will tell.
Sudden Death Syndrome is already showing up in some fields, so this may be another bad year for it. I haven’t seen any white mold yet, but suspect it will be showing up soon. Brown spot has been common on the lower leaves in many soybean fields. I have also been seeing some frogeye leafspot already, which is early for this disease. Fungicide application can help manage these two diseases. Downy mildew and bacterial blight are also showing up in some fields. Fungicides do not prevent these two diseases because they are caused by bacteria. Most soybean fields are now in the R3 stage, which is usually the best time for fungicide applications if they are going to be made. For pictures of frogeye leafspot see X.B. Yang’s website at http://ssnavi.public.iastate.edu/frogeyeleafspotpics.html.
Gray leaf spot is showing up in many fields, especially in the south. There has also been a report of southern corn leaf rust in the southern part of the area. Southern rust is much more virulent than common rust. Eyespot is more common in the north. If a fungicide is to be sprayed, it would be best if it could be done within the next week or so. Fields with hybrids that are more susceptible to foliar diseases that are showing fungal disease lesions within three leaves of the ear leaf are the fields that are most likely to respond to a fungicide application.
DEALING WITH FLOOD DAMAGED CROPS
Several thousand acres were flooded this past week along the Wapsipinicon and Maquoketa rivers. There were reports of only corn tassels showing above the floodwaters. For corn and soybean fields that were totally submerged, it is doubtful that there will be much of a crop to harvest, so the focus should be on preparing for future year’s crops. Partially submerged crops may survive, but debris will need to be removed from the field to avoid damage to harvest equipment in the fall. Pastures that were submerged for a few days will likely survive, but mud and debris will need to be removed before the pastures can be utilized.
By the time corn and soybean fields dry out enough to walk in them it will be more apparent how much of the crop will survive. One of the first things that should be done is to check out the insurance options on the crop. Even if the crop is totally destroyed, there are things that should be done this summer and fall to restore the productivity of the soil. After removing debris, any sand and gravel deposits of more than 4” will need to be removed from the fields. Silt deposits can usually be worked in to the original topsoil. Since there is now new soil in the field, the soil fertility level has likely changed so the soil should be tested for fertility before planting the next crop.
To prevent soil compaction problems, avoid running trucks and heavy equipment over the wet soils. Most soils are not dry enough for traffic until the top 5-6 inches crumble, rather than slick over or pack. A cover crop such as oats or rye can help to keep weeds under control the rest of the season and can help to dry out waterlogged soils. New weeds may have been introduced to the field in the flood, so these will need to be managed in future years. There may also have been herbicides, such as atrazine, introduced with the soil. An oat cover crop this fall would also help to identify any potential future problems with herbicides in the introduced soil. The main concern with herbicide contamination would be for fields where a late summer seeding of forages will be attempted this August. Late summer forage seedings should be done by August 10-20. For more information on reseeding flood damaged forage crops in the late summer see Steve Barnhart’s article in the ISU Extension Integrated Crop Management News at http://www.extension.iastate.edu/CropNews/2008/0710barnharts2.htm.
Forage grasses and legumes can withstand a few days to a week underwater, but will not perform well if the soil remains saturated for several weeks after the water recedes. Any flood debris that will harm livestock or equipment should be removed as soon as possible. If flooded areas are recovering slowly and there are concerns about the viability of the stand, dig random plants in several areas and evaluate the condition of the root systems. Legume plants with a firm taproot, creamy white in color with no evidence of root rot, and that have green and visually healthy crowns and crown buds have the greatest likelihood for survival. These plants need a week or more of sunshine and drying soils. Legume or grass plants with watery, mushy, textured roots, yellowish or tannish in color, and those with no evidence of active crown buds will be the least likely to survive, even with good growing conditions during the next few weeks. For other information on dealing with the aftermath of the flood see ISU Extension’s Flood Cleanup website at http://www.extension.iastate.edu/disasterrecovery/flood.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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