July 1, 2010
Year-in and year-out, the best time to apply a fungicide to corn, if needed, is from tasseling through silking (VT-R1). However, it has been our experience that in years when tasseling occurs unusually early, as is likely to be the case this year, it is often most profitable to delay the application until the blister (R2) stage of development in mid-to-late July. To make the decision about the need to apply a fungicide, Alison Robertson, ISU Extension Plant Pathologist, suggests:
1. If the corn has good genetic resistance, don’t worry, be happy.
2. If genetic resistance is moderate or poor, then just prior to spraying time,
a. go to several locations in each hybrid in each field and
b. inspect several plants at each location.
i. Check the ear leaf and the next three leaves down.
c. If the hybrid has poor resistance and if you find any lesions on the leaves in question, have a fungicide applied.
d. If the hybrid has moderate resistance, also consider
i. Did you find a few lesions or many?
ii. Is the weather forecast conducive to further development of the disease?
White Mold Prevention
I have received some questions regarding the chance of a major white mold outbreak and the wisdom of applying Cobra or Phoenix to soybeans. While mold flourishes in fields with a closed canopy with a history of white mold during cool, wet springs and with highly susceptible soybeans. So if the field is planted to a variety with lower susceptibility, that lowers the likelihood of problems in 2010. The recent high temperatures lower the risk for 2010 and if the canopy is still fairly open, that also lowers the risk. Finally, if there in not much of a history of white mold in the field, then there will not be much inoculum present, and not much chance for a white mold outbreak this year. One way to help make a decision on whether to use Cobra or Phoenix is to scout the field for the white mold mushrooms on the soil surface. The mushrooms are very small and can be confused with other mushrooms. For a picture of the mushrooms, see X.B. Yang’s article at http://www.extension.iastate.edu/CropNews/2010/0617yang.htm.
Work in 2009 by Carl Bradley of the University of Illinois suggests that Cobra is a very viable option in suppressing white mold, and Phoenix has the same active ingredient. The Cobra label calls for rates of 6-8 oz/A for white mold suppression, although X.B. Yang obtained white mold suppression in some trials in the mid-90’s with rates as low as 2-4 oz/A.
The best time to apply a soybean fungicide to control foliar diseases is at beginning pod set (R3), so we still have about a month before we should consider making a fungicide application to soybean. Some fields now have a fairly high level of brown spot on the leaves; this will provide inoculum for brown spot to recur later in the season, so fields with brown spot now should be watched especially closely as those fields approach R3 to determine the need / lack of need for a fungicide application at that time.
As producers are making the last (hopefully) herbicide application, some are again considering including an insecticide with the herbicide “just in case…” So far bean leaf beetle activity has been miniscule and there is no evidence of soybean aphids moving from field to field. If you look long enough, you will probably find a colony or two of aphids, but predators should “take them out.” So the application of an insecticide at this time will most likely only kill the predators, leaving the soybeans unprotected a few days later so that even a small infestation later in the season can quickly explode. In addition, unless a contact herbicide is being used, the coverage will probably not be good enough that an included insecticide would kill soybean aphids anyhow.
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
Making Decisions on Fungicide Applications 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.
IOWA LEARNING FARM WATERSHED IMPROVEMENT FIELD DAY
Cuddeback Farms, Washington (1690 250th St)
Soil nitrate and stalk nitrate testing as well as current crop concerns will be the focus of the tour. A complementary meal is available.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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