July 26, 2004
Gray Leaf Spot
Gray Leaf Spot has shown up in some corn fields south of Highway 92 at levels not observed in nearly a decade. It is generally unlikely that a fungicide application will be cost-effective when used in a timely fashion. Looking at some of the fungicide options, Tilt and Stratego must be applied by silking at the latest at the onset of the disease, which eliminates both products at this point in the season. Quadris only has a seven day harvest interval, but the label states to use it at the onset of the disease, which again eliminates it as being a viable option. The bottom line is that it is best to just let it run it's course this year and emphasize gray leaf spot resistance / tolerance when selecting hybrids the next time corn is grown in the field.
rootworm pressure has been surprisingly high in 2004, resulting in many
insecticide failures. If you have not yet evaluated the performance of
any rootworm insecticides used, now is a good time to dig up a few root masses,
wash them off thoroughly with a hose, and inspect the roots. To help you
evaluate what you see, go to http://www.ent.iastate.edu/pest/rootworm/nodeinjury/nodeinjury.html.
mold is occurring at high levels in some fields along and north of I-80.
At this point in the season, there are no good management options. In the
future in those fields, the best management tactics are moving away from
drilled or 15-inch rows back to 30-inch rows and selecting varieties with
greater tolerance and/or less susceptibility to white mold. In addition,
an application of Cobra or
aphid numbers remain generally very low. However, the populations can
explode very quickly, and the recent cooler weather may prompt that, so
continue to remain vigilant. Scouting and threshold information can be
found on pages 66 - 67 of the June 14, 2004 Iowa State University Integrated
Crop Management Newsletter or at http://www.ipm.iastate.edu/ipm/icm/2004/6-14-2004/soythreshold.html.
Bean Leaf Beetle
Bean leaf beetle numbers also continue to be generally low. However, numbers in some fields, especially south of Highway 78, are approaching threshold, so, again, remain vigilant. Scouting and threshold information can be found on pages 160 - 161 of the August 9, 1999 Iowa State University Integrated Crop Management Newsletter or at http://www.ipm.iastate.edu/ipm/icm/1999/8-9-1999/blbeffects.html.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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