Crops Bulletin
July 11, 2012
Issue 8    


Prepared by

Paul Kassel

Extension Field Agronomist



Serving Clay, Buena Vista, Dickinson, Emmet, Hancock, Kossuth, Palo Alto, Pocahontas, Sac and Winnebago Counties



The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Many materials can be made available in alternative formats for ADA clients. To file a complaint of discrimination, write USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964.




Spider Mites. Corn and soybean crops can be affected by spider mites. Spider mites reproduce at a greater rate and cause more crop damage when sunny conditions, dry soil and high temperatures are prevalent. Damage from spider mites is a stipling or discoloring of leaf tissue - caused by spider mites taking the moisture from the leaf tissue.


There are no exact thresholds for spider mites. The following are some guidelines.

       -treat soybeans between R1-R5 (bloom through beginning seed set) when most plants have mites, and there is heavy stipling on lower leaves.

       -Treat corn from R1-R4 (silk through dough stage) when most plants have mites at or around the ear leaf and there is 15-20 percent leaf discoloration.


Soybean Aphids. Soybean aphids have been found in Northwest Iowa. However, they are at very low levels. Continue to check fields for soybean aphid - as aphids will reproduce faster and cause more damage if dry soil conditions continue.


Fungicide Use on Soybeans. Fungicides applications for soybean should be applied at the R3 stage (3/16 inch pod at one of the four uppermost nodes). Foliar diseases of soybean are at low levels this year.   However, fungicide applications to soybean have increased soybean yields when foliar disease levels are low. Some local studies have shown the following responses.

    - 6 year study at the ISU NW Research farm - 3.0 bu/a.

    - ISU FARM research - 1.9 bu/a in 2011, 3.5 bu/a from 2006 to 2010. 


Fungicide Use on Corn. Fungicide use in corn also has the potential to increase yields by reducing leaf disease and increasing the physiological efficiency of the corn plant. Little information is available to support the idea that fungicide application will decrease the effects of moisture stress.


Corn yield responses to fungicide correlate to disease levels. Data from ISU has shown a 9.5 bu/a yield response when disease severity was more than 5% in 23 different trials (4.8 bu/a when disease severity was less than 5%). Another local study showed 13.8 bu/a response from 2007 to 2010 with low levels of leaf disease.



Prepared by Paul Kassel, Extension Field Agronomist

Phone: (712) 262-2264, Email: