October 12, 2007

If you are planning to use a seed treatment, the best way option is to place the seed order with a request for seed treatment fungicides. Every year seedling diseases affect soybeans in some area of Nebraska, with the location often changing from year to year. The history and records of the fields you manage are the most important guides in determining whether you should use a seed treatment fungicide.

The most common fungi involved in seedling diseases in Nebraska are species of Fusarium, Phytophthora, Pythium, and Rhizoctonia. All four can killi soybean seedlings or cause damage sufficient enough to affect the plant's ability to achieve its full yield potential. The most common diseases are Pythium and Rhizoctoina, each of which is favored by differening sets of conditions. Wet, high clay content soils favor Pythium, as well as Phytophthora, and drier, sandy soils favor Rhizoctonia. This does not mean you will not see the disease in all fields, but the risk is greater if the field conditions are favorable for disease development. Keep in mind those fields with a history of Phytophthora problems should be planted into a resistant variety. Fields with a long-term history of Phytophthora may require a different resistance gene if it continues to kill plants when you grow resistant varieties. In addition, fields with a history of Phytophthora will require additional levels of Mefenoxam or Metalaxyl chemistry above the standard rate. For more information, see the extension publication, Management of Phytophthora Diseases of Soybean (NF02-518).

With the trend toward earlier planting into cooler soils, seed treatments are becoming more popular. Consider the following list in assessing your need for a seed treatment fungicide. I would recommend use if you have any of the following conditions:

• Field history of seedling / emergence problems: Ideally you have identified the disease and can match the correct treatment; if not, use a good combination product.
• Early planting: If you're considering early planting, fungicide seed treatments are a necessity. Cool, wet soils are very conducive to poor stands without treatment. No-till fields will have cool soils later in the season than fields receiving some tillage. These will more commonly have seedling disease problems.
• Phytophthora history: Fields with a history of Phytophthora will need additional metalaxyl or mefenoxam treatment. Even with resistant varieties, I recommend a treatment as fields with Phytophthora generally favor Pythium and in some fields you may not be able to buy the correct resistance source. 

Most seed companies have a standard treatment they offer. In many instances this will not be sufficient for a field with Phytophthora. Based on your field history, if you know the disease you are trying to control, check with your seed salesman to ensure that you have a good match for the fight. All references in this article to seed treatment apply only to the use of seed treatment fungicides and are not intended to be applied to seed treatment insecticides. Seed treatment fungicides are not all compatible with Rhizobium inoculants. Always check the label for compatibility; many products require that seed be planted within as short a time as four hours of inoculation with some liquid based Rhizobium inoculants. For additional information please see the UNL publication, Seed Treatment Fungicides for Soybean, which is currently being updated. (The current edition is available at http://www.ianrpubs.unl.edu/sendIt/nf411.pdf

Loren J. Giesler
Extension Plant Pathologist