What the heck happened? Most farmers have had that experience, wondering why the appearance of a field can change dramatically in a week or less. But windshield surveys are easy compared to when you are wading out in to the field, then discovering something totally unexpected.

A higher percentage of yield limiting problems are found on acreage planted to the same crop every year.

Now is an excellent time to scout for existing challenges within continuous soybean fields.

The build-up of pathogens in either the soil or on crop residues may increase disease causing organisms such as Soybean cyst nematode (SCN), frogeye leafspot and Sclerotinia white mold.

Dr. Anne Dorrance, Ohio State University Extension soybean disease expert, says the key to preventing this from happening is to go look at those fields – now. The R6 growth stage is also a good period to tell if you chose the right variety for that particular field (resistance to Sudden Death Syndrome (SDS), SCN, White mold, Stem Canker, Frogeye) as well as if the pathogen population is increasing.

Drive by scouting doesn’t work in these cases, you need to walk into the field to look into the canopy.

Here is some of what you might find.

•Soybean cyst nematode (SCN) “hot spots” will mature much faster than the rest of the field, giving it a patchy appearance. For those pockets which are maturing early, dig up the plants and check the roots for the white pearls of the SCN females. They will be the size of the head of pin. If you can easily find them, rotate to wheat or corn for 2012.

•Sudden Death Syndrome (SDS) and Brown Stem Rot (BSR) have similar foliar patterns. SDS may have a brighter yellow cast to the leaves than BSR while BSR gives a “greasy” appearance to the base of the stem. The best way to separate these two is to compare the crown and the pith. For SDS the internal tissues of the crown are gray, the blue-green color of the spores are sometimes on the surface of the root tissue and the pith is white and healthy. In contrast, BSR, the crown is white, but the pith is chocolate brown color. For BSR, this discoloration of the pith may be the whole length of the plant or just at a few nodes. Soil pH plays a key role in symptom development for BSR. SCN also plays a role in the severity of the symptoms for SDS, so check those roots for signs of the SCN females.

•Frogeye leaf spot is established in Ohio and successfully overwintered again in 2011. For the most part, inoculum levels were low, but it has been found in several fields. Check the mid- and upper canopies for frogeye, if present; plant a variety with higher levels of resistance or switch to wheat or corn.

•Sclerotinia stem rot infected plants appear in locations that had cool nights and closed canopy during flowering. Dr. Dorrance found her first plant recently, with a lesion near the base of the plant. The plants will stand tall with the leaves withered and thick cottony growth on the main or side branches.
Fields infected with this disease annually should be rotated to prevent buildup of inoculum.

Under no-till conditions, sclerotia (the survival structures) will break down on the soil surface faster than if they are buried.

•Phytophthora stem canker can actually be found throughout the year, approximately 1 to 2 weeks after a heavy rain. If you find scattered dead plants or those that are wilted with a chocolate brown canker colonizing the stem, that plant was killed by Phytophthora sojae. For 2012, choose a variety with a higher level of field resistance or partial resistance. The Rps genes are still important, but no longer provide full protection across a field.

Checking fields now help you make better decisions on variety selection and rotation decisions for next year. Want to learn more? Look at the following website for information and color photos:
http://oardc.osu.edu/ohiofieldcropdisease/t01_pageview2/Home.htm

The views and opinions expressed in this column are those of the author and not necessarily those of Farm World. Readers with questions or comments for Roger Bender may write to him in care of this publication.