Add 2 bean diseases to watch list

The soybeans have emerged, have taken off, and are growing well. We don’t have to worry about diseases in them anymore, right?

In the past, that has been pretty true. Early-season root diseases were, by far, our main issue in soybeans. There was an occasional year when white mold or some late-season phytophthora appeared. But there was little to do to prevent those issues.

A few diseases are beginning to raise their ugly heads in the Dakotas. Brown stem rot and sudden death syndrome are two fungal pathogens that can cause severe yield loss when present.

Key Points

• The soybean crop in the Dakotas is not disease-free anymore.

• Sudden death syndrome is one disease to watch for.

• Brown stem rot can reduce yields without showing symptoms.

Brown stem rot

I have seen brown stem rot mainly in Cass and Richland counties in North Dakota. I don’t believe it is widespread at this point, but it is a disease that should be included in our “to watch for” list while scouting.

Visual signs of brown stem rot normally appear around mid-August. Leaves with light green to yellow blotches or a brownish cast, hanging dried or dead leaves, and a brown pith to the stem are symptoms to watch for.

BSR is normally more severe in lower-pH soils, which may help contain the spread of the disease. Temperatures over 90 degrees F halt development, as well. This is a disease that is managed well by rotation, as it does not survive well without a host crop.

There are two known strains of BSR. One produces visual symptoms, the other does not. Both are controlled by resistant varieties. Yields can be reduced by 15% even before symptoms are noticed. Genetic resistance is the best option if BSR is likely to be a problem. We do have some options of varieties with resistance to BSR in our maturities.

Sudden death syndrome

Sudden death syndrome is not believed to have made it to North Dakota yet. However, there are fields in west-central Minnesota that are infected. The visual signs of SDS and BSR are very similar and can be confused at times.

In plants affected by SDS, the leaves begin to show small yellow blotches that will increase in size, leading to necrosis of the leaf tissue. The veins typically remain green, though. The pith of the stem will not turn brown as in BSR, so this is the easiest way to tell the two diseases apart.

There are a dozen different genotypes of SDS, with no known genetic resistance for varieties in our maturity at this time. Cool soils at planting and moist soils later in the growing season help promote the disease.

Soybean cyst nematode can increase the severity of SDS, so resistant varieties should be used.

These are just two of the diseases that are on the pathogen radar for our region. Make sure to send any plants that you may question to a diagnostic lab to be identified. Early detection will allow you to be better prepared for these diseases and help you in managing them.

Spelhaug is an agronomist with Peterson Farms Seed, Harwood, N.D. Contact him at 866-481-7333 or visit www.peterson
farmsseed.com.

This article published in the July, 2010 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.