Crop Observation and Recommendation Network
provided by the Agronomic Crops Team

Establishing a Healthy, Vigorous 03 Soybean Crop

By Drs. Jim Beuerlein and Anne Dorrance
The Ohio State University

Getting a crop off to a good start and keeping it healthy and growing fast will almost always result in high yields baring a catastrophe.  When soybean seed is dropped from the planter into the soil and covered, the potential yield for that crop is at its maximum.  Yield potential decreases daily if growth conditions are less than ideal or if stresses caused by disease or insects slow plant growth.  Good crop management is protecting the crop from stress and the resulting yield reduction.  Because soybeans are grown in short crop rotations there are many stresses that must be managed. Everyone is aware of the need to provide adequate nutrition and control weeds for the crop since failing to do so becomes obvious early in the season and is paid for with a reduced harvest.  Few producers eliminate or adequately control the stress caused by disease and insects.

During the last five years we have seen a general decline in the ability of the Phytophthora genes to adequately control that disease due to the evolution of the pathogen to produce new races that can defeat the resistance genes.  The Phytophthora resistance genes currently available are effective in less than half of our soybean fields.  The level of partial resistance, which is effective against all races of Phytophthora, is present in cultivars at much lower levels than a few years ago.  Therefore, it is now easier for Phytophthora to cause serious yield reductions than in the past. Seed treatment fungicides will help control Phytophthora and other root and stem rot diseases and allow the crop to get off to a better start.

Seed companies sometime tell customers that the quality of their seed is very high and fungicides are not needed.  Seed quality and disease susceptibility are two different factors.  No matter how good the seed quality, if a variety is susceptible to a disease and the proper environmental conditions exist, it will become diseased and lower yields will be harvested.  Seed treatment fungicides can’t provide season long control of disease, but they will help the crop get off to a healthier start so it will be better able to fight off disease if it occurs later in the season.

Do fungicide seed treatments pay?  The answer from ten years of testing fungicide seed treatments on soybeans is both yes and no.  Most root rot diseases can infect the crop anytime throughout the season if conditions are suitable for infection.  Generally, the later in the season an infection occurs, the lower the yield losses.  In years with dry soil conditions during and after planting there is little disease development early in the season and the beneficial effect of fungicide seed treatment is reduced.  When the soil becomes saturated for a couple days following planting, the seed treatment fungicides pay large dividends.  When plants become infected, they don’t grow or produce well even though we may not see symptoms of the disease. By the time we can see disease symptoms, the yield has already been reduced by 10 percent.  Replanting will cost a minimum of $80 per acre due to lost production, the purchase of more seed and replanting.  Over our ten years of testing, the use of fungicide seed treatments has increased income by 3.5 times more than the treatment cost.  Over time, it pays to use a fungicide seed treatment on soybean seed regardless of variety, crop rotation, tillage, soil type, soil drainage or planting date.

Several fungicides can control some of the root rot diseases, but only two, Allegiance and Apron XL, are effective against Phytophthora root rot.  Most seed companies do not use the "full labeled rate" of these materials when treating seed.  Where severe Phytophthora exist, full label rates are needed for control.  Most seed companies will add extra Allegiance or ApronXL when requested.  The following table shows the efficacy of several seed treatment fungicides on four soybean diseases.

Relative efficacy of fungicide seed treatments for control of certain diseases of SOYBEANS in Ohio

 Efficacy based on labeled rates of active ingredient for each material.
 Efficacy rating scale: E=excellent, G=good, F=fair, P=poor, N=no activity, ND=no data
* Control of Phytophthora damping off only at the higher labeled rates. Low rates of Metalaxyl and Mefenoxam do not control Phytophthora but  they do control Pythium.

The following two tables show the economic effect of fungicide seed treatment in 2001, a growing season with lots of early disease, and 2002, a growing season with little early disease.

Effect of Fungicide Seed Treatments on Soybean Profit ($/Ac) in Ohio in 2001.

** numbers in ( ) are negative.   + Soybeans = $5.00 per Bushel.