Soybean Seed Treatments for 2000

April 7, 2000
Many years of evaluation throughout Illinois show that fungicide seed treatments on soybeans offer a 2 to 3 bushels per acre yield advantage in most fields, even when there were no obvious seedling blight problems present. While it is true that seed treatment fungicides seldom provide a yield advantage under ideal seedbed conditions, we all know how difficult it can be to predict the weather several weeks beyond planting. Given the correct environment for the pathogen, the difference in yield between treated and untreated seed can be substantial.

While research typically supports the use of soybean seed treatment fungicides, the fact remains that a lot of seed goes untreated. Why? Here are four common reasons: (1) some varieties are simply not treated by the seed companies; (2) some producers feel the cost of treating soybean seed is not justified; (3) some producers don't want to take the time to treat seed on the farm; and (4) once the seed is treated, it can only be used for seed. If you can't use the excess seed this year, you don't want to save it for next year. Unlike corn, which can be stored for several years, soyean seed will have reduced germination and vigor. Thus the problem becomes what to do with it.

Despite these limitations, a few producers do choose to treat at least some of their soybean seed each spring. In addition to the cost of the product, consider the following issues when deciding about seed treatment fungicides: (1) disease history and anticipated seedbed conditions; (2) product effectiveness; and (3) application method.

Disease History and Anticipated Seedbed Conditions

Experience shows that seed treatments are the most beneficial when seedlings are stressed during the first 10 to 14 days after planting. Examples of stress include heavy rains, crusted soils, compaction, deep planting, cool soils, improperly set planters, reduced seed quality, very dry soils, and some postemergence herbicides. Under these conditions, a number of aggressive or fairly weak pathogens can become active and cause yield losses.

Pythium and Phytophthora (often called "water molds" because they produce a swimming spore when the soil is very wet) produce a soft rotting of the seed, or damping-off, before or after emergence.


Damage by water mold fungi.

With damping-off, a dark-brownish or blackish soft rot girdles the seedling stems and plants die. There is no recovery from these infections. While you can choose varieties that limit the damage by Phytophthora, they are not all equal. For example, some varieties offer resistance, while others offer tolerance. A resistant variety has one or more resistance genes (for example, the "Rps1K" gene) that stops a great deal of the infections by certain races, or subpopulations, of the pathogen. Although this resistance is activated within a day or two after germination, it will only work against certain Phytophthora races. This is called race-specific resistance. Recent research suggests that Phytophthora races 1, 3, 4, and 54 are common in Illinois (note that the commonly used Rps1K resistance gene is not effective against race 54). If you plan to use a tolerant (sometimes referred to as "field resistance") variety, keep in mind that this general type of plant defense is not activated until 10 to 14 days after emergence. Thus, if you use either a resistant or tolerant variety and are concerned about Phytophthora damping-off, you should still use a seed treatment containing metalaxyl or mefenoxam.

Rhizoctonia produces distinct reddish lesions along one side of the stem and does not commonly girdle stems.


Damage by Rhizoctonia.

Plant growth is reduced in the early season, but death of plants is uncommon in Illinois. Rhizoctonia infections are favored by moist weather at planting, which allows the seed to germinate but which then turns hot and dry at emergence, delaying the development of a satisfactory root system. The pathogen is not particularly aggressive and causes most damage in fields where other factors affect early-season growth and development.

Fusarium infections usually cause a more generalized discoloration of the root system. Roots turn dark brown to black throughout the root mass rather than producing the lesions found with Rhizoctonia. Symptoms include a blackening or browning of the vascular system in roots and the destruction of the lower taproot area. Leaves may become chlorotic (yellow) and droop or drop off. Fusarium is not a common problem in Illinois and is not seen, unless severe stress is placed on the germinating seed and seedling. Seedlings infected with Fusarium die.

Sclerotinia is not an important seedling pathogen, but recent research has shown that it can be moved from field to field via infected seed lots. This may seem like old news, but the key finding is that the pathogen can be present in seeds that do not show symptoms.


Sclerotinia-infected seeds (with and without symptoms).

Where a seed treatment is not used, a small percentage of the Sclerotinia-infected seeds will be transformed into a survival structure called a sclerotium. Sclerotia can survive in the soil for up to 5 years. Under cool, moist conditions, these structures will become active, releasing spores into the soybean canopy to initiate the foliar disease called Sclerotinia white mold.

Sclerotinia white mold has been relatively rare in Illinois soybeans over the last few years, so the chances of finding sclerotia and "hidden infections" in Illinois-produced seed lots is remote. If a white fluffy growth and small black sclerotia should appear during a warm germination test, use an effective seed treatment.


Small black sclerotia indicating presence of Sclerotinia.

The most effective fungicides are Rival, Maxim, and thiram to control the seed infection.

Product Effectiveness

Typically, seed treatments will last only about 2 weeks beyond planting. When germination and emergence are prolonged, much of the effectiveness of the seed treatment is gone by the time plants emerge. Meanwhile, pathogens such as Phytophthora and Rhizoctonia can initiate or persist into early summer, given the correct environmental conditions. It is important to remember that, while seed treatment fungicides offer considerable protection early on, they are not designed to protect plants from these later assaults.

Common seed treatment active ingredients and the fungi they control are listed in Table 3. Table 4 lists some of the more common products used to treat soybean seed. This list is not complete and is given for illustrative purposes only. Check with local dealers to determine what products are available in your area and at what cost. Also consult the Illinois Agricultural Pest Management Handbook for further information. Note that many of these products are available as convenient premixes. Should you decide to alter or make your own combination, be sure to read and follow the labels of each product and contact the manufacturer(s) if you need clarification. Finally, if you use a soybean inoculant, apply it just before planting, as the beneficial bacteria may not survive extended contact with certain pesticides.

Application Method

Remember how you've always been told if you're going to do a job, do it right? Yes, that advice applies here, too. While we are not going to get into a debate over commercial versus on-farm seed treatment, suffice it to say that, no matter who does the treating or how it is done, you can't expect maximum benefit from the seed treatment if the product is not applied uniformly. A nonuniform application can outright kill some seeds while leaving other seeds completely unprotected. On the other hand, in the quest for perfect coverage, you may cause excessive damage to the seed. As you can see, treating seed is a bit of a balancing actusing the appropriate equipment in the first place can save you some frustration.

In addition to the traditional canister dispensing tubes used for hopper-box applications, there are several continuous-flow treaters on the market. With the latter system, the liquid product is applied to seed as it passes through an auger from the bin or truck to the planter or drills. For example, for a little over $300, producers can purchase the F.A.S.T. (Farmer Applied Seed Treater) system from Trace Chemicals (1-800-846-2980, or www.tracechemicals.com). If you desire a little more capacity and mixing flexibility, Gustafson (972-985-8877, or www.gustafson.com) offers a TST (Total Slurry Treater) system for about $1,000. If you use an air seeder, Flexi-Coil (www.flexicoil.com/rowcrop/aircarts/stu.htm) now offers an optional, on-the-go seed treatment system with their air seeders. If you are considering an on-farm seed treatment system this year, talk to your local equipment suppliers about the pros and cons of these and other seed treatment systems. Have a safe spring season!-- Bruce Paulsrud and Wayne Pedersen

Author: Bruce Paulsrud Wayne Pedersen