New Tests For Soybean Cyst Nematode (and So What?)
In 2002, the soybean cyst nematode (SCN) continued its record of being the most damaging pest of soybean in Illinois. We've known about SCN in the state since the late 1950s. We know how to control the damage it causes—with rotations and resistant varieties. We have excellent, high-yielding resistant varieties, too. So why does SCN continue to cost us over $100 million per year?
- SCN causes "invisible" losses. In high-yield environments, yield loss can be up to 30% without any sign of sick plants. You can see an SCN-resistant and an SCN-susceptible variety growing side-by-side. Yield of the resistant variety in that field was 58 bushels/acre, whereas the susceptible yielded only 42 bu/A./li>
- SCN can damage even highly resistant plants, if the number of nematodes is high enough. The reason for this is that resistance does not prevent SCN from infecting roots—it prevents them from maturing into adults. So, if you have enough nematodes making holes in the roots, you're going to see some damage. Soybean producers who don't keep track of their SCN numbers and think they are doing enough by growing resistant varieties may be surprised to learn that their SCN problem isn't gone.
- SCN populations differ in their ability to attack resistant varieties. There are no completely resistant soybean varieties. Even the most highly resistant ones will allow a few SCN to mature and produce eggs. Over time, you can select a population that renders resistance useless.
Really getting to know your enemy will help you in the battle against SCN. You need to know three main things about each infested field.
- What is the population density of SCN in the field (how
many nematodes are there)? This number will help you determine whether
you should grow a non-host (such as corn), a resistant variety, or
a susceptible variety.
- Are they able to attack resistant varieties? An SCN type test (explained below) will help you choose the best resistant varieties.
- What is the field history (how often have soybeans been planted, and what varieties were used)? If you've planted resistant varieties in a field, and especially if you've planted the same resistant variety more than once, you need an SCN type test.
Three new methods of SCN testing will get you the information you need to combat SCN: egg counts, the SCN Type test, and our screening program.
SCN Egg Counts
SCN eggs hatch into the microscopic worms that infect soybean roots. The eggs are contained within cysts, which are the dead remains of the females that produced the eggs. Many labs perform SCN cyst counts because SCN cysts are easy to extract from soil and fairly easy to see and count under low magnification. SCN populations have been assayed based on cyst counts ever since SCN was first found in Illinois.
Egg counts are harder to do and may be slightly more expensive, but they can give a much better picture of what's going on in a field than cyst counts do, especially if SCN-resistant varieties have been grown in the field.
The main reasons for the superiority of egg counts over cyst counts are that cysts are dead—they can't infect anything—and that cysts contain different numbers of eggs.
Old cysts contain fewer eggs than new cysts, and cysts produced on some resistant varieties contain fewer eggs than those produced on a susceptible. Relying on cyst counts may not give an accurate reflection of the actual SCN population.
Digital imaging technology has now improved to the point that we can count eggs electronically. We are developing a protocol for SCN egg counting based on an automated microscope and a digital image analysis system. This will make egg counts faster, easier, and more accurate than ever. Even better, electronic egg counts are no more expensive than traditional ones.
SCN Type Tests
For many years, soybean seed companies have been giving information about matching the SCN-resistant variety to the SCN problem by labeling their varieties as "resistant to race 3" or "resistant to races 3 & 14." But up until now there has been no easy (or inexpensive) way to tell whether your SCN were race 3 or something else.
We will soon be offering a test that will tell producers what kind of SCN they have so they can match them up with the right SCN-resistant varieties. This is not a race test or an HG Type test—it's a new test called an SCN Type test. It's based on the HG Type test used by nematologists to classify SCN populations, but it's streamlined for farmers, and it's much easier to understand and use.
Here's how it works: We extract the SCN from a soil sample and grow the nematodes on four soybean lines in a greenhouse for 30 days, at which time we count the females (cysts) on the roots. From the counts, we calculate a Female Index (FI)* that will indicate which lines will support the nematodes. FI numbers are usually less than 100. Anything less than 30 is considered low, and more than 60 is high.
The four lines used for the SCN Type test are a standard susceptible variety (Williams 82) and three lines called plant introductions (PI) 548402, PI 88788, and PI 437654. We use these three because 99 percent of the SCN-resistant soybean varieties in Illinois got their resistance from one or more of these three. They are often called the "sources of resistance."
Using the FI numbers to choose varieties is easy. Choose varieties that have a source of resistance that matches the lowest FI numbers, and avoid varieties that have sources of resistance that match the highest FI numbers (see examples 1 and 2). The source of resistance is easy to find on the web or in print. In 2003, the Illinois Soybean Program Operating Board will publish a useful booklet called "Soybean Varieties with Soybean Cyst Nematode Resistance," which will give the source(s) of resistance in each SCN-resistant variety.
Example 1. The SCN in this farmer's field show a high FI on PI 88788. He can expect varieties with PI 548402 and PI 437654 to have much more effective resistance in his fields. He can use a variety with either PI as a source of resistance this year, then switch to the other source the next time he grows soybeans.
|Source Of Resistance||FI||Choose Varieties With This Source Of Resistance|
Example 2. The SCN in this farmer's field have low FI on all three sources of resistance. He's in luck, and any resistant variety should do better for him than a susceptible one will. He should still switch varieties the next time he grows soybeans in this field.
|Source Of Resistance||FI||Choose Varieties With This Source Of Resistance|
An SCN Type test is not necessary every time soybeans are grown. In fact, it probably only needs to be done once. Choose a variety with the source of resistance that matches the lowest FI number, and then the next time soybeans are planted in that same field, choose a variety with a different source of resistance. In this way, SCN will be less likely to adapt to the resistant varieties.
There are two cautions to using the SCN Type test to design an SCN-management plan.
- If SCN numbers are very high (say, more than 10,000 eggs per 250
cm3 soil), then even SCN-resistant varieties may be damaged.
Always base use of the SCN Type test on knowledge of the SCN numbers
in a field.
- Base the choice of varieties on yield (of course!). Once the best source of resistance is identified, check out soybean variety trial data (http://vt.cropsci.illinois.edu/soybean.html), and pick the highest-yielding variety that has the desired source of resistance.
Screening for Resistance
When a seed producer puts "resistant to SCN race 3" or "resistant to SCN races 3 & 14" on a seed label, what does that really mean? It's a simple question, but it has a very complicated answer. The simplest answer is:
The resistant variety allowed fewer females to develop on its roots than a comparable susceptible variety. The resistant variety will usually yield more than a susceptible variety in a field with a heavy SCN infestation.
However, it does not mean that every race 3-resistant variety has the same level of resistance, or that the variety will out-perform a susceptible variety in every infested field. One of the biggest problems we have in classifying "resistance to race 3" is that not all race 3s are the same. (The same is true for any race).
In order to help producers interpret SCN-resistance labeling, we are now screening all the SCN-resistant varieties entered in the public soybean variety trials for their levels of resistance. This is being done as a collaborative effort among the University of Illinois (myself), the USDA (Dr. Greg Noel, also located at the U of I), and Southern Illinois University (Dr. Jason Bond). We cannot include data in this publication due to space considerations, but data tables will be available through the Variety Information Program for Soybeans (VIPS).
Varieties are being screened for resistance to at least three different "race 3" SCN populations, to one "race 1," and to one "race 5." These represent the most common races that occur in Illinois. The method used for screening is very similar to that described above for SCN Type tests, but we convert the FI values into resistance ratings for ease of interpretation. Notice that any variety with an FI value of less than 60 is considered to have some level of resistance.
The best way to choose varieties based on this information is, again, to start with yield. Check the yields of varieties (all of the variety test sites are infested with SCN), and choose a list of those that look the best in environments similar to the one where they will be grown. From that list, check the resistance levels from our screening test, and choose the ones with the best resistance (HR, R, or MR).
- Know how many SCN eggs you have. Submit a soil sample to a good lab for analysis. If you have more than 10,000 eggs per 250 cm3 soil, don't grow soybeans if you have any other option.
- Know your FI numbers. Choose varieties that match your lowest FI.
- The next time you grow soybeans in that same field, choose a variety with a different source of resistance if you can. At the very least, don't grow the same variety in the same field twice in a row, even with a corn crop in between.
- Choose varieties based on unbiased yield data and unbiased SCN-resistance data.