Keeping Up With the Changes In SCN Management Recommendations
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Illinois is ranked number one in a lot of things, but perhaps the distinction of being the state with the most soybean cyst nematodes (SCN) is one it could do without. Over 80 percent of the soybean fields in Illinois are infested. SCN is our most important soybean pathogen, although to hear some people tell it, we don't have as big a problem with it as we had in the past. Nothing could be further from the truth! We may not see symptoms as severe as those we've losing yields. During the next few months, you'll probably hear more about one or more of the changes in SCN assessment and management that are outlined here.
Actual Threshold vs. Damage Threshold
For many years, recommendations for SCN management have been based on an estimate of the damage threshold, or the number of nematodes that could be expected to reduce soybean yields. The damage threshold estimate that we currently use (3 cysts/ 100 cc soil) is a good estimate based on years of data collected in many parts of Illinois. The only problem with the threshold is that it's an average, which means that it does not necessarily apply to any particular field. The SCN-soybean interaction is site-specific: In some fields, an SCN population density of 3 cysts/100 cc soil would not cause any measurable yield loss, whereas in other fields, it would be devastating.
The damage threshold concept is very useful for management of crop pests such as insects, for which action can be taken during the season as the result of scouting. For SCN, however, the action threshold is a more useful concept. This threshold is the number above which some action should be taken to prevent increases in SCN populations, whether or not measurable damage is occurring. If there's anything we know for sure about SCN populations, it's susceptible varieties should be avoided in SCN- have as big problem with it as we infested fields. Therefore, we will emphasize an action threshold of one clearly identifiable soybean cyst nematode as the signal to begin a rotation with seen before, but you'll resistant varieties and nonhost crops. In addition, the field should be monitored periodically for SCN levels.
Monitoring SCN Levels
There are two reasons why the number of eggs in a field is a better measure of SCN population change over time than the number of cysts. First, a characteristic of some SCN-resistant varieties is that they allow some female SCN (cysts) to develop, but these females do not produce large numbers of eggs compared to those on susceptible varieties on which there is a high correlation between numbers of cysts and numbers of eggs. Many of the varieties produced from the Plant Introduction (PI) 88788 source of resistance will allow cyst development with low egg production. Since these are the varieties most readily available and therefore most commonly grown, the egg numbers (rather than the cyst numbers) are better measures of SCN population densities.
The second reason to measure SCN populations in terms of eggs is that labs in all our surrounding states follow that procedure. In order for us to use their information on SCN management, we need to use the same terms.
This is not to say that there is anything wrong with monitoring cyst numbers, particularly for producers who did not know previously that their fields were infested. But for more useful information over time, the egg count is preferred.
The Race Is Off
Thanks to decades of hard work and cooperation between nematologists and soybean breeders, today we have SCN-resistant varieties with excellent agronomic characteristics. Many growers have been so pleased with the results they're getting from the use of resistant varieties that they're tempted to use the same ones repeatedly. This can create bigger SCN problems, because resistant varieties don't have complete resistance; that is, they do allow a few nematodes to develop and reproduce. These successful worms are adapted to the resistant variety and can build up to yield-reducing levels. The yield reduction is often "hidden," in that soybean fields suffering SCN-induced yield loss will appear to be perfectly healthy (unless some other yield-limiting factor is stressing the plants).
The adaptation of SCN to resistant varieties has been referred to in the past as "race shift." To farmers and seed producers used to dealing with the concept of race in Phytophthora or wheat stem rust, for example, this seemed like a straightforward enough idea: Put selection pressure on the SCN population, and it will shift to a new race that ins't controlled by the resistance. Our understanding of SCN populations was at this level in 1970, when the first SCN race system was published. By the time the expanded race system (16 races) was published in 1988, our understanding of genetic diversity in SCN was much better, but no better scheme than the race system had been proposed.
A group of nematologists, geneticists, and soybean breeders, all with years of experience working with SCN, met to consider modifying the SCN race system to make it more useful. The result of the meeting was a new system, composed of the most conservative possible changes to the race system. In fact, the new system has only three major changes:
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The name will be changed to HG Type. "HG" stands for Heterodera glycines, the scientific name for soybean cyst nematode. "Type" is free of the implications of "race" as a genotype designation.
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Pickett will be dropped from the list of soybean differentials. Pickett was derived from a cross with Peking and has no demonstrably different genes for resistance to SCN.
- New differentials will be added and referred to as "indicator lines." Four new sources of SCN resistance have been getting from registered in the journal Crop Science since Peking, PI 88788, and PI 90763 were identified.
If we used the same model as the race system for designating HG Types, we would now have 128 different Types to keep track of, which would be intolerable. HG Types will be named with numbers based on the indicator lines for which their FI>=10. For example, an SCN population that produces FI>=10 on PI 88788 would be an HG Type 3. Another population with FI>=10 on Peking, PI88788, and Cloud, would be an HG Type 1.2.7. A population that produces no FI>=10 would be an HG Type 0. In addition, anyone who designates an HG Type will be required to publish the FI as well. For example, simply designating a population as an HG isn't Type 2 will be insufficient; the designation will have to be "HG Type 2 with an FI of 65."
The HG Type system probably will not come into common use in the immediate future. For the present, the HG Type system primarily is a tool to describe SCN populations better than we can with the race system. In the next few years, though, the HG Type system will replace the race system, unless a system based on the identification of SCN genotypes is proposed.
For some time, specialists have been recommending that growers with SCN infestations rotate their resistant varieties by switching sources of resistance or, for example, changing from a "Race 3 resistant" variety to a "Races 3 and 14 resistant" variety when they next grow soybeans. This recommendation is still good and will be unaffected by the HG Type system. Most growers do not know what races occur in their fields, and most will not need to know what HG Type they have. For those who do wish to know, we are working on a modified or incomplete HG Type test based on the sources of resistance available to growers. Seed companies currently labeling their soybean varieties according to race resistance will have to come up with a more informative label. It has been recommended that seed companies print the source of resistance on the label, but it may be some time before that or something similar happens.
Refined Management Recommendations
Research on cultural methods for managing SCN has been going on for years, and as cultural practices have changed, we have tested their impact on the SCN-soybean interaction. Some of the recommendations resulting from recent research are new and surprising to soybean producers, as follows:
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Susceptible varieties are no longer recommended for use in a rotation in SCN- infested fields, unless the SCN population is reduced below the detection level. So many high-yielding resistant varieties are now available that rotation of resistant varieties (with at least one intervening year of a nonhost) should be possible. Information on SCN resistance is available through the Varietal Information Program for Soybeans).
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The recommendation to rotate sources of resistance in an SCN-management rotation is a good one, but this may not be possible or even desirable if other disease pressures (such as SDS) are present. It may be as efficient simply to rotate resistant varieties, because even those that have the same source of resistance will not necessarily have the same resistance profile. It's a good idea to monitor SCN populations in any rotation scheme-especially this one.
- Specific tillage and row spacing practices have not proved generally useful for SCN management. Although certain practices such as no-till have been touted as such in the popular press, none have been shown to be useful in all environments. The only way to know whether a specific practice is useful in a particular field is to monitor its effects on SCN in that field.
The Race Isn't Off...
SCN can still cause hidden yield losses to farmers with infested fields,
so the race to prevent damage isn't been tested for SCN-test it. If
a field has SCN-monitor it. If resistant varieties are used-don't use
the same one in the next rotation.