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Breakdown of soybean resistance to soybean cyst nematode

Careful management is key to reduction.

Since the discovery of soybean cyst nematode (SCN) in Ontario in the late 1980s, SCN-resistant varieties of soybean have become available to minimize devastating yield losses. Now, there are reports that this resistance might be breaking down.  The change in field resistance, however, is difficult to determine.

According to most authorities on the subject, soybean cyst nematode is the number one disease problem in Ontario.Photo courtesy of Albert Tenuta, OMAFRA.

Several factors contribute to the appearance of reduced resistance to SCN in specific fields. Resistant soybean varieties do not have complete immunity against SCN; this variable resistance may give the appearance of resistance breakdown where it has not occurred. On the other hand, both the limited genetic diversity of plant resistance sources and crop management practices can contribute to real changes in SCN populations, affecting resistance.

 
Each SCN-resistant soybean variety available today was developed using one of several breeding lines, or sources of resistance. “Plant introduction (PI) 88788 is the source of resistance for probably more than 90 percent of the SCN-resistant soybean varieties in the Midwestern United States and Canada,” explains Dr. Greg Tylka, nematologist at Iowa State University. “This is because it is the least difficult of the sources of resistance to use and still maintain high yields.”  In the 2007 Ontario Soybean Variety Trials (www.gosoy.ca), only one of the 26 SCN-resistant varieties for which these third-party performance data are available had a non-PI 88788 source of resistance. This lack of genetic diversity contributes to concerns about the breakdown of PI 88788 resistance.

The seed industry has been working for years to introduce other sources of resistance into good-yielding varieties of soybean. “It has been really hard to break that genetic linkage between resistance and low yield,” says Don McClure, soybean breeder with Syngenta Seeds Canada. “It takes a lot of screening of progeny and many breeding generations to get up to decent yield levels.” McClure notes that several seed companies are close to releasing new varieties with non-PI 88788 resistance that are suitable for Ontario growing conditions.

The actual resistance itself is something of a misnomer because no one SCN-resistant variety has complete immunity against all SCN field populations. Albert Tenuta, plant pathologist with the Ontario Ministry of Agriculture, Food, and Rural Affairs, compares it to a soybean variety that has resistance to specific pathotypes or races of the Phytophthora fungus. “You can challenge that soybean variety with those Phytophthora pathotypes and you won’t get infection. The only way to get infection is if you get a new pathotype that bypasses that variety’s resistance.” With SCN, however, a variety that is classified as resistant can have up to 10 percent of the number of cysts found on roots of a susceptible check variety such as Lee 74. A moderately resistant variety can have 10 to 30 percent levels of cysts.

 
This variability in resistance has a lot to do with the genetic nature of the resistance. SCN resistance is linked to multiple genes, but “varieties are coming out without the full complement of genes conferring resistance,” says McClure. Because of this, a grower should be aware of the level of resistance of the specific variety being used in any given year. Depending on the field, as well as seasonal and genetic factors, a moderately resistant variety may show more severe SCN symptoms than a highly resistant variety.

However, the possibility of resistance breakdown is of concern and can occur because of factors that change the SCN population. For example, a grower might find a PI 88788 source that works well and then continue to plant it year after year in the same field. This causes the nematode population in that field to become adapted to that variety to some degree.  “The worst thing the grower can do is to keep planting the same SCN-resistant variety time after time after time,” says Tenuta. “That will promote a quicker shift to new populations of the nematode, which may overcome that source of resistance faster.

Figure 1:  A small proportion of fields have unusually large numbers of soybean cyst nematode eggs in the soil.  Source: Tom Welacky, Agriculture and Agri-Food Canada, Harrow, Ontario For further information on soybean cyst nematode see www.soybeancyst.info and www.planthealth.info/scn_basics.htm

”Still getting good yieldsTylka has conducted field experiments in which varieties with the PI 88788 source of resistance are grown yearly at nine or 10 locations throughout Iowa.  Even though some varieties are showing increasing levels of SCN, perhaps indicating some resistance breakdown, soybean yields are still quite good. “It’s not a good sign that the nematode is building up numbers on PI 88788 resistance, but I don’t think we’re on the verge of a collapse either. We’re still seeing very good yields, even though SCN reproduction is above 10 percent. I still get 55 and 60 bushel per acre yields from good PI 88788 resistance varieties,” says Tylka. 
In Ontario, Tom Welacky, research scientist with Agriculture and Agri-Food Canada, is involved with the yearly field trials that examine the agronomic performance of SCN-resistant varieties in SCN-infested fields for Table 7 of the Ontario Soybean Variety Trials.  Welacky says that the resistant varieties still have better performance than susceptible varieties in those fields based on yields and other agronomic measurements.

Welacky is also in the third year of an Ontario-wide survey to monitor changes in population numbers and races of SCN.  So far, he has tested 135 of 424 samples to see how well the nematodes reproduce on the original PI 88788. He is finding that although about one-quarter of the samples tested have greater than 10 percent reproduction on PI 88788, most of these fall between 10 and 30 percent reproduction. But he notes, “PI 88788 resistance is still more than adequate for taking care of the problem in Ontario fields as the SCN populations are now.” Also, the yields still seem to be as good as those of the average resistant variety. “In other words,” says Welacky, “We’re not getting an agronomic indication of serious breakdown in resistance, even though there are some populations that do reproduce over 10 percent on the PI 88788 varieties.”

However, Welacky has found that some fields have unusually high populations of  SCN (Figure 1), which he cautions could indicate resistance problems. Further tests on these samples are pending. Even if resistance breakdown is not severe in these populations, such high levels are not good news. “Even if you have a resistant variety, if you start getting over 10,000 eggs per 100 grams of soil, SCN can start to overcome the resistance because the population size is overwhelming.”

Integrate management options
There are a number of aspects that the grower should consider in his or her strategy to maintain yields, keep nematode populations low, and reduce the probability of decreased resistance. Detection is key. “It is important for growers to first know that they have SCN in the field,” says Tenuta. “Every grower in Ontario, whether in southwestern Ontario or Ottawa, should be scouting for SCN.”
Growers should also keep track of their soybean yields over the years. Yield decreases that are unrelated to weather or other obvious factors can appear long before other signs of SCN. In addition, changes in yield could indicate an increase or shift in SCN field populations to ones that reproduce well on that particular resistant soybean variety.

All three SCN experts recommend that once SCN has been confirmed, growers should rotate resistant soybean varieties to keep nematode populations from adapting to one particular variety.  The best practice is to rotate varieties with different sources of resistance, for example, PI 88788 and another source. “It’s extremely difficult for the nematode population to build up on two different sources of resistance,” says Tylka.  However, this practice is often difficult to follow because of the lack of availability of other sources of resistance. Despite this problem, McClure says that the seed industry is not yet hearing a big push from farmers to have alternate sources of resistance.

If growers cannot rotate sources of resistance, says Tenuta, they should at least rotate SCN-resistant varieties that have the PI 88788 source of resistance. He also recommends the increased use of crop rotation to reduce SCN population numbers.  “Our highest SCN populations have been in fields that had heavy use of soybean in the rotation,” says Tenuta, citing a corn/soybean rotation. “Any time you introduce a new crop in the rotation to extend the years between the soybean crops, that definitely has a positive effect.”
Ultimately, changes in the nematode population have a lot to do with how the grower manages SCN.  If a grower ignores the problem and continues to grow a susceptible variety, SCN population numbers can quickly become extremely high.  “Then it’ll be a tough battle to reduce that population to workable levels where it won’t be damaging to soybeans,” says Welacky.

 
Welacky emphasises that consistent monitoring is important. Growers should monitor their fields every two to three years to check SCN population levels. However, he thinks that the overall population decreases he has detected in the SCN population survey may indicate that growers are doing a good job of using resistant varieties and nonhost crops.

“All soybean growers in Ontario should be thinking about SCN and have a management plan in place because it is our number one disease problem and continues to be,” states Tenuta. As for other Canadian soybean growers, Tenuta says that so far, SCN has only been detected in Ontario, not the rest of Canada, but it is likely only a matter of time before it moves to other areas.