farmgate: Here Are Some Facts About SCN That Will Keep You Awake Tonight
What has your yield monitor been telling you about your soybean field? If you have been harvesting some of the seed company test plots which produced the results that are e-mailed around every day, you have been getting 65 to 85 bushels of beans per acre. If your yields are 30-40 bushels, you either had bin-run seed, lack of moisture, or possibly a problem with soybean cyst nematode (cue the scary Halloween music). Yes, SCN may be fleecing you, a lot like your jobless brother in law; but you can do something about the SCN problem.
At the University of Missouri, Nematology laboratory coordinator Bob Heinz says you are blaming yield loss on everything but SCN, confident that you planted SCN resistant soybean varieties. But he says based on 2005 research at the University of Illinois, 74% of SCN populations can attack most resistant soybean varieties, and while that was true for Illinois, Heinz says it is probably also true for Missouri. And regardless of your state, the statistic may be applicable as well. "More than 90% of all SCN-resistant beans planted in Missouri derive their resistance from the same source," Heinz said. "By only using one source of resistance, you're just asking for trouble. Producers think they're growing resistant beans, but really they're developing a population of nematodes that may grow well on their resistant lines."
The resistance issue is concerning as well to University of Illinois soybean breeder Brian Diers, who says, “Although there are many SCN-resistant varieties available, there is a danger that we are too dependent on the PI 88788 source of resistance. Among maturity group II to IV varieties that have an identified source of SCN resistance, PI 88788 is the sole resistance source for 93% and is at least one of the resistance sources for 95%. If we depend too much on the PI 88788 resistance source, we run the risk of SCN populations overcoming this resistance.”
One of Diers’ colleagues, University of Illinois nematologist Terry Niblack, was responsible for the 2005 survey that produced the results which concerned Missouri’s Heinz. Niblack says, “The SCN Type tests are not complete, but the preliminary data are shocking. In 1991, most of the SCN populations in Illinois were SCN Type 0, equivalent to “race 3” in the old terminology. In practical terms, this meant that most SCN problems could be controlled by the use of SCN-resistant varieties. After 15 years of almost exclusive use of SCN-resistant varieties derived from the soybean line PI 88788, we have seen a complete shift in the SCN Type: most SCN populations (~80%) are now SCN Type 2 or 1.2, which means that they can attack PI 88788 and any resistant varieties derived from PI88788. This survey demonstrated how the use of SCN-resistant varieties can change SCN populations.”
Let’s regroup and catch our breath. We used to have a problem, and we knew it. To solve that problem we used resistant varieties. Now we are back to square one, with the problem, but Missouri’s Heinz measured awareness and says farmers are in a state of denial. “The survey revealed that while 61% of the producers' samples contained SCN egg levels that exceeded the economic damage threshold, 62% of the producers did not believe they had any yield loss attributable to SCN. Nearly two-thirds of the producers had never submitted a sample for an SCN egg count test.”
Now that you have realized you should get a soil sample and have it tested for SCN, let’s put the brakes on SCN reproduction. Agronomists at Ohio State University are telling their farmers to do a better job at controlling winter annuals which provide a host to SCN. “These weeds extend the period during which SCN can reproduce during the growing season, since SCN reproduction otherwise stops when soybeans usually reach full maturity in September. For example, recent research has confirmed that SCN can infect winter annual weeds in early fall and produce new egg-containing cysts before the end of October. This is a major cause for concern since these additional cysts mean higher egg concentrations in soil and a greater likelihood of significant crop damage the next time soybeans are grown in the field. The following winter annual weeds have been identified as alternate hosts of SCN: purple deadnettle, henbit, field pennycress and shepherd’s purse.”
Deadnettle and henbit, both of which have purple blooms in the early spring are a bit late emerging this fall, giving farmers a couple extra weeks of opportunity to control them, in 2007 no-till bean fields. Specialists Kent Harrison and Mark Loux at Ohio State say, “Maximum deadnettle emergence was delayed until mid-September, which presents a good opportunity to control it effectively in early October while seedlings are small and before SCN reproduction on the weeds can occur. In SCN-infested fields, it is important to control purple deadnettle and henbit before sufficient time elapses for SCN to complete its life cycle and reproduce, typically 3 to 5 weeks after the weeds emerge. Delaying weed control measures beyond that period may be too late to prevent the deposition of new SCN cysts+eggs in the soil.”
Summary:
Unexplained yield loss in soybeans may be due to soybean cyst nematode, but you won’t know it without a soil test. While resistant soybean varieties are commonly used, SCN has been able to overcome the resistance, to the surprise of 62% of farmers. One of the factors that compounds SCN problems is the increasing number of winter annual weeds in no-till fields. Many, particularly deadnettle and henbit, are winter hosts for SCN allowing reproduction of the nematodes even when soybean roots are not present.
Posted by Stu Ellis on October 5, 2006 5:19 AM to farmgate