Phytophthora is morphing. Like many pathogens, it has evolved about 70 new races able to survive on and infect resistant soybean varieties. And some of the new races can overcome all currently available soybean variety resistance genes on the market,says Jim Kurle, University of Minnesota plant pathologist.

“The last time we had a major Phytophthora resistance gene fail (back in the late '70s) there were 300,000 acres of soybeans lost in Ohio,” says Anne Dorrance, Ohio State University plant pathologist. “We're not seeing anything that dramatic yet, but we have found isolates that could kill plants with the Rps 1k gene, the most widely used resistance gene, in the early '90s.”

The experience of Minnesota researchers illustrates the scope of the problem. Kurle has identified 23 different races, or pathotypes,of Phytophthora, up substantially from the five races identified in 1986. Many of these do not correspond with races described in the scientific literature.

Single gene resistance to Phytophthora is absolute to certain races; but there are other genes conferring partial resistance against many races, explains Craig Grau, University of Wisconsin plant pathologist. “Those are your safety net.”

Soybean varieties with Phytophthora resistance have at least one Rps (resistant to Phytophthora sojae) resistance gene. They are Rps 1a, 1b, 1c, 1k, 2, 3, 6 and 8. These genes confer absolute resistance to Phytophthora, providing you don't have a race that has mutated to overcome it. Rps 1c, 1k, 3 and 6 are the most effective and widely used genes. A newly discovered gene, Rps 8, is being used more widely. “Rps genes such as Rps 1a and Rps 1c that were generally effective across the Corn Belt may now be ineffectivein some areas,” Kurle says. “Farmers should remember that various Phytophthora races maybe present in their fields and that they can defeat most available sources of resistance. Farmers cannot rely on single-gene resistance alone to maintain yields and profitability.”

Grau thinks of resistance as a high-wire act. “Your established resistance genes (Rps genes) can sometimes be defeated by these new Phytophthora races, but your better soybean varieties have a safety net — a second tier of partial genetic tolerance,” he says.

If you are planting a Phytophthora-resistant variety and the disease still plagues your field, try a variety with a different Rps gene than the one you have used, say experts.

Resistance to established Rps genes is highly regionalized. In Wisconsin, for example, the vast number of Phytophthora races have been effectively controlled by the Rps 1k gene. By contrast, Rps 1k has failed in North Dakota and Ohio, Grau says. Minnesota, too, may fall into this category.

The introduction of varieties with the Rps 8 resistance offers new choices of resistant varieties but,inevitably, use of the Rps 8 resistance gene will select isolates that can infect cultivars with this gene.

With the evolution of more complex Phytophthora strains or races, farmers' variety selection grows trickier. Breeders have to consider new types of resistance that are more difficult to work with, such as tolerance (also called partial resistance or field tolerance). Varieties with this form of resistance reduce but do not eliminate infection by Phytophthora.

Ideally, growers will want to select a variety with both types of genetic resistance to Phytophthora.

As more strains or races of Phytophthora sojae are able to evade genetic resistance, growers will have to rely on multiple tactics against Phytophthora, including tolerance or partial genetic resistance, fungicide seed treatments and field drainage. It will become increasingly important for farmers to identify fields where Phytophthora is a problem. This is where diligent scouting of soybean seedlings in the trifoliate stage, when it is particularly easy to spot its “signature symptoms” becomes important.

About Phytophthora

  • Phytophthora sojae infection is associated with saturated soils and soil temperatures of 50-60° F.

  • Phytophthora root and stem rots are most prevalent on heavy clay soils in no-till or reduced-tillage management systems.

  • Management of Phytophthora has relied heavily on single-gene resistance.

  • Resistance (Rps) genes are used to characterize races of Phytophthora sojae. More than 50 races have been identified.

  • To simplify and clarify identification of isolates, researchers and breeders have adopted a new system, the pathotype,to characterize P. sojae isolates.

  • The pathotype designation lists the Rps genes that can be overcome by the P. sojae isolate.

  • Exposure to resistance genes is selecting populations of P. sojae that can defeat commonly used and effective resistance genes.

  • Tolerance or partial resistance allows infection to occur, but with reduced effects on yield.

  • P. sojae can be controlled by treating seed with fungicides containing metalaxyl.

  • As resistance-defeating pathotypes become more prevalent, management of Phytophthora root and stem rots will rely increasingly on the combination of resistance, tolerance, drainage and seed treatment fungicides.