Phytophthora Root Rot
  in Soybean
  ------ variety selection is key

Phytophthora root rot in soybean (click here for picture) is prominant in many area fields in 1997. Varietal differences for resistance was very evident at the Fond du Lac UW variety performance trials. Understanding varietal resistance to phytophthora root rot is somewhat complicated.

Race specific resistance

There are many races of the Phytophthora root rot fungus in Wisconsin soils. Genes can be incorporated into varieties to provide resistance to the specific races. The genes are labeled as follows: Rps1-a, Rps1-b, Rps1-c, Rps1-k, Rps3, Rps4, and Rps6. Each of the genes may provide resistance to 10 or more Phythophthora races. Varietal gene resistance data can be found in the Wisconsin Soybean Variety Test Bulletin (available at no charge from the UW Extension Office). The Rps1-a gene is not effective in most soybean fields in Wisconsin. The Rps1-c gene has value, but many fields have races that overcome its effectiveness. The Rps1-k gene is the most effective gene in Wisconsin (effective in about 82% of fields), but plants need to be monitored for disease symptoms that may indicate the development of non-targeted races in the field. Where soybean is grown often in a crop rotation, it's sometimes advisable to rotate varieties between Rps1-c and Rps1-k gene resistance to prolong the effectiveness of the Rps1-k gene.

Field tolerance or resistance

Now comes the complication. "Tolerance" or "field resistance" to Phytophthora root rot is sometimes cited for specific varieties. This is non race-specific and can help overcome a lack of gene resistance in some situations. The performance of soybean varieties with the field tolerance reaction to Phytophthora root rot is generally good unless soil conditions are highly favorable for the disease to develop. It should be noted that reliable tests for tolerance have not yet been developed. Field tolerant varieties would benefit favorably from Apron (fungicide) seed treatment in most years.

Other considerations

Unlike Brown Stem Rot, crop rotation has limited value to control Phytophthora root rot. The pathogen produces spores that enable it to survive for many years in the soil. Because the fungus is most active when soils are waterlogged, choosing fields with good internal drainage offers the best defense against Phytophthora root rot.