Integrated Crop Management

Races of Phytophthora

Phytophthora root rot is a well-known disease to Iowa soybean growers. Although the disease has been under control, with no large-scale severe outbreaks, it continues to be a production concern and causes damage in scattered areas because of the development of new races. In the last 2 years, reports have increased for Phytophthora root rot from varieties with the Rps1k gene (or k-gene). Despite the relatively dry spring thus far, we have received reports of Phytophthora root rot on varieties with the Rps1k gene.

At least 45 races of Phytophthora have been identified. However, there are only a few races of concern for Iowa soybean growers. Races commonly found in Iowa include 1, 3, 4, 13, 15, and 25. Races 13 and 15 may comprise a very small portion of the total.

The table lists genes that are resistant to infection by Phytophthora. Varieties carrying the Rps1k gene are resistant except for race 25 and this gene is effective in controlling Phytophthora. You should pay attention to Phytophthora problems found on varieties with the Rps1k gene. An increased number of such reports indicates the possibility of increasing populations of race 25 or other races that can defeat the Rps1k gene. In Iowa, race 40 was found 5 years ago and it would not be surprising to find other races that defeat the Rsp1k gene.

If you see Phytophthora damping-off on varieties with the Rps1k gene, consider the use of a variety with the Rps6 gene. If varieties with the Rps6 gene are not available, consider varieties with the Rps1c gene together with seed treatment with Apron. The Rps1c gene has very good tolerance to Phytophthora.

Resistance gene reaction to races of Phytophthora.

Resistance gene
Race Rps1a Rps1b Rps1c Rps1d Rps1k Rps6
1 R R R R R R
3 S R R R R R
4 S R S R R R
8 S R R S R S
13 R R R R R S
15 R R R R R R
25 S S S R S R

S, susceptible; R, resistant.

This article originally appeared on page 106 of the IC-484(14) -- June 19, 2000 issue.

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