Phytophthora race 25 and soybean 1k gene strategy

Soil moisture in many regions of Iowa is at field capacity or more so planting will occur in wet soils this spring. Wet soils increase the risk of damping-off in soybean by the fungi Phytophthora and Pythium. Seedling disease concerns are further compounded this year by low-to-poor seed quality. If damping-off occurs this year, growers may have difficulty finding soybean seed for replanting and may want to consider the following strategies for controlling Phytophthora and Pythium damping-off.

Planting date

Because of poor seed quality this year, it may be wise to wait until the soil temperature is above 60°F before planting soybean. Pythium activity decreases as soils warm, and low-vigor seeds germinate and emerge more rapidly in warm soils. In contrast, damping-off caused by Phytophthora is more prevalent in warm soils.

Seed treatments

Seed treatments that contain Apron or metalaxyl are effective in controlling damping-off caused by both Pythium and Phytophthora and are a good investment in wet, poorly drained soils and in fields with a history of damping-off. We caution that Apron helps with controlling damping-off but does not last long enough to protect against root rot and stem rot caused by Phytophthora. In addition to Apron or metalaxyl, many formulas also have compounds effective against Fusarium and Rhizoctonia.

Resistant varieties

Resistance is the most effective strategy for control of diseases, unfortunately, there is no good resistance for damping-off caused by Pythium. Damping-off caused by Phytophthora can be reduced by planting soybean varieties with good resistance genes. Several races of Phytophthora are found in Iowa fields with races 1, 3, 4, and 25 the most common. Soybean varieties with the Rps-1C gene withstand races 1 and 3. The more common Rps-1k gene withstands races 1, 3, and 4 but not 25. The Rps-1k gene has been widely bred into soybean to manage Phytophthora root rot and has been very effective in management. However, for the past 2 years we have observed an increased prevalence of race 25 Phytophthora that has overcome the 1k gene.

Problem fields

Fields with a history of damping-off, Phytophthora root rot, or both require planning resistant varieties. If damping-off occurs with a soybean variety with the Rps-1C gene, replant with a soybean variety with the Rps-1k gene. If the Rps-1k gene fails then try to find a soybean variety with the Rps-6 gene, which should control race 25. Unfortunately, few soybean varieties have the Rps-6 gene. If soybean with the Rps-1k gene shows damping-off and replant seed is unavailable, consider planting continuous corn or a nonhost crop until breeders incorporate the Rps-6 gene into high-yielding varieties. If planting varieties with the 1k gene again, wait until the soil warms and dries and use Apron seed treatment.

This article originally appeared on pages 48-49 of the IC-486 (6) -- April 23, 2001 issue.

Updated 04/22/2001 - 1:00pm