Scout for sudden death syndrome

It is time to scout for sudden death syndrome (SDS) in soybean fields. With the help of growers, extension staff, and ag business personnel last year, SDS was found in 13 Iowa counties. They were Black Hawk, Boone, Clinton, Des Moines, Floyd, Johnson, Jones, Henry, Marshall, Scott, Story, Washington, and Wright. The most severely infested fields were in eastern Iowa.

The first signs of sudden death syndrome: scattered yellow or white spots.

Sudden death syndrome is a fungal disease, first found in Iowa in 1993. It occurs after flowering, most often after the pod-fill stage. In Iowa, SDS generally appears in late July and early August, although delayed planting does reduce disease development. When scouting, pay special attention to early planted fields, especially in eastern Iowa, and fields that are flat, fertile and expected to have high yields.

SDS in an intermediate stage, interveinal necrosis.

SDS symptoms are fairly easy to identify. Leaves on infected plants first have scattered yellow or white spots between leaf veins. These spots start to die and enlarge to form brown streaks between the veins (interveinal necrosis). Only mid-vein and major lateral veins remain green. Eventually, infected leaves drop but petioles remain on the stem.

The plant on the left has root rot casued by SDS, compared to a healthy plant (right).

Diseased plants are easily pulled out of the ground because the taproots and lateral roots have deteriorated. The root cortex is light gray-brown and the discoloration may extend up into the stem. If infections are severe during the early reproductive stage, flower and pod abortion may occur. For a more detailed description, see an ISU Extension publication on this topic, Soybean Sudden Death Syndrome, Pm-1570.

It is possible to mistake two common diseases--the foliar-type brown stem rot (BSR) and stem canker--for sudden death syndrome because these diseases show similar leaf symptoms. However, SDS has symptoms on both leaves and roots. BSR shows only root rot (discoloration is brown and limited to the pith); whereas, SDS also may have a light gray-brown discoloration in the vascular tissue. A big difference between SDS and stem canker is that stem canker has a brown canker on the main stem and stems become brittle. Unlike brown stem rot or stem canker, severe SDS root rot makes soybean plants easy to pull.

Because sudden death syndrome is new to Iowa, laboratory isolation is necessary to confirm its presence. You may want to send suspected plants to the ISU Plant Disease Clinic at 351 Bessey Hall, Iowa State University, Ames, IA 50011. There is no charge for SDS identification and work is supported by soybean check-off dollars.

This article originally appeared on pages 152-153 of the IC-476(22) -- August 19, 1996 issue.

Updated 08/18/1996 - 1:00pm