Pest Patrol: Soybean cyst nematode (SCN) and sudden death syndrome (SDS)

#PestPatrol with Albert Tenuta, OMAFRA

Figure 3. Interveinal yellowing and browning typical of SDS infection.

The dry conditions in 2020 have been favourable for soybean cyst nematode (SCN) infection. A reminder that you can look for SCN by digging up plants with a shovel and gently removing soil and examining roots for the presence of the small white to yellow cysts (Figure 1 below).

Figure 1. Small white to yellow lemon-shaped SCN cysts on root. Figure 2. Note the small size (1 mm) of SCN cysts compared to much larger Rhizobium nodules. photo: Supplied

Cysts are considerably smaller (1 mm) compared to Rhizobium nodules (Figure 2 above). If you find SCN, take a soil sample at the end of the season to determine population levels and plan to use an SCN-resistant variety in the future. If you are growing an SCN-resistant variety and have high levels of cysts on the root, this could indicate the SCN PI88788 resistance gene is not as effective as in the past. Therefore, growers should use another source of variety resistance such as Peking.

Sudden death syndrome (SDS) has become more noticeable during the 2020 season due to the much-needed rain events, especially in the southwest. Symptoms are yellowing and browning with green veins (Figure 3 at top of page). Leaves can drop prematurely but the petioles will remain. To confirm SDS, cut stems and look for brown discoloured areas along the outer stem layer while the centre (pith) will remain white (Figure 4 below).

Figure 4. Browning of the stem due to SDS infection. Note the pith remains white. photo: Supplied

In plants infected with brown stem rot, the stem pith will be brown while the outer stem layer will remain healthy. Late season manganese deficiency produces bright yellow leaves in the upper canopy between the veins with small brown spots and can be confused with SDS.

SDS-tolerant varieties as well as seed treatments are available and should be used in the future.

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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