Mid-to-Late Season Soybean Diseases

Beginning pod (R3) through full seed (R6) are critical soybean growth stages. Soybean diseases occurring during this time may affect yield potential. The impact of fungal diseases, for example Septoria brown spot, may be reduced with foliar fungicide application. However, foliar fungicide applications have no activity on bacterial diseases, such as bacterial leaf blight or bacterial pustule, or stem diseases such as sudden death syndrome (SDS) or brown stem rot. Soybean products differ in their tolerance or resistance to foliar and stem diseases.

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Figure 1

Figure 1. Bacterial leaf blight (left) and Bacterial pustule (right).

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Bacterial leaf blight (Figure 1) symptoms include small, angular, water soaked spots which turn yellow; then brown surrounded by a yellow halo as the tissue dies. Several spots may merge with the dead tissue dropping out giving the leaves a ragged appearance. Leaves usually remain on the plant. Disease incidence is commonly observed in the mid to lower canopy. Cool, wet weather favors the disease which is caused by bacteria spread by wind, rain, and cultivation when the foliage is wet.

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Bacterial pustule (Figure 1) begins with tiny, pale-green spots with raised centers which appear on upper and/or lower leaf surfaces. Small, light-colored pustules later form in the center of lesions, often on the lower leaf surface. Symptoms can vary from small, individual lesions to large, irregular, brown areas which form when lesions coalesce. Leaves become ragged when wind and rain tear away dead areas. Severe infection can result in some defoliation. Warm, wet weather contribute to incidence of this bacterial disease.

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Septoria brown spot lesions are angular to irregular, dark brown, and caused by a fungus. Leaf tissue surrounding lesions becomes yellow. This is usually one of the first foliar diseases to occur and symptoms are generally mild during early growth stages. Late in the growing season, leaves become rusty brown or yellow and drop prematurely. Symptoms first appear on lower leaves during warm, wet conditions and then progress to the upper leaves.

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Figure 2

Figure 2. Soybean stem split lengthwise showing the brown pith in the stem from BSR.

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Brown stem rot (BSR) can cause either foliar symptoms and internal stem symptoms or only internal stem symptoms. Foliar symptoms may include yellow and brown discoloration appearing between the leaf veins which are very similar to sudden death syndrome (SDS). To diagnose BSR from SDS, split the stem lengthwise and view the internal stem pith. BSR infected stems have dark chocolate-brown pith, sometimes only at the nodes (Figure 2). Soybean products differ in tolerance or resistance to BSR.

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Sudden death syndrome (SDS) usually appears mid to late season. Symptoms begin as yellow spots scattered on the leaves. The spots become necrotic, and merge, leaving the leaf veins green. Infected leaves drop off, however the leaf petioles remain attached.

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SDS infected plants have a reduced root mass and can easily be pulled from the soil. Foliar symptoms may be similar to BSR, however, the pith inside the stem remains white compared to brown, dead-like pith exhibited by infected BSR plants.

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SDS is favored by high yield environments and may be more prevalent in years with cool temperatures and moist soil throughout the first half of the growing season.

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The SDS fungus enters through plant roots and infested fields have been associated with moderate to high levels of soybean cyst nematode (SCN) populations. SDS infected fields should be screened for SCN population densities. Soybean products differ in SDS tolerance.