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 such as Cercospora leaf blight, downy mildew, frogeye leaf spot, and Septoria brown spot may be reduced with foliar fungicide application. Applications for fungal diseases are generally made between R3 and R6 growth stages. Application after R6 is usually not recommended and foliar fungicides are generally active from 14 to 21 days.

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Foliar fungicide applications have no activity on bacterial diseases such as bacterial leaf blight 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 7

Figure 7. Bacterial leaf blight (left) and Cercospora leaf blight (right).

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Bacterial leaf blight (Figure 7) 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.

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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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Cercospora leaf Blight (Figure 7) is caused by a fungus with initial symptoms including mottled purple-to-orange discoloration of the uppermost leaves. Later the leaves have a leathery appearance highlighted with bronzing. Severe infection may result in defoliation, however, green leaves often remain below the defoliated area. Symptoms usually first appear at the beginning of seed set. The same fungus causes purple seed stain.

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Figure 8

Figure 8. Downy mildew (left) and Frogeye leaf spot (right)

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Downy mildew (Figure 8) is caused by a fungus and appears as small, yellow-green areas on the upper leaf surface. These areas enlarge and become grayish to dark brown with yellow-green margins. During moist weather, tufts of gray to purple fungal growth can be seen protruding from lesions on the lower leaf surface.

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Infected seeds, narrow rows and cool, moist conditions favor disease development.

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Frogeye leaf spot (Figure 8) appears as round, brown to gray lesions on the leaves which are surrounded by a thin, dark reddish-colored ring. The center of the lesions typically have small, black, fungal structures that can be seen with a hand lens. Lesions are prominent and can be up to 1/8 inch diameter. Infected seeds often have gray or brown discoloration. Infection is caused by a fungus and occurs usually after flowering.

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Figure 9

Figure 9. Septoria brown spot beginning symptoms (left) and fully developed symptoms (right).

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Septoria brown spot (Figure 9) lesions are angular to irregular and dark brown and caused by a fungus. Leaf tissue surrounding lesions become 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 10

Figure 10. 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 10). Soybean products differ in tolerance or resistance to BSR.

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Figure 11

Figure 11. SDS initial leaf symptoms (left) and severe leaf symptoms with a split stem exhibiting white pith (right).

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Sudden death syndrome 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 (Figure 11). Infected leaves drop off, however the leaf petioles remain attached. 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.