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Timely scouting and disease identification during the reproductive phase of development can provide information to facilitate crop management.
Figure 1. Soybean stem affected by white mold.
White mold produces white, cottony mycelial growth on the outside of the stem and pods, wilted leaves, stems that appear “bleached�? and shredded (Figure 1). Sclerotia, small black structures that resemble mouse or rat droppings, can be found on and inside plants that have been infected by white mold. White mold development is favored by cool, cloudy, wet, humid weather at flowering. The disease is more problematic in soybeans in high-yield environments where high plant populations, narrow row spacing, and an early-closing canopy are commonly used. Some foliar fungicides are available for white mold management but timing is critical.
Figure 2. Leaf and stem symptoms of phytophthora.
Plants infected with phytophthora have mid- to late-season symptoms that include: brown lesions on the roots, root rot and a brown discoloration of the stem that can extend from below the soil line up into the plant (Figure 2). Leaves turn yellow, wilt, and stay attached to the plant. Plants may die throughout the season. The pathogen can infest plants early in the season but not show symptoms until mid-season or later if seedlings survive the damping-off stage of the disease. Wet weather, poorly drained soils, and compaction favor the disease.
Figure 3. Interveinal chlorosis and white stem pith of SDS.
Soybean cyst nematode (SCN) infestations may not be detected for several years because yield reductions often happen slowly and symptoms can be easily confused with other disease and management problems. SCN symptoms include irregular patches of stunted, yellow plants, and white female nematodes on roots. Plants under stress are more susceptible. SCN feeding causes wounds increasing susceptibility to soil-borne pathogens.
Figure 4. Internal browning of the stem from brown stem rot.
Sudden death syndrome (SDS) has not been confirmed in North Dakota but known infestations are in Otter Tail county Minnesota. SDS can interact with SCN to cause significant loss of yield potential.1 Foliar symptoms of the SDS include chlorosis and necrosis between the leaf veins (Figure 3). The stem has white pith.
Brown stem rot has similar leaf symptoms as SDS and to distinguish between SDS and brown stem rot (BSR), split the stem and look at pith coloration (Figure 4). The pith will be discolored with BSR but white with SDS.
Charcoal rot has shown up in some late season soybeans recently. Charcoal rot is caused by a pathogen that can infect soybeans, corn, and sunflowers. Premature plant death with leaves still attached is the most common symptom. A gray discoloration of root and stem tissue develops below the outer tissues. Hot, dry weather, during the reproductive growth stages in the driest areas of the field. Black speckling can be found on the lower stem.
1Markell, S. 2015. Late season soybean stem diseases. North Dakota State University Crop & Pest Report. 160629201418