MU Plant Diagnostic Clinic: The Soybean Sorrows of 2016

Published: March 2, 2017

Picture 1: Soybean seedlings with rot at the necks. Damping off initiates at the soil line, sometimes plants push upward before symptoms of rot are apparent. Picture by Patti Hosack.

Picture 2: Pythium oospores observed in symptomatic root tissues. Picture by Patti Hosack.

Picture 3: Lower stem tissues with microsclerotia, indicative of charcoal rot. Picture by Patti Hosack.

Soybeans made up 73 of the 168 field crops submitted to the diagnostic clinic in 2016, the most of any single crop. Overall, 2016 weather was quite unusual. A mild winter and early start to spring was followed by a cool May and hot, dry June. Heavy rainfall events in July brought drought relief to many Missouri regions, but late July/August flooding for some. Environmental disorders, such as lack of or too much water, and turbulent environmental conditions predisposed soybeans to a variety of diseases.

Early in the season, both seedlings and young plants were submitted with damping off or root rots. Species of Pythium are the typical causal agents of these types of disease, and are often associated with low lying areas prone to flooding or prolonged soil saturation. Symptoms include root and/or crown rot, foliar discoloration, similar to nutrient deficiency, stunting, or foliar necrosis (Picture 1). Pythium is identified by the oospores made in rotted tissues (Picture 2). Besides Pythium, other soilborne fungal pathogens can be cause the same type of symptoms. These include species of Rhizoctonia, Fusarium, and Phytophthora. Diagnosis of the causal agent can be helpful in determining management recommendations, such as crop rotation or seed treatments. Also awareness is raised to other potential issues that can correlate with early season root rots, such as stalk rots or vascular wilts. Species of Pythium are able to move from the roots into the stalk and cause crown and/or stalk rots later in the season. While species of Fusarium can be associated with minor root rots, then later develop into vascular wilts.

The hot, dry June decreased plant vigor through both heat and drought stress, and increased soybean susceptibility to charcoal rot. Other than abiotic issues such as drought stress or chemical injury, charcoal rot was the most often diagnosed problem on soybean in 2016. Charcoal rot is caused by the fungal pathogen, Macrophomina phaseolina, and typically shows up at flowering. Symptoms of charcoal rot include small leaflets, chlorotic new growth, mid-day wilting and often a light silvery-gray discoloration on the lower stems and taproot. The diagnostic sign is microsclerotia in the roots and lower stem cortex and pith tissues (Picture 3). The fungus survives as these microsclerotia on crop residue, and has a broad host range. As mentioned earlier, plant health and the environmental precursor of a hot, dry June were crucial to the prevalence of charcoal rot in 2016.

Phytophthora stem rot was the second most common disease diagnosed. Phytophthora species prefer wet conditions, and when soils are saturated produce a motile spore (zoospore) to spread from plant to plant. The disease was severe in July and August, particularly in areas prone to flooding or prolonged soil saturation. Symptoms of this disease are stunted, chlorotic plants and in severe cases, plant death (Picture 4). Stem lesions can be observed girdling stems and often blighting entire stems (Picture 5). Lesions are not superficial to the surface but cause discoloration (dry rot) through the cortex tissues. The MU plant clinic confirms Phytophthora presence by culture plating or serological testing.

Picture 4: (left) Plants showing symptoms of Phytophthora stem rot. Picture by Patti Hosack. Picture 5: (right) Stem lesion caused by Phytophthora stem rot. Picture by Patti Hosack.

Other diseases common to soybeans were also diagnosed in 2016. A list of confirmed diseases and pests diagnosed from soybean samples is below:

• Anthracnose stem blight / leaf spot (Colletotrichum spp.)
• Brown stem rot (Cadophora gregata)
• Charcoal rot (Macrophomina phaesolina)
• Fusarium root rot
• Phomopsis seed decay
• Phytophthora stem rot
• Pod and/or stem blight (Diaporthe spp.)
• Cercospora leaf blight
• Pythium root rot
• Rhizoctonia stem and/or root rot
• Stem borer (Dectes texanus)
• Sudden death syndrome (Fusarium virguliforme)
• Insect damage: thrips & spider mites