No. 14 Article 6/June 30, 2006

Brown-Speckled Yellow Soybeans

A few producers in the central Illinois area noticed yellowed beans over the last week. While yellowed beans have become a summer norm following postemergence herbicide applications, the symptoms observed recently do not seem consistent with the "yellow flash" typically experienced after an application of glyphosate. The leaves in these fields appeared yellow while the veins remained green, as if plants were suffering a nutrient deficiency (manganese or iron). In addition, leaves were often speckled with small brown spots, as if the plants were suffering from sun scorch or scald. In one particular field, the producer noted "green" tire tracks in otherwise yellowed beans. The fields in question likely were suffering from a nutrient deficiency (manganese appeared most probable), and the speckling likely resulted from a nutrient deficiency as well. Finding the underlying "true" cause means that one needs to dig a little deeper . . . literally.

Tire tracks devoid of rhizoctonia root rot symptoms (photo-Matt Montgomery, Sangamon-Menard Extension).

An examination of root material in one of the piqued fields showed roots that were "leopard spotted," with orange, orange-yellow, slightly brown, or occasionally brick red lesions. Roots displaying more obvious lesions often shriveled up, from the soil line down, when left in the open air for just a few minutes. In other words, a few minutes in the open air transformed root tissue into a darkened, sickly version of its former self. Root symptoms similar to those described are consistent with Rhizoctonia root rot. While manganese availability may have been marginal in the yellowed bean field, the real issue working these plants over appeared to be a lack of root material, caused when Rhizoctonia compromised the root system, affecting the plant's ability to procure micronutrients.

Lesions caused by Rhizoctonia solani (photo-Matt Montgomery, Sangamon-Menard Extension).

The disease we call Rhizoctonia root rot is caused by the fungus Rhizoctonia solani. A very hardy fungus that survives as either fungal fruiting bodies (sclerotia) or fungal hairs (mycelia), R. solani has a very wide host range. An excellent saprophyte, the fungus is known to reside and feed on field residue. However, cool, wet weather followed by warm to hot weather and semidry soil conditions encourages infection of seedlings. The tire tracks observed in the aforementioned field appeared because compaction maintained just enough moisture during warm to hot weather to deter the disease. All major Midwest field crops can be infected by this fungus, which can slash stands by half and reduce yields a couple score in the most severe cases. Following infection, roots sometimes develop the red to red-brown colored lesions near the soil surface. These lesions may eventually girdle the root, bringing the plant to its knees. Plant stress and heavy soils also encourage the onset of this disease.

Several options, used in combination, can reduce problems with Rhizoctonia root rot. First, maintaining or creating adequate drainage may decrease the likelihood of encountering the wet conditions initially needed to jump-start Rhizoctonia solani. Second, any practice that decreases plant stress should narrow the window of opportunity available for this pathogen to infect seedlings. Such decreased stress lets plants outpace this rotting fungus. Third, seed treatments may be a useful management option where historic Rhizoctonia problems occur. Finally, producers should use soybean varieties displaying high vigor and rapid germination. While residue may encourage the presence of this saprophyte, the benefits of minimum, strip, and no-till systems outshine occasional problems with Rhizoctonia. Infrequent problems with this disease should not precipitate a shift in tillage.--Matt Montgomery

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