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June 6, 2003
Source: American Phytopathology Society, DISEASE NOTES [edited]

First Report of Charcoal Rot on Soybean Caused by Macrophomina phaseolina in North Dakota
C. A. Bradley and L. E. del Rio, Department of Plant Pathology, North Dakota State University, Fargo,
ND 58105. Plant Dis. 87:601, 2003; published on-line as D-2003-0317-  01N, 2003. Accepted for publication 4 Mar 2003.

In late August 2002, patches of soybean (_Glycine max_) in a field in Richland County, ND exhibited symptoms and signs of charcoal rot and died prematurely. Dead plants had a silvery-gray appearance, and black microsclerotia (76 +/- 28 micrometers in diameter) were present in the vascular tissue of the lower stems and roots.

Gray hyphae grew from stem tissue cultured from stem pieces placed on PDA which subsequently turned black and formed microsclerotia (188 x 139 +/- 48 micrometers). The fungus was identified as _Macrophomina phaseolina_ [Mp] (Tassi) Goid. based on colony color, and morphology and size of microsclerotia (1).

To confirm pathogenicity, soybean plants (cv. Garst D 041 RR) were grown in a greenhouse and inoculated with the Mp isolate. Soybean stems at the V2 stage were excised just below the third node.

Mycelia plugs of a 1-week-old culture of Mp were placed into the large end of disposable micropipette tips (200 microliters). The micropipette tips containing the Mp culture were subsequently placed over 10 excised soybean stems. 10 excised soybean stems, inoculated with micropipette tips containing plugs of noninfested PDA, served as controls.

30 days after inoculation, the micropipette tips were removed, and lesions were measured. The mean length of Mp lesions was 7 mm; no lesions developed on control plants. Mp was reisolated from infected
tissue of inoculated plants placed on PDA.

Charcoal rot was only observed in the Richland County field; however, no surveys were conducted to determine the prevalence of the disease throughout the soybean production area of North Dakota. Because it is now known that this disease occurs in North Dakota, growers, extension personnel, and crop consultants must scout for the disease and practice recommended management strategies.

Reference:
(1) G. S. Smith and T. D. Wyllie. Charcoal rot. Pages 29-31 in: Compendium of Soybean Diseases, 4th ed. G. L. Hartman, J. B. Sinclair, and J. C. Rupe, eds. The American Phytopathological Society, St. Paul, MN, 1999.

[Mp continues to spread in the USA. It is a weak pathogen of soybean, killing plants that are stressed, especially by high temperatures. It is an important pathogen in southern USA , Mexico, and Africa. Mp has a very wide host range, including maize and sunflower. Sclerotia survive in soil, thus providing inoculum for future infections of the crop. Charcoal rot is the major fungal disease in Kansas.

A survey during 1996-98 of 23 soybean diseases in the USA provides information relevant to charcoal rot.
- total production was 7.9 billion bu.; average was 2.6 billion bu.
- total disease losses were 1.358 billion bu; average was 453 million bu.

Average disease losses (million bu.) due to specific pathogens:
- soybean charcoal rot - 22.4.
- phytophthora root rot - 45.5
- soybean sudden death - 16.0
- soybean cyst nematode - 237.0
- various viruses - 4.6

Additional references:
<http://aes.missouri.edu/delta/research/soyloss.stm>
<http://www.gsf99.uiuc.edu/invited/2_5_01.pdf> - Mod.DH]