April 16, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of charcoal rot epidemics caused by Macrophomina phaseolina in soybean in Iowa
X. B. Yang and S. S. Navi, 351 Bessey Hall, Department of Plant Pathology, Iowa State University, Ames 50011. Plant Dis. 89:526, 2005; published on-line as DOI: 10.1094/PD-89-0526B. Accepted for publication 17 Feb 2005.

_Macrophomina phaseolina_, the causal agent of soybean charcoal rot, is widely present in soil and has been reported to cause yield losses of 30 to 50 percent, mainly in southern soybean production regions of the United
States. (2). Charcoal rot was first reported in North Dakota during 2002, suggesting a range expansion for _M. phaseolina_ (1). Charcoal rot has been occasionally observed in Iowa in individual soybean plants, but epidemics
of the disease have not been recorded.

During the 2003 growing season, a severe epidemic of charcoal rot was observed throughout the state. Diseased plants were first noticed in late July and by late August, patches of diseased plants wilted and died prematurely in many fields. The pith of diseased plants had a brown discoloration in taproots and lower stems. Symptoms were observed up to the 4th or 5th nodes, typical of charcoal rot. In some plants, no discoloration was evident. In discolored plants, microsclerotia of _M. phaseolina_ were commonly observed in the epidermis, just beneath the epidermis, and inside taproots and lower stems of wilted plants.

A systematic survey was conducted between late August and early September, 2003 to determine the prevalence and severity of charcoal rot in Iowa. The disease was observed in 60 percent of fields surveyed in northern Iowa
(north of latitude 42.5 deg N), 90 percent in central Iowa (latitude 41.6 to 42.5 deg N), and 20 percent in southern Iowa (south of latitude 41.6 deg N). Incidence in surveyed fields ranged from 10 to 80 percent. One 80-ha field in Hampton (northern Iowa) had 50 percent disease incidence. An incidence of 80 percent, with extensive premature death prior to growth stage R6 (full seed), was observed in a 16-ha field east of Huxley (central Iowa). The fungus was isolated by splitting open the tap roots of a few representative symptomatic plants from 18 fields.

Tissue colonized with microsclerotia of _M. phaseolina_ was scraped, collected in sterile petri dishes, surface sterilized in 1 percent sodium hypochlorite, washed in distilled sterile water, and transferred to potato dextrose agar. The plates were incubated for 2 weeks at 22 ± 1 deg C on laboratory benches with a 12-h photoperiod. All resulting cultures produced abundant microscleorotia of _M. phaseolina_ similar to those described by Smith and Wyllie (3). Pycnidia were observed in 2 cultures. August 2003 was the driest month on record in Iowa, which may have contributed to the outbreaks of this disease. Statewide, yield in Iowa soybean during 2003 was 1976.1 kg/ha (2798 kg/ha in 2002).

The large-scale epidemic of charcoal rot may have contributed to the overall reduction in soybean productivity in Iowa in 2003 (4). Since _M. phaseolina_ also infects corn (2), and corn/soybean rotation is the most common cropping system in Iowa, efforts are needed to address the future risk of _M. phaseolina_ to corn and soybean.

References:
(1) C. A. Bradley and L. E. del Rio. Plant Dis. 87:601, 2003.
(2) D. C. McGee. Soybean Diseases: A Reference Source for Seed Technologists. The American Phytopathological Society, St. Paul, MN, 1992;
(3) G. S. Smith and T. D. Wyllie. Charcoal rot. Pages 29-31 in: Compendium of Soybean Diseases. 4th ed. G. L. Hartman et al., eds. The American Phytopathological Society, St. Paul, MN, 1999.
(4). X. B. Yang et al. Biology and management of soybean charcoal rot. Pages 55-60. in: Proc. 15th Integrated Crop Management Conf. Iowa State University, Ames. 2003.

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[The charcoal rot pathogen _Macrophomina phaseolina_ (Mp), also known as dry-weather wilt or summer wilt, is a warm-weather pathogen, causing stress on crops under unfavorable environmental conditions. Mp is widely
distributed, and the disease occurs worldwide. Under severe disease conditions, the disease reduces yields and seed quality. Mp has a wide host range and geographic distribution, infecting more than 500 crop and weed
species. It is highly variable, with isolates differing in microsclerotial size and the ability to produce pycnidia. Microsclerotial morphology is a key taxonomic characteristic in its identification. Cultural and morphological characteristics can vary as a result of continuous sub-culturing. Plants moderately resistant to cvs. such as Delta-Pineland 3478, Hamilton, or Jackson II, or the tolerant cvs. Davis or Asgrow 3715 are recommended. Select early maturing cvs. that do not have late reproductive growth stages that might coincide with periods of drought
stress and high temperatures.

Links:
<http://www.cornandsoybeandigest.com/mag/soybean_charcoal_rot_worse/>
<http://www.soydiseases.uiuc.edu/index.cfm?category=diseases&disease=94>
<http://www.scielo.br/scielo.php?pid=S0100-41582003000200002&script=sci_arttext&tlng=en>
- Mod.DH]