May 17, 2004
From: American Phytopathological Society, Plant Disease Notes [edited]

Occurrence of soybean stem canker (Diaporthe phaseolorum var. meridionalis) in Wisconsin
S Li, National Soybean Pathogen Collection Center, Department of Crop Sciences, University of Illinois, Urbana; NC Kurtzweil, CR Grau, Department of Plant Pathology, University of Wisconsin-Madison; GL Hartman, USDA-ARS,
Department of Crop Sciences, University of Illinois, Urbana. Plant Dis 2004; 88: 576, published online as D-2004-0315-01N, 2004. Accepted for publication 29 Jan 2004.

Soybean (_Glycine max_) developed symptoms characteristic of stem canker during the 2000 to 2003 growing seasons in Wisconsin. Symptoms were widespread in 2003 and were associated with yield losses of about 1 per
cent statewide and as much as 25 per cent in individual fields. Affected plants expressed dieback of foliage beginning at growth stage R3 and progressed until the R6 growth stage. Dark brown lesions were frequently
observed at a single node on the lower portion of stems of plants expressing foliage dieback.

Fungi were isolated from symptomatic plants collected from 7 growers' fields in Rock, Sauk, Veron, and Walworth counties and the Arlington and Marshfield Agricultural Research Stations. Stems with lesions were cut into approximately 5 mm pieces, surface disinfested with a 0.5 per cent NaOCl solution for 3 min, rinsed 3 times in sterile distilled water, and placed on water agar (WA) or potato dextrose agar (PDA) at pH 4.5.

Hyphal tips from colonies of interest were excised and placed on acidified PDA at 25 deg C under continuous light for 25 to 30 days. In addition to _Diaporthe phaseolorum_ var. _caulivora_ (the cause of northern stem canker), 4 isolates of _D. phaseolorum_ var. _meridionalis_ (the cause of southern stem canker) were isolated.

Colonies of _D. phaseolorum_ var. _meridionalis_ isolates were white, lanose, and became tan with age as previously described for _D. phaseolorum_ var. _meridionalis_ (1). Pycnidia with alpha conidia (no beta conidia) and perithecia with 3.1 to 3.4 x 9.5 to 9.8 micrometers ascospores formed on oat flakes on acidified WA after 30 days. Stromata were brown to black and irregularly shaped.

4 isolates of _D. phaseolorum_ var. _meridionalis_ were tested for pathogenicity in a controlled environment using a cut stem inoculation method (2). Stems of 3-week old seedlings of cv. Sturdy were cut at the midpoint between the 2nd and 3rd node, and a PDA mycelial plug (4 mm diameter) was placed on the surface of the cut stems. This method was used to inoculate 15 plants in 3 replicates for each isolate tested. Inoculated plants were placed in a mist chamber in the dark at 25 deg C for 4 days and later moved to a greenhouse with a 16 h photoperiod at 24 ± 3 deg C for 3 days.

All plants challenged by this method exhibited stem lesions that were 2 to 3 cm long and of similar color to lesions observed in field-grown plants. For each isolate tested, _D. phaseolorum_ var. _meridionalis_ was reisolated from 3 randomly selected symptomatic plants. Negative controls with a PDA plug did not produce lesions.

To our knowledge, this is the first report of _D. phaseolorum_ var. meridionalis on soybean in Wisconsin. The significance of this report relates to the potential spread of _D. phaseolorum_ var. _meridionalis_ beyond its known southern range in the United States.

References:
(1) FA Fernandez, et al. Stem canker. In: GL Hartman, et al (eds). Compendium of soybean diseases, 4th ed. St Paul, MN: American Phytopathological Society, 1999: pp 33-35.
(2) S Li, et al. Plant Dis 2001; 85: 1031.

[Stem canker has been reported in Europe, South America and in most soybean regions of Canada and the United States. During the early 1950s, northern stem canker was prevalent in the north-central US, causing losses of up to 50 per cent. Southern stem canker was first observed in the southern US in 1973, caused widespread losses in the early 1980s, and is now considered endemic throughout the south, where losses of 100 per cent have been reported. There does not appear to be any overlap in geographical range of these 2 stem canker diseases.

There are 2 different types of stem canker, northern and southern. Northern stem canker is caused by the fungus _Diaporthe phaseolorum_ var. _caulivora_ [Dpc], and southern stem canker is caused by _D. phaseolorum_ var. _merdionalis_ [Dpm]. Both pathogens can overwinter in infested soybean residue and may be spread with infested seed. Stem canker is closely related to pod and stem blight, which is caused by the related pathogen
_Diaporthe phaseolorum_ var. _sojae_.

Stem canker is best managed by planting resistant or moderately resistant varieties. Although resistance genes are known for southern stem canker, apparently none is available for northern stem canker. Delayed planting may be helpful, and tillage may reduce disease in fields where stem canker has been a problem. Foliar fungicides may be useful in managing the disease.

Additional reference: <http://www.soydiseases.uiuc.edu/diseases.php?id=13> - Mod.DH]