2º Congreso Argentino De Fitopatología Libro de Resúmenes

2 º Congreso Argentino de Fitopatología
loss estimates of up to 50-65% were reported by some soybean
growers.
Several additional cases were seen in the early 2000’s and
numerous putative cultures of Diaporthe/Phomopsis were isolated.
The overwhelming majority of isolates had cultural characteristics
consistent with Diaporthe phaseolorum var. caulivora (Dpc), the causal
agent of Northern stem canker. Isolates of Phomopis longicolla were
also recovered from cankers. Inoculation trials, utilizing the toothpick
method (Crall, 1952) established pathogenicity of these putative Dpc
isolates. Phomopsis longicolla isolates, when similarly inoculated, did
not cause canker symptoms or death of inoculated plants.
Additional outbreaks of NSC were documented over the next several
years with Dpc type isolates consistently being isolated from cankers.
The identity of isolates was confirmed using the PCR-RFLP technique
developed by Zhang et al. (1998). Cultures of the two RFLP types
Dpc A and Dpc B (Zhang et al., 1998) were isolated 61% and 16
% respectively from cankers. Phomopsis longicolla composed 14.5%
of total isolates. No isolates of the Southern stem canker pathogen,
Diaporthe phaseolorum var. meridionalis (Dpm) have been identified
in South Dakota to date.
These damaging high-incidence outbreaks of NSC in South Dakota
have been relatively rare. Usually only one or two such outbreaks
have been documented each year. Characteristically, disease is
evenly distributed throughout these fields, disease incidence reaches
50-90% and yield losses can be as high as 65%.
One of the characteristic and truly puzzling aspects of high-incidence
outbreaks is the cropping history of the impacted fields. In nearly every
case, the field in which a high-incidence outbreak has occurred has
not had soybeans as a crop for several years immediately preceding.
Often these fields have been in a corn/alfalfa rotation. In 2009 a
field with a high-incidence field was documented near the town of
Castlewood, South Dakota. Disease incidence reached ca. 90% with
nearly each soybean stem completely girdled and killed. A yield of
approximately 2.7 MT/hectare was expected based on normal growing
conditions. Actual yield was 1 MT/hectare. Remarkably, this field had
been in enrolled in the Conservation Reserve Program (Anonymous,
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