August 23, 2004
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited]

Recent outbreak of soybean sudden death syndrome caused by Fusarium virguliforme and F. tucumaniae in Argentina

M. Scandiani and D. Ruberti, Laboratorio Agricola Rio Parana, Ruiz Moreno 225 (2930) San Pedro, Argentina; K. O'Donnell, Microbial Genomics and Bioprocessing Research Unit, NCAUR, USDA, ARS, Peoria, IL 61604; T. Aoki,
Genetic Diversity Department, NIAS, 2-1-2 Kannondai, Tsukuba, Ibaraki, 305-8602, Japan; R. Pioli, Facultad Ciencias Agrarias, (2123) Zavalla, Argentina; L. Giorda, EEA, INTA Manfredi, (5988) Manfredi, Argentina; and A. Luque and M. Biasoli, CEREMIC, Facultad Ciencias Bioquimicas y Farmaceuticas, UNR, (2000) Rosario, Argentina. Plant Dis. 88:1044, 2004; published on-line as D-2004-0714-01N, 2004. Accepted for publication 22 Jun 2004.

Sudden death syndrome (SDS) of soybean was detected initially in Argentina during 1991-1992 in the Pampas Region and 1992-1993 in the Northwest Region. The 1st report of the fulfillment of Koch's postulates of SDS
caused by _Fusarium solani_ f. sp. _glycines_ in Argentina was published in 2003 (3). Subsequently, analyses have shown that _F. solani_ f. sp. _glycines_ represents several morphologically and phylogenetically distinct species, including _F. tucumaniae_ in Argentina and _F. virguliforme_ in the United States (1).

Isolations were made from plants that exhibited typical SDS symptoms (interveinal foliar chlorosis and necrosis leading to defoliation of the leaflets but not the petioles) from fields in Santa Fe and Buenos Aires provinces in 2001, 2002, and 2003.

To determine which species are responsible for SDS in Argentina, cultures of 8 slow-growing isolates that developed bluish pigmentation and produced abundant macroconidia in sporodochia on potato dextrose agar were subjected to morphological and molecular phylogenetic analyses and pathogenicity tests. Morphological analyses demonstrated that 3 of the isolates were _F. virguliforme_ and 5 were _F. tucumaniae_. Isolates of _F. tucumaniae_ produced long and narrow sporodochial conidia while _F. virguliforme_ produced diagnostic comma-shaped conidia.

Molecular phylogenetic analyses of DNA sequences from multiple loci confirmed morphology-based identifications and showed that the soybean SDS pathogen in the United States, _F. virguliforme_, was also present in Argentina. To our knowledge, this is the 1st report of _F. virguliforme_ in Argentina and of this pathogen outside the United States. 5 isolates of _F. tucumaniae_ and 3 isolates of _F. virguliforme_ were used for pathogenicity tests. _F. virguliforme_ isolate 171 provided by J. Rupe (University of Arkansas, Fayetteville) was used as a positive control.

Soybean cultivars Ripley, RA 702, Pioneer 9492RR, Spencer, and A-6445RG were inoculated with each of the isolates tested in a greenhouse assay using soil infestation and toothpick methods (2).

All 8 isolates produced typical foliar SDS symptoms 15 to 25 days after inoculation. Severity of foliar symptoms averaged 3.3 for F. virguliforme, 2.6 for _F. tucumaniae_, and 3.3 for the positive control using a disease severity scale in which 1 = no symptoms and 5 = severely infected or dead plants. Under these conditions, _F. virguliforme_ appeared to be more virulent than _F. tucumaniae_. Noninoculated plants remained symptomless.

Koch's postulates were confirmed with soybean cultivars RA 702 and A6445RG. Isolates recovered from symptomatic plants inoculated by the soil infestation and toothpick methods were identical to those used to inoculate the plant. Strains were recovered at frequencies of 100 and 60 percent from plants inoculated by the toothpick and soil infestation methods, respectively. To our knowledge, this is the 1st report of the fulfillment of Koch's postulates for _F. tucumaniae_ and _F. virguliforme_ in Argentina.

References:
(1) T. Aoki et al. Mycologia 95:660, 2003.
(2) K. W. Roy et al. Plant Dis. 81:1100, 1997
(3) M. Scandiani et al. Plant Dis. 87:447, 2003.

[Sudden death syndrome (SDS) of soybean has reached epidemic proportions in North and South America during the past decade. A molecular diagnostic test to rapidly detect and distinguish the 2 SDS pathogens is being developed by USDA scientists. Results of this study are important to plant pathologists, plant breeders and quarantine officials who need to know that soybean SDS is caused by at least 2 different pathogens. Factors affecting SDS include the choice of cultivar, planting date, growth state at the time of infection, and field environment. Selection of soybean varieties that are the most resistant to SDS is crucial to maintaining a healthy soybean crop. Disease progress is reduced in mid-May through early June plantings, compared to those in early May, most likely because soil is warmer and drier in later plantings than in earlier plantings. Crop rotation on a regular regimen also lessens disease severity.

Additional references:
<http://www.indianasoybeanboard.com/SuddenDeath.shtml>
<http://www.nal.usda.gov/ttic/tektran/data/000013/63/0000136399.html>
<http://www.mycologia.org/cgi/content/abstract/95/4/660>
- Mod.DH]