RESUMO
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 first 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 eight 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 three of the isolates were F. virguliforme and five 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 first report of F. virguliforme in Argentina and of this pathogen outside the United States. Five isolates of F. tucumaniae and three 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 eight 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% from plants inoculated by the toothpick and soil infestation methods, respectively. To our knowledge, this is the first 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.
RESUMO
Foliage symptoms on soybean resembling those of sudden death syndrome were detected in Argentina during 1991 and 1992 in the Pampas Region and during 1992 and 1993 in the Northwest Region. Isolations were made in 1999, 2000, and 2001 from soybean plants (Glycine max (L.) Merr.) showing these symptoms. Five isolates of fungi obtained from taproot tissue and blue sporulation on taproot exteriors were selected for further evaluation. These isolates were plated on potato dextrose agar supplemented with streptomycin (PDAS). Based on the spore morphology, colony growth rate, morphology and pigmentation on PDAS, and lack of microconidia (1) five isolates were identified as Fusarium solani f. sp. glycines. Soybean cvs. Ripley, Spencer, Pioneer 9492RR, and A6445 RG were inoculated in greenhouse tests with each of the isolates using toothpick and soil infestation methods for a total of six experiments. Isolate 171 provided by J. Rupe (University of Arkansas, Fayetteville) was tested as a positive control. Foliar symptoms typical of sudden death syndrome and similar to those in the field were observed 14 and 25 days, respectively, after inoculations using the toothpick and soil infestation methods. Lesions produced on leaves averaged 3.6 for all five isolates and 4 for the reference strain using a disease severity scale where: 1 = no symptoms; 2 = slight symptom development with mottling and mosaic on leaves (1 to 20% foliage affected); 3 = moderate symptom development with interveinal chlorosis and necrosis on foliage (21 to 50% foliage affected); 4 = heavy symptom development with interveinal chlorosis and necrosis (51 to 80% foliage affected); and 5 = severe interveinal chlorosis and necrosis (81 to 100% foliage affected). Noninoculated controls were symptomless. Differences in virulence were observed among the isolates. Based on disease symptoms in the greenhouse and cultural morphology on PDAS, the isolates were classified as Fusarium solani f. sp. glycines. Isolates recovered from symptomatic plants resembled Fusarium solani f. sp. glycines on PDAS and peptone/p-chloro-nitrobenzene agar amended with streptomycin, confirming Koch's postulates. Fusarium solani f. sp. glycines was recovered from 60% of inoculated plants. Reference: (1) K. W. Roy et al. Plant Dis. 81:1100,1997.
