ABSTRACT
Asian soybean rust, caused by Phakopsora pachyrhizi, is regarded as one of the most destructive diseases of soybean (Glycine max (L.) Merr.). In Argentina, it was first detected in the province of Misiones in the northeast near Paraguay and Brazil during the 2001-02 growing season (2). The following season, it also was found in the neighboring province of Corrientes. However, it did not reach major soybean production areas in northern Argentina until the end of the 2003-04 season. During April 2004, as soybean crops were nearing maturity, the disease was found throughout the region of northwestern Argentina, which includes the provinces of Tucumán, Salta, Jujuy, Catamarca, and Santiago del Estero, where approximately 6% of the soybean crop of Argentina is produced. During February and March, the area had a severe drought and above average temperatures, but in April, rainfall was abundant, particularly during the first half of the month. Soybean rust was first observed on 16 April in several locations of the departments (counties) of Moreno and Jiménez in the province of Santiago del Estero, and the following week in the departments of Alberdi, Burruyacú, Cruz Alta, Famaillá, La Cocha, and Leales in the province of Tucumán, in the department of Santa Rosa in the province of Catamarca, and in the departments of Anta, Metán, Rosario de la Frontera, and San Martín in the province of Salta. In those fields where the disease was detected, nearly all plants showed symptoms. Affected crops were mostly in growth stages R7 to R8, except for a few fields that had been planted late and were in a late R5 stage. Yield losses as much as 28% and premature defoliation occurred in these fields only. Disease severity, measured as percentage of affected leaf area, ranged from 45 to 50% in untreated fields and 0.9 to 39% in fungicide-treated fields. Leaf lesions were reddish brown, irregularly shaped, and were more abundant on the abaxial surface. Under the dissecting microscope, uredinia were observed as erumpent pustules with a conspicuous central pore. Masses of urediniospores were expelled through the pore and covered the pustules. Urediniospores were hyaline to pale yellow-brown, sub globose to ovoid, with finely echinulate, hyaline walls, and an average size of 27.8 × 18.5 µm. Because there are two morphologically similar species of Phakopsora that infect soybean, P. pachyrhizi (the Asian species) and P. meibomiae (the New World species), a molecular differentiation was carried out using the polymerase chain reaction (PCR) assay described by Frederick et al. (1). DNA extracted from 37 samples from different locations was amplified with specific primers for both species of Phakopsora and specific primers for P. pachyrhizi and for P. meibomiae. Twenty-eight samples amplified with the two species primers and with the P. pachyrhizi primer. None of the samples amplified with the P. meibomiae primer. Specimens have been deposited at Instituto Miguel Lillo, Tucumán, Argentina. These results confirmed the presence of P. pachyrhizi in the provinces of Catamarca, Tucumán, Salta, and Santiago del Estero, Argentina. References: (1) R. D. Frederick et al. Phytopathology 92:217, 2002. (2) R. L. Rossi. Plant Dis. 87:102, 2003.
ABSTRACT
Frogeye leaf spot of soybean (Glycine max (L.) Merr.), caused by Cercospora sojina Hara, was first detected during the 1997-98 growing season at low incidence and severity (<1% of the leaf diseased) levels in the provinces of Tucumán, Salta, Jujuy, Catamarca, and Santiago del Estero in northwestern Argentina. During the 1998-1999 growing season, disease incidence increased and disease severity grew to 10% of the leaf surface diseased on highly susceptible cultivars in a few locations. An outbreak of frogeye leaf spot occurred throughout northwestern Argentina during the 1999-2000 growing season. Frogeye leaf spot was severe on susceptible cultivars in the provinces of Salta, Santiago del Estero and Catamarca with the greatest intensity in the northeastern part of the Province of Tucumán. Symptoms on leaves were circular lesions that ranged in size from 1 to 5 mm, were reddish-brown to gray or tan, and were bordered by a narrow, reddish-brown to purple margin. Conidiophores and conidia of C. sojina developed on the abaxial leaf surface (1,2). Severely diseased leaves were desiccated and dropped during the R6 stage of growth. Lesions also developed on stems, pods, and seeds. Field surveys indicated that this disease reduced the yields of the highly susceptible cultivars Anta 82 RR, Coker 6738, and A 6445 RG by 48, 34, and 25%, respectively. C. sojina was cultured from diseased tissue on PDA acidified with 0.2% lactic acid and maintained on V-8 juice agar amended with streptomycin sulfate (100 mg/l). Conidia were elongated, dark, 38 to 62 × 5 to 9 µm, with 2 to 6 septa, and borne on dark conidiophores with 1 to 4 septa. Pathogenicity tests were conducted on seedlings of the susceptible cultivars A 6445 RG and Coker 6738 and on the resistant cultivars A 8000 RG and Shulka. Seedlings were inoculated at the V3 growth stage by spraying the leaves with a conidial suspension (4 × 104 conidia/ml) using a hand-held atomizer. Control plants were sprayed with sterile distilled water. Plants were placed in a moist chamber at 26°C for 2 days and then transferred to a greenhouse bench where they were kept at 25 to 30°C. Symptoms identical to those observed in the field became visible after 7 to 10 days. Ratings were made 14 days after inoculation by estimating the percentage of leaf area affected using a standard area diagram. Lesions covered 60 to 65% of the leaf area of susceptible cultivars, but less than 2% on resistant cultivars. Control plants remained healthy. C. sojina was reisolated from lesions on leaves of susceptible plants. Above-average rainfall and high relative humidity in northwestern Argentina during the first three months of 2000 may have encouraged the severe outbreak of frogeye leaf spot of soybean. The outbreak was aggravated by the widespread use of notillage systems in the region and the large hectarage planted with susceptible cultivars. References: (1) S.G. Lehman J. Agric. Res. 36:811-833, 1928. (2) D. V. Philips and J. T. Yorinori. 1989. Frogeye leaf spot. Pages 19-21 in: Compendium of Soybean Diseases, 3rd ed. APS Press, St. Paul, MN.