Severe Outbreaks of Soybean Frogeye Leaf Spot Caused by Cercospora sojina in the Pampean Region, Argentina

Abstract

Frogeye leaf spot of soybean (Glycine max (L.) Merr.) caused by Cercospora sojina Hara was reported to be severe from 1998 to 1999 in northwest Argentina (2). Although the disease was detected at low prevalence (5 to 25%), incidence, and severity in the Pampean Region from 2005 to 2008, no severe outbreaks have been recorded in the provinces of Córdoba, Santa Fe, and Buenos Aires. During the 2008–2009 growing season, disease spread rapidly throughout most soybean-growing areas of the Pampean Region. Disease was observed on almost all varieties of maturity group (MG) III, IV, and V. Symptoms on leaves were circular, reddish brown-to-gray spots (1 to 6 mm) and bordered by typical, narrow, reddish purple margins. Morphology of the fungi was examined on infected tissues. Conidiophores were light-to-dark brown, fasciculate, geniculate, and measured 110 to 203 μm long. Conidia were 1 to 9 septate, hyaline, elongate to fusiform, and measured 26 to 111 (47.3 ± 14.7) × 5.2 to 7.4 μm (6.1 ± 0.7). Pathogenicity tests were conducted on seedlings of a susceptible cultivar by spraying leaves of each of 80 plants at the V3 growth stage with 18 ml of a conidial suspension (3 × 104 conidia/ml) with a hand-held atomizer. Plants were covered with plastic bags and placed in a greenhouse at 28 to 30°C for 48 h. The plastic bags were removed and plants were maintained in high humidity at the same temperature. The same number of noninoculated plants was used as controls. After 10 to 12 days, all inoculated plants showed typical symptoms. Koch's postulates were fulfilled by isolating C. sojina from inoculated plants. Control plants remained healthy. Foliar lesions and morphological characteristics of the pathogen were consistent with C. sojina (1). Disease assessments were made for the middle and upper canopy from 15 arbitrarily collected plants. Soybean plants were in growth stages between R3 and R5 during the survey. Incidence (percentage of plants affected) and severity (percentage of leaf area affected with lesions) were visually estimated from each of the 30 soybean-production fields located in Monte Cristo, Alta Gracia, Jesús María, W. Escalante, Monte Buey, (10 fields, Córdoba Province), Venado Tuerto, Villa Cañás, Cristophersen, María Teresa, (12 fields, Santa Fe Province), Pergamino, Rojas, and Salto (8 fields, Buenos Aires Province). Incidence was 100% in all fields from Córdoba and Santa Fe. Incidence in Buenos Aires was 0 to 100%. Highest severity levels were quantified from fields in Córdoba (severity of 30 to 60%). Lesions also developed on stems and pods. In samples from Buenos Aires, severity levels were ≤10% in the eight soybean fields. Number of lesions per leaflet was recorded from central leaflets in samples from Monte Cristo, Alta Gracia, Venado Tuerto, and María Teresa with 20 to 55 typical lesions per leaflet. Since the disease was always more important in northwest Argentina, genetic resistance is more commonly available in varieties of MG VII to VIII, so most of the varieties of MG III, IV, and V frequently planted in Pampean Region are susceptible. This fact in combination with rainfall, warm temperatures, and high relative humidity in no-till fields during this summer have encouraged the severe outbreak of frogeye leaf spot, especially in the province of Córdoba and in some regions of Santa Fe. References: (1) D. V. Phillips. Page 20 in: Compendium of Soybean Diseases. 4th ed. The American Phytopathological Society. St. Paul, MN, 1999. (2) D. L. Ploper et al. Plant Dis. 85:801, 2001.