Marcelo Carmona

Modelling of wheat black point incidence based on meteorological variables in the southern Argentinean Pampas region

Studies were undertaken during 3 growing seasons at several locations on the Argentinean Pampas to investigate the relationships between environmental factors and black point incidence, and to develop predictive models. The strongest associations were observed throughout the critical period starting at 543 degree-days from heading to 861 degree-days (base temperature = 0°C). After a selection process, the best regression equation was: PI % = –6.50 + 0.07 DPrDDTd + 0.23 DRH, where PI is predicted disease incidence, DPrDDTd is a product of days with precipitation and the total degree-day accumulation of mean daily temperatures greater than 17°C (DDTd), and DRH is the total days with relative humidity above 62%. The equation accounted for 87% of the total variance in the disease incidence. Using logistic regression techniques, a model including precipitation frequency and DDTd could satisfactorily explain the probability of occurrence of severe, moderate, and light epidemics.

Evaluation of native bacteria and manganese phosphite for alternative control of charcoal root rot of soybean.

Plant growth promoting rhizobacteria (PGPR) are potential agents to control plant pathogens and their combined use with biopesticides such as phosphites may constitute a novel strategy to incorporate in disease management programs. In the present study, 11 bacterial isolates were selected on the basis of their antagonistic activity against Macrophomina phaseolina in dual-culture tests, and their plant growth promoting traits. Selected isolates were characterised on the basis of auxin and siderophore production, phosphate solubilisation and rep-PCR genomic fingerprinting. Two of these isolates, identified as Pseudomonas fluorescens 9 and Bacillus subtilis 54, were further evaluated for their inhibitory capacity against M. phaseolina using in vitro (on soybean seeds) and in vivo (greenhouse assay) tests. Both bacteria were applied individually as well as in combined treatment with manganese phosphite as seed treatments. Damage severity on soybean seeds was significantly reduced, compared with the untreated control, by both bacterial strains; however, the individual application of phosphite showed to be least effective in controlling M. phaseolina. Interestingly, the phosphite treatment improved its performance under greenhouse conditions compared to the results from the in vitro assays. In the greenhouse trials, the greatest reductions in disease severity were achieved when strain P. fluorescens 9 was applied singly or when strain B. subtilis 54 was combined with manganese phosphite, achieving 82% of control in both cases. This work is the first to report the control of M. phaseolina using combined treatment with PGPR and phosphite under greenhouse conditions.

Detection and transmission of Drechslera avenae from oat seed

El presente trabajo tuvo como objetivos detectar y cuantificar a Drechslera avenae en muestras de semillas y evaluar su transmision hacia los coleoptiles y plumulas de la avena (Avena sativa). Un total de trece muestras resultaron infectadas por D. avenae, representando un 54% de prevalencia. Los valores maximos y minimos de infeccion obtenidos fueron 0 y 52% con un promedio de 7%. Sobre las semillas, D. avenae produjo ademas de conidioforos, conidios y pseudotecios inmaduros, picnidios pequenos, esfericos y con forma de pera conteniendo conidios diminutos. El patogeno fue detectado en los coleoptiles, sin sintomas en las plumulas con una eficiencia de transmision del 55%. La tasa de transmision sintomatica fue del 12,5%. Se discuten la importancia de la semilla infectada y su papel epidemiologico.

Comparison of methods to assess severity of common rust caused by Puccinia sorghi in maize

Disease severity evaluation is an important decision support for adoption of strategies and tactics for disease control. The most commonly used method to assess disease severity is visual, but the problem is repeatability, due to subjectivity and imprecision of estimates. For Puccinia sorghi, a threshold of action of 1% severity was determined, so high precision is required in disease quantification. The aim of this study was to compare different assessment methods and analyze their association. Two diagrammatic scales were used to estimate severity, the Peterson and Amorim scales. Pustules were counted with the naked eye and with a 20x magnification hand lens. Software for disease quantification, Assess 2.0, was used to determine actual percentage area and lesion count. No significant differences were found between naked-eyed count and with magnifier. Lesion count with Assess 2.0 gave an imprecise result. Significant differences were found between diagrammatic scales. Compared with Assess 2.0, severity using Peterson was 2% higher, showing widely scattered differences (R 2 =0.48). Overestimation with visual scales was suggested, especially at low severity levels. Counting pustules was more objective, precise and reproducible. Thus, a calibration curve was constructed (R 2 =0.79), which will allow calculation of severity from

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

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. Kochs 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.

Protection of canola (Brassica napus) against fungal pathogens by strains of biocontrol rhizobacteria

Abstract Strains of Pseudomonas fluorescens and Bacillus amyloliquefaciens, isolated from soybean rhizosphere, inhibited mycelial growth of Botrytis cinerea and Sclerotinia sclerotiorum, in vitro. Leaves from Brassica napus seedlings, pre-inoculated with either of these bacteria, exhibited systemic protection against fungal pathogens.

First Report of Sexual Reproduction by the Soybean Sudden Death Syndrome Pathogen Fusarium tucumaniae in Nature

Of the four fusaria that have been shown to cause soybean sudden death syndrome (SDS), field surveys indicate that Fusarium tucumaniae is the most important and genetically diverse SDS pathogen in Argentina. Although none of the SDS fusaria have been shown to produce perithecia in nature, a heterothallic sexual cycle has been demonstrated for F. tucumaniae via laboratory crosses. Herein we report on the discovery of perithecia of F. tucumaniae on soybean in Argentina. Ascospores derived from these perithecia gave rise to colonies that produced sporodochial conidia diagnostic of F. tucumaniae. Sporodochial conidia were longer and narrower than those produced by the other SDS fusaria; these conidia also possessed a diagnostic acuate apical cell and a distinctly foot-shaped basal cell. Sixteen strains derived from single ascospores subjected to a validated multilocus genotyping assay (MLGT) for SDS species determination, together with 16 conidial isolates from two sites where teleomorphs were collected, independently confirmed the morphological identification as F. tucumaniae. This study represents the first authentic report of sexual reproduction by a soybean SDS pathogen in nature.

Evaluation of indigenous bacterial strains for biocontrol of the frogeye leaf spot of soya bean caused by Cercospora sojina.

Aims:  Assessment of biological control of Cercospora sojina, causal agent of frogeye leaf spot (FLS) of soya bean, using three indigenous bacterial strains, BNM297 (Pseudomonas fluorescens), BNM340 and BNM122 (Bacillus amyloliquefaciens).

Relationship between IC50determined in vitro/in vivoand the fungicide rate used in the Field

Published data containing fungicide concentrations that control 50% (IC50) of a given fungus were analyzed. In the analysis we considered: (i) the IC50 determined in vitroand in vivofor a given fungicide and for a specific fungus; (ii) the concentration (g/ha) of active ingredient for the fungicide indicated to control a specific disease in the field; (iii) water volume of 120/L used in the spray; (iv) the fungicide a.i. concentration (mg/L) in 120 L volume; (v) and the ratio of the concentration used in the field with that determined in the laboratory. The analysis were performed by using IC50 data for DMIs, QoIs, a carbamate and a benzimidazol against the following fungi Bipolaris sorokiniana, Drechslera tritici-repentis, D. siccans, Fusarium graminearum, Puccinia triticina, Exserohilum turcicum, Phakopsora pachyrhiziand Corynespora cassiicola. The fungicide concentrations sprayed in the field were 33.9 (D. siccansand trifloxystrobin) to 500,000.0 (E. turcicumand iprodione) times higher than that determined in the laboratory. It was concluded that the IC50 was not related to the concentration used in the field and therefore should be used to compare the power among fungicides and to monitor the fungal sensitivity shift towards fungicides

First Report of Ascochyta rabiei Causing Ascochyta Blight of Chickpea in Argentina

In November 2011, lesions similar to those reported for Ascochyta blight (1) were observed on Cicer arietinum L. (chickpea) plants growing in three commercial fields located at Río Primero and Río Segundo (Cordoba Province) and Lobería (Buenos Aires Province), Argentina. Disease incidence (percentage of plants affected) was 100% in all fields surveyed. Plants showed leaves, petioles, stems, and pods with brown lesions. Symptoms on leaves and pods were circular to oval (2 to 14 mm) while in the stems the lesions were elongated (2 to 30 mm). Seeds appeared small and shriveled with brown discoloration. Morphology of the fungi was examined on infected tissues. Numerous black pycnidia measuring 94.6 to 217.9 μm (145.9 ± 28.8 μm), arranged in concentric rings, were observed within of all the lesions. Conidia were predominantly aseptate, straight, hyaline with blunt ends, and measured 9.3 to 12.9 (11.3 ± 1.12) × 3.3 to 5.0 μm (4.2 ± 0.51). Morphological characteristics of the pathogen were similar to those described for Ascochyta rabiei (Pass.) Labrousse (teleomorph Didymella rabiei (Kovacheski) v. Arx (= Mycosphaerella rabiei Kovacheski)) (2). Fungus from infected leaf tissues was isolated on potato dextrose agar. Pathogenicity tests were conducted on seedlings of the susceptible cultivar by spraying leaves of each of 100 seedling plants with 10 ml of a conidial suspension (2 × 104 conidia/ml) of the isolated pathogen with a handheld atomizer. Plants were covered with plastic bags and placed in a growing chamber at 20 to 25°C for 3 days. The plastic bags were removed and the plants were maintained in high humidity at the same temperature. Noninoculated plants were used as controls. After 5 days, all inoculated plants showed typical symptoms. Foliar and stem lesions symptoms were similar to those originally observed in the field. Control plants remained healthy. Kochs postulates were fulfilled by isolating A. rabiei from inoculated plants. The colonies and the morphology of conidia were the same as those of the original isolates. To our knowledge, this is the first report of A. rabiei infecting chickpeas in Argentina. The outbreak of Ascochyta blight in Argentina is of concern because of its severity and the possibility that the pathogen was introduced on seed. This report underscores the need for further research on effective management programs for Ascochyta blight. References: (1) B. Bayaa and W. Chen. Compendium of Chickpea and Lentil Diseases and Pests The American Phytopathological Society, St. Paul, MN, 2011. (2) E. Punithalingam and P. Holliday. Page 337 in: CMI Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1972.