Pythium aphanidermatum has a cosmopolitan distribution in the warmer parts of the world. The aim of this study was to identify the causal agent of damping off in soybean seedlings on late planting dates under hot conditions, from different locations in Buenos Aires province. The isolates induced different levels of plant and seedling death and growth rate at different incubation temperatures (15, 25 and 35°C). In accordance with morphological, cultural and molecular characteristics, the pathogen was identified as P. aphanidermatum, and this is the first report of this oomycete causing soybean root and stem rot in Buenos Aires province, Argentina.

Pythium aphanidermatum presenta una distribución cosmopolita, principalmente en las regiones cálidas del mundo. El objetivo de este trabajo fue identificar el agente causal del damping off en plántulas de soja en fechas de siembra tardía bajo condiciones cálidas, en diferentes localidades de la provincia de Buenos Aires. Los aislamientos indujeron diferentes niveles de mortandad de plantas y plántulas y en la tasa de crecimiento a diferentes temperaturas de incubación (15, 25 and 35°C). De acuerdo con la morfología, las características culturales y moleculares, el patógeno fue identificado como P. aphanidermatum, y esta es la primera cita de este oomycete causando podredumbre de tallos y raíces en soja en la provincia de Buenos Aires, Argentina.

RESUMO Em soja Pythium spp. causa apodrecimento de sementes, da raiz e “tombamento”. O objetivo deste estudo foi verificar a relação do gene de resistência Rps 1k de Phytophthora sojae com Pythium ultimum e P. irregulare. Quatro genótipos de soja foram semeados em solo inoculado ou não com Pythium spp, dois deles com o gene Rps 1k (SE+1 e SE+2) e os outros dois sem ele (SE-1 e SE-2). Os genótipos SE-1 e SE-2 não diferiram entre si, não importa se eles foram inoculados ou não, nem eram diferentes de SE + 2. Por outro lado, genótipos com Rps 1k diferiam uns dos outros, sendo inoculados ou não. SE + 2 apresenta uma emergência de 95% enquanto que a SE + 1 tinha 78.35% de emergência, mas em ambos os casos a emergência diminuiu quando foram inoculados, com 63.35% e 34.15% respectivamente. Nas condições analisadas o gene Rps 1k de resistência a Ph. sojae não mostrou aumento do número de plântulas emergidas testados contra as espécies de P. ultimum ou P. irregulare.

ABSTRACT In soybean Pythium spp. produces seed and root rot and “damping off”. The aim of this study was to verify the relationship between resistant gene Rps 1k to Phytophthora sojae with Pythium ultimum and P. irregulare. Four genotypes of soybean were sown on inoculated soil, two of them with gene Rps 1k and the other two without it. Genotypes without the gene did not differ among themselves, no matter whether they were inoculated or not. On the other hand, Rps1k genotypes differed from each other, whether or not they were inoculated. SE+2 presented 95% of emergence whereas SE+1 presented 78.35% of emergence, but in both cases the emergence decreased when genotypes were inoculated with 63.35% and 34.15% respectively. Under the assayed conditions the gene Rps1k resistant to Ph. sojae did not show a greater number of emerged seedlings against the tested strains of P. ultimum or P. irregulare.

RESUMEN En el cultivo de soja Pythium spp. produce la pudrición de semillas y raíces, y “damping off”. El objetivo del presente trabajo fue verificar la relación entre el gen Rps 1k de resistencia a Phytophthora sojae y P. ultimum y P. irregulare. Cuatro genotipos de soja, dos con el gen Rps 1k (SE+1y SE+2) y dos sin el gen (SE-1 y SE-2) se sembraron en suelo inoculado y sin inocular. Los genotipos SE-1 y SE-2 no difirieron entre sí, ya sea que estuviesen inoculados o no, ni tampoco difirieron del SE+2. En cambio, los genotipos con el gen Rps1k difirieron entre sí, tanto los que estuvieron inoculados como los no inoculados. SE+2 presentó un 95% de emergencia mientras que SE+1 presentó un 78.35% de emergencia pero en ambos casos la emergencia disminuyó cuando se inoculó, con 63.35% y 34.15% respectivamente. Bajo las condiciones del presente ensayo las variedades utilizadas conteniendo el gen Rps 1k a Ph. sojae no presentaron mayor número de plántulas emergidas frente a las cepas probadas de P. ultimum ni de P. irregulare.

In roots of adult tomato plants a wet and brown lesion, extending 2-4 cm above the soil was observed. Diseased plants withered and died. The objective of this paper was to isolate and to identify the causal organism of these symptoms. General and selective culture media were used. After 24 hours colonies with white cottony mycelium were developed. The morphology of the colony and the production, morphology and size of reproductive structures were evaluated. The rDNA extraction was made from mycelium of pure cultures 7-10 days old grown in agarized media. The ITS region was amplified using ITS4 and ITS5 primers, sequenced and compared in gene bank - NCBI BLAST server to verify their sequence similarity to the type or holotype available. Coenocytic mycelium, spherical-globular sporangium and aplerotic oospores, characteristic of the genus Pythium were observed in microscopic slides. Pathogenicity tests were conducted with positive results; a microorganism identical to the original was re-isolated. The characterization achieved by traditional techniques, molecular studies and the results of the pathogenicity tests support the conclusion that Pythium aphanidermatum is the causal agent of basal rot on mature plants of tomato.

En raíces de plantas adultas de tomate se observó una lesión castaña de aspecto húmedo, que se extendía 2-4 cm por encima del suelo. Las plantas atacadas se marchitaron y murieron. El objetivo del trabajo fue aislar e identificar al organismo causal de esta sintomatología. Se usaron medios de cultivos generales y selectivos. A las 24 horas desarrollaron colonias con micelio blanco algodonoso. Se evaluó la morfología de la colonia y la producción, morfología y dimensiones de estructuras reproductivas. El rDNA del micelio de un aislamiento fue extraído de cultivos puros con 7-10 días de crecimiento en medios agarizados. La región ITS se amplificó usando primers ITS4 e ITS5, se secuenció y comparó en banco de genes del servidor BLAST- NCBI para verificar su similitud con las secuencias tipo u holotipo disponibles. Al microscopio se observó micelio cenocítico, esporangios esfericos-globulados y oosporas apleroticas característicos del género Pythium. Se efectuaron pruebas de patogenicidad con resultados positivos reaislándose un microorganismo de características idénticas. La caracterización lograda por las técnicas tradicionales, los estudios moleculares y el resultado de las pruebas de patogenicidad permiten concluir que Pythium aphanidermatum es el causante de la Podredumbre basal de plantas adultas de Tomate.

Pathogens within the Diaporthe complex cause seed decay, stem blight and stem canker on soybean, representing a serious threat for this crop species. We herein utilize worldwide sequence data retrieved from Genbank in order to assess the species boundaries between the soybean stem canker causal agents, and define whether or not they should be regarded as members of the same biological species. These studies were complemented with compatibility tests, in order to validate our findings from a biological standpoint. Species delimitation assays supported the occurrence of a speciation event between D. caulivora and D. phaseolourm var. meridionalis. A speciation hypothesis between D. aspalathi and D. phaseolourm var. meridionalis was also supported, based on three reciprocally monophyletic substitutions at locus EF1-α. Compatibility tests further validated species delimitation assays indicating that D. caulivora has developed barriers to gene exchange with D. phaseolorum var. meridionalis. Clarification of the specific boundaries of the SSC pathogens and related entities will be an important asset to future research in soybean pathology, epidemiology and breeding.

Stem canker caused by Diaporthe phaseolorum var. caulivora is an important disease of soybean in Argentina. The objective of this study was to determine its survival ability in artificially infested straw under laboratory and field conditions. In laboratory, stem pieces of soybean, maize, sorghum, sunflower, potato and wheat were autoclaved, placed in petri dishes on Potato Dextrose Agar and Water Agar, and inoculated with a 7-day-old pathogen culture. All crop residues were colonized and produced perithecia. Debris artificially infested with D. phaseolorum var. caulivora were placed in plastic net bags and transferred to an un-cropped area in a field plot at the University of Buenos Aires. Straws were left on the ground from winter to spring season in both 2007 and 2008 years. After 6 months abundant perithecia were developed in all straws. However, a higher number of perithecia on soybean and sunflower compared to maize, sorghum and potato was determined. These findings suggest that other crops, besides soybean, could maintain alive the inoculum of Diaporthe phaseolorum var. caulivora from soybean for at least 6-7 months.

The genus Pythium (Pythiaceae (Peronosporomycetes - ex Oomycetes), Kingdom Straminipila) includes important pathogens, affecting a wide range of hosts of economic value, causing damping-off and decline of herbaceous and wooded plants due to rootlets rot. In order to acquire a more comprehensive vision of Pythium in Argentina, a review and an updated report of recent progress in this matter was carried out since the first reports in the late XIX century till October 2009. Information was taken from printed and on line primary and secondary sources such as Proceedings of national and international Scientific Meetings, Bulletins from National Institutions and Universities, periodical Journals, books and data bases. The information was analyzed and categorized, thus updating the number of species of this genus, their geographical distribution, hosts affected, and symptoms. So far 18 species have been cited affecting, 247 hosts. The greatest number of hosts is affected by P. ultimum and P. debaryanum, followed by P. irregulare and P. aphanidermatum. The diversity of Pythium species in the world suggests that perhaps a wider variety of species, still not cited, could be present in Argentina. Projects dedicated to surveying the presence and identification of Pythium spp are necessary.

El género Pythium (Pythiaceae (Peronosporomycetes-ex Oomycetes), Straminipila) incluye patógenos que afectan hospedantes de importancia económica, ocasionando damping off en pre y postemergencia de plántulas de almácigo y el declinamiento de plantas herbáceas y adultas por destrucción de las raíces absorbentes. Con la finalidad de conocer el estado actual de la información disponible sobre este género en Argentina, se llevó a cabo una revisión desde los primeros reportes a fines del siglo XIX hasta octubre de 2009. Se consultaron fuentes primarias y secundarias de información escrita y electrónica: presentaciones en congresos nacionales e internacionales, publicaciones periódicas nacionales e internacionales, boletines de instituciones y universidades, libros y bases de datos. La información obtenida fue categorizada y analizada permitiendo conocer el número de especies de este género, su distribución geográfica y por tipo de cultivos, relaciones hospedante-patógeno involucradas y la sintomatología observada. Se encuentran citadas 18 especies que afectan a 247 hospedantes. Un elevado número de hospedantes fueron afectados por P. ultimum y P. debaryanum, seguidos por P. irregulare y P. aphanidermatum. La diversidad de especies de Pythium en el mundo sugiere que un mayor número de especies podrían estar presentes en el país, siendo necesario su relevamiento e identificación.

Dieffenbachia picta ("comigo-ninguém-pode") é uma espécie difundida em todo o mundo devido ao seu valor ornamental. Na Argentina, é cultivada nas províncias de clima mais quente e nos arredores da cidade de Buenos Aires. Na primavera de 2007 um grande número de plantas apresentou sintomas de murcha e presença de lesões encharcadas (anasarca) nas folhas inferiores, principalmente em plantas de 4 a 6 meses de idade. Em seguida, constatou-se murcha da planta devido ao apodrecimento do caule e raiz. Um organismo peronosporomycete foi isolado de tecidos afetados. O patógeno foi identificado como Phytophthora nicotianae com base em características culturais, morfologia das estruturas vegetativas e reprodutivas, e testes moleculares baseados na sequência da região ITS do DNA ribossômico. Os testes de patogenicidade foram conduzidos e os postulados de Koch atendidos. Em estudos complementares, as variedades de D. picta Alex, Compact, Marianne e Camilla foram suscetíveis, enquanto a variedade Tropic snow (D. amoena) foi resistente. Este é o primeiro relato de P. nicotianae causando podridão radicular de Dieffenbachia picta na Argentina e nas Américas.

Dieffenbachia picta is a worldwide appreciated crop due to its ornamental value. In Argentina it is cultivated in warm provinces and in the outskirts of Buenos Aires city, where in spring 2007 a great amount of plants were lost in commercial greenhouses. Symptoms such as wilting and soaked lesions in the basal leaves began in four- to six-month-old plants causing plants to wilt due to basal stem and root rot processes. A Peronosporomycete was consistently isolated from diseased tissues. The pathogen was identified as Phytophthora nicotianae based on cultural characteristics, morphology of vegetative and reproductive structures, and on the analysis of the sequence of the nuclear ribosomal internal transcribed spacer (ITS) region. Pathogenicity tests were carried out and Koch's postulates were fulfilled. In complementary studies, the dieffenbachia varieties Alex, Compact, Camilla and Marianne were susceptible to the infection caused by P. nicotianae, whereas Tropic snow (D. amonea) was resistant. This is the first report of P. nicotianae causing stem and root rot of D. picta in Argentina and in the Americas.