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The effects of phosphorus (P) application on the relationship between the arbuscular mycorrhizal fungus (AMF) Rhizophagus intraradices and the pathogen Macrophomina phaseolina (charcoal rot) affecting soybean (Glycine max L.) are unknown. We evaluated the effects of P on both the severity of the pathogen and the AMF protection against the charcoal rot in soybean. We conducted greenhouse experiments with a randomized multifactorial design with ten replications. The treatments were: two concentrations of P as superphosphate (0 and 50 kg of P ha−1), inoculated and non‐inoculated with the AMF R. intraradices, and infected and non‐infected with M. phaseolina. Soybean was seeded in pots containing 1 kg of sterilized substrate soil : sand : perlite (7 : 3 : 2). When soybean completed pod formation (R4 phenological stage), the plants were harvested. Plant parameters, mycorrhizal colonization, and disease severity were measured. The presence of M. phaseolina negatively affected soybean biomass, but AMF inoculation improved it. Phosphorus reduced AMF colonization but not arbuscules percentage. Moreover, both P and AMF inoculation had a negative effect on disease severity, although P also reduced mycorrhizal protection. These results suggest that phosphorus application could reduce disease severity, but can simultaneously partially reduce the AMF protection against the pathogen. These effects should be considered in agricultural integrated management practices of soybean.

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The capacity of arbuscular mycorrhizal fungi (AMF) to alleviate the negative effects incited by root pathogens in a range of plant hosts has been established. On the other hand, accumulated evidence also shows that fertilization practices can negatively impact AMF. Nevertheless, the interaction between AMF, pathogens and fertilizers, especially nitrogen (N) fertilizers, has not been previously reported. In this work, the effect of nitrogen on both the severity of the pathogen Macrophomina phaseolina (charcoal root rot) and the protection by the arbuscular mycorrhiza fungi (AMF) Rhizophagus intraradices was investigated in greenhouse experiments using soybean (Glycine max) as a host. The treatments were two levels of N (0 and 92 kg of urea ha −1 ), inoculation and non-inoculation with the AMF, and infection and non-infection with the pathogen. Soybean was harvested at R4 phenological stage (completed pod formation). Plant biomass, numbers of pods and leaves, plant height, root length, greenness index, mycorrhizal colonization and disease severity were measured. Pathogen infection reduced soybean biomass and negatively affected the greenness index, but co-inoculation with AMF improved these parameters. Nitrogen fertilization reduced AMF colonization but not arbuscules percentage. N fertilization increased disease severity but mycorrhizal symbiosis was able to reduce it. These results demonstrate that severity of charcoal root-rot disease in N fertilized soybean can be reduced by AMF inoculation. The implication of these results is that N fertilization could increase the risk of diseases in soybean but mycorrhiza could contribute to soybean charcoal root rot control even if the crop is under N fertilization.

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Ferns, which are usually colonizing different environments and their roots frequently present mycorrhization, have two adult stages in their life cycle, the sporophytic and the gametophytic phase. This paper describes the experimental mycorrhizal association between Pteris vittata leptosporangiate fern and a strain of Glomus intraradices during the life cycle of the fern, from spore germination to the development of a mature sporophyte. The aim of this study was to compare the colonization pattern of in vitro cultures of G. intraradices along the fern life cycle with those found in nature. For this, mature spores were obtained from fertile P. vittata fronds growing in walls of Buenos Aires city, Argentina. Roots were stained and observed under the light microscope for arbuscular mycorrhizal colonization. Approximately, 75 fern spores were cultured in each pot filled with a sterile substrate and G. intraradices (BAFC N° 51.331) as inoculum on the surface. After germination took place, samples were taken every 15 days until the fern cycle was completed. In order to determine colonization dynamics each sample was observed under optical and confocal microscope after staining. Gametophyte was classified as Adiantum type. Male and female gametangia were limited to the lower face, mycorrhizal colonization started when they were differentiated and took place through the rhizoids. Spores and vesicles were not found in this cycle stage. Paris-type mycorrhizal colonization was established in the midrib and in the embrionary foot. It was colonized by external mycelium. When the first root was developed soil inoculum colonized de novo this structure and Arum-type colonization was observed. This study proves that the type of colonization is determined by the structure of the host, not by the fungus. Both the gametophyte and embryo foot have determined growth and Paris-type colonization, while, sporophyte roots have undetermined growth and Arum-type colonization. The structures found in vitro cultures were highly similar to those found under natural conditions. Rev. Biol. Trop. 60 (2): 857-865. Epub 2012 June 01.Key words: Pteris vittata, Glomus intraradices, Pteridophyta-gametophyte-sporophyte, Arum colonization, Paris colonization.Over 90% of terrestrial plant groups have some type of symbiosis with soil fungi, and/or some mycorrhizal form. During the Devonic period, the first plants with roots appeared on land, the ferns belonging to Pteridophyta, Filicales groups, still exist today. They are widely distributed, particularly in tropical environments, and many of them have roots colonized by arbuscular mycorrhizal fungi (AM) (Brundrett 2002). Fern species with fine roots and long absorbent hairs sometimes limit mycorrhizal colonization. This facultative association is considered to be a feature of more evolved ferns (Fernandez et al. 2010). Cairney (2000 and Brundrett (2002) suggested that mycorrhizal symbiosis probably enabled plants to colonize land, conferring advantages such as increased fitness and resistance ...