O. Andrade, R. Campillo, A. Peyrelongue, and L. Barrientos. 2011. Soils suppressive against Gaeumannomyces graminis var. tritici identified under wheat crop monoculture in southern Chile. Cien. Inv. Agr. 38(3):345-356. Improved knowledge of the biological phenomenon of soil suppressiveness is critical for the management and biological control of soilborne pathogens. Andisols, which are located in southern Chile, show very high conduciveness to the take-all disease of wheat caused by the fungal soil-borne pathogen Gaeumannomyces graminis var. tritici. However, no previous reports have investigated suppressive soils in this important wheat-producing area. The first part of this study was conducted to identify soils suppressive to the take-all disease, and will be followed by a characterization of its microflora to identify potential bio-control agents against the fungal pathogen. Based on the transferability of suppressiveness into the same sterile soil background, 20 soils were collected from different wheat-growing areas in southern Chile and were classified as either suppressive or conducive to the take-all disease under artificial inoculation in a greenhouse environment. Five soils were found to have highly suppressive properties to the take-all disease of wheat, and suppressiveness was observed in soils with a long history of wheat monoculture. Suppressive and conducive soils were found to have overlapping physicochemical characteristics. This is the first report of soils suppressive to take-all of wheat in Chile.

The fungus Thecaphora solani (syn.: Angiosorus solani), the causal agent of potato smut, was cultivated in vitro for the first time. Teliospores obtained from galls of infected potato plants were used to inoculate commonly used solid and liquid media. The teliospores produced two kinds of vegetative tissue depending on the nutrient status of the media. A very slow radial-growing, hyaline, and septate mycelium, as usually seen in most of the in vitro-cultivated filamentous fungi, was obtained in wateragar medium after 30 to 40 days. On the other hand, a white, sponge-like mycelial mass was obtained in HCM + 1% activated charcoal, and on common potato dextrose agar or malt-yeast-peptone solid or liquid media, after 40 to 50 days under lab conditions. The identity among teliospores and the sponge-like mycelial mass was corroborated by DNA fingerprinting and partial sequencing of the large subunit (LSU) rDNA region. The sexual cycle of the pathogen was completed under lab conditions based on the development of teliospores on the sponge-like mycelial mass. The first attempt to reproduce the disease under controlled conditions was successful, inducing a gall in a cv. Desirée potato explant cultivated in vitro inoculated with radial-growing mycelia. Phylogenetic analysis of LSU rDNA data of the genus Thecaphora and other smut fungi confirmed the initial classification of the pathogen as T. solani.

This research was initiated to determine whether soils suppressive to take-all of wheat caused by Gaeumannomyces graminis var. tritici (Ggt) occur in Montana, and to identify the organisms most likely involved in this suppression. From an initial screening of eight soils collected from different wheat growing areas of Montana, two were highly suppressive to take-all. Microbial characterization of these soils indicated that different mechanisms were involved in the suppression. In Larslan soil, mycoparasitism appeared to be the main mechanism. Two different fungi with exceptional ability to reduce the severity of take-all were isolated from this soil. One of these fungi could parasitize the hyphae of Ggt. Field tests with these fungi in Ggt infested soil showed increases of over 100% in both harvestable tillers and grain yield as compared to treatments without these two fungi. In tests with 48 different bacteria and 10 actinomycetes from Larslan soil, none were able to consistently reduce severity of take-all alone, or in mixtures. In Toston soil, antibiosis by actinomycetes and perhaps the involvement of Pseudomonas spp. in production of antibiotics and/or siderophores appeared to be the most likely mechanisms involved in take-all suppression. Increases in shoot dry weight over that in the Ggt infested control using mixtures of pseudomonads and actinomycetes ranged from 25% to 87%. Actinomycetes added individually or in mixtures to soil infested with Ggt consistently reduced the severity of the disease to a greater extent than did mixtures of Pseudomonas spp.

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