Biological Control of Take-All and Growth Promotion in Wheat by Pseudomonas chlororaphis YB-10

Wheat is a worldwide staple food crop, and take-all caused by Gaeumannomyces graminis var. tritici can lead to a tremendous decrease in wheat yield and quality. In this study, strain YB-10 was isolated from wheat rhizospheric soil and identified as Pseudomonas chlororaphis by morphology and 16S rRNA gene sequencing. Pseudomonas chlororaphis YB-10 had extracellular protease and cellulase activities and strongly inhibited the mycelium growth of Gaeumannomyces graminis var. tritici in dual cultures. Up to 87% efficacy of Pseudomonas chlororaphis YB-10 in controlling the take-all of seedlings was observed in pot experiments when wheat seed was coated with the bacterium. Pseudomonas chlororaphis YB-10 was also positive for indole acetic acid (IAA) and siderophore production, and coating wheat seed with the bacterium significantly promoted the growth of seedlings at 107 and 108 CFU/mL. Furthermore, treatment with Pseudomonas chlororaphis YB-10 increased activities of the wheat defense-related enzymes POD, SOD, CAT, PAL and PPO in seedlings, indicating induced resistance against pathogens. Overall, Pseudomonas chlororaphis YB-10 is a promising new seed-coating agent to both promote wheat growth and suppress take-all.

Antagonistic Activity of Chilean Strains of Pseudomonas protegens Against Fungi Causing Crown and Root Rot of Wheat (Triticum aestivum L.)

Seed treatments with antagonistic bacteria could reduce the severity of crown and root rot diseases in wheat crops. The objective of this study was to evaluate the potential antagonistic activity of a bacterial consortium of three Chilean strains of Pseudomonas protegens against the wheat crown and root rot pathogens Gaeumannomyces graminis var. tritici, Rhizoctonia cerealis, and Fusarium culmorum. Two field experiments were carried out on artificially infested soil during two consecutive seasons (2016-2017 and 2017-2018) in an Andisol soil of southern Chile. Control treatments (not inoculated with fungi) were also included. Each treatment included a seed treatment of spring wheat cv. Pantera-INIA with and without the bacterial consortium. Both phytosanitary damage (incidence and severity) and agronomic components were evaluated. Bacterial populations with the phlD+ gene in the wheat plant rhizosphere during anthesis state (Z6) were also quantified. In both seasons, infection severity decreased by an average of 16.8% in seeds treated with P. protegens consortium, while yield components such as spikes m^-1 and number of grains per spike increased. The use of antagonistic bacteria resulted in a total yield increase only during the first experimental season (P < 0.05). In general, accumulated rainfall influenced the antagonistic effect of the consortium of P. protegens strains, accounting for the differences observed between the two seasons. The results suggest that this P. protegens consortium applied on seeds can promote plant growth and protect wheat crops against crown and root rot pathogens in Southern Chile under field conditions.

Characterization of the Wheat-Psathyrostachys huashania Keng 2Ns/2D Substitution Line H139: A Novel Germplasm With Enhanced Resistance to Wheat Take-All

Take-all is a devastating soil-borne disease that affects wheat production. The continuous generation of disease-resistance germplasm is an important aspect of the management of this pathogen. In this study, we characterized the wheat-Psathyrostachys huashania Keng (P. huashania)-derived progeny H139 that exhibits significantly improved resistance to wheat take-all disease compared with its susceptible parent 7182. Sequential genomic in situ hybridization (GISH) and multicolor fluorescence in situ hybridization (mc-FISH) analyses revealed that H139 is a stable wheat-P. huashania disomic substitution line lacking wheat chromosome 2D. Expressed sequence tag-sequence tagged site (EST-STS) marker and Wheat Axiom 660K Genotyping Array analysis further revealed that H139 was a novel wheat-P. huashania 2Ns/2D substitution line. In addition, the H139 line was shown to be cytologically stable with a dwarf phenotype and increased spikelet number. These results indicate that H139, with its enhanced wheat take-all disease resistance and desirable agronomic traits, provides valuable genetic resources for wheat chromosome engineering breeding.

Microbial Community Composition in Take-All Suppressive Soils

Gaeumannomyces graminis var. tritici (Ggt) is the main soilborne factor that affects wheat production around the world. Recently we reported the occurrence of six suppressive soils in monoculture areas from indigenous "Mapuche" communities, and evidenced that the suppression relied on the biotic component of those soils. Here, we compare the rhizosphere and endosphere microbial community structure (total bacteria, actinomycetes, total fungi, and ascomycetes) of wheat plants grown in suppressive and conducive soils. Our results suggested that Ggt suppression could be mediated mostly by bacterial endophytes, rather than rhizosphere microorganisms, since the community structure was similar in all suppressive soils as compared with conducive. Interestingly, we found that despite the lower incidence of take-all disease in suppressive soils, the Ggt concentration in roots was not significantly reduced in all suppressive soils compared to those growing in conducive soil. Therefore, the disease suppression is not always related to a reduction of the pathogen biomass. Furthermore, we isolated endophytic bacteria from wheat roots growing in suppressive soils. Among them we identified Serratia spp. and Enterobacter spp. able to inhibit Ggt growth in vitro. Since the disease, but not always pathogen amount, was reduced in the suppressive soils, we propose that take all disease suppressiveness is not only related to direct antagonism to the pathogen.

Generic names in Magnaporthales

The order Magnaporthales comprises about 200 species and includes the economically and scientifically important rice blast fungus and the take-all pathogen of cereals, as well as saprotrophs and endophytes. Recent advances in phylogenetic analyses of these fungi resulted in taxonomic revisions. In this paper we list the 28 currently accepted genera in Magnaporthales with their type species and available gene and genome resources. The polyphyletic Magnaporthe 1972 is proposed for suppression, and Pyricularia 1880 and Nakataea 1939 are recommended for protection as the generic names for the rice blast fungus and the rice stem rot fungus, respectively. The rationale for the recommended names is also provided. These recommendations are made by the Pyricularia/Magnaporthe Working Group established under the auspices of the International Commission on the Taxonomy of Fungi (ICTF).

tritici) in succeeding crops of winter wheat

Experiments on the Rothamsted and Woburn Experimental Farms studied the effects on take-all of different break crops and of set-aside/conservation covers that interrupted sequences of winter wheat. There was no evidence for different effects on take-all of the break crops per se but the presence of volunteers, in crops of oilseed rape, increased the amounts of take-all in the following wheat. Severity of take-all was closely related to the numbers of volunteers in the preceding break crops and covers, and was affected by the date of their destruction. Early destruction of set-aside/conservation covers was usually effective in preventing damaging take-all in the following wheat except, sometimes, when populations of volunteers were very large. The experiments were not designed to test the effects of sowing dates but different amounts of take-all in the first wheats after breaks or covers apparently affected the severity of take-all in the following (second) wheats only where the latter were relatively late sown. In earlier-sown second wheats, take-all was consistently severe and unrelated to the severity of the disease in the preceding (first) wheats. Results from two very simple experiments suggested that substituting set-aside/conservation covers for winter wheat, for 1 year only, did not seriously interfere with the development of take-all disease or with the development or maintenance of take-all decline (TAD). With further research, it might be possible for growers wishing to exploit TAD to incorporate set-aside/conservation covers into their cropping strategies, and especially to avoid the worst effects of the disease on grain yield during the early stages of epidemics.

Suppression of Wheat Growth and Yield by Pratylenchus neglectus in the Pacific Northwest

Many wheat (Triticum aestivum) fields planted annually in the Pacific Northwest are infested by high populations of the lesion nematode, Pratylenchus neglectus. Spring wheat cultivars varying in tolerance and resistance to P. neglectus were treated or not treated with aldicarb to examine relationships between the nematode and growth and yield of annual direct-seeded (no-till) wheat. Increasing initial density of P. neglectus in soil was more strongly associated with declining growth and yield of intolerant (Machete and Spear) than moderately tolerant (Frame and Krichauff) cultivars. Yield suppression by P. neglectus was generally 8 to 36% for intolerant cultivars, but reached 71% in soil also harboring Heterodera avenae, Rhizoctonia solani AG-8, and Gaeumannomyces graminis var. tritici. Intolerant cultivars had lower yields than Krichauff in rainfed but not in irrigated experiments. Density of P. neglectus in mature roots was generally lower for moderately resistant Krichauff than for susceptible Machete and Spear. Aldicarb improved yields in irrigated but not in rainfed experiments, and increased plant height and reduced variability in tiller height, canopy temperature, and density of P. neglectus in roots. This is the first report of damage to wheat by P. neglectus in the Pacific Northwest. Breeding wheat for tolerance and resistance to P. neglectus is suggested.

Comparison of Gaeumannomyces- and Phialophora-like fungal pathogens from maize and other plants using DNA methods

Several DNA-based techniques, developed for identifying and differentiating fungi in the Gaeumannomyces-Phialophora complex associated with take-all diseases of cereals and grasses, were used to compare fungi from maize. Maize isolates obtained as G. graminis (Sacc.) Arx & H Olivier var. tritici Walker, from the UK, having been identified by ascospore morphology and in pathogenicity tests on wheat, were indistinguishable from isolates of the same variety obtained from wheat. Isolates of G. graminis (Sacc.) Arx & H Olivier var. maydis Yao et al., recently described as the maize take-all fungus from China, were identical in DNA tests to the anamorphic fungus Phialophora radicicola Cain and almost identical to Phialophora zeicola Deacon & Scott, whose description was originally based on isolates from South Africa and France. These species appear to represent the holomorph of the same fungus. The late wilt pathogen of maize, from India and Egypt, commonly known as Cephalosporium maydis Samra et al., but suggested as being the Phialophora anamorph of a Gaeumannomyces species, was closely related to other Gaeumannomyces species included in the tests.