Aphanomyces macrosporus sp. nov. Causing Root Rot in Barley and Some Other Plants.

In recent years, a new root rot disease in barley, which is caused by an Aphanomyces species, was found in field surveys in Southern Sweden and Denmark. Its symptoms occurred at the early tillering stage, around the BBCH 21 growth stage, and included the yellowing of leaves, brown coleoptiles, and the discolouration of roots. Prolonged soil wetness after rainfall favoured disease development, which sometimes advanced the yellowing patches to entire fields, resulting in lower yields. Oospores were found in the fine roots of diseased plants, and Aphanomyces isolates were obtained from these roots, as well as from the roots of barley plants grown in the greenhouse in soil samples from infected fields. Based on morphological analysis, we found that the new isolates were similar to those already obtained from barley and spinach roots in the 1990s in the same growing area. The morphological and molecular analyses performed in this study clearly separated and distinguished these barley isolates from other known Aphanomyces, and hereby Aphanomyces macrosporus sp. nov. is proposed as a new plant pathogenic species. It has larger oogonia and oospores than A. euteiches, A. cochlioides, and A. cladogamus, with one up to eight diclinous antheridia per oogonium. The phylogenetic analysis of the ITS rDNA region sequences grouped these new Aphanomyces isolates in a monophyletic clade, which was clearly distinguished from other plant pathogenic Aphanomyces species. The further pathogenicity of A. macrosporus on other plants is currently under investigation, but it is clear that it can at least infect barley, spinach, and sugar beet, indicating a wide host range for this species. The widespread presence and presumably broad host range of this new pathogenic Aphanomyces species must be considered in crop rotations.

Occurrence and Anastomosis Grouping of Rhizoctonia spp. Inducing Black Scurf and Greyish-White Felt-Like Mycelium on Carrot in Sweden.

Carrots with different Rhizoctonia-like symptoms were found in the main Swedish carrot production areas from 2001-2020. The most commonly observed symptoms were a greyish-white felt-like mycelium and black scurf, the latter often associated with Rhizoctonia solani anastomosis group (AG) 3-PT on potato. An overall increase in disease incidence in all studied fields over time was observed for both symptoms. The majority of Rhizoctonia isolates sampled from carrot in the period 2015-2020 were identified as AG 3 (45%) and AG 5 (24%), followed by AG 1-IB (13%), AG 11 (5%), AG-E (5%), AG BI (3%), AG-K (3%) and AG 4-HGII (2%). To our knowledge, this is the first report describing AG 5 in Sweden as well as AG 3, AG 11 and AG-E inducing Rhizoctonia-like symptoms on carrot. Secondly, we report for the first time that R. solani AG 3, and the less observed AGs: AG 1-IB and AG 5 can induce black scurf symptoms on the taproot of carrots. Due to a widely used carrot-potato crop rotation in Swedish areas, a possible cross-over from potato to carrot is suggested. This information is of high importance to reduce Rhizoctonia inoculum in soils, since avoiding carrot-potato crop rotations needs to be considered.

Verticillium Wilt in Oilseed Rape-the Microbiome is Crucial for Disease Outbreaks as Well as for Efficient Suppression.

Microbiome management is a promising way to suppress verticillium wilt, a severe disease in Brassica caused by Verticillium longisporum. In order to improve current biocontrol strategies, we compared bacterial Verticillium antagonists in different assays using a hierarchical selection and evaluation scheme, and we integrated outcomes of our previous studies. The result was strongly dependent on the assessment method chosen (in vitro, in vivo, in situ), on the growth conditions of the plants and their genotype. The most promising biocontrol candidate identified was a Brassica endophyte Serratia plymuthica F20. Positive results were confirmed in field trials and by microscopically visualizing the three-way interaction. Applying antagonists in seed treatment contributes to an exceptionally low ecological footprint, supporting efficient economic and ecological solutions to controlling verticillium wilt. Indigenous microbiome, especially soil and seed microbiome, has been identified as key to understanding disease outbreaks and suppression. We suggest that verticillium wilt is a microbiome-driven disease caused by a reduction in microbial diversity within seeds and in the soil surrounding them. We strongly recommend integrating microbiome data in the development of new biocontrol and breeding strategies and combining both strategies with the aim of designing healthy microbiomes, thus making plants more resilient toward soil-borne pathogens.

The structure of the Brassica napus seed microbiome is cultivar-dependent and affects the interactions of symbionts and pathogens.

BACKGROUND: Although the plant microbiome is crucial for plant health, little is known about the significance of the seed microbiome. Here, we studied indigenous bacterial communities associated with the seeds in different cultivars of oilseed rape and their interactions with symbiotic and pathogenic microorganisms. RESULTS: We found a high bacterial diversity expressed by tight bacterial co-occurrence networks within the rape seed microbiome, as identified by llumina MiSeq amplicon sequencing. In total, 8362 operational taxonomic units (OTUs) of 40 bacterial phyla with a predominance of Proteobacteria (56%) were found. The three cultivars that were analyzed shared only one third of the OTUs. The shared core of OTUs consisted mainly of Alphaproteobacteria (33%). Each cultivar was characterized by having its own unique bacterial structure, diversity, and proportion of unique microorganisms (25%). The cultivar with the lowest bacterial abundance, diversity, and the highest predicted bacterial metabolic activity rate contained the highest abundance of potential pathogens within the seed. This data corresponded with the observation that seedlings belonging to this cultivar responded more strongly to the seed treatments with bacterial inoculants than other cultivars. Cultivars containing higher indigenous diversity were characterized as having a higher colonization resistance against beneficial and pathogenic microorganisms. Our results were confirmed by microscopic images of the seed microbiota. CONCLUSIONS: The structure of the seed microbiome is an important factor in the development of colonization resistance against pathogens. It also has a strong influence on the response of seedlings to biological seed treatments. These novel insights into seed microbiome structure will enable the development of next generation strategies combining both biocontrol and breeding approaches to address world agricultural challenges.