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.

Three species of Aphanomyces, A. euteiches, A. cochlioides and A. cladogamus are devastating agricultural plant pathogens causing root rot disease on pea, sugar beet and spinach, respectively. Here we investigated the distribution of the three Aphanomyces species in parts of Sweden, Denmark and Lithuania using climate chamber soil bioassays, and tested for correlations with macronutrient levels in the soil. Aphanomyces euteiches was the most widespread pathogen found in 28% of the tested samples in Sweden and Denmark, while A. cochlioides had a more limited distribution, and was found in 22% of the samples. Aphanomyces cladogamus was only detected in Sweden and was present in 16% of the tested samples. Aphanomyces euteiches only infected pea plants, while A. cochlioides and A. cladogamus infected both sugar beet and spinach. There were significant ( P < 0.001) differences in disease severity index (DSI) between regions, pathogens and for the interaction between regions and crops. Soil calcium (Ca) content ranged from 82 to 3600 mg/100 g dry soil. No species of Aphanomyces was detected in soils with Ca levels above 250 mg/100 g soil, indicating a pathogen-suppressive effect from calcium. Similar results were observed from 1350 soil samples collected from commercial pea growing fields, as DSI values on pea remained very low (below 20 on a 0–100 scale) in soil samples with confirmed presence of A. euteiches and a Ca value above 210 mg/100 g soil. High Ca levels was a significant ( P < 0.05) predictor for low probability of presence of Aphanomyces spp. in soil.

ISHS II Modelling Plant Growth, Environmental Control and Farm Management in Protected Cultivation ADAPTING A PREDICTION MODEL FOR FLOWER DEVELOPMENT IN CHRYSANTHEMUM TO NEW CULTIVARS

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.

Rhizoctonia solani is a soil-borne pathogen with a broad host range and considerable genetic diversity, yet little is known about its population structure in Sweden or Northern Europe. This study presents the most comprehensive survey to date of R. solani and related fungi across major field crops in Sweden, focusing on the southern agricultural region Scania. A total of 202 isolates were collected and characterized using rDNA-ITS sequencing for anastomosis group (AG) identification, geographic mapping, and multivariate analysis. The isolates primarily belonged to multinucleate AG-5 (81), followed by AG-3 (32), AG-2-1 (30), and AG-1 IB and AG-4 HGII (both 13), among other AGs, binucleate isolates, and Waitea species. AG-5 was clearly dominant, particularly on legumes, showing mild aggressiveness on pea and faba bean and high intraspecific ITS diversity. In contrast, AG-4 HGII and AG-2-1 displayed higher virulence, while AG-3 was non-pathogenic on the same hosts. Multivariate analyses revealed crop- and region-specific AG distributions. AG-3 was strongly associated with carrot and potato in Northeastern Scania, whereas across the rest of the region, AG-5 dominated legumes and was also found on sugar beet, together with AG-4 HGII and AG-2-1, while AG-2-1 occurred mainly on Brassicaceae. These findings have direct implications for crop rotation planning and disease management, highlighting potential risks in rotations involving legumes, Brassicaceae, and sugar beet, while rotations with carrot or potato may present lower risk. Overall, this study underscores the ecological complexity of Rhizoctonia in Swedish agroecosystems, emphasizing the need for localized surveillance and tailored management strategies to mitigate disease risk.

This study focuses on eighty-one Rhizoctonia solani Kühn. anastomosis group (AG)-5 isolates collected from various field crops, primarily Pisum sativum L. (pea), in southern Sweden and other (Nordic) countries since 2015. Using rDNA internal transcribed spacer (rDNA-ITS) sequencing and classical hyphal anastomosis assays, two distinct subgroups of AG-5, named AG-5 I and AG-5 II, were identified. ITS analysis revealed that the thirty-four AG-5 II isolates (42% of all AG-5 isolates) exhibited higher intra-isolate polymorphism, with numerous insertions and double peaks, suggesting ongoing evolutionary changes and a greater diversity of ITS regions within nuclei. In contrast, tef1-α sequences of a representative subset of AG-5 isolates showed less polymorphisms, higher bootstrap support, and clearer clustering of the two subgroups, highlighting the stability of this marker for phylogenetic inference. Pathogenicity assays showed that AG-5 II caused more severe epicotyl discoloration in pea and faba bean, while AG-5 I was associated with higher incidence of damping-off in sugar beet. AG-5 II also displayed higher tolerance to fludioxonil, but the broader ecological and evolutionary significance of this observation requires further investigation. In conclusion, this work provides the first report on the subdivision of AG-5 into two subgroups, AG-5 I and the evolving AG-5 II. rDNA-ITS and tef1-α markers together allow robust delineation of these subgroups, which differ in pathogenicity and fungicide tolerance, emphasizing their relevance for crop disease management.