Understanding the sugar beet holobiont for sustainable agriculture

The importance of crop-associated microbiomes for the health and field performance of plants has been demonstrated in the last decades. Sugar beet is the most important source of sucrose in temperate climates, and—as a root crop—yield heavily depends on genetics as well as on the soil and rhizosphere microbiomes. Bacteria, fungi, and archaea are found in all organs and life stages of the plant, and research on sugar beet microbiomes contributed to our understanding of the plant microbiome in general, especially of microbiome-based control strategies against phytopathogens. Attempts to make sugar beet cultivation more sustainable are increasing, raising the interest in biocontrol of plant pathogens and pests, biofertilization and –stimulation as well as microbiome-assisted breeding. This review first summarizes already achieved results on sugar beet-associated microbiomes and their unique traits, correlating to their physical, chemical, and biological peculiarities. Temporal and spatial microbiome dynamics during sugar beet ontogenesis are discussed, emphasizing the rhizosphere formation and highlighting knowledge gaps. Secondly, potential or already tested biocontrol agents and application strategies are discussed, providing an overview of how microbiome-based sugar beet farming could be performed in the future. Thus, this review is intended as a reference and baseline for further sugar beet-microbiome research, aiming to promote investigations in rhizosphere modulation-based biocontrol options

Influence of Sugarbeet Tillage Systems on the Rhizoctonia-Bacterial Root Rot Complex

The Rhizoctonia-bacterial root rot complex in sugarbeet caused by Rhizoctonia solani and Leuconostoc mesenteroides can cause significant yield losses. To investigate the impact of different tillage systems on this complex, field studies were conducted from 2009 to 2011. Split blocks with conventional and strip tillage as main plot treatments were arranged in a randomized complete block design with four replications. Within main plots, there were seven treatments (non-inoculated check and six R. solani AG-2-2 IIIB strains). Regardless of tillage, the roots responded in a similar manner for fungal rot (conventional 8% versus strip 7%), bacterial rot (26% versus 34%), total rot (33% versus 41%), neighboring roots infected (1.7 roots versus 1.5 roots), distance spread (157 mm versus 150 mm), and the number of dead plants (12% versus 14%). Most R. solani strains also responded in a similar manner for disease variables. Strip tillage resulted in 6% more root yield in 2009 (P = 0.087), while conventional tillage resulted in 7% and 27% more root yield in 2010 (P = 0.063) and 2011 (P = 0.012), respectively. The tillage systems influenced disease variables in a similar manner, but more studies will be needed to determine their impact on yield

Influence of Rhizoctonia-bacterial Root Rot Complex on Storability of Sugarbeet

The Rhizoctonia-bacterial root rot complex can lead to yield loss in the field but may also lead to problems with sucrose loss in storage. Thus, studies were conducted to investigate if placing sugarbeet roots suffering from root rot together with healthy roots could compromise the ability of the healthy roots to retain sucrose. Over a three year period, root samples from three commercial cultivars were compared in storage as a healthy (eight healthy roots) or rotted (eight healthy roots + one rotted root) treatment inside an outdoor storage pile. The experiment was arranged as a split block (healthy in one half of block and rotted in the other) with the whole blocks arranged in a randomized complete block design with four replications. Samples were retrieved from storage in December, January, and February and evaluated for discolored and frozen root area, weight loss, and sucrose reduction and recovery. When comparing the healthy to the rotted treatment over the nine year x sampling date combinations, the Wilcoxon signed-rank test indicated the median change for discoloration (7% increase), frozen area (14% increase), sucrose loss (5% loss), and recoverable sucrose (689 kg/ha less or 8% reduction) were significantly different from zero (P = 0.008, 0.031, 0.007, and 0.008, respectively). These data indicate that the Rhizoctonia-bacterial root rot complex not only leads to yield loss in the field but can also negatively affect neighboring healthy roots in storage leading to additional sucrose losses

Understanding the Molecular Basis of Fusarium solani Mediated Root Rot in Pisum sativum

Pea (Pisum sativum) is an important cool-season crop, which is gaining renewed prominence due to an increased interest in plant-derived proteins. Its high nutritional value, low production costs, and short life cycle make pea ideal for the plant-derived protein market. However, pea yields are jeopardized by the root rot fungus, Fusarium solani f. sp. pisi (Fsp). Green-seeded cultivars are susceptible to Fsp, while most of the wild, purple-seeded pea genotypes are highly resistant. A time course RNA-sequencing approach was undertaken to identify Fsp-responsive genes in four partially resistant and four susceptible green-seeded pea genotypes. Gene expression analysis resulted in the identification of 42,905 differentially expressed contigs (DECs). Fsp challenge produced a more intense and diverse overexpression of genes in the susceptible genotypes, while the partially resistant genotypes showed fewer changes in the expression of defense-related genes and a faster reset to a basal metabolic state. Genes involved in exocytosis, the anthocyanin synthesis, as well as the DRR230 pathogenesis-related gene were overexpressed in the partially resistant genotypes. Genes involved in endocytosis, sugar transport, salicylic acid synthesis, and cell death were overexpressed in the susceptible genotypes. Analysis of the 42,905 DECs resulted in the identification of 769 predicted Single Nucleotide Polymorphisms (SNPs), which were validated and used to screen two segregating populations and to perform quantitative trait loci (QTL) analyses. A new QTL WB.Fsp-Ps 5.1 explained 14.76% of the resistance to Fsp, while four other QTLs explained 5.26-8.05% of the variance. Lastly, the association between the anthocyanin biosynthesis pathway and Fsp resistance was studied via ectopic overexpression, antisense expression, and CRISPR/Cas9-mediated gene editing of the flavanone 3-hydroxylase gene in a highly resistant purple-seeded line. We report an efficient pea transformation protocol with a mean efficiency of 2.9%. Transgenic events exhibiting a range of variation in petal pigmentation were obtained representing CRISPR/Cas9, overexpression, and silencing constructs. To the best of our knowledge, this is the first demonstration of CRISPR/Cas9 mediated gene editing in pea. The transgenic lines will be used in subsequent pathogen challenge assays to determine if the pea anthocyanin biosynthesis pathway is critical for Fsp resistance

Summaries of Papers and Posters Presented at the Eighth Hellenic Phytopathological Congress, Heraklion, Crete, Greece, october 22-24, 1998

The 8th National Phytopathological Congress, organized every two years by the Hellenic Phytopathological Society (HPS), was held in Heraklion, Crete, October 22-24, 1998 and it was attended by 400 participants. At this meeting, 44 oral presentations and 55 posters were presented dealing with plant diseases caused by fungi, bacteria, viruses, non-parasitic disorders and disease control. Moreover, two round table discussions were held. The first was on “Modern Methods for Disease Diagnosis and Identification of Plant Pathogens” and the second on “New Groups of Fungicides. Advantages over Older Compounds”. Also, a special session was held for review and discussion about the poster presentations. Abstracts of the papers and posters of this congress are presented in this issue

Investigation of resistance against Ditylenchus dipsaci on sugar beet

The stem and bulb nematode Ditylenchus dipsaci (Kuhn 1857) Filipjev 1936 is a migratory endoparasite ranked in the top ten plant-parasitic nematodes worldwide. Ditylenchus dipsaci has emerged as an economically threatening pest in the European sugar beet (Beta vulgaris L.) production. In Germany and Switzerland, some major sugar beet growing regions are particularly affected by D. dipsaci. The nematode migrates into the plant in the spring and reproduces in the hypocotyl during the growing season. Soil-borne pathogens introduced by D. dipsaci leads to crown root rotting and plant death. The broad range of host plants of D. dipsaci hinders crop rotation strategies for a successful management of this nematode. To date, no sugar beet cultivars carrying resistance towards D. dipsaci are available for sugar beet producers, depriving them of effective measures against this nematode. The lack of control measures and the growing public demand for sustainable sugar production have encouraged breeders to develop resistant cultivars. For this reason, this thesis aimed to investigate resistance against D. dipsaci on sugar beet. Before investigating the interaction between sugar beet and the nematode, the development of an in vivo test system was required. It aimed to replace above-ground D. dipsaci inoculation with a soil inoculation more closely related to field conditions. The most suitable inoculation time point, inoculum level, and positioning on sugar beets, as well as rearing process on carrots, were determined. At a 15:8°C day:night temperature regime, penetration rates of D. dipsaci into sugar beet seedlings were at maximum following soil inoculation at plant emergence. High soil moisture increased nematode migration into seedlings when D. dipsaci inoculation was carried out in four holes 1 cm from the plant base. The nematode suspension was previously reared for 35 days on carrot discs to obtain active D. dipsaci inoculum. To find potentially resistant sugar beet restricting reproduction and penetration of D. dipsaci, in vivo bioassays were carried out with 15 pre‑breeding populations and 79 breeding lines. It could be demonstrated that none of the genotypes showed complete resistance towards D. dipsaci. However, a high variation of the penetration rate by D. dipsaci was observed among the genotypes. They also responded differently to the fresh biomass reduction caused by the nematode combined with soil-borne pathogens. Based on these results, candidates for partial resistance were further investigated in microplot experiments conducted in the Rhineland (DE) and Seeland (CH) regions. The sugar beet genotype effect on D. dipsaci penetration could not be validated. The genotypes did not show a sufficient tolerance towards the rotting of the plant tissue. Nematode pathogenicity and virulence differed depending on experiment locations and years. Finally, virulence and pathogenicity of four D. dipsaci populations were investigated under in vivo conditions. No difference was found in D. dipsaci penetration rate into sugar beet seedlings. However, Seeland (CH) population showed a significantly higher reproduction on sugar beets than the others populations, validating observations obtained in microplot experiments.2022-01-2

Investigation of resistance against Ditylenchus dipsaci on sugar beet

The stem and bulb nematode Ditylenchus dipsaci (Kuhn 1857) Filipjev 1936 is a migratory endoparasite ranked in the top ten plant-parasitic nematodes worldwide. Ditylenchus dipsaci has emerged as an economically threatening pest in the European sugar beet (Beta vulgaris L.) production. In Germany and Switzerland, some major sugar beet growing regions are particularly affected by D. dipsaci. The nematode migrates into the plant in the spring and reproduces in the hypocotyl during the growing season. Soil-borne pathogens introduced by D. dipsaci leads to crown root rotting and plant death. The broad range of host plants of D. dipsaci hinders crop rotation strategies for a successful management of this nematode. To date, no sugar beet cultivars carrying resistance towards D. dipsaci are available for sugar beet producers, depriving them of effective measures against this nematode. The lack of control measures and the growing public demand for sustainable sugar production have encouraged breeders to develop resistant cultivars. For this reason, this thesis aimed to investigate resistance against D. dipsaci on sugar beet. Before investigating the interaction between sugar beet and the nematode, the development of an in vivo test system was required. It aimed to replace above-ground D. dipsaci inoculation with a soil inoculation more closely related to field conditions. The most suitable inoculation time point, inoculum level, and positioning on sugar beets, as well as rearing process on carrots, were determined. At a 15:8°C day:night temperature regime, penetration rates of D. dipsaci into sugar beet seedlings were at maximum following soil inoculation at plant emergence. High soil moisture increased nematode migration into seedlings when D. dipsaci inoculation was carried out in four holes 1 cm from the plant base. The nematode suspension was previously reared for 35 days on carrot discs to obtain active D. dipsaci inoculum. To find potentially resistant sugar beet restricting reproduction and penetration of D. dipsaci, in vivo bioassays were carried out with 15 pre‑breeding populations and 79 breeding lines. It could be demonstrated that none of the genotypes showed complete resistance towards D. dipsaci. However, a high variation of the penetration rate by D. dipsaci was observed among the genotypes. They also responded differently to the fresh biomass reduction caused by the nematode combined with soil-borne pathogens. Based on these results, candidates for partial resistance were further investigated in microplot experiments conducted in the Rhineland (DE) and Seeland (CH) regions. The sugar beet genotype effect on D. dipsaci penetration could not be validated. The genotypes did not show a sufficient tolerance towards the rotting of the plant tissue. Nematode pathogenicity and virulence differed depending on experiment locations and years. Finally, virulence and pathogenicity of four D. dipsaci populations were investigated under in vivo conditions. No difference was found in D. dipsaci penetration rate into sugar beet seedlings. However, Seeland (CH) population showed a significantly higher reproduction on sugar beets than the others populations, validating observations obtained in microplot experiments