Previous projects have demonstrated that compost and management residues are good peat replacers for container cultivation of ornamentals. Composts and management residues have a specific microbiology that potentially enhances plant resilience and disease resistance. Our goal is to further increase the sustainability of the ornamental sector by an optimal working microbiology of the growing medium and by the replacement of peat in growing media by local and sustainable alternatives such as compost and management residues, e.g., from heathlands. This leads to a lower use of chemical crop protection products and less nutrient losses. A first step is the assessment of the stability of these materials. Different batches of composts and management residues were tested for their chemical and biochemical composition, were incubated to assess the N fixation risk, and the C mineralization was assessed by CO2 flux measurements. Results on the potential for predicting the N fixation risk and the stability based on the biochemical and chemical characteristics indicate that management residues behave different from composts. The methods for stability assessment should be selected and/or combined as a function of type of material, the dry bulk density, and the risk for N immobilization.

ReGrow4C aims at substantially extending the life span of peat- and perlite-based growing media. The extraction of peat from pristine peatlands threatens these sensitive ecosystems and carbon sinks, meanwhile resulting in increased emissions of greenhouse gases. Each batch of peat that is reused as growing medium results in a clear reduction in CO2 emissions and a lower impact on the climate. After using growing media for one cultivation, we aim to reuse the spent material as substrate for another crop. Spent peat and perlite based growing media from strawberry and cucumber cultivation were tested for upcycling after steam treatment. The first focus was on assessing the stability of the spent growing media, in comparison with values for a range of composts. In this study C mineralization was assessed by CO2 flux measurements in the lab during a 30 day period. We compared the CO2 flux measurements with four other indicators for assessing stability of the materials: oxygen uptake rate (OUR), biodegradation potential, mineral N content and risk for N immobilization. The spent growing medium had a low decomposition rate, and CO2 flux measurements were found to be applicable for stability assessment for spent growing media and composts.

To optimize the pH of management residues for optimal use in growing media, liming or acidification can be used.Previous research has shown the effectiveness of these treatments to increase or decrease the pH of management residues.However, the effect of these treatments on the microbiome of management residues remains unclear.In this study, two types of management residues, forest sods and heath chopper, were limed, while a third type, soft rush, was acidified.We assessed the effect of these two optimization treatments on the microbial community of the management residues using 16S rRNA and ITS2 gene metabarcoding, PLFA analysis and Biolog EcoPlates.Results indicate that the type of management residue is important to determine the effect of liming.In forest sods, liming seemed to have a negative effect for the use in growing media, with a decrease in bacterial and fungal diversity, and microbial biomass.In heath chopper, liming seemed to have a positive effect for the use in growing media, with an increase in bacterial diversity, microbial biomass and activity, and functional diversity.Acidification of soft rush increased fungal diversity, but decreased microbial biomass and activity and functional diversity, which makes it unclear whether it is an appropriate strategy for the use in growing media.

Soil microorganisms maintain vital soil processes in agroecosystems, simultaneously affected by agricultural intensification and climate change in a negative way.Organic amendments such as biochar and compost have the potential to counteract these adverse effects on soil microorganisms.However, often only a snapshot is studied while seasonal changes also play an important role.To reveal the temporal dynamics of the soil microorganisms after organic application, we have studied two fields treated with either a single dose of biochar (Field A: 0 vs. 10,900 kg C ha -1 ) or a yearly compost application (Field B: 0 vs. 2,000 kg C ha -1 year -1 ).Soil samples were taken every five weeks for over a year.Using metabarcoding, we showed that biochar did not affect the bacterial community of the soil.This can be explained by the fact that the soil provided a stable environment for the bacterial community by which biochar had minor effects, or that the single-application was insufficient and a repeated application is necessary.Compost application on the other hand shifted the relative abundance of 16 bacterial families.In this study, the families of the Bacteroidetes and Chloroflexi were enriched and proteobacterial and acidobacterial families mostly declined.

Our current understanding of the microbial communities inhabiting growing media is limited.However, techniques such as phospholipid fatty acid analysis, metabarcoding and shotgun metagenomics are increasingly used to analyze and understand microbial life in growing media and therefore they are starting to fill the knowledge gap.Using these techniques in the interreg2seas project Horti-BlueC (www.horti-bluec.eu),we try to understand the microbiological processes involved in sustainable growing media based on plant fibers, chitin and biochar.In sustainable growing media, peat is (partially) replaced and/or the use of chemical fertilizers and plant protection products is reduced.Plant fibers include defibrated miscanthus straw, flax shives and reed.Chitin is produced from shell fish waste such as crab and shrimps shells.Biochar (charred material) is rich in carbon and is produced from organic material such as wood, spent growing media and the woody fraction of green waste.We showed that adding biochar to peat mainly changes the bacterial community, whereas plant fibers and chitin mainly change the fungal community of the growing media.By changing the microbiome of the growing media, these new amendments might reduce the impact of horticulture on the environment and contribute to a circular-based economy.

ISHS II International Symposium on Growing Media, Soilless Cultivation, and Compost Utilization in Horticulture Nature conservation management residues as alternative for farm yard manure: effects on nutrients, carbon and disease suppression

Strawberry cultivation is associated with high mineral fertilizer doses and extensive use of chemical plant protection products. Based on previous research, we expected that chitin application to peat substrate would increase the nutrient availability and activate the plant systemic defense response, resulting in higher strawberry yields and fewer disease symptoms. We set up two experiments in which the temporal variability and differences in initial nutrient concentrations of the growing media were taken into account. Chitin treatment resulted in the attraction of plant growth–promoting fungi toward the plant root, such as species from genera Mortierella and Umbelopsis. In addition, by the end of the experiments 87 mg of mineral nitrogen (N) per liter of substrate was mineralized, which can be related to the observed increase in plant shoot biomass. This, however, led to nutrient imbalances in plant shoots and fruit; N concentration in the leaves increased over 30%, exceeding the optimal range, while phosphorous (P) and potassium (K) deficiencies occurred, with concentrations lower than 50% of the optimal range. This may explain the decreased fruit yield and disease resistance of the fruit toward Botrytis cinerea. In contrast, chitin caused a clear defense priming effect in the strawberry leaves, with a strong induction of the jasmonic acid response, resulting in fewer foliar disease symptoms. Chitin causes positive effects on shoot growth and foliar disease resistance, but caution needs to be taken for nutrient imbalances leading to negative influences on root growth, fruit production, and disease susceptibility toward B. cinerea. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Previous research showed that adding biochar at the onset of the composting process alleviates this process and reduces emissions. The objective of this study was to evaluate in a field trial whether this compost with biochar applied during the process (i.e., biochar-blended compost) has added value compared to compost or biochar alone on chemical, physical and biological soil properties and crop yield. A single application of biochar, compost and biochar-blended (BB) compost increased the C content of the top soil in the long term, but only compost and biochar-blended compost had a lasting effect on pH and the K content. Two to 4 years after a single application of all three amendments in the field, the soil microbial biomass, richness, diversity and community composition remained unchanged. Due to intensive soil tillage, biochar migrated to subsoil already after 2 growing seasons, thus increasing the C sequestration in subsoil. We conclude that in general, the biochar-blended compost outperformed biochar and had a similar effect as the compost.

Adding biochar at the onset of composting has proven to be beneficial for this process, and when added to the soil, this biochar-blended (BB) compost has outperformed biochar on soil quality but had a similar effect as the compost. Because soil quality gets more and more attention as an important tool for soil health, pot trials were conducted to evaluate the effect of compost and biochar alone, and biochar-blended (BB) compost on disease suppression. In addition, it was investigated whether the disease suppression was associated with changes in soil quality (chemical, physical and biological soil properties). The two selected bio-assays were Rhizoctonia solani (basal rot) on lettuce and potato cyst nematodes (PCN) on potato. Both compost and BB compost released mineral N and thus increased lettuce and potato yield but biochar did not. None of the amendments affected the susceptibly of lettuce leaves to basal rot. Compost suppressed PCN while biochar had no effect on PCN reproduction. Moreover, BB compost reduced the suppressing effect of compost. We suggest that the inhibitory effect of biochar blended in compost might be attributed to the fungal biomass which was not increased in the BB compost-amended soil compared to compost-amended soil, and/or the absorbance of suppressive compounds by the high surface area of biochar. Interestingly, all three amendments reduced the water evaporation from the soil surface, which can have important implications for drought stress of plants.

Chitin is a valuable peat substrate amendment by increasing lettuce growth and reducing the survival of the zoonotic pathogen Salmonella enterica on lettuce leaves. The production of chitin-catabolic enzymes (chitinases) play a crucial role and are mediated through the microbial community. A higher abundance of plant-growth promoting microorganisms and genera involved in N and chitin metabolism are present in a chitin-enriched substrate. In this study, we hypothesize that chitin addition to peat substrate stimulates the microbial chitinase production. The degradation of chitin leads to nutrient release and the production of small chitin oligomers that are related to plant growth promotion and activation of the plant's defense response. First a shotgun metagenomics approach was used to decipher the potential rhizosphere microbial functions then the nutritional content of the peat substrate was measured. Our results show that chitin addition increases chitin-catabolic enzymes, bacterial ammonium oxidizing and siderophore genes. Lettuce growth promotion can be explained by a cascade degradation of chitin to N-acetylglucosamine and eventually ammonium. The occurrence of increased ammonium oxidizing bacteria, Nitrosospira, and amoA genes results in an elevated concentration of plant-available nitrate. In addition, the increase in chitinase and siderophore genes may have stimulated the plant's systemic resistance.

Abstract Bacterial inoculation of soybean seeds to improve biological nitrogen fixation is a well-known practice to achieve higher seed and protein yield with reduced fertilization. The optimal inoculation strategy in temperate regions is unknown because soybeans are rarely cultivated under colder growing conditions. The aim of the present work was to determine the most suitable inoculation strategy for soybean cultivation in Belgium. Field trials were set up with four Bradyrhizobium inoculants (HiStick, Force 48, Biodoz and Optimize) at two locations over 2 years (2014–2015) and compared with a non-inoculated control treatment. In addition, HiStick was tested at three doses and Optimize at two time periods prior to sowing. Under Belgian conditions, all inoculants were effective in establishing rhizobial symbiosis, resulting in increased yield, protein content, protein yield and thousand-grain weight compared with the non-inoculated control. A single dose of HiStick was sufficient to establish symbiosis. Pre-inoculation with Optimize 2 weeks before sowing gave an intermediate performance for most parameters between the non-inoculated control treatment and inoculation with Optimize 24 h prior to sowing. Among the four products tested, Biodoz seemed the best product for inoculation under cool growing conditions. Based on the atpD gene, the bacterial strain of Biodoz showed complete similarity with Bradyrhizobium diazoefficiens, while strains of other products were identified as Bradyrhizobium japonicum. In vitro growing capacity of the Biodoz strain at 8 °C was higher compared with the other strains. Better cold adaptation of the Biodoz strain might be a possible explanation for the better performance of Biodoz in Belgium.

Several materials to be used in growing media were tested for immobilization of mineral N by adding 350 mg N L-1 substrate as KNO3 followed by one week of incubation at 37°C. Based on the difference between the theoretical and actual mineral N content after incubation, the % N immobilization was calculated. The robustness and reproducibility of the protocol was tested by changing incubation circumstances, the applied N sources and the amount of N added. The relevance of the indicator was tested (a) for a range of green waste composts, (b) during the composting of four mixtures, and (c) in pot trials with growing media including different composts and lignocellulosic materials. Higher microbial activity in green waste composts (indicated by higher oxygen consumption) and thus a lower compost stability resulted in higher N immobilization. N immobilization decreased due to stabilization during the composting process, with a distinct effect of the specific feedstock composition. The protocol was tested for woody materials and for Miscanthus straw, spent coffee grounds and chopped reed straw. The applied source of mineral N in the test affected the pH of the material, and may thus also affect the microbial activity and thus the related N immobilisation. Growing media including materials with high N immobilization resulted in lower plant growth. We conclude that quality of composts and lignocellulosic materials for use in growing media could be successfully assessed based on this protocol.

Healthy soils are of vital importance for organic greenhouse cultivation. Due to the intensity of the cropping system, soil-borne diseases are of great concern. To achieve a more resilient system, growers use multiple strategies, including resistant rootstocks, organic soil amendments, biological soil disinfestation and plant stimulants. Soil-borne pathogens that form (micro) sclerotia are particularly difficult to manage due to their persistence in soil. Previous studies have shown that incorporation of lignin-rich crop residues can decrease the viability of microsclerotia of Verticillium longisporum in cauliflower fields. The purpose of this study was to evaluate the effect of lignin-rich amendments on the microsclerotia of V. dahliae in soils from organic greenhouses with a history of Verticillium wilt in solanaceous crops. In the laboratory, two greenhouse soils (river and sea clay) were mixed with 10% (w/w) broccoli (stem), bristle oat (whole plant), reed (stalks or leaves), Ethiopian mustard (whole plant) and corn (stalks or leaves). In the river clay soil, corn stalks, broccoli stem, reed and bristle oat significantly decreased the amount of viable microsclerotia. In the sea clay soil, only reed leaves and stalks had a significant effect. Only in the sea clay soil, a significant negative correlation was found between the amount of lignin added and the amount of viable microsclerotia detected. These results indicate that in the river clay soil, mechanisms other than lignin may be involved in Verticillium control. Surprisingly, real-time PCR analysis showed that the amount of Verticillium tricorpus was much higher than that of V. dahliae in both soils. Sequencing of a Verticillium strain isolated from the sea clay soil showed that it should be re-classified as Verticillium isaacii (formerly V. tricorpus). It is necessary to investigate the role of these different Verticillium species, before new control measures against Verticillium are further developed.

Anthracnose, caused by C olletotrichum acutatum (species complex), has become a troublesome problem in strawberry production worldwide. This paper reports (i) an optimized sampling method combined with a real‐time PCR technique to detect the latent presence of C . acutatum in cold‐stored strawberry plants used as planting material in several European countries, and (ii) a study of the spread of C . acutatum following a point inoculation under field conditions. Screening of different parts of planting material suggested that C . acutatum is most likely to be present on runners and old petioles. In addition, in seven out of nine batches of planting material from different nurseries, latent infection by C . acutatum was detected in at least one of five replicate samples. Field experiments in 2009 and 2010 showed extensive latent within‐field spread of the pathogen on strawberry leaves, with a within‐row dispersal distance up to at least 1·75 m in 1 week. A straw ground cover between the rows did not decrease C . acutatum spread, probably because introduced (and/or subsequent) inoculum was confined to the plant bed (within the row) and was not present between the beds. Moreover, the number of C . acutatum spores on the symptomless leaves, as estimated using a real‐time PCR method, was significantly ( P < 0·05) correlated with the incidence of fruit rot at harvest and post‐harvest ( r = 0·56–0·66). These results illustrate the importance of detecting latent infections in planting material and strawberry leaves in the field.

AIMS:To investigate the interaction between cauliflower and the isolate VerticilliumVt305, obtained from a field suppressive to Verticillium wilt of cauliflower, and to evaluate the ability of VerticilliumVt305 to control Verticillium wilt of cauliflower caused by V. longisporum.METHODS AND RESULTS:Single and combined inoculations of VerticilliumVt305 and V. longisporum were performed on cauliflower seedlings. Symptom development was evaluated, and fungal colonization was measured in the roots, hypocotyl and stem with real-time PCR. No symptoms were observed after single inoculation of VerticilliumVt305, although it colonized the plant tissues. Pre-inoculation of VerticilliumVt305 reduced symptom development and colonization of plant tissues by V. longisporum.CONCLUSIONS:VerticilliumVt305 is an endophyte on cauliflower plants and showed effective biological control of V. longisporum in controlled conditions.SIGNIFICANCE AND IMPACT OF THE STUDY:This work can contribute to the development of a sustainable control measure of V. longisporum in Brassicaceae hosts, which is currently not available. Additionally, this study provides evidence for the different roles of Verticillium species present in the agro-ecosystem.

In April 2010, pink-orange spore masses that later turned brown were observed on 7 to 50% of the transplant lots during a routine screening of Belgian strawberry (Fragaria × ananassa, cv. Elsanta) for the latent presence of Colletotrichum acutatum using the petiole freeze method (4). These spore masses contained hyaline, canoe-shaped to allantoid conidia (mean size 7.5 × 1.8 μm), which is not consistent with C. acutatum spore morphology. Subsequently, a spore mass was transferred onto potato dextrose agar (PDA) and a gray-to-brown colony with whitish, aerial mycelium was produced, which is also not consistent with C. acutatum isolates. To identify the fungus, the ITS1-5.8S-ITS2 rDNA region was amplified by PCR and sequenced. The 485-bp region was 100% identical to that of Pilidium concavum specimen voucher BPI 1107275 (GenBank Accession No. AY487094). P. concavum (Desm.) Höhn. (synanamorph Hainesia lythri; teleomorph Discohainesia oenotherae) is a pathogen of strawberry causing tan-brown rot of fruit and is a common secondary invader of roots and dead strawberry plant parts (3). A recent strain of P. concavum from strawberry, isolate UPL 50, obtained from Brazil (L. Zambolim, Univ. Fed. de Viçosa, personal communication) showed similar colony, microscopic (mean spore size of 6.8 × 1.8 μm), and molecular (ITS sequence 98% identical to that of P. concavum specimen voucher BPI 1107275) features as the Belgian isolate. Pathogenicity tests were conducted on mature strawberry fruits by submerging 15 fruits per isolate for 3 min in a conidial suspension (2 × 106 conidia ml-1 of water) obtained from a 2-week-old colony on PDA. Controls were submerged in sterile distilled water. The inoculated fruits were incubated in a moist chamber at 25°C. Sunken, yellowish brown lesions with pink and later orange-brown spore masses were observed starting 3 days after inoculation on 88 and 94% of the fruit for the Brazilian and Belgian isolate, respectively. The control fruits remained healthy. The fungal isolates were reisolated from symptomatic fruits and their identity was confirmed based on morphological features. During a strawberry field survey in July 2010 in Sint-Truiden (Belgium), lesions typical of those described above were observed on eight strawberry fruits (cv. Elsanta). The fungus was isolated from the symptomatic tissue of two fruits and characterized as described above. Since P. concavum was latently present on strawberry transplants and caused disease on the fruits in the field, we conclude that P. concavum is a potential threat for Belgian strawberry production. Moreover, no strawberry cultivars with resistance to the pathogen have been reported. The disease has previously been reported on strawberry in South America and Poland (1,2), but to our knowledge, this is the first report of P. concavum on strawberry in Belgium. Although the spore and colony morphology of P. concavum is different from C. acutatum, the spore masses of P. concavum can easily be confused with the spore masses of C. acutatum when using the freeze method. This suggests the need for microscopic analysis of these spore masses during routine analyses. References: (1) L. Cedeno et al. Interciencia 26:113, 2001. (2) U. P. Lopes et al. New Dis. Rep. 21:7, 2010. (3) J. L. Maas. Compendium of Strawberry Diseases. The American Phytopathological Society St. Paul, MN, 1998. (4) J. C. Mertely and D. E. Legard. Plant Dis. 88:407, 2004.

Wet sieving of soil samples, followed by plating on semi-selective medium and microscopic analysis, is the most commonly used technique to quantify microsclerotia-forming Verticillium species in soil. However, the method is restricted to small samples, does not allow easy differentiation between species, and takes several weeks to complete. This study describes an alternative method to test 100-g soil samples for three Verticillium species (V. tricorpus, V. dahliae, and V. longisporum) using density flotation-based extraction of microsclerotia followed by new real-time polymerase chain reaction (PCR) assays. Primers for these real-time PCR assays were designed to the ribosomal DNA internal transcribed spacer for V. tricorpus and the β-tubulin gene for V. dahliae + V. longisporum and V. longisporum. Tests with artificially and naturally infested soils showed that the new method is reproducible and sensitive (0.1 to 0.5 microsclerotia/g soil), allows differentiation among the three species, and can be completed in one day. The results of the new method and the wet-sieving method were highly correlated for V. tricorpus (R2 = 0.78), but not for V. dahliae/V. longisporum, probably due to the loss of germinability of V. dahliae/V. longisporum microsclerotia during prolonged dry storage of the soil.