Research Highlights: Mycorrhizal helper bacteria (MHB) promote mycorrhization processes and are commonly found in the mycorrhizosphere of fungi, such as the edible hypogeous fungus Tuber melanosporum Vittad. Background and Objectives: The effectiveness of MHB in promoting the mycorrhization process and the root development of Portuguese oak (Quercus faginea Lam.) seedlings destined for truffle plantations has not been determined. The main aim of this study was to shed light on the effect of bacterial inoculation on fungal root tip colonization and seedling root traits. Material and methods: We performed a co-inoculation trial using three bacteria naturally present in the T. melanosporum niche (i.e., Pseudomonas fluorescens, Pseudomonas putida, and Bacillus amyloliquefaciens) and two different bacterial inoculation times (one month and nine months after fungal inoculation) under glasshouse conditions. Results: Only P. fluorescens had a significant mycorrhizal promoter effect, increasing the truffle inoculation rates of root tips by more than 10% compared with seedlings that received non-bacterial inoculation treatments. Simultaneously, the co-inoculation of P. fluorescens with T. melanosporum improved seedling root growth parameters compared with those of seedlings that received non-bacterial inoculation treatments. The different bacterial inoculation times and applications of uninoculated bacterial growth media did not affect the root traits analyzed or the root mycorrhization rates. Conclusions: These results suggest that P. fluorescens bacteria have a potential commercial application as a treatment for truffle-inoculated seedlings to improve both seedling quality and mycorrhizal colonization under nursery conditions.

Aim of study: We aimed to i) evaluate the effects of mulching on Tuber melanosporum mycelium biomass and seedling growth (i.e. root collar diameter and seedling height) and ii) unravel the relationship between growth in root collar diameter and mycelium abundance, in a T. melanosporum plantation.Area of study: The experimental plantation is located in the Pre-Pyrenees mountains in Catalonia, Spain.Material and methods: The experimental plantation was established in 2010 using one-year-old T. melanosporum inoculated Quercus ilex seedlings. Double-layered mulch materials were placed around the seedlings. The mulch materials were removed from randomly selected seedlings in 2015 and 2018. Soil samples were collected in 2018 at 40 and 80 cm distances from seedlings that had mulching during five and eight years, and T. melanosporum mycelium biomass was estimated by quantitative Polymerase Chain Reaction (qPCR). Seedling root collar diameter and height were measured simultaneously when mulch materials were removed.Main results: Mulch removal time did not have significant effects on T. melanosporum mycelium biomass or seedling growth. However, mycelium biomass at 40 cm distance tended to be higher on seedlings after eight-year mulching with 0.9 mg/g soil whereas mycelium biomass was 0.4 mg/g soil after five-year mulching. A positive relationship between mycelium biomass and seedling root collar diameter was also found.Research highlights: Mulching seems to have a positive effect on truffle mycelium biomass, with nearly two times higher quantity of mycelium after eight-years compared with five-years mulching usage. Seedling root collar diameter is a good indicator of mycelium expansion in the plantation.Keywords: Black truffle; Quercus ilex; mulching; tree growth; truffle cultivation.

Understanding the soil fungal community in vineyards sheds light on the interactions between plants and their associated microorganisms. For example, identifying arbuscular mycorrhizal fungi (AMF), which are beneficial to grapevine growth, is a good indicator of soil health. In contrast, other fungi, such as the pathogen group, can be detrimental to vine growth. The present study aimed to characterize the soil fungal community and the fungal diversity present at six Mediterranean vineyards located in Burgos (Spain), delving into fungal functional guilds and focusing on AMF and pathogenic fungal groups. The fungal structure was investigated using DNA metabarcoding in three soil samples taken from each vineyard, and differences in the abundance of functional guilds were assessed. Similar soil fungal community structures were observed among soil sample repetitions within vineyards. In contrast, adjacent vineyards presented differences in their microbial composition. Saprophytes followed by pathogens were the dominant fungal functional guilds across all vineyards. However, no differences in the relative abundance of the different fungal functional groups were observed among sites. The vineyard with the highest relative abundance of AMF (0.5%) also had the lowest pathogen relative abundance from all the sites (29.76%). Also, sites presenting a high relative abundance of pathogens in soil (>35%) had a low relative abundance of AMF (<0.05%). Our results suggest that the fungal community is affected by the intrinsic properties of the soil and the characteristics of each vineyard’s microsite over the effect of the geographical proximity. In addition, to improve our understanding of the soil microbial ecology, we highlight the necessity of prospecting soil fungal analyses into functional groups, interpreting diversity results within taxonomic groups alongside the total abundance of target groups/species.