Pattern of Tuber melanosporum extramatrical mycelium expansion over a 20-year chronosequence in Quercus ilex-truffle orchards

Successful cultivation of black truffle (Tuber melanosporum) requires a long-term investment and the maintenance of the symbiosis throughout its preproductive and productive years. Monitoring the symbiosis over time is challenging, as it requires methods that can detect the belowground proliferation of the fungus associated with its host tree. In this study, we used a chronosequence design to study the expansion pattern of this fungus as the host tree grows. We hypothesize that this expansion can be estimated by monitoring T. melanosporum DNA from soil beneath host trees of different ages (3, 5, 7, 10, 14, and 20 years old) and at different distances from the trunk of the trees (40, 100, and 200 cm). We also wished to evaluate the presences of Tuber brumale and Tuber indicum, potentially problematic truffle species, in these plantations. To detect the mycelium of T. melanosporum in these soils, we extracted DNA and performed polymerase chain reaction (PCR) with Tuber species-specific primers, and to estimate DNA amount, we measured relative band intensities from the amplicons in agarose gels. Both age and distance were related to T. melanosporum DNA quantity, which was more abundant in the oldest age classes, reaching a plateau in 5–7 years. At 40 cm from the tree, there were no differences in T. melanosporum DNA amounts in orchards of different ages, but at 100 and 200 cm, younger orchards had less T. melanosporum DNA. We did not detect DNA from T. brumale or T. indicum in any of our samples.