ENDOPHYTES AND OAK DECLINE IN SOUTHERN EUROPE - THE ROLE OF HYPOXYLON MEDITERRANEUM
UniversitÓ della Tuscia, Viterbo, Italy
According to the current definition, endophytes comprise all organisms that spend at least part of their life cycles asymptomatically in plant tissues. For fungi, this definition could embrace not only mutualistic species but also those that can cause disease following an asymptomatic phase, conventionally termed 'latent infection'. Such pathogenesis may contribute to host mortality and is especially important in woody hosts. For example, oak forests worldwide are affected by oak decline, a loosely-defined syndrome which occurs cyclically with the onset of both abiotic and biotic stress factors. Aspects of forest management and severe seasonal droughts are thought to be the most important predisposing and concomitant factors in oak decline. Several fungal species isolated from declining oaks are known or suspected to be pathogenic on stressed hosts; these include: Phytophthora spp., Diplodia mutila, Fusarium eumartii, Cytospora spp., Hypoxylon mediterraneum (=Biscogniauxia mediterranea), Apiognomonia quercina, Colpoma quercinum and Aureobasidium apocryptum. Of these, at least H. ;mediterraneum, A. ;quercina, C. ;quercina and A. ;apocryptum are described in the literature as endophytes in asymptomatic living tissues of several oak species. The life cycles and ecological adaptations of most of these fungi are still to be defined but current data indicate a possible range of roles in the regulation of host development and survival at the level of the individual plant and, as described below, of the whole forest stand.
The life cycle and the interaction with host trees and local environmental conditions was studied for H. ;mediterraneum, known to cause 'charcoal diseases' of several broadleaved genera, particularly Quercus. This fungus can be detected in healthy vigorous oaks, as early as the seedling stage, living asymptomatically in bark tissues at hundreds of colonization points. It can also be isolated from most other above-ground tissues, including leaves and acorns. On Q. ;cerris in Italy, it can be considered the dominant endophyte. When the tree or part of it is subjected to severe water stress, the fungus develops rapidly within the woody tissues, preferentially spreading through the large air-embolized xylem vessels and apparently killing the adjacent parenchyma. The entire host may soon die, probably owing to synergistic colonization of tissues from several infection points where the fungus was latently present. It is known that Quercus species differ in their susceptibility to damage by this fungus, depending on their resistance to water stress.
The symptomatic phase results in reproduction and dispersal. The sexual fruit bodies, the perithecia, are embedded in black stromata which appear along the dead branches and stem. The ascospores released from the perithecia are thought to be the main means of transmission to new hosts; being carried by wind and water or by insects which commonly inhabit the declining hosts. Recent data do not support the hypothesis of vertical transmission of H. ;mediterraneum via acorns. Wide genetic variability, consistent with the heterothallic mating system of this fungus, has been detected within its populations by the use of molecular markers. This is extremely important for the epidemiology of the fungus, providing it with the genetic flexibility needed for the long-term survival both of endophytic populations within individual hosts and of progenies from symptomatic host tissue within the wider forest environment.
In healthy oak stands, H. ;mediterraneum develops overtly only on suppressed individuals or on the lower branches of dominant ones. In such conditions, endophytes such as this fungus could play a role at the single-tree level, for example in the self-pruning of suppressed branches. In mixed stands frequently subjected to drought and predisposed by unsuitable anthropic pressure on species selection, the pathogenic development of H. ;mediterraneum in dominant individuals has an effect on stand composition. In effect, it counteracts artificial selection by killing individuals of the less adapted species and contributes to the re-establishment of the original forest composition. In this context, some endophytes could act as regulating factors of the biocenosis.