Ecology of Hypoxylon


Elements of ecology of Hypoxylon

Hypoxylon taxa are saprophytes of angiospermous plants. Known European taxa are restricted to woody dicotyledons, while a few tropical taxa are known from monocotyledons, especially bamboo. Some of them are highly host specific but most of them are plurivorous, and most of commonly encountered trees and shrubs are hosts for Hypoxylon. It is noteworthy that, in our region, Hypoxylon was never found on some common woody plants such as Buxus sempervirens, Euonymus latifolia, Robinia pseudoacacia, Sambucus nigra, on which other Pyrenomycetes are not uncommon. These observations might be refuted by observations from other areas, as, for instance, a collection of H. ticinense on Sambucus nigra reported from western France (Leroy & Surault, 1999). Two recent collections of H. julianii and H. perforatum on Robinia likewise show this tree a possible host for Hypoxylon.

They appear to be mostly weak pathogens with high saprophytic capacity (Ju & Rogers, 1996), and many species proved to grow endophytically in a wide range of various hosts without producing the teleomorphs (Petrini & Petrini, 1985). Hypoxylon fragiforme or H. cohaerens developing rapidly on killed wood of Fagus prove at least a latent infection of the host by the fungus (Chapela & Boddy, 1988a; 1988b). Dying branches of Fraxinus are soon invaded by H. cercidicolum or H. intermedium before they fall onto the ground, as does H. laschii on Populus, but the pathogen role of these species remains obscure.

Where to collect Hypoxylon?

All European Hypoxylon species are active saprophytes depending mostly upon humidity. They grow on bark or decorticated wood of trunks or branches lying on the ground in coombs, in alluvial forests, on the banks of streams or rivers, in damp woods, usually in shadowy places. Some species can grow on branches still attached to the tree or broken branches hanging above the soil level, but always in locations with a high and rather constant level of humidity of the atmosphere. Low temperatures favour a few species known to have boreal or hemiboreal distribution, which will be found in mountains or northern regions of Europe. Most of Hypoxylon prefer mild temperatures and humidity that they rather find in regions of western Europe under oceanic influence.

When to collect Hypoxylon?

They develop slowly in natural conditions, needing several months (usually 3-6) from the formation of the first coloured stromatal tissue or anamorph on the surface of the substrate to the maturation of stromata with ascigerous perithecia. A too long period of drought is frequently fatal to their development. Low temperatures encountered during winter for short periods apparently only stop or slow down the development of Hypoxylon taxa that have started in autumn, but apparently do not kill them. Therefore Hypoxylon taxa can be found throughout the year, providing that their substrate remained without long interruption in a humid enough environment, with more favourable periods in summer and autumn.

Most of them are perennial, at least for a few following years. In favourable conditions, it is not rare to find the anamorph or young stromata growing close to hardly mature stromata appeared a few months ago.

Some taxa are frequently found growing associated on the same substrate, for instance H. carneum and H. rubiginosum on rotten wood or H. cercidicolum, H. intermedium, H. perforatum, H. petriniae and H. rubiginosum on a same branch of Fraxinus. Nethertheless, all species usually grow solitary and no association between Hypoxylon taxa appears obligatory. These associations are rather a challenge to field taxonomists!

Ecology of Hypoxylon remains poorly known, and above data need to be completed, corroborated or refuted by observations from other regions of Europe.