Petrakia liobae leaf spot of common beech

P. liobae is a sac (ascomycete) fungus that occurs on the leaves of the common beech (Fagus sylvatica). The infection leads to the development of conspicuous leaf browning in the form of irregularly shaped patches of dead tissue, so-called necroses, with dark brown edges (Fig. 1).

From summer to autumn, small white diaspores appear on these patches, serving the asexual dispersal of seeds. At around 150 μm in diameter, these are clearly visible to the naked eye. If you look at them under a magnifying glass (Fig. 2) or microscope, they appear as short-stalked spheres with long hairs.

During winter and spring, the sexual fruiting bodies form on fallen leaves in the leaf litter (Fig. 3). These lenticular, dark brown or black fruiting bodies are around 0.2 mm in diameter and often have a sunken centre. Infested leaves are often found in the lower canopy or on young trees in the undergrowth.

In spring, the blackish brown sexual fruiting bodies of the fungus mature on the dead leaves from the previous year (Fig. 3). They release their spores from April to May – just at the time when the new leaves of the beech are also emerging. The spores are spread through the air, leading to the initial infection of the young leaves. The presence of the fruiting bodies in the litter explains why it is mainly the lower canopy and young beech trees that are colonised.

Brown spots develop on the living leaves infected by the fungus. The small white diaspores (Fig. 2) eventually appear on these, developing between June and October. These asexually formed diaspores serve – as do the sexual spores – to propagate the fungus and lead to secondary infections of the beech leaves. The initial infection of the leaves is favoured by wet spring days; wet summers in turn lead to increased secondary infections.

The fungus has now been found throughout Switzerland, in southern and central Germany, in Austria, Slovakia and Slovenia, and in the French Pyrenees. There is no evidence confirming its presence in northern and southern Europe as yet.

The fungus prefers the common beech (Fagus sylvatica) as host, but in the Botanical Garden in Munich it has also been found on the oriental beech (F. orientalis), which some botanists consider to be a subspecies of the common beech. The natural range of the oriental beech lies south-east of the range of the European beech (Fig. 4). In Slovenia, many hornbeams (Carpinus betulus) growing next to infected common beech trees also showed signs of diseased leaves. In trials, sessile oaks (Quercus petraea) and sweet chestnuts (Castanea sativa) also became diseased.

Infection mainly affects young beech trees in the understorey and low-hanging beech branches on shady, humid sites. Trees on sunny and sparse, light sites seem to be infected less frequently. In the course of infection trials it was shown that, because they have thicker cell walls, sun-exposed leaves of the common beech are more resistant to fungal infection than the softer leaves hanging in the shade. However, this protective effect is lost again by sun-exposed leaves it they are damaged mechanically or by insect herbivores such as the beech leaf-miner beetle (Orchestes fagi).