PEZICULA: ENDOPYHTE, MUTUALIST AND WEAK PATHOGEN
Institut fur Mikrobiologic, Technische Universitat Braunschweig, Spielmannstrasse 7, D-38106 Braunschweig, Germany
Background and objectives
One of the most commonly isolated endophytic fungal genera from both deciduous and coniferous trees is Pezicula, as well as its anamorphic genus, Cryptosporiopsis . However, Pezicula may also be a weak pathogen, causing cankers of stressed or weakened trees . Thus, the questions arise: (1) what role does Pezicula (Cryptosporiopsis) play within its hosts, and (2) are the endophytic isolates identical with those isolated as weak pathogens from necrotic lesions?
Results and conclusions
The roots of axenically cultured larch seedlings were inoculated either with one of the endophytes, Cryptosporiopsis sp. or Phialophora sp., or with the pathogen, Heterobasidion annosum. The endophytes had previously been isolated from the roots of mature larch trees growing in a mixed forest. Infections with either of the endophytes were not only symptomless, but also resulted in better growth of the host plants compared with the controls. In contrast, infection with the pathogen resulted in disease and diminished growth. If the seedlings were stressed with excessive nitrogen (4 times the concentration in the controls), infections with either of the endophytes were initially able to compensate for the stress symptoms and poor growth observed in the uninfected control plants. However, the seedlings infected with Cryptosporiopsis sp., in contrast to those infected with Phialophora, soon developed disease symptoms. Thus, whereas in nonstressed plants, by improving growth of their hosts both endophytes assumed the roles of mutualists; in plants stressed with nitrogen only the role of the Phialophora isolate was mutualistic and that of Cryptosporiopsis was as a weak pathogen.
The above experiments were conducted with one isolate from each genus. It is conceivable that individual isolates of a particular taxon may be phenotypically very variable. We compared the phenotypic characteristics of morphology and secondary metabolization profiles with genetic parameters, as analysed using RAPD investigations of 65 endophytic and 10 pathogenic (from necroses) isolates of Pezicula livida and P. cinnamomea. For each of the two species and for each of their two morphology groups, there was a 89% correlation between the phenotypic and genetic parameters for the strains isolated as endophytes, but no correlation of these parameters for those strains isolated as pathogens - neither among the pathogenic strains nor of the individual pathogenic strains with the endophytic strains. These results show that the endophytic isolates of Pezicula/Cryptosporiopsis seem to form uniform groups in which morphology, chemotaxonomy and RAPD patterns correlate. The results also suggest that the pathogenic isolates, as has already been discussed for other phytopathogenic species, may vary considerably in their aggressiveness and metabolism.
Further experiments are being conducted to study the role of the pathogenic isolates when infected into axenically grown larch seedlings under stressed and nonstressed conditions.
1. Kehr RD 1991, European Journal of Forest Pathology 21, 218-233.
2. Kowalsld T, Kehr RD, 1992. Sydowia 44, 137-168.