Characterization of the strawberry pathogen Gnomonia fragariae, and biocontrol possibilities

The strawberry root rot complex or black root rot is common and increasing problem in perennial strawberry plantings worldwide. In many cases the causes of root rot are not detected or it is referred to several pathogens. During the survey on strawberry decline in Latvia and Sweden the root rot complex was found to be the major problem in the surveyed fields. Isolations from diseased plants showed that several pathogens such as Cylindrocarpon spp., Fusarium spp., Phoma spp., Rhizoctonia spp. and Pythium spp. were involved. Among these well known pathogenic fungi a poorly studied ascomycetous fungus, Gnomonia fragariae, was repeatedly found in association with severely diseased plants. An overall aim of the work described in this thesis was then to characterize G. fragariae as a possible pathogen involved in the root rot complex of strawberry, and to investigate biological control possibilities of the disease caaused. In several pathogenicity tests on strawberry plants G. fragariae was proved to be an aggressive pathogen on strawberry plants. The pathogenicity of G. fragariae has been evidently demonstrated for the first time, and the disease it causes was named as strawberry root rot and petiole blight. In order to investigate the process how G. fragariae invades and colonizes the roots and petioles of strawberry, the fungus was genetically transformed with the green fluorescent protein (GFP) gene. As revealed by studies with GFP-tagged strain, G. fragariae is an aggressive invader of both strawberry root and petiole tissues, and the cortex are the most affected tissues. Observations of the pathogen development in root and petiole tissues showed that the petioles are more sensitive to the pathogen infection resulting in more severe damages in shorter period of time. Gnomonia fragariae was also able to infect vascular tissues of petioles in later stages of the disease development. Ascospores were found to be highly infective suggesting that they could be one of the major means of disease spread in the field. Gnomonia fragariae isolates were further characterized and phylogenetic relationships inferred with other Gnomonia species and members of Diaporthales based on the nucleotide sequence analyses of two loci (nuLSU rDNA and ITS/5.8S region) of the ribosomal rDNA array. In addition to molecular methods, morphological examination was also performed. Results showed that G. fragariae is genetically distinct from the genus type species, G. gnomon, and other members of Gnomoniaceae, although morphological evidences to support this discrepancy were not found. It was suggested, that G. fragariae together with G. rubi, G. rosae and Hapalocystis represents a genetically distinct group, possibly a new family, within Diaporthales. In a series of biocontrol and root inoculation experiments carried out with two non-pathogenic Fusarium sp. strains originated from strawberry roots it was shown, that these strains have beneficial effect on strawberry plants and can reduce root rot on strawberry caused by G. fragariae. However, the evaluation of disease suppression efficiency and effect on strawberry productivity in actual culturing systems and elucidation of the modes of action of these Fusarium sp. strains need to be further studied