European or Persian Walnut (Juglans regia) is an important and healthy food as well as base material of timber industry. Several pests (pathogens and insect pests) may cause serious damages on walnut. These are less known on the crop land of the tree. Results of some years of our experiments including bacteriological and mycological studies, are presented in this paper. The optimum time of chemical protection against the walnut blight (Xanthomonas arboricola pv. juglandis) was determined. Occurrences of pathogenic fungi were surveyed in an orchard and on home garden trees in Hungary (18 fungus species were identified). The following experimental results are reported on the pathogenic fungi: cultivar resistance to walnut anthracnose (Gnomonia leptostyla), dying of wood parts in the cultivar collection, application of the spore trap, in vitro fungicide testing against Phomopsis juglandina.