Sukachev Institute of Forest Research, Akademgorodok, Krasnoyarsk, 860036, Russia

Phytopathogenic fungi were studied in two forest nurseries, beginning from 1985 to 1993. The nurseries were located in southern taiga and at the border between forest and steppe.

Materials and methods
The qualitative and quantitative composition of micromycetes was studied on Chapek's medium and potato agar. The virulence of the phytopathogenic fungi was determined by the method of artificial inoculation. Microconidia of Fusarium spp. were obtained by immersion of the fungus culture into modified potato medium. Spores of Alternaria alternata were obtained from the sunface of its culture on Chapek's medium. Sterile quarts sand in containers was moistened with water suspensions of spores and then sown with Scots pine seeds. Seedlings grew on standard medium of Walker.

Results and conclusions
The survey of forest nurseries indicated that over 60 percent of conifer seeds and seedlings die of contagious damping-off. Pathogens have been specified in samples taken from the zones with different soil and climatic conditions. Nine species of Fusarium were isolated from the soil of the nurseries. Most widespread were F. solani, F. oxysporum, F. moniliforme, F. bulbigenum and F. sporotrichiella. Selected isolates possessed strong virulent characteristics that caused high (up to 89.5%) mortality among Scots pine seedlings. Species like F. moniliforme and F. oxysporum infect seeds and germinants, causing damping-off before emergence. Other representatives of this genus damaged almost exclusively young seedlings and caused damping-off after emergence. Studies of isolates of A. alternata indicated that this fungus damages photosynthetic organs of the seedlings. The conducted studies have shown that the structure of populations of phytopathogenic fungi is formed under the influence of conifer seedlings directed to the selection of clones for their virulent characteristics. The increase of contagious ability of species is closely allied to their population density. Correlation coefficient was 0.89 for dark grey, weakly podzolized soil and 0.91 for podzolized chernozem.