All-Russian Institute of Plant Protection, 189620, St Petersburg, Pushkin, Russia

Background and objectives
For more than 20 years my collegues I and have carried out population research of plant pathogenic fungi. The aim of this research is to study the population structure, to determine the population areas and to estimate the influence of ecological, geographical and different anthropogenic factors on polymorphism and dynamics of populations. Puccinia recondita, Pyrenophora teres, Bipolaris sorokiniana, Fusarium graminearum and F. oxysporum have been used as the objects of this population research.

Materials and methods
The use of morphological, biochemical (isozymes) and physiological (pathogenicity) markers allowed analysis of the natural population structure. As the result, the index of genetic similarity of populations, genetic distance and the degree of population localization were determined.

Results and conclusions
P. recondita has large natural populations which occupy vast territories. Five isolated populations of the fungus were revealed in the former USSR. The influence of natural selection and spore migration on the population variability were determined[1]. P. teres exists in the form of local populations [2]. Clones collected from adjacent sites differed significantly in morphological characters and virulence gene frequency. The high degree of polymorphism of the local populations is caused by fungi genome instability and heterokaryosis.

Populations of B.sorokiniana have significantly larger areas compared with P. teres. The differences in the population structure were observed only between geographically distant populations; for example, between the populations of north-western Russia and the Urals. Analysis of F. graminearum populations showed that geographically distant populations from the North Caucasus and Far East differ in morphological characteristics, isozyme spectra, aggressiveness and the ability to produce mycotoxins. Soil populations of F. oxysporum are very local because soil conditions prevent active migration of the fungus. Therefore, the contacts between the populations are limited.

The population structure of fungi is a dynamic system. Investigation of changes in the population structure of P. teres showed the following. (1) Under the influence of host-plant genotype, the virulence of the population increases to the end of the vegetative period at the expense of accumulation on the susceptible cultivars of complicated races with many virulence genes; however, on the cultivars with high levels of nonspecific resistance, simple races are accumulated at the expense of the elimination of unnecessary virulence genes. (2) During over-wintering, unnecessary virulence genes are also eliminated; however, in P. recondita most of the unnecessary genes are neutral for influence on competitive pathogen ability, but the frequency of some differ significantly during the vegetation period.

These population studies have practical application in breeding for disease resistant cultivars and their territorial distribution.

1. Mikhailova LA 1996. Mycology and Phytopathology 30, 84-90 (in Russian).
2. Afanasenko O, Hartleb H, Guseva NN, Minarikova V, Janosheva M 1995. Journal of Phytopathology 143, 501-507.