OBSERVATIONS ON THE POPULATION BIOLOGY OF THE GRAPE POWDERY MILDEW FUNGUS UNCINULA NECATOR

Abstract

Random Amplified Polymorphic DNA (RAPD) analysis and mating type distribution were used to investigate the population biology of the phytopathogenic fungus Uncinula necator (Schw.) Burr., the causal agent of powdery mildew of grapevine. It is known that the fungus has two overwintering strategies, mycelium and conidia in dormant buds or cleistothecia, but their relative importance in disease epidemiology is still undetermined. Recently, the existence has been hypothesized of two genetically separated biotypes in U. necator that would be related with its overwintering modes: a biotype overwintering as conidia and mycelium in buds would infect shoots and leaves early in the season; the other biotype would overwinter as cleistothecia and infect bunches. RAPD analysis was carried out on 374 isolates of U. necator collected in Southern Italy. Statistical analysis of variation clustered the isolates into two major groups according to the time in the season when they were sampled in vineyards, confirming the existence of two different biotypes into the fungal species. Similar proportions of the two mating types were found in the pathogen’s populations, as well as into each biotype, even in single vineyards. Therefore, the two mating types of the fungus are not separated either in space or time. Sexual crosses between isolates belonging to the two biotypes were fertile and yielded viable ascospore progeny. These results suggest that meiotic recombination can be an important source of genetic variation in U. necator and cleistothecia can play an important role in its overwintering.