Rapid change in the genetic diversity of Botrytis cinerea populations after the introduction of strains in a tomato glasshouse.

In tomato glasshouses, the population structure of airborne inoculum of Botrytis cinerea depends on the production of endogenous inoculum on diseased plants as well as on incoming exogenous inoculum. Both types of inocula may contribute differently to the development of epidemics. Two strains of B. cinerea were introduced in each of four separate compartments of an experimental tomato glasshouse. We monitored their impact on disease development and on the genetic diversity of B. cinerea populations using microsatellite markers. The naturally occurring airborne inoculum of B. cinerea displayed a high level of genetic diversity and was rapidly displaced in the glasshouse, as isolates with microsatellite profiles identical to the introduced strains amounted to 66% of the inoculum sampled from the air 14 days after inoculation and 91% of those collected from stem lesions 60 days after inoculation. This suggested an important role of secondary inoculum in disease development, which is compatible with the hypothesis of a polycyclic development of gray mold epidemics in tomato glasshouses. In controlled-environment tests on tomatoes, a wide range of aggressiveness levels was observed, both for isolates sampled from the air and from lesions on plants. Hypotheses are proposed to explain the negligible impact of naturally incoming isolates on the epidemics observed inside the four glasshouse compartments.

Oidium neolycopersici: intraspecific variability inferred from amplified fragment length polymorphism analysis and relationship with closely related powdery mildew fungi infecting various plant species.

Previous works indicated a considerable variation in the pathogenicity, virulence, and host range of Oidium neolycopersici isolates causing tomato powdery mildew epidemics in many parts of the world. In this study, rDNA internal transcribed spacer (ITS) sequences, and amplified fragment length polymorphism (AFLP) patterns were analyzed in 17 O. neolycopersici samples collected in Europe, North America, and Japan, including those which overcame some of the tomato major resistance genes. The ITS sequences were identical in all 10 samples tested and were also identical to ITS sequences of eight previously studied O. neolycopersici specimens. The AFLP analysis revealed a high genetic diversity in O. neolycopersici and indicated that all 17 samples represented different genotypes. This might suggest the existence of either a yet unrevealed sexual reproduction or other genetic mechanisms that maintain a high genetic variability in O. neolycopersici. No clear correlation was found between the virulence and the AFLP patterns of the O. neolycopersici isolates studied. The relationship between O. neolycopersici and powdery mildew anamorphs infecting Aquilegia vulgaris, Chelidonium majus, Passiflora caerulea, and Sedum alboroseum was also investigated. These anamorphs are morphologically indistinguishable from and phylogenetically closely related to O. neolycopersici. The cross-inoculation tests and the analyses of ITS sequences and AFLP patterns jointly indicated that the powdery mildew anamorphs collected from the above mentioned plant species all represent distinct, but closely related species according to the phylogenetic species recognition. All these species were pathogenic only to their original host plant species, except O. neolycopersici which infected S. alboroseum, tobacco, petunia, and Arabidopsis thaliana, in addition to tomato, in cross-inoculation tests. This is the first genome-wide study that investigates the relationships among powdery mildews that are closely related based on ITS sequences and morphology. The results indicate that morphologically indistinguishable powdery mildews that differed in only one to five single nucleotide positions in their ITS region are to be considered as different taxa with distinct host ranges.