Review of tomato powdery mildew – a challenging problem for researchers, breeders and growers

Tomato powdery mildew (Pseudoidium neolycopersici (syn. Oidium neolycopersici)) is one of the most devastating diseases of cultivated tomatoes worldwide. Although the first epidemics were recorded nearly 30 years ago, many aspects of the host-pathogen interaction are still not well understood. Detailed morphological and molecular studies of the anamorphs confirmed that P. neolycopersici is phylogenetically close to Erysiphe aquilegiae var. ranunculi. Host range is rather broad; apart from the family Solanaceae, hosts were found in very distant plant families. Screening of wild Solanum sect. Lycopersicon spp. (previous Lycopersicon spp.) germplasm revealed valuable sources of resistance (Solanum habrochaites, Solanum pennellii, Solanum cheesmaniae, Solanum chilense, Solanum peruvianum). The main resistance mechanism was found to be a hypersensitive response (HR), in some cases followed by limited development of the pathogen. However, there is a broad variation in resistance response at the histological and cytological levels. Biochemical studies focusing on production of reactive oxygen species (ROS), reactive nitrogen species (RNS) and corresponding enzymes during infection of P. neolycopersici showed that all these play a crucial role in activation of defense responses in genotypes of wild Solanum sect. Lycopersicon.