Biology of cymoxanil action against Phytophthora infestans infection of tomato and potato

We have conducted a qualitative and quantitative cytological analysis on the effects of cymoxanil on the host-pathogen interaction between the Late Blight pathogen (Phytophthora infestans) and leaf tissue of tomato. Qualitative observations were also made in potato. Both preventative and curative treatments of cymoxanil were employed. Preventative treatments of tomato affected development of the pathogen prior to penetration of the host surface, as evidenced by a reduced size of appressoria. After penetration, haustorium development was especially limited in the presence of cymoxanil relative to the numbers observed in untreated tissues. Both preventative and postinfection treatments with cymoxanil abated spread of the pathogen beyond the initial sites of infection, apparently by inducing a set of hypersensitive-type host responses. These host cell responses included a rapid onset of granulation, plasmolysis and yellowing of cytoplasm in penetrated epidermal cells, formation of host cell wall appositions (papillae), browning of mesophyll cells and walls of epidermal cells, and collapse of invaded host cells. As a consequence of cymoxanil action, sporulation by the pathogen was also prevented.