Plants of Cucumis sativus were either preinoculated with TNV or treated with drops of a 5 % solution of CuSO4 on the cotyledons, and 5 days later challenge-inoculated with Sphaerotheca fuliginea on the first true leaf. The induced systemic resistance was assessed by evaluating the percentage of conidial germination, the length of the hyphae derived from single conidia, the number of cells with lignified walls and that of cells containing haustoria. The number of necrotic cells was also recorded. Compared with controls, in TNV-preinoculated plants conidial germination was lower, hyphal length shorter and the number of haustoria much reduced. The majority of haustoria was found in cells with lignified walls. Pretreatment with CuSO4, although inducing a slight increase of cell wall lignification, did not appreciably reduce the infection process. The number of necrotic cells was very low in all cases. These results suggest that, in this host/pathogen interaction, hypersensitive cell death has no part in the induced defence reaction, although a major role is played by the lignification process.
