Tobacco necrosis virus induces systemic resistance in cucumbers against Sphaerotheca fuliginea

Cucumber (Cucumus sativum) plants locally infected with tobacco necrosis virus (TNV) in cotyledons or lower true leaves were partially protected against infection by Sphaerotheca fuliginea, on the upper leaves. On TNV-infected plants both colony development and conidial production of S. fuliginea was reduced by about 65 and 85%, respectively, compared to TNV-free plants. Protection in leaf 2 was evident between 48-96 h after inoculating leaf 1 with TNV. Of the five cultivars tested, Poinsett, which was partially resistant to powdery mildew, expressed the highest degree of induced resistance. In two separate field trials, cucumber plants were partially protected against subsequent infection by S. fuliginea by a prior inoculation with TNV. Histochemical observation indicated that lignification was more frequent in challenged-protected than in challenged-unprotected plants. Localized infection with TNV failed to protect cucumbers against downy mildew incited by Pseudoperonospora cubensis. TNV did not induce resistance to powdery mildew in cantaloupe (Cucumis melo) nor in squash (Cucurbita moschata) plants.