INDUCTION OF RESISTANCE TO PERONOSPORA-TABACINA IN TOBACCO LEAF-DISKSBY LEAF-DISKS WITH INDUCED RESISTANCE

Tobacco leaf disks with lesions caused by tobacco mosaic virus (TMV) o r Peronospora tabacina and lesion-free disks from symptomless leaves o f TMV-infected plants' induced resistance to P. tabacina and Botrytis cinerea in leaf disks in an agar medium. The tobacco cultivar used in all experiments contained the N gene for resistance to TMV. Resistance was not induced by sodium salicylate (NaSA)-treated leaf disks, nor b y NaSA added to the medium. The phytoalexins rishitin, lubimin and phy tuberol were detected in the medium containing disks with lesions and they were associated with the inhibition of spore germination and grow th of three fungi in the medium. However, phytoalexins were not requir ed for the induction of resistance as they were not detected in the me dium containing lesion-free disks from TMV-infected plants, and the me dium containing these disks did not inhibit spore germination and grow th of the fungi tested. These results provide evidence that disks cont aining lesions caused by TMV and P. tabacina and lesion-free disks fro m leaves of TMV-infected plants produce a volatile and/or release a di ffusible signal(s), which is not a phytoalexin or NaSA, and this signa l(s) induces resistance to P. tabacina. The putative signal is present in uninfected disks that have induced resistance. (C) 1997 Academic P ress Limited.