Leaf surface chemicals from Nicotiana affecting germination of Peronospora tabacina (Adam) sporangia

A bioassay was used to evaluate the effects of cuticular leaf components, isolated from N. tabacum, N. glutinosa (accessions 24 and 24a), and 23 other Nicotiana species, on germination of P. tabacina (blue mold). The leaf surface compounds included alpha- and beta-4,8,13,-duvatriene-1,3-diols (DVT-diols), (13-E)-labda-13-ene-8 alpha-,15-diol (labdenediol), (12-Z)-labda-12,14-diene-8 alpha-ol (cis-abienol), (13-R)-labda-8,14-diene-13-ol (manool), 2-hydroxymanool, a mixture of (13-R)-labda-14-ene-8 alpha,13-diol (sclareol), and (13-S)-labda-14-8 alpha,13-diol (episclareol), and various glucose and/or sucrose ester isolates. The above in acetone were applied onto leaf disks of the blue mold-susceptible N. tabacum cv. TI 1406, which was then inoculated with blue mold sporangia. Estimated IC50 values (inhibitory concentration) were 3.0 micrograms/cm2 for alpha-DVT-diol, 2.9 micrograms/cm2 for beta-DVT-diol, 0.4 micrograms/cm2 for labdenediol and 4.7 micrograms/cm2 for the sclareol mixture. Manool, 2-hydroxymanool, and cis-abienol at application rates up to 30 micrograms/cm2 had little or no effect on sporangium germination. Glucose and/or sucrose ester isolates from the cuticular leaf extracts of 23 Nicotiana species and three different fractions from N. bigelovii were also evaluated for antimicrobial activity at a concentration of 30 micrograms/cm2. Germination was inhibited by 20% when exposed to sugar esters isolated from N. acuminata, N. benthamiana, N. attenuata, N. clevelandii, and N. miersii, and accessions 10 and 12 of N. bigelovii. These results imply that a number of compounds may influence resistance to blue mold in tobacco