Bioactivity of certain monoterpenoids against potato Soft-Rot bacteria

Evaluation the inhibitor activity of five monoterpenoides namely carvacrol, p-cymene, eugenol, isoeugenol and thymol against growth of Potato soft rot pathogenic bacteria (Pectobacterium carotovorum subsp. carotovorum) studied in different concentrations under laboratory & field conditions. carvacrol and thymol showed the strongest effect against P. c. subsp. carotovorum isolates. The growth of tested bacteria decreased significantly by increasing the concentrations of carvacrol and thymol. (Eugenol+ Thymol) and (carvacrol+thymol) mixtures showed equal significance against P. c. subsp. carotovorum isolates. Experiments to enhance the antimicrobial efficacy by mixing with the four synergists: Triphenyl phosphate, Dimethyl Meleate, Piperonyl butoxide and Triton X-100 reached.Thymol+Triton X-100 in 1:2 a ratio showed the highest effect against Pcc160 isolate with MIC 125 μg/mL. Scanning electron microscope showed distinct inhibitory effect of carvacrol, (eugenol+ thymol) mixture and [carvacrol + Triton X-100 in 1:1 (v/v) ratio] mixture on the growth and survival of P.c. subsp. carotovorum (Pcc 160).In field conditions, tested monoterpenoides, their combinations and joint action effects of the most active monoterpenoids, carvacrol, eugenol and thymol with Triphenyl phosphate and Triton X100 had significant reduced the percentage of soft rot infection on potato plants, the combination eugenol with Triphenyl phosphate (TPP) was the most effective, where it caused a reduction of the relative disease control (RDC) with 90. 54 % in season1and 89% RDC in season 2.