Investigations on resistance mechanism of sunflower against to downy mildew (Plasmopara halstedii (Farlow) berl. et de toni)

Downy mildew caused by Plasmopara halstedii (Farlow) Berl. Et de Toni) is one of the most destructive disease of sunflower (Helianthus annuus L.).The use of resistant varieties in the control of disease is still in high demand today due to resistance problem of the pathogen to the fungicides used. The aim of this study is to characterize the morphological and biochemical resistance mechanism occured in different sunflower inbred-lines against the disease agent. Twelve inbred-lines (9 resistant and 3 sensitive of them to the pathogen based on symptomatic observations and presence or absence of the resistance genes) were used throughout the study. Sporangium numbers after secondary infection by leaf inoculation in addition to morphological characteristics (width and length of stomata and aperture size) of the lines were determined. Moreover, anti-oomycete effects of the fractions raised after Thin Layer Chromatography (TLC) analysis of the extracts obtained during and after root inoculation were investigated. The content of volatile compounds in fractions representing inhibition of >50% for sporangial discharge were detected by Gas Chromatography – Mass Spectrometry (GC/MS), whereas the content of the non-volatile or easily degradable compounds in the same fractions were detected by Liquid Chromatography – Mass Spectrometry (LC-MS/MS). As a result, among the all lines tested, the resistant lines CL 217 R, HAR 5, K5 R SN:11/2014 and K-9 R-SN:3/2014 had the important resistance indications for secondary infection (low sporangium number), morphological (narrow stomatal aperture) and biochemical mechanism (having the fractions with anti-oomycete effects on sporangia and phenolic compounds at high amount. It was determined that 2-ethyl-2-methyl-tridecanol (an alcohol); nonanal (an aldehyde), 2,6,10,15-tetramethylheptadecane (an alkane); 2-(1-Ethoxy-ethoxy)-1,1,1-trifluoro-4-iodobutane (an alkyl halide); 9-octadecenoic acid (Z)- (a fatty acid); 2-hydroxy-3-[(1-oxooctadecyl)oxy]propyl ester (an ester) were present in anti-oomycete fractions obtained from resistance lines only, both during and after inoculation of the pathogen. This study is the first report for the examination of resistance mechanism for sunflower downy mildew disease and it was thought that the obtained results would notably contribute to alternative control methods against this disease.