Introducing a cut-stem inoculation method for fast evaluation of sunflower resistance to Macrophomina phaseolina

The fast adaptation to different growing conditions of a fungus Macrophomina phaseolina, led to its becoming one of the sunflower (Helianthus annuus L.) disease causal agents in regions with a temperate climate. Methods currently used to determine sunflower resistance require laborious manual inoculation and confirmation of pathogen appearance, due to the late stage of testing. The paper proposes a cut-stem method for inoculating sunflower plants in the controlled conditions and the possibility of early-stage disease evaluation. A set of 15 sunflower inbred lines was inoculated using M. phaselolina isolate in the growth chamber and the obtained data were analysed using Cut-stem Disease Severity (CSDS) and compared with disease severity obtained from field experiments using traditional inoculation methods (toothpick, Unwounded Stem Base Inoculation (USBI) and non-inoculated plants). The results showed that, based on CSDS, inbred lines infected with the cut-stem inoculation method significantly differed regarding resistance to M. phaseolina. None of the inbred lines exhibited complete resistance but three lines could be proposed as a source of resistance to this pathogen. Ranking of inbred lines which was based on resistance to M.phaseolina was similar in all inoculation methods and in non-inoculated plants. There were highly significant correlations between the values obtained from growth chamber experiment and disease severity scores from field evaluations. Thus, the obtained results indicate that the cut-stem method could potentially complement field testing methods and be valuable tool in sunflower breeding for resistance to M. phaseolina.