Variation in aggressiveness of Phoma macdonaldii isolates from three Balkan countries and Ukraine

Abstract

Phoma macdonaldii is a ubiquitous pathogen, affecting sunflower by premature leaf senescence, stem cortical tissue necrosis and basal stem girdling. Sunflower genotypes expressing partial resistance have been reported. Currently, disease has reemerged as a threat to sunflower production in some sunflower cropping areas. To estimate variability in pathogen population, sunflower stems with disease symptoms from Serbia, Turkey, Romania and Ukraine were sampled. Total of 54 isolates was used for inoculation of four inbred lines differing in resistance to phoma black stem. Results from scoring of sunflower inbred lines seven days after inoculation showed significant differences in disease severity measured by percentage of cotyledon petiole necrosis. Significant difference was detected both among isolates and genotypes. Isolates were segregated in five clusters. Five isolates were found to be highly aggressive based on disease severity. The least aggressive were seven isolates, producing mild symptoms on all tested genotypes. Majority of tested isolates lead to complete necrosis of inoculated plant part of the most susceptible genotype and mild symptoms of other three genotypes. Isolate aggressiveness was not correlated with geographic origin. In conclusion, significant variability among pathogen isolates was confirmed with several isolates distinguished as highly aggressive. This research could assist in breeding process for resistance to phoma black stem.