Pathogenicity variation of Pythium ultimum on safflower under lab, greenhouse and field conditions

Seed rot caused by Pythium ultimum is an important disease of safflower in Iran and other countries. Therefore, finding the genetic sources of resistance and releasing of resistant cultivar is considered as important objective by safflower breeders. This disease will spread when air humidity and the moisture in the soil are sufficiently prepared and this situation occurs exactly in the time of safflower planting. So far, the damage caused by Pythium spp has been reported on seeds and seedlings of safflower, sorghum, wheat, alfalfa and sugar beet. Evaluation of seed rot for fifteen safflower genotypes for their response to causal agent was performed at laboratory, greenhouse and field conditions at Gorgan University of Agricultural Sciences and Natural Resources during 2008 and 2009. The experiments were conducted as split plot design in a completely randomized design in which sterile and infected media were included as main plot and fifteen safflower genotypes were considered as sub plot in four replicates. Artificial inoculation in lab was conducted with 10⁵ per mL zoospore suspension and in greenhouse and field infected soil with fungi agent were used. Results showed that there was significant difference among the genotypes for percent of seed rot in all conditions including laboratory, greenhouse and field. In the sterile media, the changing trends of genotypes were similar in laboratory, greenhouse and field conditions. The highest number of non-germinated seeds was observed in a greenhouse, laboratory and field conditions, respectively. The lowest seed germination in greenhouse, laboratory and field conditions were respectively belonged to genotypes Syrian, Aceteria and LRV-5151, but the results were different at the inoculated media. Presence of genotypic variation for percent of non-germinated seeds in infected media indicated that there is a good chance for finding genotypes with low rotted seeds. Difference between two media i.e. sterile and infected to the pathogen at laboratory and greenhouse was significant, that showed the applied pathogen is the main reason for decrease in percent of germinated seeds. None of genotypes was superior to others in all conditions, so it's not possible to recognize one genotype as the best for all conditions. So, three genotypes including PI-250537, Hartman and Arak-2811 have the lowest seed rot at greenhouse, laboratory and field, respectively. No considerable correlation was between laboratory, greenhouse and field for the percent of germinated seeds in inoculated media. The only significant correlation was observed between the percent of non-germinated seeds at sterile and inoculated media in laboratory condition, so that the lowest genotype × medium interaction was occurred at laboratory condition that it seems quite logical because of non-variable environmental factors in these circumstances. In overall, the effects of pathogen were greater at laboratory and greenhouse than field condition. The highest percent of safflower seed rot was observed at greenhouse.