Relative susceptibility of fourteen promising local apple genotypes to fruit black rot Diplodia seriata and its relationship with some physicochemical traits

Diplodia seriata is the causal agent of apple limb and trunk cankers as well as fruit black rot. Disease was detected in the Iranian apple collection located at horticultural research station of Kamal-shahr, Karaj. Fourteen apple promising genotypes and two commercial cultivars (Golden Delicious and Fuji) were tested in the field and laboratory for their relative susceptibility to the pathogen, during 2016-2017. The fruits were inoculated in the field at 2 to 3 weeks before harvest with fungal mycelia from 11-day-old cultures of PDA medium. In the laboratory, detached fruits were inoculated similarly. Genotypes were classified for susceptibility to the fungus by determining disease severity of attached fruits in the field based on lesion expansion (mm/degree-day), and on detached inoculated fruits in laboratory according to the mean lesion diameter, both 5 days after inoculation. Based on the laboratory and field data, Iranian genotypes were classified into three relative susceptibility groups: most susceptible: TR-1, SSB, and H1-SH; moderately susceptible: MD-N2, GO-N3, B-K-KH, DO-P-SH, MN8, T5 and Golden Delicious; least susceptible: SBA, MN10, S-O-G, KH2, ME6 and Fuji. There was a negative correlation between disease severity and flesh firmness, but total solid soluble (TSS) and les