Relative susceptibility of selected apple cultivars to Colletotrichum acutatum

Eighteen apple cultivars were tested in the field and laboratory for their relative susceptibility to one of the bitter rot pathogens, Colletotrichum acutatum. Fruit were inoculated in the field at 3 to 4 weeks preharvest wit h cheesecloth strips soaked in a conidia suspension. in the laboratory, det ached fruit were inoculated using a conidia suspension in capped, sterile m icrocentrifuge tubes attached to the fruit surface with modeling clay. The same fruit as above also were inoculated over a wound on the side opposite the nonwounded inoculation. Fruit were tested for relative susceptibility t o the fungus with five criteria: disease incidence and severity of attached fruit in the field, disease incidence and severity of detached fruit in la boratory inoculations of nonwounded fruit, and disease severity in laborato ry inoculations of wounded fruit. Relative cultivar ranks from field tests were not reproducible in the 2 years studied, whereas laboratory tests show ed moderate reproducibility with nonparametric rank correlation tests. Base d on the laboratory data from 2 years of study, cultivars were classified i nto four relative-susceptibility groups: most susceptible: Pristine, Honeyc risp, and Ginger Gold; highly susceptible Yataka, Sansa, Arlet, and Enterpr ise; moderately susceptible: Sunrise, Golden Supreme, PioneerMac, GoldRush, Golden Delicious, and Creston; and least susceptible: Fuji. Compared to pr evious cultivar rankings, the results of the present study indicate that ne w apple cultivars from the first NE-183 planting show no improvement in res istance to C. acutatum.