A SSR Marker Linked to the Susceptibility to Alternaria Mali in Apple

Altemaria mali is a leaf pathogen in apple, and causes leaf spots, and premature drop when serious. This disease is of worldwide importance, but is most serious in the East Asia (Japan, Korea and China). The wide use of fungicides not only adds cost to farmers, but also pollutes the environment. The objective of this study was to characterize a 5-year F₁ population from a cross of a resistant cultivar (‘Huacui’) and a susceptible cultivar (‘Golden Delicious’), consisting of 110 individuals, along with 10 each of mature parent trees (14 years old). A field survey of disease severity was conducted in 2008 and 2009 under the natural conditions in the experimental orchard of the Xingcheng Institute of Pomology (Xingcheng, Liaoning Province, P.R. China, 40°37’N, 120°44’ E). The average disease index of the F₁ population was higher than the mid-parent value, suggesting that resistance to Alternaria may be controlled by a major gene in combination with some minor genes. Artificial inoculation was also done both on the living trees and on the detached leaves in 2009. The different inoculation procedures produced different types of disease spots and color features. Based on the field data, 110 F₁ plants were divided into susceptible and resistant groups, and genomic DNA were extracted and pooled. Eighty primer pairs (Simple sequence repeat-SSR markers) were screened against the two DNA pools, and one pair, namely CH05g07, was showed to be linked to the susceptible DNA pool. This primer pair was used to screen all individual 110 F₁ progenies and two parent trees. The differentiation of 93 individuals (84.5%) with the marker matched with the field disease resistance rating. This marker was further screened with 20 cultivars with known susceptibility or resistance and its linkage to susceptibility was validated. These results suggest that this marker can be used in marker-assisted selection for resistance/susceptibility to Alternaria leaf spot disease.