Thirty-three half-sib apple clonal rootstocks tolerant to crown rot produced in a 14-year pathology breeding program

Dominant challenges of Iranian apple cultivation soils consist of heavy texture, poor organic matter, high alkalinity and so calcium and iron deficiency. In addition, traditional submerged irrigation systems and superficial manure distribution predispose the apple trees to crown rot disease. To produce clonal rootstocks the process of parent selection was achieved based on lack of foliar chlorosis, weak vigor, and general adaptness to regional pedoclimatic conditions. ‘Northern Spy’ was selected as the carrier of resistance gen to <em>Phytophthora cactorum</em> together with other native three native crab apples during 2004-2007. The obtained 5000 progenies of 1-year-old half-sib seedlings were transferred into the thermostatic greenhouse where afterward they were inoculated by the agent of soil disease pathogen in 2008-2010. The screening resulted in the individuation of 99 crown rot tolerant genotypes which were transplanted in the nursery. After a biennial growth outward till 2012, clonal propagation by layering as second screening led to 33 easy rooting promising clonal apple rootstocks in 2013-2014. ‘Golden Delicious’ and ‘Red Delicious’ were grafted on the newly produced rooted shoots of each clone through further layering. A selection plot was established in an orchard containing rootstock scions combinations besides more gradual dwarfing rootstocks M9, M7, MM111, Support 4, and P22 as planted in Karaj, in the winter of 2013. Promising clonal stocks affected different tree height, rootstock diameter, graft union and trunk diameters, internode length and diameter of annual branches, total branch length, growth habit, number and mean length of root suckers in 2015.