Selection of apple rootstock breeding families for Phytophthora crown rot resistance

Crown and root rot of apple rootstocks associated with Phytophthora species is an important disease causing major losses in apple production areas. Crown and root rots are often associated with major abiotic stresses like prolonged water submergence and poorly drained or compacted soils. Phytophthora species are also implicated in the replant disease complex. The Geneva (R) apple rootstock breeding program has been active in the selection for crown rot resistance within its germplasm. In 2009 we conducted a replicated experiment featuring 16 full-sib families representing crosses between elite rootstocks and wild Malus species to validate the reliability of the selection method used. The method established in the 1970s required the inoculation of young seedlings, two weeks after emergence, with a mixture of several Phytophthora cactorum strains collected throughout the USA and subsequent root submergence with cool water for 76 h. The experiments were set up with four replicates of 40 full-sib seedlings per flood basin. Full sib family percent survival and flood images were collected three weeks after inoculation. The inoculation was successful in displaying differences between both control treatments (flooded non-inoculated [similar to 83% survival] and non-flooded/non-inoculated [similar to 87% survival]) and the mean survival of all inoculated treatments (similar to 26% survival). There were, for the most part, no significant differences between full-sib family survival means, probably caused by localized variation within inoculation bins and between bins where one full-sib family would display. Only one family ('G.41' x 'Malus sieversii pool 4') displayed higher than normal survival (similar to 47% survival). This may indicate an improved source of resistance to crown rot within the pollen pool of that family. However, the variance within inoculated family replications may indicate potential escapes of up to 15% of the total survivors. More research is needed to improve the reproducibility of this important selection parameter within Geneva (R) apple rootstocks.