|ALDWINCKLE, HERBERT - Cornell University|
|ROBINSON, TERENCE - Cornell University|
Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2022
Publication Date: N/A
Interpretive Summary: Apple orchards planted in heavy, wet soils often succumb to a root (rootstock) disease called "crown and root rot" caused by a fungus like organism called Phytophthora. Certain apple trees are able to resist this disease because of their genetic composition whereas others are very sensitive to it. Our breeding program needed to ascertain the genetic nature of the resistance and conducted several inoculation experiments on apple seedling families derived from various combinations of resistant and susceptible parents. The percentage of seedling survival was taken as an indicator of how effective the parents were in conferring resistance to their progeny. In the process, we identified a family of seedlings derived from a wild domesticated apple ancestor which showed very good survival (about half) compared to the rest of the families (about a quarter survival). This work will facilitate breeding of new rootstocks that will apple growers that have problems with this disease.
Technical Abstract: Crown and root rot of apple rootstocks caused by Phytophthora species is an important disease that causes 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. Resistance to crown and root rots caused by Phytophthora species exists within the apple rootstock germplasm. The Geneva® 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 that we had been using. The method established in the 1970’s, required the inoculation of young seedlings, 2 weeks after emergence, with a mixture of several Phytophthora cactorum strains collected throughout the U.S.A. and subsequent root submergence with cool water for 76 hours. 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 results indicate that in general the inoculation was successful as the difference between the control flooded non-inoculated treatment (~83% survival), the control non-flooded/non-inoculated (~87% survival) and the mean survival of all treatments (~26% survival). While the treatment seemed to work overall, 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.41xMalus sieversii pool 4) displayed higher than normal survival (~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 up to 15% of the total survivors. More research is needed to improve the reproducibility of this important selection parameter within Geneva® apple rootstocks.