Breeding program for novel disease-resistant cider apple cultivars
The year 2015 saw challenging conditons in the apple nursery for the seedling crosses of ‘Golden Russet’ x ‘GoldRush’, being subjected to a variety of pest and disease pressures. Seedling progeny were transplanted in springtime from the previously-described ‘Scab Chamber’ to field rows in 2015. Field rows were maintained unsprayed and weed-free by hand hoeing and were adjacent to an unsprayed orchard block in order to allow free movement of pests and disease innoculum. Fireblight conditions were prevalent in the early summer, although such stress did not result in increased succeptibilty to other diseases. A limited amount (9.5%) of seedling mortality due to fireblight (Erwinia amylovora) was observed and any seedling showing signs of fireblight was removed from the trial. An infestation of Green Apple Aphid (Aphis pomi) at mid-summer proved to be difficult to eradicate and likely contributed to less-than-optimal development of the affected seedlings. Conversely, new cedar apple rust and scab infections were at practically non-existent observable levels. Vast differences in seedling vigor were observed, with some progeny exhibiting optimal growth and structural development, whike others failed to reach targeted growth levels. Overall, observable disease levels continued to be lower than predicted. Although it is theoretically possible that the seedling progeny posess exemplary disease resistance, a conservative approach recommends that more extensive evaluation be carried out. Further field trials are recommended in order to ascertain disease resistance and to induce early fruting by grafting on to highly dwarfing rootstock as well as on to mature trees in unsprayed orchard conditions.
Performance targets for 2015 remain below expected rates of disease succeptibility despite the introduction of fireblight into the seedling orchard. Of the remainng 116 seedlings which showed no sign of disease, 11 (approximately 9.5%) succumbed to fireblight during the growing season and were removed from the trial. Evaluation of scab and cedar apple rust lesions was confounded by the leaf distortion caused by Green Apple Aphid and thus no seedlings were removed from the trial on this basis.
Despite the ongonig exposure of the seedling crosses to disease-inducing innoculum and conditions under field conditions, and despite modest 2015 mortality from fireblight infection, apple scab and cedar rust infection continues to be greatly lower than predicted. This remains puzzling, but it may be possibly explained by higher juvenile resistance to diseases, as suggested by Dan Cooley, University of Massachusetts Plant Pathologist (personal communication). The objective of inducing disease in approximately 80%-90% of the seedlings seems to be falling far short of the mark at this stage. It is uncertain if field conditions encountered thus far are representative of average field conditions in New England or if disease pressure was somehow lower despite the efforts of this farmer to induce such conditions favorable to rampant spread of disease. Continued evaluation of seedlings is recommended to assess disease succeptibility in order to rule out the potential ‘false negatives’ that may be the result of this evaluation to date.
Impacts and Contributions/Outcomes
The final phase of this research project was projected to be the dissemination of research outcomes and the offering of budwood for orchard trials by like-minded apple growers. Unfortunately, due to observed rates of apple scab and other disease succeptibilty which are drastically lower than predicted, it is recommended that research continue until seedling selections are identified which are proven to resist common apple diseases over a broader range of orchard conditions than those which have been provided thus far in this research project in order to ascertain true disease resistance. Several more years of evaluation following the grafting of scionwood from the seedling crosses to both a dwarf trellis orchard and a mature semi-dwarf orchard block should provide further data from which to effectively evaluate these crosses.
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