In an effort to increase the availability of new disease-resistant apple cultivars suited to hard cider production, we crossed the traditional apple cultivar ‘Golden Russet’ with the disease-resistant cv. ‘GoldRush’. In 2013, 200 apple blossoms of ‘GoldRush’ were emasculated and subsequently hand-pollinated with ‘Golden Russet’ pollen, and from the resultant fruit, a total of 534 seeds were obtained. In 2014, a 66% germination rate was achieved, and of those seedlings, initial mortality was quite high (35%) in the high-humidity and close spacing of the propagation greenhouse. Through the end of 2014, surviving progeny showed a 32% incidence of apple scab and a 17% incidence of cedar apple rust, which was lower than predicted. Those diseased seedlings were removed from the trial.
In 2015, field rows of seedlings were established to continue to evaluate disease susceptibility, vigor, and tree form. Counter to all expectations, very little apple scab or cedar apple rust was observed in 2015, which indicates that there may have been abnormally low presence of disease innoculum in the research orchard. 10% of the apple trees were infected with fireblight and were removed from the trial. Of the remaining 104 seedlings, 82% grew to at least 48″, with 45% exceeding 72″. A visual evaluation of tree form categorized 89% as acceptable (strong leader, adequate feathering).
Concern remains that the surviving progeny in the trial have not been adequately screened for apple scab, despite the best efforts of the investigator to promote disease-inducing conditions. More research and time is needed to continue the work with these crosses before they are deemed ready for wider evaluation (in grower trials, etc.). To this end, we have decided to postpone the outreach plan until specific seedlings are determined without doubt to be truly disease resistant, hopefully by 2017.
Many heirloom hard cider apples, such as ‘Golden Russet’ are susceptible to diseases such as apple scab. They require copious amounts of fungicidal sprays to ensure the productivity of the tree. In contrast, modern desert cultivars have been developed in various university initiatives to successfully resist these diseases. We cross-bred two of these respective cultivars, ‘Golden Russet’, the esteemed traditional hard cider apple, and ‘GoldRush’, a grower-friendly modern apple (which bears considerable resistance to apple scab) because we dearly wish for an apple that combines the juice quality of the heirloom apple with the disease resistance and productivity of the modern apple. Because the hard cider industry is a fast-growing market segment, riding on the success of the craft-brewing movement and the growing consumer preference for locally-grown food, we believe that this type of research benefits both apple growers and consumers alike, as well as potentially reducing environmental impacts associated with spraying. Our relevant experience is owning and running a small 10 acre apple orchard as well as an apple nursery with 3000 trees for planting out another 10 acres. Our cultivars include Golden Russet, Yarlington Mill, Dabinett, Kingston Black, and GoldRush. We employ very low-spray methods, press, and ferment our fruit to produce a variety of hard ciders to sell at Massachusetts farmer’s markets under the label of Carr’s Ciderhouse. My academic experience includes studying graduate pomology at UMass Amherst with Dr. Duane Greene, who is the technical advisor of this project.
Our objectives for this research project were to successfully cross breed the ‘Golden Russet’ and ‘GoldRush’ apple cultivars, collect the seeds, grow out the seedlings in conditions subjecting them to severe disease pressure, and then to select the most promising seedlings on the demonstrated basis of health and excellent disease-resistance. Evaluation criteria for this trial included ongoing disease resistance, growth habit, and vigor, with a particular emphasis on the first criterion.
We were forced to substitute a different cultivar of apple, ‘GoldRush’, for the intended cultivar ‘Liberty’. This was due to severe deer browsing in our young ‘Liberty’ orchard block. When it became apparent that the number of ‘Liberty’ blooms was likely insufficient for the project needs, we decided to use our ‘GoldRush’ trees as a very good alternative. ‘GoldRush’ is an excellent orchard performer which we think highly of, and it also posseses the genes for disease resistance that we had hoped to introduce from ‘Liberty’. For this reason, we feel fortunate to have had a very worthy back-up cultivar like ‘GoldRush’ to substitute in this case, but more importantly, we can still achieve the project goal of introducing additional disease resistance into a ‘Golden Russet’ type apple suited to high-quality cider production.
1. Selection of the parent genetic material. We chose to cross the apples ‘Golden Russet’ and ‘GoldRush’ hoping to combine their valuable characteristics. In our original proposal, we specified use of the cultivar ‘Liberty’ instead of ‘GoldRush’, but there was a scarcity of fruit buds in our orchard for that variety so the plan was modified to substitute cv. ‘GoldRush’. ‘GoldRush’ is a rugged cultivar, described as highly resistant to apple scab, as well as having moderate resistance to other important diseases such as cedar apple rust, downy mildew, and fire blight (cf. Cummins Nursery Website). It is a very productive variety with moderate vigor and a spur-type growth habit, making it a grower-friendly tree. Its disease resistance is due to a genetic lineage which includes Malus floribunda, the hardy Asian crab apple. Hard cider made from ‘GoldRush’ fruit is good, but not nearly as distinctive or refined as that which is made from ‘Golden Russet’. The cultivar ‘Golden Russet’, on the other hand, is a vigorous, rangy tree which is prone to tip bearing and is susceptible to defoliating levels of apple scab. However, it produces some of the finest fruit for hard cider making, with high sugar levels and a distinctive, delicious flavor profile. Other positive attributes of this cultivar include firm flesh and russeted skin, which contribute to insect and disease resistance.
2. Cross Pollination of ‘Golden Russet’ x ‘GoldRush’. Controlled cross pollination of the two cultivars took place in April 2013 at the Carr orchard in Hadley, Massachusetts. The bloom period for the two cultivars overlaps slightly, taking place in the early-to-middle bloom period window. Petals and anthers were removed from 200 ‘GoldRush’ blossoms (’emasculation’) just prior to open bloom, and hand-pollinated with pollen collected from anthers of ‘Golden Russet’. Allowing for some incomplete pollination and some crop loss due to weather, etc., the goal was to obtain 500 viable seeds.
3. Tending of Cross-Pollinated Fruit to Maturity. Pollinated, emasculated blooms were bagged after hand pollination with zip-loc bags in order to protect the developing fruit from pests and diseases. 131 of 200 fruits survived the initial ‘June drop’ period, indicating that they had been pollinated.
4. Harvest and Storage of Seed. 108 mature fruits were harvested in early November of 2013. 534 seeds were carefully removed and stored in moist sphagnum moss in a sealed container held at 34 degrees, in order to provide cold, moist stratification for 60 days prior to germination. Seeds were observed to germinate in late January and were transferred to 10″x20″ growing flats, sown 3/4″ deep in sterile growing medium. Seedlings were grown out until 6-8″ in height, at which point they were transplanted to the ‘Scab Chamber’ propagation greenhouse.
5. Greenhouse trials of hybridized seedlings. In 2014, following techniques developed by Hough et al. (1970), Aldwinckle et al. (1976) and Lamb et al. (1978) for selection of ‘Liberty’, apple seedlings were grown on in a humid greenhouse environment (‘Scab Screening Chamber’) where disease conditions were created by the placement of apple scab infected leaves (collected from unmanaged apple trees in our orchard) in the seedling growing environment. The maintenance of high humidity, warm temperatures, and leaf-wetting was intended to stimulate scab ascospore production and infection. After a season of maintaining such conditions, apple scab and cedar apple rust infection occurred and became visible on the leaves of seedlings which did not have immunity.
6. Field Trials of Scab-Immune Seedlings. Initially, field trials were to start in the summer of 2014, but were delayed to allow for a full growing season in an environment more conducive to disease development. In 2015, seedlings deemed to show scab immunity by visual inspection were grown on in unmanaged (i.e. unsprayed, to allow diseases to attack the seedlings) field conditions. Seedlings were periodically (every 2 weeks) evaluated for signs of disease, as well as evaluated for vigor and form at the end of the growing season, after leaf fall.
In 2013, the initial project goal of apple seed production was not quite met. The yield of seeds from 108 fruits was much lower than expected. We were hoping to get 1000 seeds, but ended up with 534. This may have been due to partial or inadequate pollination.
Out of 534 seeds obtained from fruit cross-pollinated in 2013, 352 were successfully stored, germinated and grown out in 2014. This represents a germination rate of almost 66%, which was well within our expected target range of 60-80% germination.
In 2014, apple seedling leaves developed lesions from both apple scab and cedar apple rust, although scab infection was lower than predicted. Due to these lower rates of scab infection in the spring and early summer of 2014, we elected to keep growing the seedlings in the Scab Chamber over the full course of the summer in order to maximize potential for scab infection rather than transplanting them for field trials, as we determined this was the most crucial phase of the screening process. Even with this modification, apple scab infection rates were still far lower than predicted.
Mortality data for the Scab Chamber Phase in 2014 covers mortality by category. This includes: 1. Unknown cause of death (15 or 4.3%), 2. Accidental weeding (9 or 2.5%), and 3. Pythium infection (99 or 28%). Out of 352 initial seedlings, overall mortality rates were quite high, at 123 total, or 34.9%. The loss of seedlings to Pythium was no doubt caused by the same conditions that were intentionally created to foster other apple diseases. It is recommended in future trials to allow the seedlings to grow well past “first true leaf” stage (when the stem becomes woodier) before exposing them to disease-inducing conditions, as well as allowing 6″ of spacing between seedlings in the “scab chamber”, as this will likely reduce incidence of early Pythium infection.
Of the surviving seedlings, 229 in number, 39 or 17% showed symptoms of cedar apple rust and 74 or 32.3% showed symptoms of apple scab. These infected seedlings were removed from the breeding program, leaving 50.6% or 116 seedlings which showed no sign of apple scab or other leaf diseases. These resistant seedlings were given durable metal tags with identification numbers. The number of resistant seedlings as determined by the screening process is a much higher percentage than was predicted in the research literature (10-15%), so it is possible that either: 1. The ‘GoldRush’ x ‘GoldenRusset’ cross produces a very high percentage of disease resistant progeny or 2. Apple scab-inducing conditions were not optimal and therefore some of the seedlings which did not show any sign of disease may in fact be susceptible to apple scab. To eliminate the second possibility, further field trials are needed.
Performance targets for 2015 remain below expected rates of disease susceptibility despite the introduction of fireblight into the seedling orchard. Of the remaining 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 ongoing exposure of the seedling crosses to disease-inducing innoculum and conditions under field conditions in 2015, and despite modest 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 susceptibility in order to rule out the potential ‘false negatives’ that may be the result of this evaluation to date.
Education & Outreach Activities and Participation Summary
Outreach efforts have been suspended at present, pending further evaluation of disease resistance in the apple progeny. Disease levels throughout the trial were lower than expected, leading to the hypothesis that the seedlings have not been properly screened for disease susceptibility or that disease pressure in the years of the trial was much lower than average and does not reflect usual orchard conditions encountered by growers. Although there is a minor chance that our results may be due to an extraordinarily high level of conferred resistance in the seedlings, the data is significantly out of line with the results that are predicted in the research literature (cf. Lamb et al. 1978.) With this concern in mind, it does not seem prudent or fair to disseminate findings until we are absolutely certain that we can recommend planting of these potential new cultivars. We hope that subsequent years will reveal the disease resistance of the seedlings in such a way that we are fully confident about the results, at which time we intend to carry out the outreach component of this project.
The project now continues onward with the likelihood of achieving its aim of completing identification of new disease-resistant apple cultivars in the near future. We anticipate moving forward with the outreach portion of the project when we are satisfied with the reliability and value of the research. In future, I would recommend allowing a minimum of 4-5 years for projects of this type involving woody plant materials with a multi-year maturity cycle.