Breeding program for novel disease-resistant cider apple cultivars

Breeding program for novel disease-resistant cider apple cultivars

Summary

Our project seeks to breed new disease-resistant varieties of apples suited for hard cider production. For apple cultivars in hard cider production systems, disease resistant traits can be of major benefit to growers. Such benefits include improving overall tree health and fruit quality, and lessening spray costs and environmental impacts. The aim of our breeding program is to produce a novel apple cultivar which has both the superior juice characteristics of the ‘Golden Russet’ and the grower-friendly productivity and disease resistance of ‘GoldRush’.

During 2014, we successfully continued work on our apple breeding program by growing out 352 ‘GoldRush’ x ‘Golden Russet’ seedling crosses in an environment specifically designed to promote disease. This ‘Scab Screening Chamber’ was designed as a small high tunnel, covered with greenhouse plastic, and where our seedling crosses spent the summer crowded into a high-humidity environment with a heavy mulch of diseased apple leaves gathered from an unmanaged apple orchard as per protocol laid out by Aldwinkle et al. The objective was to induce disease in seedlings which did not posess resistant genes in order to screen them out from further evaluation. 

Objectives/Performance Targets

Performance targets for 2014 were met in most respects, although some outcomes were different from parameters predicted by our literature search.

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. 

Mortality data for the Greenhouse 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 conditons that were intentionally created to foster other apple diseases. It is uncertain how to avoid these losses.

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 succeptible to apple scab. To eliminate the second possibility, further field trials are needed.

Accomplishments/Milestones

Milestones achieved in 2014 for our breeding program include the successful inducement of disease in apple seedlings and the selection of seedlings which were not affected by disease. In this broad sense, we have achieved our main goal. Overall, the disease screening process was effective, but the humid Scab Chamber conditons proved stressful to seedlings in ways we did not anticipate. The main unanticipated result was significant seedling mortality from oomycete pathogen Pythium spp., likely due to natural populations in greenhouse soil.

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.

Impacts and Contributions/Outcomes

Collaborators: