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Mark K Ehlenfeldt and Allan W. Stretch

Previous studies demonstrated a strong positive correlation between early spring shoot growth and susceptibility to the blighting phase of mummy berry. Plants with slow shoot growth derive resistance from avoidance but also may have biochemical resistance. Shoot growth of six highly resistant cultivars was artificially advanced, then plants were exposed to natural infection to assess disease response. Increased susceptibility with increasing shoot length was observed in most cultivars, although degree of response was variable. Within cultivars, there were increases in susceptibility up to shoot lengths of ≈20 mm, beyond which susceptibility decreased. Studies of fruit infection in 48 cultivars found a range of susceptibility and resistance, but little correlation between blighting and fruit infection resistance.

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Mark K. Ehlenfeldt, Allan W. Stretch, and Vickie Brewster

Thirty-three Vaccinium corymbosum selections and cultivars were artificially inoculated with spores of Colletotrichum gloeosporioides in the green fruit stage. Fruit was harvested when ripe and incubated under high-humidity conditions for 1 week, before evaluation. A wide range of susceptibility to anthracnose fruit-rot was found, ranging from 8% to 85%. Among the most-resistant cultivars were: `Elliott' (8%), `Murphy' (8.3%), `Stanley' (13%), and `Weymouth' (16.9%). Among the most-susceptible cultivars were: `Bluetta' (85%), `Spartan' (82.7%), `June' (69.9%), and `Northblue' (69.5%). Uninoculated checks had a maximum of 6% infection.

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Mark K. Ehlenfeldt, Vickie Brewster, and Allan W. Stretch

Potted plants of 53 highbush and half-high blueberry cultivars were screened for resistance to the blighting phase of mummyberry, Monilinia vaccini-corymbosii under controlled nursery conditions over an 18 day infection period. Significant differences were observed in the susceptibility of different cultivars, with `Bluehaven', `Bluegold', and `Blueray' being among the most susceptible, and `Bluejay', `Jersey', and `Duke' being among the most resistant. Differences were also observed in the latent periods and rate of disease progress which may have a bearing on the severity of the secondary fruit infection phase. Preliminary observations suggest that shoot blighting and fruit infection frequencies are not strongly correlated under high inoculum conditions.

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Mark K. Ehlenfeldt, James J. Polashock, Allan W. Stretch, and Matthew Kramer

Response to foliar infection by Colletotrichum acutatum Simmonds ex Simmonds was assayed in a diverse group of 149 blueberry cultivars and selections using a detached leaf-disk assay. Disks were inoculated and incubated for one week, then were digitally imaged, and images analyzed for percent leaf decay. Infection percentages across cultivars averaged 32%, and ranged from 8% to 79%. The lowest levels of foliar infection were seen in the cultivars, Burlington, Sharpblue, and Berkeley. Foliar responses were compared to anthracnose fruit rot susceptibility data from a previous study. Several clones were observed to have low levels of both foliar and fruit infection. Cultivars with particularly good resistance to both phases included `Sharpblue', `Sunshine Blue', `Legacy', `Little Giant', `Flordablue', `Elliott', `Blue Ridge', `Blue Rose', and `November Glow'. Little correlation was observed between foliar response and fruit response to anthracnose infection (r = 0.15). Since C. acutatum overwinters primarily in vegetative tissue, breeding new cultivars with foliar resistance may assist in reducing inoculum levels of this disease under field conditions.

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Mark K. Ehlenfeldt, James J. Polashock, Allan W. Stretch, and Matthew Kramer

Mummy berry disease of blueberry has two distinct phases: a blighting phase that infects emerging shoots and leaves early in the spring and a flower infection phase that ultimately leads to infected (mummified) fruit. Cultivated blueberry (Vaccinium spp.) genotypes that are resistant to one phase are not necessarily resistant to the other phase. The resistance of cultivated blueberry (Vaccinium spp.) genotypes to each phase of the disease is different. A large number of cultivars were screened for resistance to each phase. Cultivar standards (cultivars with well-documented responses to the disease) were used in the screening to evaluate long-term variation and aid comparisons across years. Using nine standards for the blight phase, 125 cultivars were tested and ranked for relative resistance using a ranking system based on resampling and principal component analysis. Similarly, using six standards for the flower/fruit infection stage, 110 blueberry cultivars were tested and ranked for relative resistance. Cultivar rankings show that lowbush cultivars and other types possessing high percentages of lowbush germplasm are generally more resistant to both phases of the disease. Among highbush cultivars, Bluejay is reliably resistant to both phases. Documentation of resistance to each phase will allow selection of cultivars for planting in affected areas and will facilitate the development of breeding strategies to produce cultivars with superior resistance.

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Mark K. Ehlenfeldt, James J. Polashock, Allan W. Stretch, and Matthew Kramer

Mummy berry (Monilinia vaccinii-corymbosi) is an important disease of cultivated blueberry (Vaccinium spp.). The disease has two distinct phases: a blighting phase initiated by ascospores and a fruit infection stage initiated by conidia during bloom. In this study, we investigated, in a nursery setting, blueberry cultivar resistance to both phases of the disease and, using multiple “standards” with a range of susceptibilities, examined, over 9 to 12 years, factors affecting disease incidence in controlled inoculations. The analyses of our data show that a minimum of 8 years of testing is necessary to obtain stable rankings of cultivar susceptibility for the fruit infection phase of the disease. Insufficient years of data were available to estimate this for the blight phase. Eight years are necessary largely as a result of uncertainty arising from the large environment × genotype interaction, estimated to be more than double any other source of observed variation, other than that resulting from sampling/individual plants. For individual cultivars, temperature and the amount and frequency of precipitation in January to March (when neither plant nor pathogen were presumed active and when both were in cold frames somewhat protected from environmental conditions) were predictive of later disease incidence. For most cultivars, the same weather variables at the same time period were found to be predictive for independently modeled cultivars. Additional cultivars, with only a few years' data, were grouped with the standard with which they shared similar environmental (year) responses and possibly similar disease predictive models.

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Mark K. Ehlenfeldt, Allan W. Stretch, Nicholi Vorsa, and Arlen D. Draper

'Cara's Choice' is a mid-season ripening, tetraploid, hybrid blueberry (Vaccinium × 'Cara's Choice') that was developed by the cooperative breeding program of the Agricultural Research Service of the U.S. Department of Agriculture (USDA) and the New Jersey Agricultural Experiment Station (NJAES). 'Cara's Choice' was given its name in recognition of its excellent fruit quality with improved sweetness, firmness, and flavor.

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Mark K. Ehlenfeldt, Allan W. Stretch, Nicholi Vorsa, and Arlen D. Draper

'Hannah's Choice' is an early-ripening, tetraploid, highbush blueberry (Vaccinium corymbosum L.) that was developed by the cooperative breeding program of the Agricultural Research Service of the U.S. Department of Agriculture (USDA) and the New Jersey Agricultural Experiment Station (NJAES). It was named because it represents an improvement in sweetness, firmness, and flavor over currently grown early cultivars.