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M.K. Ehlenfeldt and A.W. Stretch

Resistance to blighting by Monilinia vaccinii-corymbosi (Reade) Honey was evaluated under greenhouse conditions in multiple populations of the diploid species Vaccinium boreale Hall & Aalders, V. corymbosum L., V. darrowi Camp, V. elliottii Chapm., V. myrtilloides Michx., V. myrtillus L., V. pallidum Ait., and V. tenellum Ait., as well as in accessions of the polyploid species 4x V. hirsutum Buckley and 6x V. corymbosum f. amoenum Aiton. Significant species differences were found in mean blighting levels averaged over 2 years, with values ranging from 3.5% for V. boreale to 49.2% for 2x V. corymbosum, compared with 27.5% for the resistant 4x V. corymbosum check, `Bluejay', and 64.3% for the susceptible 4x V. corymbosum check, `Blueray'. Wild Vaccinium species may serve as new sources of resistance to blighting, if resistance can be transferred easily and horticultural type recovered.

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A.W. Stretch, M.K. Ehlenfeldt, and V. Brewster

In vitro conidia production by Monilinia vaccinii-corymbosi (Reade) Honey, the cause of mummy berry disease in blueberry, was significantly enhanced by cellulose acetate membranes placed on the surface of V-8 juice agar for most of the pathogen isolates tested, compared to V-8 juice agar alone. Temperature and light affected conidia production, but the effects were not consistent. Higher temperature (22 vs. 15 °C) yielded better sporulation, but the effects of light environment were variable. When 55 isolates from various sources were rated visually for sporulation on cellulose acetate membranes at 22 °C under ambient light/dark cycles, a wide range of conidium production was observed, and three of 55 isolates (6%) were identified as having very high conidia production.

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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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A.W. Stretch and M.K. Ehlenfeldt

Sixty-eight highbush blueberry (Vaccinium corymbosum L.) cultivars and selections were evaluated over 3 years for their resistance to the fruit infection phase of mummy berry disease [Monilinia vaccinii-corymbosi (Reade) Honey]. Average incidence of fruit infection under test conditions was 34.3% in 1995, 14.4% in 1996, and 27.9% in 1997, with significant differences occurring among clones in all 3 test years. Several cultivars exhibited consistent resistance to mummy berry fruit infection across all years of testing. `Northsky', `Reka', `Northblue', `Cape Fear', `Bluegold', `Puru', and `Bluejay' were among the most resistant, and `Atlantic', `Berkeley', `Herbert', and `E-176' were among the most susceptible. The consistent resistant reaction of certain cultivars indicates that they may be suitable as parents for introducing resistance into a breeding program. No significant correlation was observed between blighting resistance and fruit infection resistance.

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

Monilinia vaccinii-corymbosi (Reade) Honey. There are two distinct infection phases, each of which must be completed for the disease to persist. The primary phase results in blighting of shoots and sometimes flower clusters. This phase is initiated by

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

Kramer, 2006 ). Another important disease of blueberry is mummy berry caused by the ascomycete Monilinia vaccinii-corymbosi (Reade) Honey ( Hildebrand et al., 1995 ). Mummy berry disease is unique in that it has two distinct phases. The first phase is

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Liming Chen, Matthew Wallhead, Michael Reding, Leona Horst, and Heping Zhu

pannosa , is another common disease when peach is grown near apple that is susceptible to powdery mildew ( Blake et al., 2016 ). Mummy berry caused by the fungus Monilinia vaccinii-corymbosi is one of the most serious diseases of blueberry. When mummy

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A.W. Stretch, M.K. Ehlenfeldt, and V. Brewster

Fifty-five highbush blueberry (Vaccinium corymbosum L.) cultivars and selections were evaluated over 2 years for their resistance to the shoot blighting phase of mummy berry disease [Monilinia vaccinii-corymbosi (Reade) Honey]. Blight incidence in 1993 ranged from 1% to 78% and differences among cultivars were significant. In 1994, infection levels were lower and ranged from 0% to 43%, again with significant differences among the entries. Several cultivars exhibited mummy berry blight resistance in both years. Ranking most resistant to less resistant were `Jersey', `Elliott', `Bluejay', `Duke', `Stanley', `Darrow', `Meader', and `Angola'. Among the cultivars consistently blightsusceptible were `Bluehaven', `Bluegold', `Northblue', `Croatan', `Northsky', `Sierra', `Harrison', `Coville', and `Murphy'. The consistent resistant reaction of certain cultivars indicates that they may be suitable as parents for introducing resistance into a breeding program. The evaluation methodology developed in these tests should be useful in screening germplasm for new sources of resistance and evaluating segregating progeny from crosses.

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M.K. Ehlenfeldt, A.W. Stretch, and V. Brewster

The resistance of 48 highbush blueberry cultivars and selections to the blight phase of mummy berry disease, incited by the fungus Monilinia vaccinii-corymbosi (Reade) Honey, was examined in relation to percent Vaccinium angustifolium Ait. ancestry, season of fruit maturity, and shoot growth during the primary infection phase. Correlations of percent blighting with percent V. angustifolium ancestry were significant across 3 years, but correlations with fruit maturity were significant in only 2 of 3 years. Correlations of percent blighting with early shoot growth were significant in both years measured, with r values of 0.54 in 1994, 0.83 in 1995, and 0.83 across years. A multiple regression found only shoot growth highly significant for susceptibility and rendered V. angustifolium ancestry and season of fruit maturity nonsignificant. Resistant cultivars exhibiting early shoot elongation suggest that resistance can be either biochemically or escape based.

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M.K. Ehlenfeldt and A.W. Stretch

The resistance of 26 rabbiteye blueberry (Vaccinium ashei Reade) cultivars to the blighting phase of mummy berry disease was evaluated under controlled conditions. In 1997, blight levels ranged from 31% to 84%, and averaged 61.3% across all cultivars. In 1998, blight levels ranged from 71% to 99%, and averaged 89.9%. Several cultivars, including `Coastal', `Delite', `Centurion', `Walker', `Callaway', and `Garden Blue', exhibited significantly lower levels of mummy berry blight infection in both years. Blighting levels were significantly correlated with new shoot length in 1997, but not in 1998. Rabbiteye blueberry, in general, is less resistant to mummy berry blight than is highbush blueberry (V. corymbosum L.), but several options exist for potential improvement.