Graduate Thesis Or Dissertation
 

Frost hardiness of buds, flowers and fruit of pear (Pyrus communis L.)

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kk91fp83z

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  • Spring frost damage to pears in the Pacific Northwest frequently results in substantial crop losses. This research was undertaken to investigate the effects of frost on pear buds, flowers, and fruit through controlled freezing tests and field studies in order to better understand the frost phenomenon and refine frost protection decisions. Controlled freezing studies on 'Bartlett' pear (Pyrus communis L.) showed that the percent of florets injured by frost increased with decreasing temperature, advancing developmental stage, and increasing duration at minimum temperatures of -2, -3, and -4° C. Increases in injury occurred with exposures of 30 or 60 minutes at all stages except the small fruit stage, in which injury continued to increase for 2 hours at -2° C. No significant effects of freezing rate were found at -2, -3, or -4° C. However, there was a significant effect of freezing rate at -5° C. No hardiness differences were found between comparable floral developmental stages from weak and vigorous trees. Bloom delay through evaporative cooling resulted in a loss of hardiness beyond that found earlier in the season on non-misted trees for similar stages of development. However, under field conditions a certain amount of frost protection was gained through bloom delay. Simulated frost injury to small fruit ovaries at intervals after full bloom significantly-increased fruit malformation, reduced fruit weight, and increased fruit drop of 'Bartlett', 'Bosc', and 'Comice' pear trees. Time of injury did not affect fruit weight and malformation in most cases, but early injury did significantly increase fruit drop. Significant positive correlations were found between fruit weight and seed content, while negative correlations were found between fruit malformation and seed content for all cultivars. Crop density was correlated significantly with sum of percent floral injury from frost, orchard design, and height in the tree for 'Bartlett', 'Bosc', and 'Anjou' pear. Regression models for crop density, regressed on sum of the percent injury, orchard design, and height in the tree differed between cultivars indicating that one model cannot be used to estimate crop density at harvest for all cultivars. Crop density was greater at low levels of frost injury in free standing than in hedgerow trees, and at greater elevations in the trees. Data suggest that 30 percent frost injury in hedgerow and 60 percent frost injury in free standing pear orchards are reasonable injury levels to accept without incurring crop losses.
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