- 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.