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WSU-TFREC Orchard Management Forum

Spring temperatures & fruit size

Degree days after bloom (fruit cell division rate?)

Tim Smith -- WSU Extension, North Central Washington

There are a number of ways to measure the amount of heat the fruit has been exposed to during the cell division stage after bloom. One relatively crude, but somewhat effective, way is to total the daily degree days over the base of 40 F for the first 40 days after bloom. From this number, you can compare the potential for cell division across seasons. In general, warmer post bloom periods lead to larger fruit size, all other things being equal, (which they rarely are. Pruining, thinning and fruit load also make a major difference in fruit size, but these things often even out for any given season over the entire industry).

The table below was calculated using the following formula:

daily high temperature - 40 = daily degree days.

The daily amount was totaled for the 40 days following each years full bloom date at the WSU Tree Fruit Extension and Research Center, in Wenatchee.

average of all years, with April 28 as full bloom date, and the 30 year daily average temps

Average: 1405

1997-----1326   --normal
1996-----1119   --cold
1995-----1428   --normal
1994-----1392   --normal
1993-----1577   --hot
1992-----1388   --normal
1991-----1218   --cool
1990-----1180   --cold
1989-----1384   --normal
1988-----1240   --cool
1987-----1502   --warm
1986-----1408   --normal
1985-----1399   --normal
1984-----1105   --cold
1983-----1422   --normal
1982-----1408   --normal
1981-----1243   --cool
1980-----1272   --cool
coldest springs: 1984, 1996, and 1990

Fruit was generally smaller than usual in 1984, and blossom return was not as good as usual the next season.

warmest springs: 1993 and 1987

So, you think 1996 had a cool spring?? You're right. Perhaps we will see somewhat smaller average fruit size????? What happened in 1990 and 1991?

--Tim Smith

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Wenatchee WA, 22 August 1996