MN Fruit & Vegetable IPM News - Vol. 2, No. 2

An apple grower emailed in two questions regarding freeze damage and thinning. It appears, from data collected, that there have been several events (4/22-4/23 and 5/2-5/3, to name a few) where the night time temperature in many parts of MN has fallen below 32F and stayed there for several hours each time. I contacted Dr. Emily Hoover, Professor of Horticulture at the University of Minnesota (email: hoove001@umn.edu) and asked if she would respond to the grower’s questions.

She began by prefacing her answers with the following introduction: There are a lot of caveats to consider such as the cultivar/rootstock combination (vigor level), age of the tree, total # of flowers, an estimate of the # of flowers damaged, weather conditions during bloom, estimating potential fruit set, past cropping history - to name just a few.

With this in mind, here are her answers to the two questions.

Grower question: I have blooms with obvious freeze damage and later opening ones that look healthy. What can I expect for fruit set with these varied conditions?

Dr. Hoover’s response: The estimate for the percent of flowers that need to set to make a commercial crop, assuming extensive bloom, is about 5%. So, trees can lose a lot of flowers and still produce a commercial crop. If an orchard has obvious freeze damage to some flowers, they will probably not set fruit, and fall (maybe already having done so). The flowers opening later, with no visible browning caused by death of cells due to freezing of intercellular water, should set normally. Fruit set may be reduced, but the crop may still require thinning as secondary and tertiary flowers may set. However, as the MSU article points out, flowers can be damaged, but set fruit, with the resultant damage not clearly seen until harvest.

Grower question: What does all this mean for my post bloom thinning program this year?

Dr. Hoover’s response: All of this then can have major impacts on the fruit thinning program. I would wait as long as possible to thin fruit this year, after a guesstimate can be made as to the amount of set there will be within a cluster. If clusters are setting only one fruit, then thinning may not be needed, or the crop load should be assessed as the season progresses. If multiple fruit are setting per cluster, then fruit thinning should proceed as planned for. Frost may kill only a few flowers low in the tree but injure leaves throughout the tree. Absorption of thinning chemicals into injured fruit and leaves is likely to be greater than into non-injured fruit and leaves, leading to a greater thinning response. Frost injury also can impair photosynthesis leading to a reduced carbohydrate supply thus accentuating thinning.

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison ( hutch002@umn.edu), Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness ( woldx018@umn.edu), Department of Entomology, University of Minnesota

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jeanne Ciborowski, 651-297-3217, jeanne.ciborowski@state.mn.us , MDA, 90 W. Plato Blvd., St. Paul, MN 55107-2094. You can access the Newsletter at the U of MN web site in htm format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf format at: www.mda.state.mn.us/biocon/fruitreports/

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