Q: I have two full-grown (7-foot and 10-foot) orange trees that were loaded with oranges, but as many as 10 (total from both trees) fall off each day. We find each of these on the ground with a large greyish-brown spot that is softer than the rest of the orange.

Oranges have been on the trees since September, and for the last year I have treated the trees every three months with a citrus-avocado fertilizer. (We bought the property in 2009 and they probably weren't fertilized by previous owner.)

Do you have any ideas about how I can stop the oranges from falling off the trees and remain on the trees until ripe and picked?

I also have a 7-foot tall tangerine tree that is loaded with fruit that tastes more like lemons than tangerines. Any ideas about sweetening the tangerines?

Also, should citrus trees be pruned, and if so, what month? Your information and expertise will be appreciated.

– Bob Annecone, Carmichael A: According to UC Master Gardener Bill Pierce, your orange trees are most likely infected with brown rot. Fungus are splashed on the fruit by rainfall and this causes the soft spots and premature fruit drop.

Fruit rot begins at the spots and can have a pungent, rancid odor. Remove diseased fruit on the tree and on the ground. Don't store infected fruit with clean fruit.

To overcome the problem, the tree and the soil can be sprayed with a fungicide. That's usually applied in October or November, before the rains set in. In cases of severe infestation or prolonged wet weather, a second application in December or January may be necessary.

"My guess is that your sour tangerine is really a Rangpur lime, or sour mandarin," Pierce said.

This variety is prized for the great "limeade" that can be made from the juice. From time to time, Sunset Magazine has published recipes using this fruit; these can be obtained through Readers Guide and back files of the magazine at the public library.

"I found several on the Internet, too," Pierce said.

Citrus can be pruned in the spring and summer. As pruning stimulates growth, you do not want to prune too early or too late to avoid new growth being damaged by frost.

Things to remember in pruning citrus are:

• The bulk of the crop is produced on the lower half of the tree. How much do you want to reduce your crop?

• The bark of citrus is easily sunburned and this leads to disease or insect entry into the tree. If you expose bark, it should be painted with white interior latex paint diluted 50 percent with water.

Your trees are probably dwarf or semidwarf varieties, Pierce said. Most folks just thin the tops on these trees.

Learn more with the Cooperative Extension's environmental horticultural note on citrus. For a copy of EHN No. 12, send a self- addressed stamped business-sized envelope to: EHN 12, UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827.

For more on fruit tree care including citrus, consider attending the Master Gardeners free winter open garden from 9 a.m. to noon Jan. 21 at the Fair Oaks Horticulture Center in Fair Oaks Park.

GARDEN QUESTIONS?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties.

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