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 grayish-brown spot, which 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 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?

Should citrus trees be pruned, and if so, during what month?

Your information and expertise will be appreciated.

– Bob Annecone, Carmichael

Your description of the fruit drop and grayish- brown spots on the fruit from your orange tree sounds like brown rot, according to UC Master Gardener Carol Rogala.

Brown rot is a fungus that occurs primarily on fruit borne near the ground during wet weather. The fungus spores on the ground are splashed onto the fruit on the lower branches by rain or irrigation water.

To cut down on the spores, remove the diseased fruit and pick up any off the ground. Also, rake up any fallen leaves and change the mulch under the tree.

This fall, a preventive Bordeaux treatment can be applied – before the first rains – to tree skirts up to 4 feet and to the ground beneath the tree.

A complete description of the Bordeaux treatment can be found in Pest Note Publication 7481.

This is available online at www.ipm.ucdavis.edu. It is also available by sending a self-addressed, stamped business-size envelope to: PN 7481, UC Cooperative Extension, 4145 Branch Center Road, Sacramento CA 95827.

As for feeding, average-size mature orange trees (15 to 20 feet high) should be fertilized at 1 pound of actual nitrogen per tree per year. Because adequate levels of nitrogen are required during flowering and fruit setting, later winter or early spring broadcast fertilizer applications to the soil can provide the required nitrogen supply. UC Cooperative Extension specialists advise that high levels of nitrogen fertilizer are to be avoided during the summer and fall as they contribute to thicker rind, lower juice content and re-greening of Valencia oranges.

Orange trees require very little pruning. Light pruning in the top of the tree to promote growth of inside fruitwood may be helpful.

As the tree ages, the top branches are usually the first to decline in productivity and fruit quality. Most deadwood and weak non-productive wood should be removed; however, severe pruning will adversely affect the production of fruit.

Tangerines that taste like lemons are certainly not what you want. Your tangerines may look ripe; however they may not be ripe. The change in color in the fruit is associated with loss of chlorophyll and an increase in carotenes (yellow, orange and red pigments).

This change in coloration may not correlate with fruit edibility. Despite the mature color of the fruit rind, chilling may be required to increase sugar content. Depending on the variety of your tangerine, tree harvest can be between November and January for most, and as late as March for the Dancy variety.

It is also possible you have a Rangpur (a mandarin-lemon hybrid) or a naturally sour mandarin.

GARDEN QUESTIONS?

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

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