Garden Detective: Satsuma mandarin tree

Last winter, the fruit on our Satsuma mandarin tree developed odd black spots. The black part of the fruit is very hard and when I cut it open the part attached to the black area is a light-green color. The rest of the fruit is normal. Not all the fruit has the black area. Some of the fruit is tart, but other fruit is OK.

I have had the tree many years but this never happened before. Could it be sunburn? I cannot seem to find an answer as to what is causing the problem.

I would appreciate any suggestions as to what the problem might be.

– Maryann Lorson, North Highlands

According to UC master gardener Cindy Gause, this sounds like a case of citrus brown rot fungus.

The symptoms are primarily found on ripe or ripening citrus fruit. At first, the fruit will have firm, leathery lesions with a water-soaked appearance, but they soon turn soft and have a tan to olive-brown color and a pungent odor. Eventually, the infected fruit will drop.

Citrus brown rot fungus is caused by a Phytophthora fungus species and is found on the soil beneath the tree. When it rains, the spores are splashed onto the tree skirts, which is why you will see most of the infected fruit on the lower half of the tree.

If the infected fruit goes unnoticed during harvest, it will likely infect other fruit during storage.

Cultural controls can significantly reduce the occurrence of brown rot. That includes pruning the tree 24 or more inches above the ground and applying organic mulch 4 to 6 inches thick on the ground below the tree, leaving a 1-foot area bare around the trunk.

One spray of a copper fungicide – preferably a Bordeaux mixture – in mid-October before or, at least, after the first rain can provide protection through the wet season.

A Bordeaux mixture is a combination of copper sulfate, lime and water that naturally fights fungal diseases. It can be used in fall and winter to manage fire blight on pears and apples; leaf curl and shot hole on peaches and nectarines; downy mildew and powdery mildew on grapes; peacock spot on olives; walnut blight on walnut; and black spot on roses.

If rainfall is excessive, a second treatment can be applied in mid-December followed by a third treatment in mid-January.

For the best protection, thoroughly coat all parts of the tree and the ground beneath the tree with the fungicide.

Photos of brown rot can be viewed at www.ipm.ucdavis.edu. To obtain directions to prepare a Bordeaux mixture, please send a self-addressed, stamped business-size envelope to: EHN 12, PN 7481, UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827.

This information can also be found on the university's integrated pest management website referenced above. Look for "Pest Note No. 7481: Bordeaux Mixture."

GARDEN QUESTIONS?

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

Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&g@sacbee.com. Please put "Garden Detective" in the subject field and include your postal address.

Order Reprint