Garden Guy: What is causing citrus tree to die?


My citrus tree (orange tree) has been slowly dying. The branches are drying and turning black. Whereas about 25 feet away, my other citrus tree (lemon) is doing fine. What may be the cause of this? Aloha Jerry.

Even though two citrus trees grow relatively close together, they are notably different species, an orange and a lemon. Thus, they have varying degrees of susceptibility to an assortment of disorders, especially if the rootstocks are different.

The “branches turning black” is an interesting comment. If this black is actually sooty mold, it would, of course, indicate a heavy insect infestation, perhaps scale or whitefly causing leaves to drop with subsequent twig dieback. Apply at least two applications of an oil/soap spray, or harsher material if you choose. Once the insects are gone new leaves will sprout. Prune out the dead twigs and the tree should resume good growth.

If, however, the blackening is an indication that the bark is dead, then a disease is possible. One common virus is Tristeza (meaning sadness). In relating to your situation, the virus can kill orange trees, depending on the rootstock, but lemons do not succumb to the disease. There is no treatment. If this is the case, when you buy another tree, make sure it is on a resistant rootstock. Tristeza is spread by aphids along with cutting tools, but does not reside in the soil.

Therefore, no soil treatment is needed.

The other possibility is a fungal disease known as Phytophthora. This disorder is in the soil and can cause root rot as well as a crown rot. When it does attack the tree trunk, usually near the soil line, an amber ooze is often seen. Later the bark will dry and crack at the infection site. This disease is usually caused by overwatering or lots of rain and/or poor soil drainage. For more a more detailed description of this disorder, known as “gummosis,’ see gardenguyhawaii.com.

If citrus trees are not planted in well-drained soil, this type of disease may ensue.

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For guidance in learning about turf pests and diseases, there are two good CTAHR publication to consult, “Turf and Ornamental Pest Control” and “Destructive Turf Caterpillars in Hawaii.” Both can be found online at http://www.ctahr.hawaii.edu/site/info.aspx.

Sometimes, though, problems in the lawn are not caused by an insect, by fungi or bacteria. But rather they are what we refer to as environmental disorders.

These types of disorders are often seen as spots that suddenly appear on the leaf blade or perhaps the entire blade has burned to a light brown color. Symptoms will appear rather suddenly, they will not increase in size and eventually the lawn recovers. Entities causing this type of malady include, over fertilization, herbicide sprays, spilled gasoline from a lawnmower and other lawn equipment, unknown chemical spills and perhaps even vog.

Contrary to these environmental disorders, when pathogens such as bacteria or fungi are involved, spots on the leaf are often surrounded by a yellow halo or the entire leaf blade may decay. In addition, the spots on the lawn will grow larger or

more numerous; this is true with an insect pest also.

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I will be teaching a class, “Tropical Fruit Trees,” from 9:30 a.m.-12 p.m. on Saturday, June 14, at the University of Hawaii at Hilo. Learn the basics of how to successfully grow tropical fruit trees in the home garden. This workshop will cover plant nutrition, soil preparation, propagation, flowering and production problems including common disease and insect pests. Some of the trees to be discussed are avocado, papaya, mango, banana and lychee. Call 974-7664 to register, or go online at http://hilo.hawaii.edu/academics/ccecs/fitness/. There is a fee.

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at askthegardenguy@earthlink.net. You also can visit his website at www.gardenguyhawaii.com.

 

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