Update on Avocado and Citrus

Over the past few years, Richard Lyons’ Nursery has reported from time to time on disease problems in avocado and citrus that have imperiled the future of both fruits as commercial crops in Florida. It is time for another update.

Avocados have been hit hard by laurel wilt, a disease spread by Xyleborus glabratus, the redbay ambrosia beetle. The insect, native to Asia, was first detected in Georgia in 2002, and has spread in all directions rapidly since then. Though the beetle’s primary interest is redbay trees, once it decimated those populations, it began to attack avocado, another member of the laurel family.  Originally, it was thought that the ambrosia beetle was the only beetle spreading the laurel wilt disease, but recently several other beetles have been identified in the spread of this fungus.

The quarterly magazine Modern Farmer recently reported that about 40,000 avocado trees have been lost in Florida. That is particularly ominous for Miami-Dade County, where traditionally 98% of the state’s commercial avocados have been grown.

Avocado trees are sitting ducks in the assault by laurel wilt. That’s because their root systems grow shallowly and radiate laterally. It means that in a commercial grove, where the root systems of trees touch—and, in fact, graft with—one another, a pathogen like the fungus that causes laurel wilt is able to spread through the plot fairly quickly. In addition, by the time a tree exhibits a physical manifestation of laurel wilt disease, such as the browning of leaves, it is too late to save the tree.

Accordingly, early detection of laurel wilt has come to be recognized as highly important to the effort to slow the spread of the disease. Research funded by the Florida Department of Agriculture and Consumer Services has led to the use of dogs trained to detect the scent of the laurel wilt fungus before an infected tree shows outward signs. First a drone is flown over an orchard to spot trees with browning canopies; those trees are removed, and the dogs are then released into the orchard to detect any trees in the early stages of infection. Those trees undergo a program of treatment to attempt to save them. However, the cost of identifying and treating diseased trees is high, so while detection via canine olfactory sensitivities is quite helpful, it nevertheless is best recognized as a mechanism that doesn’t stop the spread of laurel wilt disease, but, rather, slows it until a cure can be developed.

Meanwhile, the outlook for citrus in Florida, particularly the southern part of the state, is also tenuous. In recent decades, the family has encountered the one-two punch of citrus canker and citrus greening. The latter scourge, though more recent, is the more threatening, because it is much more likely to kill afflicted trees.

Citrus greening, also known as Huanglongbing (HLB), is caused by a bacterium that was discovered in southern Florida in August 2005 and has proved to be a formidable threat. Leaves of infected trees become chlorotic, or yellow; the visible symptoms of this chlorosis are typically referred to as “blotchy mottle.” In fact, that is probably the most reliable diagnostic symptom of citrus greening disease. The yellowing may show up in a single branch, particularly in a younger tree, and then spread throughout the plant in a year’s time. Developing fruits are misshapen and remain green instead of ripening, and their taste is rendered bitter. The disease, which, like citrus canker, is not harmful to humans, is incurable in plants, and most affected trees die within several years. The drop in orange production has been precipitous. The State of Florida last February lowered its prediction for the 2017-18 season to 45 million boxes; the high-water mark in orange production was 244 million boxes in 1997-98.

Researchers in citrus-growing regions around the world have been working earnestly for methods of cure or treatment of citrus greening. One proposal several years ago involved infusing an infected tree with antibiotics. But that required placing a cover or tent over the entire tree, a labor-intensive and expensive process. There doesn’t appear to be any recent writing on the subject.

In April 2018 it was reported that more traditional applications of bactericides—two brands of oxytetracycline and a streptomycin compound—had had variable results following three years of testing, and success appeared related to the number of annual doses. Other potential treatments have emerged in recent years. One is Benzbromarone, a drug developed to treat gout. Another is Zinkicide, a bactericide specifically created to work systemically within affected trees.

It must be emphasized that even if one or more of these chemicals makes it through the testing and regulatory processes to become commercially available to growers at reasonable prices, they are not cures for citrus greening. They are management techniques that might keep infected trees healthy enough to increase yields. A true cure for HLB does not yet appear on the horizon.

If all the foregoing obstacles weren’t enough, there is an additional challenge for backyard and commercial citrus growers in southern Florida—our alkaline soil. Upstate a number of desirable cultivars are grafted onto rootstocks that perform well in that region’s soils. Unfortunately, those rootstocks are incompatible with our limestone-based soils, and the fruits produced here are pithy, dry, and bitter.

For all the reasons described above, Richard Lyons’ Nursery recommends against planting avocado and citrus species in southern Florida, and it will not again stock them until a cure is found for laurel wilt and citrus greening.

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