Diseases of Citrus and Avocado Trees

As just about everyone knows, southern Florida’s climate permits homeowners to grow
a vast and impressive array of fruit trees.  But the lower end of the peninsula is also a
sitting duck for the introduction of diseases and detrimental insects.  Lamentably,
Richard Lyons’ Nursery must now recommend that you no longer plant citrus and
avocado trees here.

What has led to this sad conclusion?  With citrus, it took a double whammy to make us
throw in the towel.  First came citrus canker, a disease that causes lesions, or wounds,
on the fruit, stems and leaves of many citrus species.  The responsible bacterium does
not carry a risk to human health, but it makes the fruit unsightly and weakens trees.
Dade County first experienced citrus canker a century ago, and it took many years to
eradicate it throughout the southeastern states.  The disease again popped up on the
western side of Florida in the mid-1980s, but was overcome in less than a decade.
Unfortunately, citrus canker made another serious incursion into Miami-Dade County in
1995.  After initial delay, the state took some controversial steps toward thwarting the
disease, but in the wake of the hurricanes of 2005 that spread the bacterium into many
new areas, the federal government ended support for the eradication program.

The second attack on citrus has been infinitely more ominous.  Citrus greening, or
Huanglongbing, also a bacterial disease, is spread primarily by a couple of species of
psyllids (so-called jumping plant lice).  An Asian strain of the bacterium 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.  Florida’s Institute
of Food and Agricultural Sciences (IFAS) advises that the psyllids can be treated by
insectides, but warns that “elimination of the disease from an area has never been
successful.”  And, finally, the agency adds that “[b]ecause of the highly infectious
nature of these citrus diseases fruit crops other than citrus should be considered for
planting in the home landscape.”

When it comes to avocados, the origin of the problem is unique:  It is the only plant
disease or insect pest to invade from north of Florida instead of from the tropics.  It
is known as laurel wilt disease, and it is a fungus that hones in on members of the
lauraceae, or laurel family, which include redbay, swamp bay and, alas, avocado.  It
works by destroying the vascular system which transports water throughout the plant.
The fungus is spread from tree to tree by the redbay ambrosia beetle, an Asian native.
(The beetle itself would likely not kill trees without the disease-causing fungus it
carries.)  Unfortunately, the beetle is commonly found residing in firewood, so it can
be introduced to an area by simply transporting cut wood of laurel family species.

The redbay ambrosia beetle was first detected in the U.S. at Savannah, GA in 2002, where it
probably arrived in solid wood packing materials, and it was found in Miami-Dade
County in swamp bay trees in February 2011.  The bad news outweighs the good news
about laurel wilt disease.  The good news is that avocados are not the preferred host
species for the beetle.  The bad news is that, nevertheless, the beetle has been found in
the commercial avocado grove area of southern Miami-Dade County, and there is as yet
no known effective biological or synthetic chemical treatment for laurel wilt disease.

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