Citrus greening

Leo A. Martin
Tue, 14 Oct 2014 14:11:45 PDT
> if you have citrus in your yard
> please be very vigilant in checking
> your trees for [the citrus psyllid
> vector of citrus greening.] DO NOT
> transport citrus
> into or out of your area.

Most here won't know what this is. Citrus greening is a disease caused by a bacterium
lacking a cell wall. For this reason the bacterium does not tolerate drying and can only
be spread wet. It infects the fluid transport system of plants in family Rutaceae. This
includes citrus, white sapote (Casimiroa edulis), the herb rue (Ruta graveolens), and
tropical shrubs commonly called mock oranges (Murraya paniculata), though not the
temperate "mock orange" which is genus Philadelphus. Similar bacteria cause human
disease, known in English as "walking pneumonia" but citrus greening does not infect

The disease and its vector, the aforementioned psyllid, are native to eastern Asia. The
psyllid feeds on most members of Rutaceae and transmits the bacterium on its proboscis.
This is the reason Rutaceae have been on the USA no-import list for some years.

A psyllid in English is colloquially known as a leafhopper (which is different from and
smaller than a grasshopper.) The greening psyllid is so small, and so similar to other
psyllids, that it cannot be identified without a microscope. Many county agriculture
departments provide study traps to commercial and home orchardists with citrus trees.

Genetic analysis shows the psyllid was brought to the New World in the 1500s or 1600s,
probably with cultivated citrus or Murraya shrubs. The disease arrived much later; South
America some time ago, and Miami, Florida in the 1990s. The disease in the USA is
genetically distinct from that in Brazil, and is more closely related to strains in
China; for this reason, it is thought it was introduced to Miami from China. Vector and
disease quickly spread through citrus groves throughout Florida despite herculean and
sometimes shockingly, lethally violent government attempts to control it. It arrived in
Los Angeles in the early 2000s and continues spreading.

Citrus greening kills citrus trees. There is now no successful treatment for infected
trees. Trees are infected for 3-4 years before showing signs, and psyllids feeding on
infected but asymptomatic trees can spread the disease to other trees. Seeds may bear
the disease because most citrus have polyembryonic seeds, formed by parthenogenesis.

Symptoms are those of inadequate fluid transport: Stunted growth, and fruit which falls
off the tree with any water stress. Almost all fruits are affected. They bear skin scars
and are bitter. It is predicted that if a control method is not found, the citrus
industry in Florida will disapper soon.

As of now the only method of control is prevention of spread of the vector. USDA citrus
germplasm repositories now house collections in containers in triple-screened
greenhouses to prevent entrance of the psyllid. The bacterium is quite susceptible to
antibiotics but there is no effective way to disperse antibiotics through tree vascular
tissues. Study of insects possibly parasitic on the psyllid are underway. Current
thinking is that the US citrus industry will only be saved by genetically engineering
citrus to be resistant to the bacterium.

There has been some research showing the common guava (Psidium littorale) repels the
psyllid, and some growers are interplanting citrus orchards with guava.

Recommendations are also to remove members of Rutaceae from landscapes at risk, and not
to plant other plants in this family.

Arizona is the last state in the US that can grow organic citrus without pesticides.
Those of us active in fruit gardening clubs in Arizona have been following this issue
for some time. The psyllid is in the citrus fields just across the Colorado River from
Arizona near Blythe and Yuma, as well as Hermosillo, Sonora, Mexico, just a few hundred
miles south. The intervening desert has so far prevented spread into metro Phoenix but
summer storms could bring the psyllid, the way hurricanes brought certain kinds of
soybean rust from the Yucutan Peninsula to the US midwest some years ago. Some of us are
currently interplanting or preparing to interplant our orchards with guava.

Leo Martin
Phoenix Arizona USA

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