CITRUS GREENING
Dr. Mongi Zekri
Multi-County Citrus Extension Agent
University of Florida, IFAS
Citrus greening or Huanglongbing (yellow
dragon disease) was first detected in Florida
in late August 2005. It is caused by systemic
phloem-inhabiting bacterium, Candidatus
liberobacter. There are three forms of citrus
greening. Each form has a similar host range
but they differ in the temperature under which
they express strongest symptoms. The African
form, Candidatus L. africanus causes
symptoms under cool conditions while the
Asian form, Candidatus L. asiaticus causes
symptoms under warm conditions. The third
form (Candidatus Liberibacter americanus)
was found in Brazil in March 2004. The
bacterium infects all citrus species, cultivars
and hybrids and some citrus relatives.
Symptoms of Asian form are leaf chlorosis
with a blotchy mottle.
Twig dieback occurs, and the affected trees
decline to a non-productive state. Fruit is
small, lopsided, with the basal end often
remaining green, and the seeds are usually
aborted. The fruit has a bitter, salty taste.
Citrus greening is graft transmissible. The
distribution of the bacterium within an
infected tree can be irregular so not all buds
contain the bacterium or transmit the disease.
Citrus greening bacteria are transmitted by the
citrus psyllid. The Asian citrus psyllids,
Diaphorina citri, is adapted to warm humid
climates and occurs in many areas including
Florida. The bacterium is transmitted in a
persistent manner with a latent period
occurring after the psyllid acquires the
bacterium. The bacterium multiplies in the
psyllid. Because of the latent period, most
psyllids capable of transmitting citrus
greening are either late-stage nymphs or
adults. The psyllids remain capable of
transmitting the disease for the duration of
their life once the bacterium has been
acquired. Citrus species is the primary host
for feeding of the psyllids. The psyllids prefer
to feed on young flush tissue. Hosts, which
are vigorous and always flushing, such as
lemon and lime, are ideal hosts for the psyllid.
The best control for citrus greening is
exclusion. It may take awhile before citrus
greening infections are noticed. Bacterium
may be undetectable in tissues without
symptoms and incubation time may be long.
After infection, it may take more than 3 years
for the visual symptoms to show up. In areas
where citrus greening has been established,
management to reduce losses includes
propagation of citrus greening-free trees for
planting, reduction of the psyllid populations,
and by removing infected trees. The use of
parasites to control the psyllids population has
been successful in many areas.
Citrus greening is one of the most destructive
diseases of citrus. Once established, the
management of the disease to achieve
continued production of citrus is difficult and
expensive. Citrus greening can rapidly
destroy productive citrus plantings. If trees
are infected while young, they often have no
fruit production.
Replanting with healthy plants, introducing
parasites and spraying against psylla showed
that citrus production is still feasible in areas
with citrus greening.
For more information,
http://flcitrus.ifas.ufl.edu/
http://spdn.ifas.ufl.edu/Citrus%20_Greeni
ng.htm
http://www.doacs.state.fl.us/pi/enpp/ento/ci
trusgreening.html
http://www.aphis.usda.gov/ppq/ep/citrus_g
reening/index.html
Blotchy mottle
Lopsided fruit
THE CITRUS PSYLLID
The Asian citrus psylla or psyllid, Diaphorina
citri can be a serious pest of citrus. The citrus
psyllid is similar to aphids and the citrus
leafminer in requiring young leaves for
reproduction. However, unlike aphids and the
citrus leafminer, adult psyllids can survive on
hardened leaves and move to new flush as it
becomes available. Young trees should be
monitored early in flush cycles to detect
aggregations of adults on expanding
terminals. High populations of adults should
be treated before they reproduce.
Immature psyllid feeding on growing citrus
terminals cause permanent damage to young
leaves and shoots. They cause leaf distortion
and curling of tender growth (flush).
Badly-damaged leaves will die and fall off.
Control of the citrus psyllid should be taken
seriously. The citrus psyllid breeds
exclusively on young flush and has a very
high reproductive rate. Multiple, overlapping
generations can lead to very high populations.
Eggs are laid in the late winter and spring on
young leaves in the buds or in leaf axils. The
egg stalk is forced into the leaf tissue by the
ovipositor of the female. Each female may lay
up to 800 eggs during her two-month lifespan.
The life cycle takes about 20 days and there
may be up to 30 overlapping generations per
year. Adults are about the size of aphids (2.5
mm).
The citrus psyllid is an efficient vector of
greening disease, which is considered the
most serious citrus disease. Infected trees or
branches with the greening disease will suffer
heavy leaf drop, out-of-season flushing and
flowering, reduced yield, and dieback. Fruit
on infected trees will be small in size and low
in juice quality. Many fruit will fall
prematurely, while those that remain on the
tree will not color properly, but will remain
green on the shaded side, hence the name of
the disease “greening”. Root systems
including feeder roots will poorly develop.
The psyllid also attacks landscape plants
(jasmine orange, Murraya paniculata).
Generalist predators such as lacewings,
syrphid flies, lady beetles, and spiders attack
psyllids. These native natural enemies are not
expected to suppress the pest populations to a
non-economic level. Two parasitoids,
Diaphorencyrtus aligarhensis and Tamarixia
radiata of the pest have been imported in
Florida and are being released in a classical
biological control program. The use of oil is
less disruptive and should be used. Petroleum
oil at the concentration of 5% should provide
adequate control.
Tests with petroleum oil sprays against citrus
psylla indicated that oil was most toxic to first
and second instar nymphs and provided good
control if applied frequently. Insects do not
develop resistance to oil, oil has a low toxicity
to vertebrates, and oil breaks down readily in
the environment.
For young citrus trees, use of systemic
pesticides, such as imidacloprid (Admire), are
effective for suppressing psyllid populations.
Foliar sprays can also be used for psyllid
control. Some of the products that can be used
are Lorsban, Danitol, Provado, and
Malathion. Read the label and use the label
rate.
For a complete list of pesticides, get to the
2006 Florida Citrus Pest Management Guide:
Soft-Bodied Insects Attacking Foliage and
Fruit at http://edis.ifas.ufl.edu/CG004
By: Dr. Mongi Zekri
Multi-County Citrus Extension Agent
University of Florida, IFAS
Phone: 863 674 4092
Adult psyllid
Nymph
