• Huanglongbing (citrus greening) is a bacterial disease that is threatening
the citrus industry worldwide. An insect known as the Asian citrus psyllid,
Diaphorina citri, carries the organism that causes citrus greening,
Candidatus Liberibacter asiaticus. Candidatus Liberibacter spp. are
phloem-limited plant pathogenic bacteria.The phloem system of the plant
transports sugars, which are the food source of the plant, bidirectionally
through the plant. The phloem system of the plants transports the sugars
from sources of photosynthetic activity (leaves) in the plant to flowers,
fruits, roots, seeds.
• Psyllids are the main means of transmission to plants. An infected psyllid
feeds on a healthy tree and injects the bacterium into the phloem .
Candidatus Liberibacter asiaticus enters in sieve tube and uses sugar which
are transported by phloem thus changes the shape of phloem cell. It also
created gap between phloem cell. In our modeling we have calculated the
fractal dimension of normal phloem cell and the fractal dimension of same
cell after HLB and we have found that as the disease increases fractal
dimension of the cell is decreasing. It decreases from 1.9209 to 1.8541.
Huanglongbing (HLB, formerly
known as citrus greening) is one of
the most destructive diseases of
citrus, debilitating the productive
capacity of citrus trees . The HLB
disease is caused by a fastidious α
Liberibacter spp. The bacterium is
phloem limited, transmitted by
psyllid vectors, and has not been
cultured yet .
Asian Citrus Psyllid
The Asian citrus psyllid is a small insect
that feeds on the leaves and stems of
citrus trees. Once a tree is infected with
HLB,there are some cures but the psyllid
could have made a loss in economy
before you see the symptoms.It’s better
to prevent it. The best way to prevent
the disease from killing citrus trees is
to stop the Asian citrus psyllid by some
It is of three types:- 1. Ca. L. asiaticus
2. Ca. L. africanus
3. Ca. L. americanus
Scientific Information About Asian
Species: D. citri
Binomial name Diaphorina citri
Psyllids are the main means of transmission to
plants. An infected psyllid feeds on a healthy
tree and injects the bacterium into the
phloem. Once a tree is infected with the
bacterium, there is no known cure for the
disease. This in part is because the bacterium
is inside the vascular system of the plant
(systemic) and is therefore very difficult to
Life Cycle Of Psyllid
Psyllid’s are small insects about 3 to 4 mm in
length with a simple life cycle that processes
from egg through 5 nymphal instars to adult
Psyllid females lay their bright yellow-orange
eggs on tip of shoots.Number of eggs laid
depends on the host plant as well as
Example : 1. Mean of 857 eggs were deposited on
grapefruit,whereas a mean of 527 eggs
were deposited on rough lemon.
2. At 25 C eggs hatch in 2 to 4 days whereas
7 to 9 days at 20 C .
It has an egg stage,
5 wingless intermediate stages called nymphs, and
(insects molt to grow bigger)
The eggs are yellow-orange, tucked into the tips of
tiny new leaves, and they are difficult to see because
they are so small
Modes of Transport For Psyllid
The Asian citrus psyllid can fly short distances
and be carried by the wind. However, a main
way the Asian citrus psyllid spreads
throughout the state is by people transporting
infested plants or plant material. For this
reason, everyone in California is asked to not
move citrus and to only purchase citrus trees
from reputable, licensed nurseries in your
How does the psyllid get around?
The psyllid can spread naturally by flying or
it can hitch a ride on plants into new areas of California
Photos courtesy of the CDFA
Psyllid-infested curry leaves
shipped in boxes from Hawaii
Unprocessed fruit from Mexico
On ornamentals in floral bouquets
Citrus riding across the border in
passenger or cargo vans
Candidatus Liberibacter spp. are phloemlimited plant pathogenic bacteria.The phloem
system of the plant transports sugars, which
are the food source of the plant,bidirectionally
through the plant. The phloem system of the
plants transports the sugars from sources of
photosynthetic activity (leaves) in the plant to
flowers, fruits, roots, seeds.
Working Of Psyllid
The asian citrus psyllid is a phloem limited
bacteria which when attacks a tree creates
space between compactly arranged phloem
cells.After that it starts taking food from the
phloem cell and does not allow the minerals to
reach the leaves which ultimately results in
dying and turning back of leaf.
The peptidoglycane layer of the Gram negative cell wall
can be visualized (PG) in between the inner membrane
cytoplasmic membrane (CM) and the outer cell wall
. Electron micrograph of Ca. Liberibacter cells in a sieve tube of sweet
orange leaf in Saudi Arabia
• Causes worldwide crop loss.
• Reduced profitability for citrus growers.
• Mature trees, when infected, decline and become
non - productive.
• Young infected trees may never bear fruit.
Primary leaf symptoms include :
Yellowing of leaves along veins
Asymmetrical blotchy mottle.
Newly formed leaves become small and upright
Shows a variety of chlorotic patterns that resemble various
Nutrient deficiency symptoms such as zinc and manganese
As infection progresses, characteristic symptoms are :
• Lopsided fruit
• Color inversion and
• Aborted seeds
Symptoms do not always occur together on the same tree. Trees may be infected for
long periods of time without showing distinct symptoms.
Lack of specificity in disease symptoms, unknown latent period of the disease in field
trees, and our poor understanding of the effect of environment conditions on
symptom development point to the need for field-based diagnostic tests for early
An early sign of the disease is uneven
(asymmetrical) yellowing of the leaves
Leaves with HLB disease have a
blotchy mottled yellow pattern
that is not the same on both
sides of the leaf.
Leaves with nutrient deficiencies
(Zinc is an example) have the same
yellow pattern on both sides of the
Symptoms may not show up in the tree until
1-2 years after it becomes infected
Figure 25. Blotchy chlorosis throughout leaf
Within 3-5 years after infection, the tree stops
bearing fruit and eventually dies.
This citrus tree in
a backyard in
sick, with few
leaves and no
The disease is widespread in :1. Asian countries
2. South California.
6. South and Central America
Where did Asian citrus psyllid and
the HLB disease come from?
Most likely ACP and HLB came from India or Asia. Both the psyllid and
disease are affecting citrus production in Brazil, Cuba and Florida.
California has the psyllid in 2 counties in southern California but does
not yet have the disease.
HLB Disease found in
Florida in 2005 and Cuba in 2007
the pest and
Figure 34. Map courtesy of UFL IFAS Extension
Map out of date per Brian
Both the psyllid and the HLB disease
Asian citrus psyllid, but not the disease
How can I help prevent the pest and disease from
•Buy only certified disease-free trees from a reputable
•Don’t bring plant material from other states or
•Learn to recognize the pest and disease symptoms
•Check flush foliage of citrus and citrus relatives wherever you go
•Call your County Agricultural Commissioners office or
the CDFA hotline immediately, if you suspect you have
either the pest or the disease
Some Other Ways To Control Citrus
• Using temperature-controlled growth chambers, we
evaluated the time duration and temperature required
to suppress or eliminate one type of psyllid i.e. ‘Ca. L.
• Spraying trees weekly with horticulture grade oils.
• Use of soap citrus save-r.
• Use of highly toxic pyrethoid. Eg. cyfluthrin (Tempo)
• Imidacloprid (Merit) a systemic neonicotinoid
• Use of RSVP plant scrub.