Modeling huanglongbing transmission within
a citrus tree
Christinah Chiyakaa,1, Burton H. Singera,1, Susan E. Halbertb, J. Glenn Morris, Jr.a, and Ariena H. C. van Bruggena,c,1
a
Emerging Pathogens Institute, University of Florida, Gainesville, FL 32610; bDivision of Plant Industry, Florida Department of Agriculture and Consumer
Services, Gainesville, FL 32614; and cDepartment of Plant Pathology, Institute of Food and Agricultural Services, University of Florida, Gainesville, FL 32611
greening
| Candidatus Liberibacter asiaticus | Diaphorina citri
H
uanglongbing (HLB) or citrus greening has been responsible
for the near destruction of citrus industries in Asia and
Africa (1). In 1998, the psyllid vector Diaphorina citri Kuwayama
was first detected in Florida and by 2000 this pest had spread to
31 Florida counties (2). HLB was first detected in Florida in 2005
and has spread through most of Florida since then (3). In recent
years, the disease has occasionally been found in other southeastern states of the United States, and recently in California.
HLB is considered the most serious problem of citrus worldwide,
and in the United States, a committee of the National Research
Council investigated research priorities to control this disease
(4). Since the introduction of the disease into the Americas, a
lot of research has been conducted on the epidemiology of the
disease and on the vector, but results of these two lines of inquiry
have not been integrated.
HLB is associated with three noncultivable Gram-negative
bacterial species belonging to Candidatus Liberibacter, recognized
on the basis of 16S rDNA sequence analysis (1). The species observed in the United States is Candidatus Liberibacter asiaticus
(CLas). The putative greening pathogens are fastidious phloeminhabiting bacteria (2), but they could possibly occur in the xylem
as well, because psyllids sometimes feed from the xylem (5). All
HLB-associated species of Ca. Liberibacter are transmitted from
infected to healthy plants through grafting or by citrus psyllids,
particularly the Asian citrus psyllid (ACP) D. citri in Asia and the
Americas (6). The transmission process through grafting depends
on the plant part, amount of tissue used, and the pathogen strain
(1). Ca. Liberibacter spp. can be transported both upward and
downward throughout the tree, but their distribution is highly
patchy (7). The highest concentrations can be found in stem and
www.pnas.org/cgi/doi/10.1073/pnas.1208326109
midribs of flush. Flush is a newly developing cluster of very young
leaves on the expanding terminal end of a shoot.
The main symptoms on HLB-infected citrus trees are yellow
shoots, leaves with blotchy mottle, and small lopsided fruits (1,
2). Ultimately, infected branches die back and the tree dies. The
time from infection to symptom appearance is variable, depending
on the time of year, environmental conditions, tree age, host
species/cultivar, and horticultural health, ranging from less than
1 y to several years (3).
Population densities of D. citri on host plants correlate with
quantity and nutritional quality of plant flush because eggs are
laid on young flush and nymphs develop exclusively on tender
flush (8). By the time flush expand and harden, turning dark
green, nymphs have completed their development. Nymphs,
which are always found on new growth, spend most of their time
feeding on phloem juice close to where they were born, using
piercing-sucking mouth parts. Adults may be found on leaves
and along the stem, almost exclusively on new growth, except in
the winter when there is no new growth. They leap when disturbed and may fly a short distance to other flush on the same or
neighboring trees (9). Epidemic spread occurs more frequently
within trees and rows in a grove than between rows, and longdistance spread occurs occasionally (3).
Disease management is complicated, because incubation
periods are long and diagnosis is difficult. In Florida, there has
been a three-pronged approach to HLB management: (i) production of disease-free nursery stock (mandatory for commercial
citrus production in Florida); (ii) psyllid control by insecticides
and removal of symptomatic trees that test positive for CLas in
a qPCR test (adopted in large citrus groves with limited HLB
incidence); and (iii) psyllid control and application of foliar
nutritional sprays, often combined with salicylic acid and/or
phosphite (widely adopted in areas with high HLB incidence).
There is a fierce debate about the effectiveness of strategies ii
and iii, but there is agreement that none of these control strategies completely eliminates HLB transmission.
Mathematical models have played an important role in understanding the epidemiology of vector-transmitted plant pathogens, in particular viral pathogens (10–12). Analytical models
have also been developed for the spread of citrus canker (13),
but models for vector-transmitted bacterial pathogens are still
preliminary (14). Some existing vector-borne plant disease models
have been used as experimental tools with which to investigate the
relative effects of management practices on the spread of diseases
(10) and of different modes of virus transmission on epidemic
development (15).
Author contributions: C.C., S.E.H., J.G.M., and A.H.C.v.B. designed research; C.C. and
A.H.C.v.B. performed research; C.C. and B.H.S. contributed new analytic tools; C.C.
and A.H.C.v.B. analyzed data; S.E.H. provided information on psyllids; and C.C., B.H.S.,
and A.H.C.v.B. wrote the paper.
The authors declare no conflict of interest.
Freely available online through the PNAS open access option.
1
To whom correspondence may be addressed. E-mail: christinah@epi.ufl.edu, bhsinger@
epi.ufl.edu, or ahcvanbruggen@ufl.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1208326109/-/DCSupplemental.
PNAS | July 24, 2012 | vol. 109 | no. 30 | 12213–12218
APPLIED
MATHEMATICS
The citrus disease huanglongbing (HLB), associated with an uncultured bacterial pathogen, is threatening the citrus industry
worldwide. A mathematical model of the transmission of HLB between its psyllid vector and citrus host has been developed to
characterize the dynamics of the vector and disease development,
focusing on the spread of the pathogen from flush to flush (a
newly developing cluster of very young leaves on the expanding
terminal end of a shoot) within a tree. This approach differs from
that of prior models for vector-transmitted plant diseases where
the entire plant is the unit of analysis. Dynamics of vector and host
populations are simulated realistically as the flush population
approaches complete infection. Model analysis indicates that vector activity is essential for initial infection but is not necessary for
continued infection because infection can occur from flush to flush
through internal movement in the tree. Flush production, withintree spread, and latent period are the most important parameters
influencing HLB development. The model shows that the effect of
spraying of psyllids depends on time of initial spraying, frequency,
and efficacy of the insecticides. Similarly, effects of removal of symptomatic flush depend on the frequency of removal and the time of
initiation of this practice since the start of the epidemic. Within-tree
resistance to spread, possibly affected by inherent or induced resistance, is a major factor affecting epidemic development, supporting
the notion that alternate routes of transmission besides that by the
vector can be important for epidemic development.
PLANT BIOLOGY
Contributed by Burton H. Singer, May 22, 2012 (sent for review November 7, 2011)
Recently, a spatially explicit simulation model was constructed
to predict the potential dynamics and spread of citrus psyllids in
Australia, taking climate change into account (16). The model
indicated that the flush period has a major influence on psyllid
dynamics. However, infection of citrus by Ca. Liberibacter sp.
was not considered. The report by the National Research
Council stated “the construction of a mathematical model for the
CLas/ ACP/ citrus-HLB system should be a long-term goal of
HLB research” (ref. 4, p. 124). As the vector spends most of its
life time on a particular flush, and adults move primarily from
one flush to a nearby flush, a flush is the central epidemiological
unit, and a tree is considered as a population of flush, similar to
the model of Hassell et al. for whiteflies on a single plant (17).
Thus, the main objective of this study was to develop a mathematical model of the dynamics of the developmental stages of D.
citri, citrus flush development, and infection by CLas to obtain
a clearer understanding of the interactions between the pathogen,
the vector, and the tree. Additional objectives are to assess the
epidemiological parameters that have a strong influence on HLB
development, to evaluate effects of different management strategies on HLB development, and to identify the parameters that
contribute to the effectiveness of these strategies.
Methods and Results
Differential Equations for Transmission Dynamics. The psyllid life cycle has
seven distinct nonoverlapping stages: (i) egg, (ii–vi) five nymphal instars, and
(vii) adults (2). We group these stages into three developmental stages,
namely the egg (E), nymph (N), and adult (A) stages. The populations of
nymphs and adults are divided into five compartments, namely uninfected
nymphs Nu , infected nymphs Ni , uninfected adult psyllids Au , infected adults
that acquired CLas during the nymphal stage Ai2 , and infected adults that
acquired CLas during the adult stage Ai1 . The flush population is categorized
into four compartments: uninfected and healthy H, infected and asymptomatic but not yet infectious (latent) W, infectious and asymptomatic X,
and infectious and symptomatic Y. Fourth- to fifth-instar nymphs and adults
can acquire the pathogen and adults emerging from those nymphs can
transmit the pathogen in a shorter period than psyllids that feed on infected
plants only as adults (18, 19). The difference in infectivity of these two
groups of infected adults is modeled by the parameter θ1 . Initially, transovarial transmission was thought to be absent (19), but recently, there has
been some evidence of transovarial transmission (20). Nevertheless, we assume there is no transovarial transmission because it is low, variable, and
dependent on environmental conditions (20). Of most concern are the
nymph and adult stages because acquisition feeding can occur during the
last nymph stages and the adult stage. For the egg stage, the only concern is
survival to the nymph stage. Therefore, we define α as the rate of oviposition, δe as the rate at which an egg becomes nonviable, and σ −1 as the duration from egg to first nymph stage. The parameter σ=ðσ þ δe Þ is the
probability that a viable egg survives into a first instar nymph. It is assumed
that CLas is not pathogenic to psyllids but makes infected psyllids more
fertile and makes them lay more eggs than their noninfected counterparts
(20). The rate at which susceptible plants are infected, βp can be described as
βp ¼ ρπ p ω, where ω is the average time spent per visit on a plant, π p−1 is the
mean time a psyllid must feed on a plant for inoculation to occur, and ρ is
the number of flushes visited per day per vector. The product πω is the
probability of inoculation of a healthy plant during a visit by an infectious
vector, and ρω is interpreted as a measure of vector activity. Similarly, the
rate at which susceptible adult vectors are infected is βa ¼ ρπ a ω, and the rate
at which uninfected nymphs acquire the pathogen is βn ¼ ρn π n ω, where π a− 1
and π n− 1 are the mean times required for acquisition by the adult and
nymph, respectively. The infected adults retain infectivity after acquisition
throughout their lives and adult vectors emigrate from a flush at a rate κ.
For the flush population, the rate of growth of new healthy flush ξðtÞ is
assumed periodic with
ξðtÞ ¼ ξ0 ð1 þ υ sin ϖtÞ;
where ξ0 is the baseline rate of growth, υ is the strength of seasonal forcing,
and and ϖ is the period. Growth of new foliage on mature citrus trees occurs
approximately twice a year, a major flush in spring and a rebound in fall
(21). Healthy flush are infected by infectious vectors that acquired the
pathogen during the nymphal stages and those that acquire it during
the adult stage. Healthy flush also acquire the pathogen directly from both
the asymptomatic and the symptomatic infected flush at rates λ and λθ2 ,
12214 | www.pnas.org/cgi/doi/10.1073/pnas.1208326109
respectively. The flush mature at a rate δp . η1− 1 is the flush latent period, η2 is
the rate at which an infectious flush becomes symptomatic, and P is the flush
population size (density). The asymptomatic flush become symptomatic due
to an increase in concentration of the pathogen, making the symptomatic
flush more infectious to vectors than the asymptomatic flush (22). In one
study more active bacterial cells were observed in presymptomatic than in
highly symptomatic leaf samples (23), suggesting that the bacteria are
dormant or dead in highly symptomatic tissue. However, a positive correlation between concentration of CLas and HLB symptom expression has also
been shown (22). In addition, symptomatic trees (yellow color) may be more
attractive for the vector (24). Therefore, an increase in infectivity of early
symptomatic flush is modeled by the parameter θ2 . Incubation (time to
symptom expression) and latency (time to infectivity) are two concurrent
and related temporal processes, both beginning at infection (3). In most
plant–pathogen systems, the incubation period is shorter than the latent
period but in other systems like citrus HLB, latency ends when infectivity
begins and is followed by the end of the incubation period. The flow diagram for the model is given in Fig. 1.
The following system of differential equations specifies the model.
H
dE
¼ α Au þ Ai1 þ Ai2
− ðδe þ σÞE
dt
P
u
dN
X þ θ2 Y
− ðδn þ γÞNu
¼ σE − βn Nu
P
dt
dNi
X þ θ2 Y
− ðδn þ γÞNi
¼ βn Nu
P
dt
dAu
X þ θ2 Y
− ðδa þ κÞAu
¼ γNu − βa Au
P
dt
dAi1
X þ θ2 Y
¼ βa Au
− ðδa þ κÞAi1
P
dt
dAi2
¼ γNi − ðδa þ κÞAi2
dt
9
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=
[1]
>
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>
>
>
>
>
>
i
i
>
>
A1 þ θ1 A2
>
dH
ðX þ θ2 YÞ
>
¼ ξðtÞH − βp H
− δp H >
− λH
>
>
dt
P
>
>
P
>
>
>
i
i
A
þ
θ
A
1 2
>
>
1
dW
ðX þ θ2 YÞ
>
þ λH
¼ βp H
− η1 þ δp W >
>
>
>
P
dt
P
>
>
>
>
>
dX
>
>
¼ η1 W − η2 þ δp X
>
>
dt
>
>
>
>
>
dY
>
>
>
¼ η2 X − δp Y:
>
;
dt
Analysis of the system (1) is simplified if we work with the nondimensionalized system (Eq. S3 in SI Text), where ξðtÞ is interpreted as
a maturation rate.
Parameter Values. We used parameter values from ref. 6, assuming a temperature of 28 °C. Table 2 in ref. 6 shows the survivorship of immature stages
of D. citri at different temperatures (Table S1). The value for the mortality
rate of nymphs per day δn is therefore estimated from table 2 in ref. 6. This
table was also used to estimate the rate at which eggs become nonviable.
On average D. citri takes about 2.5 h to prepare the stylet pathway and
around 3.5 h to feed in the phloem, which represents a predominant activity
(5). Effective time spent per visit on a flush is assumed to be 2.5–3.5 h, and it
takes 15–30 min for adults to acquire the bacteria (25). Acquisition time for
nymphs π n is assumed to be 2 h. The maturation rate of flush was deduced
from the fact that by the time flush expanded and hardened, nymphs would
have completed their development (8). Thus, the time taken by the flush to
harden is the same as the development time of the psyllid (6).
We assume that CLas bacteria are more numerous in symptomatic than in
asymptomatic flush (22). We assume that the chance of picking up infective
bacteria is three times more from symptomatic than from asymptomatic
flush. Thus, infectiousness of symptomatic flush is given by θ2 = 3. Approximately 78% of the psyllids that fed as fifth-instar nymphs on CLas-infected
trees and 13% of the psyllids that acquired the pathogen as adults were CLas
positive at the end of an inoculation access period (18). The factor by which
infectiousness of infected adults that acquired the pathogen in the nymphal
stages θ1 is therefore assumed to be 6.
General Dynamics of the Model. The graphs in Fig. 2 A–D are obtained by
using the following initial conditions: E = 5, Nu ¼ 1, Ni ¼ 0, Au ¼ 0:5,
Ai1 ¼ 0:01, Ai2 ¼ 0:0, H ¼ 1:0, W ¼ 0:0, X ¼ 0:0, and Y ¼ 0:0. The parameter
values used are given in Table 1 with ω ¼ 0:1, η1 ¼ 0:0177767, η2 ¼ 0:005,
Chiyaka et al.
modeled in the present framework. When the value of the transmission rate
of the bacteria from flush to flush, λ, is decreased from 0.33 to 0.25, while
keeping the other parameters and initial conditions the same as those used
in Fig. 2, the psyllid populations increased to about 60 nymphs and 5 adults
per flush (Fig. 3 A and B). In this situation, the different flush categories
continue to oscillate and flush never become 100% symptomatic (Fig. 3 C and
D). In this second scenario, both the uninfected and the infected components
of psyllids and flush are at equilibrium. Other values of λ always result in either scenario 1 or scenario 2. Thus, the model has two distinct equilibrium
states with respect to disease, in addition to a disease-free equilibrium: (i) the
state with ultimately 100% symptomatic flush represented by simulations
with λ = 0.33 and (ii) the state with all components continuously present
represented by λ = 0.25.
Sensitivity Analysis. The ratio of the relative change in a variable to that in
a parameter is an index of the sensitivity of the variable to that parameter (26).
Sensitivity analysis of the parameters of model Eq. S3 was carried out relative
to the reproductive number R0 because variation in R0 is considered whenever optimal intervention strategies are sought. Equations for R0 and the
calculation of sensitivity indexes are given in SI Text. The expression for the
reproductive number is given by
R0 ¼ Q1 þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q2 þ Q21 ;
η λðξ þ η2 θ2 Þ
;
Q1 ¼ 1
2ξðη1 þ ξÞðη2 þ ξÞ
u
^ u þ βa γ þ δn þ ξ − δp A
^
βp η1 ðη2 θ2 þ ξÞ βn γθ1 N
Q2 ¼
ξ γ þ δn þ ξ − δp ðη1 þ ξÞðη2 þ ξÞ κ þ δa þ ξ − δp
and π a ¼ 48. During the first 2–3 y, the different flush categories oscillate
due to the forced oscillatory input of healthy flush twice a year. After about
3 y a saturation curve of symptomatic flush sets in and all of the flush become infected and symptomatic in about 5 y (Fig. 2 A and B). In accordance
with the availability of young flush, the different stages of the vector
population fluctuate with a large peak after the first peak in healthy flush.
When healthy flush is diminished, the vector population decreases to a very
low level, i.e., less than 0.5 psyllids (nymphs plus adults) per flush on average.
Such low populations are realistic for a severely infected and symptomatic
tree where little new healthy flush is produced.
There are also field situations when psyllid populations remain relatively
high even when a tree is infected and symptomatic. Parameter estimates
were therefore adjusted to investigate whether such situations could be
B
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
500
1500
2000
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Uninfected
nymphs
Infected
nymphs
Chiyaka et al.
200
400
600
Time (days)
Asymptomatic
flush
Symptomatic
flush
0
500
800
1000
1000
1500
2000
2500
Time (days)
D
peak at 55
peak at 35
0
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2500
Time (days)
C
Population of nymphs/flush
1000
Proportion of flush
Healthy flush
Latent flush
Population of adults/flush
Proportion of flush
A
u
^ ,A
^ as values at the disease-free equilibrium.
with N
Parameters with negative sensitivity indexes decrease the value of R0 as
their values increase, whereas those with positive values increase the value
of R0 as their values increase. The HLB model system is most sensitive to ξ
followed by λ and η1 (Table 2). The parameter that has the most effect on
the dynamics of the HLB infection is the maturation rate ξ. An increase in ξ
of 10% decreases R0 by about 20% and decreasing ω or ρ by 10% decreases
R0 by 0.6%.
Other parameters that show a great influence on HLB are rate of pathogen
transfer from infectious flush to healthy flush through plant material λ and
the latent period 1=η1 . When λ is reduced, the dynamics of HLB development
change drastically from continued expansion of HLB symptoms to a dynamic
equilibrium between healthy, asymptomatic, and symptomatic flush (SI
Text). When the latent period is varied between 30 d and 180 d, there are
large differences in the maximal proportions of asymptomatic flush attained
(35–70%), but there are only slight differences in the time needed to reach
100% symptomatic flush (Fig. S1).
0.5
peak at 4.4
peak at 2.9
0.4
Uninfected adult
psyllids
Infected adult
psyllids - A1
infected adult
psyllids - A2
0.3
0.2
0.1
0
0
200
400
600
Time (days)
800
1000
Fig. 2. (A–D) Proportions of healthy and latent flush (A),
proportions of asymptomatic and symptomatic flush (B),
populations of uninfected and infected nymphs per flush
(C), and populations of uninfected and infected adult
psyllids per flush (D). The latent flush is infected but not yet
infectious. The asymptomatic and symptomatic flush are
both infected and infectious. Infected adult psyllids are
infectious and consist of categories A1 = Ai1 and A2 = Ai2 .
Input rate of healthy flush follows a periodic function with
two peaks in 1 y. The parameter for internal movement of
the pathogen in the tree, λ ¼ 0:33.
PNAS | July 24, 2012 | vol. 109 | no. 30 | 12215
APPLIED
MATHEMATICS
u
PLANT BIOLOGY
where
Fig. 1. Model flow diagram showing the transmission of Ca. Liberibacter
asiaticus between the psyllids and the host flush. The flush compartments
are uninfected and healthy H, infected and asymptomatic but not yet infectious (latent) W, infectious and asymptomatic X, and infectious and
symptomatic Y. The vector compartments are uninfected nymphs Nu ,
infected nymphs Ni , uninfected adult psyllids Au , infected adults that acquired CLas during the adult stage Ai1 , and infected adults that acquired
CLas during the nymphal stage Ai2 .
Table 1. Parameter definitions and values used for numerical simulations of the transmission of huanglongbing
from infectious adult psyllids to citrus flush and from infectious flush to psyllid nymphs and adults
Parameter
δn
ω
πp
δe
η1
η2
ρ
πa
πn
γ
δa
α
σ
δp
θ2
θ1
κ
λ
ξ0
R0
ξ
Definition
Value
Units
Reference*
Mortality rate of nymphs
Effective time spent per visit on a flush
Feeding rate for inoculation to occur
Rate at which eggs become non viable
Rate at which infected flush becomes infectious
Rate at which asymptomatic infected flush becomes symptomatic
Number of flushes visited per day
Feeding rate by adult vectors for acquisition to occur
Feeding rate by nymphs for acquisition
Rate at which nymphs become adults
Mortality rate of adult
Rate of oviposition
Rate at which eggs become nymphs
Maturation rate of flush‡
Infectiousness of symptomatic flush
Increased infectiousness of Ai2
Emigration rate of adult psyllids
Internal transmission rate among flushes
Baseline input rate of healthy flush
Strength of seasonality
Period
0.0035
0.10–0.15
4.8
0.011
0.005–0.033
0.005–0.033
1.0
48–96
12
0.078
0.025
15.77
0.24
0.071
3
6
0.4
0.2–0.35
0.5
0.9
π/90
d
d
d−1
d−1
d−1
d−1
d−1
d−1
d−1
d−1
d−1
d−1
d−1
d−1
—
—
d−1
d−1
d−1
1
(6)
(5)
(5, 18)
(6)
Estimate (3)
Estimate (3)
Estimate
(2)
Estimate†
(6)
(6)
(6)
(6)
(6, 8)
(22)
(18)
Estimate
Estimate
Estimate
Estimate
Estimate
−1
*Parameter values from ref. 6 were at an average temperature of 28 °C. See SI Text.
The assumption made is based on the fact that nymphs have a shorter piercing mouth; therefore, it would take longer to obtain Las
for nymphs than for adults.
‡
The rate was deduced from the fact that Pluke et al. (8) noted that by the time flushes expanded and hardened, nymphs would have
completed their development. (Value of development from egg to adult used is from ref. 6.)
†
Effects of Insecticide Applications. When the psyllid death rate is increased to
0.004 d−1 to simulate the effect of insecticide spraying, while keeping the
same initial conditions and other parameter values as used for Fig. 2, results
vary, depending on the start of the spray program, namely 360 d, 450 d, and
540 d after initial inoculation, spraying twice a year with an insecticide that
has an effectiveness of 75% (Fig. 4 A and B). If insecticide is applied starting
from 360 d or 450 d after initial infection, increases in the proportion of
healthy flush and decreases in the proportion of asymptomatic infected flush
A
Uninfected nymphs
Infected nymphs
5
Adults/flush
60
Nymphs/flush
B
70
50
40
30
20
2
0
0
200
400
600
800
1000
0
Time (days)
Healthy flush
Latent flush
0.8
200
400
600
800
1000
Time (days)
D
1
Prop'n of flush
Prop'n of flush
Effects of Flush Removal. When 90% of symptomatic flush is removed every 3
mo, although the latent period is 56 d, a stable equilibrium sets in after about
3
1
10
0
C
Uninfected adult psyllids
Infected adult psyllids - A1
Infected adult psyllids - A2
4
ensue (Fig. 4 A and B). The resulting oscillations in the different categories are
ultimately very similar for the two spray starting dates and continue indefinitely. On the other hand, if spraying is started after 540 d, the dynamics
of the different flush categories follow a similar pattern to that observed
without spraying. Although early spraying reduces the temporal increase in
infected flush, it does not eliminate the pathogen from a tree (Fig. 4B). As
expected, spraying the vectors reduces the psyllid populations in all categories, but they continue to oscillate (Fig. S2).
If insecticides are applied when λ = 0.25, all flush and vector categories
continue to oscillate similar to the nonsprayed scenario (Fig. S3). However,
the total population of adult psyllids is reduced from a maximum of 4.5 to
4.3 individuals per flush (Fig. S3A) and the maximum proportion of symptomatic flush is reduced from 0.35 to 0.29 (Fig. S3B).
0.6
0.4
0.2
1
Asymptomatic flush
Symptomatic flush
0.8
Parameter
0.6
0.4
0.2
0
0
0
200
400
600
Time (days)
800
1000
0
200
400
600
800
1000
Time (days)
Fig. 3. (A–D) Populations of uninfected and infected nymphs per flush (A),
populations of uninfected and infected adult psyllids per flush (B), proportions of healthy and latent flush (C), and proportions of asymptomatic
and symptomatic flush (D). The latent flush is infected but not yet infectious. The asymptomatic and symptomatic flush are both infected and
infectious. Infected adult psyllids are infectious and consist of categories
A1 = Ai1 and A2 = Ai2 . Input rate of healthy flush follows a periodic function
with two peaks in 1 y. Internal movement parameter of the pathogen in the
tree, λ ¼ 0:25.
12216 | www.pnas.org/cgi/doi/10.1073/pnas.1208326109
Table 2. Sensitivity indexes for the
reproductive number R0 (SI Text) of
the development of huanglongbing
in a citrus tree
δn
δa
λ
θ1
η1
ρ
πn
πp
ξ
γ
κ
θ2
η2
ω
πa
Index
−0.0003529
−0.001153
0.9373
0.02229
0.9354
0.06265
0.02229
0.03132
−1.9962
0.02346
−0.01844
0.02820
0.01861
0.06265
0.0090303
Chiyaka et al.
0.6
0.4
0.2
0
0
500
1000
1500
2000
2500
1
360 days
450 days
540 days
0.8
0.6
0.4
0.2
0
0
500
Time (days)
360 days
540 days
720 days
0.8
0.6
0.4
0.2
0
0
500
1000
1500
Time (days)
2000
2500
Prop'n of symptomatic flush
D
1
Prop'n of symptomatic flush
C
1000
1500
2000
2500
2000
2500
Time (days)
1
360 days
540 days
720 days
0.8
0.6
0.4
0.2
0
0
500
1000
1500
Time (days)
Fig. 4. (A–D) Proportion of healthy flush (A) and of asymptomatic flush (B)
when psyllids are sprayed twice every year with an insecticide that is 0.75
effective, 360 d, 450 d, and 540 d after initial infection and proportions of
symptomatic flush when symptomatic flush are removed every 3 mo (C)
and every 6 mo (D) starting 360 d, 540 d, and 720 d after inoculation.
2 y for the proportion of symptomatic flush, oscillating between 0 and 21%,
no matter if the flush removal is started 360 d, 540 d, or 720 d after inoculation (Fig. 4C). On the contrary, when symptomatic flush is removed by
90% every 6 mo, stable oscillations in symptomatic flush (0–30%) occur if
flush removal is initiated after 360 d. A two-state equilibrium is reached with
flush removal starting after 540 d. A long-term high plateau (about 92%) is
reached when flush removal every 6 mo is started after 720 d (Fig. 4D).
Discussion
A mathematical model is presented to describe and analyze the
transmission of a bacterial vector-borne disease within a tree.
The model considers the transmission of the pathogen from flush
to flush through the vector as well as by movement within a tree.
An analogous comparison of multiple infection routes of human
pathogens has been of growing interest, for example in the case
of pneumonic plague (27). Selection of flush as the fundamental
unit constitutes a vector-centered rather than a host-centered approach. This is similar to the approach taken by Hassell et al. (17),
who modeled populations of whiteflies on individual leaves of a
single plant to account for the patchy distribution within a plant.
There are several assumptions on which the model is based: (i)
We assume that psyllid eggs are laid on healthy flush. This choice
is based on the assumption that new flush emerges healthy and
that one psyllid generation takes as much time as the maturation
period of a flush (8). Therefore, eggs would need to be laid on
very young flush to be able to complete development to adulthood. In reality, eggs also are laid on already infected flush and
there is some indication that female psyllids might have some
preference for infected flush (24), but an infected flush cannot
become more infected than it already is. Also, a female psyllid
does not lose infectivity after feeding on a flush. There are no
epidemiological consequences if a female lays eggs on a recently
infected rather than a healthy flush. (ii) Nymphs are not subdivided over five stages (2) in the model. Fourth- and fifth-instar
nymphs acquire Liberibacter cells more readily than younger
nymphs (18, 20), and the pathogen likely multiplies inside
the nymphs (28), but multiplication of Liberibacter is not part of
this model, because not enough is known about acquisition (3).
Therefore, the only thing that matters for the outcome of the
model is whether adult psyllids acquired the pathogen as nymphs
or as adults, because they are more infectious in the first case
(18, 20). (iii) The growth, death, and development rates are assumed to be similar for psyllids with and without CLas. In reality,
the generation time is shorter for psyllids with the pathogen than
for those without (20) and female psyllids with Liberibacter lay
more eggs than those without (20). Moreover, the vectors may be
Chiyaka et al.
more attracted to symptomatic than to asymptomatic flush (29),
and this complex interrelationship is not addressed in this model.
(iv) Symptomatic flush is assumed to be more infectious than
asymptomatic flush on the basis of higher CLas DNA densities in
symptomatic flush (22). Recently, large numbers of active CLas
cells were found in tissue samples from presymptomatic young
flushes whereas more inactive bacteria were observed in highly
symptomatic samples (23). If we assume that the symptomatic
flush is less infectious than the asymptomatic one, then the
number of subsequent infections will be reduced. Similar conclusions can be drawn in cases where some of the infected psyllids fail to transmit the pathogen to a healthy flush after feeding.
Finally, it is assumed that the psyllids are randomly distributed
among the flush, which is not true (30), and that the development of the psyllid and flush populations is synchronous,
which is partially true due to the environmental forcing of flush
production (8, 9). However, this model gives realistic results and
provides a first tool for comparison of management strategies.
The results of the model show that when internal movement of
CLas in a tree is relatively fast (λ = 0.33) and insecticides are not
applied, an infected tree will become 100% symptomatic and die
after about 5 y. This time frame is in agreement with observations when trees become infected in an established grove. The
time taken for an infected tree to become unproductive and die
depends on the tree age, because multiplication of CLas is fastest
in young citrus plants (22). In graft-inoculated young trees,
production of additional new growth decreased until about 9 mo
after inoculation, when severely affected branches stopped producing new growth (23). Most of the infected plants eventually
died over the following year.
The model indicates that the psyllid populations drop to very
low levels once the number of infected flush has increased. This
might be attributed to the fact that as much of the tree becomes
symptomatic, growth of new flush decreases (23), so that the
vectors will search for other trees with flush to lay their eggs (31).
Indeed, the parameter reflecting the flush maturation rate and
thus the availability of flush (ξ) has the most effect on the dynamics
of the HLB infection. Increasing the maturation rate or decreasing
the growth rate of flush leads to a reduction in the spread of infection in a tree. This reduction is in agreement with decreased
vector activity in a grove with diminished flush production (31).
Another parameter that shows a great influence on the expansion of HLB infection is the rate of pathogen transfer from
infectious flush to healthy flush through the tree ðλÞ. When λ is
reduced to 0.25 d−1, the dynamics of HLB development change
drastically from a situation of continued expansion of HLB symptoms to a situation of a dynamic equilibrium between healthy,
infected asymptomatic and symptomatic flush (Fig. S4). In
practice, such a situation may be present when nutritional
products and systemic acquired resistance inducers are used to
ameliorate the effect of symptoms produced by HLB in citrus
groves (3) or when a resistant rootstock or interstock is used
(32). The actual movement of CLas through a tree is evident
from systemic spread of the pathogen in plants that are inoculated through grafting in greenhouses where vectors are absent (23). However, the rates of movement and multiplication of
the pathogen inside trees are not well understood (3). As a result, models that would take the concentration of the pathogen
into account are as yet nonexistent. This finding of the importance
of internal flush-to-flush spread has broader implications as well.
For example, the spread of pneumonic plague through humanto-human aerosol contact (analogous to flush-to-flush spread) is
more important than the spread by its vector, fleas (27).
Insecticidal control of the psyllids is another method adopted
to reduce HLB spread (10). Effectiveness of this method depends on time of initial spraying, frequency of spraying, and efficacy of the insecticides. According to the model, spraying does
not curb the infection after about half of the flush has become
symptomatic. If spraying is initiated earlier, it will have beneficial
effects but will not eliminate the bacteria from the tree. To
achieve an effective reduction in disease spread, frequency and
PNAS | July 24, 2012 | vol. 109 | no. 30 | 12217
PLANT BIOLOGY
B
360 days
450 days
540 days
APPLIED
MATHEMATICS
Prop'n of healthy flush
1
0.8
Prop'n of symptomatic flush
A
efficacy of insecticide sprays should be quite high, which can be
expensive and may have potential side effects, including insecticide resistance. In addition, insecticide applications may
have little or no effect when the resistance to internal movement
is great (small λ) and a stable equilibrium exists between infected
and noninfected flush (Fig. S3).
Effects of flush initiation and removal are quite complex.
Periodic forcing of flush initiation affects the dynamics of the
model (Fig. S5). Effects of flush removal depend on the type of
flush removed (healthy or infected, asymptomatic or symptomatic), flush removal initiation relative to initial infection, and the
removal frequency relative to the latent and incubation periods.
According to our model simulations, removal of symptomatic
flush could reduce the percentage of flush that becomes symptomatic, but the effect greatly depends on the frequency of flush
removal and the time of initiation of this treatment. Flush removal every 6 mo initiated 2 y after initial infection does not
reduce symptom development. Ineffectiveness of flush removal
from highly infected trees is in agreement with experimental
results (33). Rigorous psyllid control combined with removal of
symptomatic trees from slightly infected, large groves has slowed
down epidemic development (3). These practices may be comparable to frequent flush removal in the beginning of an epidemic
simulated with our model, but spatial spread from tree to tree
would need to be included in our model to determine effects of
tree removal on epidemic development under various conditions,
in particular different latent and incubation periods.
For mathematical tractability of the model several assumptions were made. Nevertheless, there is sufficient realism in our
specifications that we were able to gain valuable insights into
effects of spraying and flush removal and into crucial parameters
to be considered when implementing intervention strategies. A
better understanding of the transmission dynamics of the pathogen between the psyllid and a citrus tree and of the spread from
tree to tree will provide further insight in planning and assessing
the impact of current control strategies and development of
effective control measures.
1. Bové JM (2006) Huanglongbing: A destructive, newly emerging, century-old disease
of citrus. J Plant Pathol 88:7–37.
2. Halbert SE, Manjunath KL (2004) Asian citrus psyllids (Sternorrhyncha: Psyllidae) and
greening disease of citrus: A literature review and assessment of risk in Florida. Fla
Entomol 87:330–353.
3. Gottwald TR (2010) Current epidemiological understanding of citrus Huanglongbing.
Annu Rev Phytopathol 48:119–139.
4. Committee on the Strategic Planning for the Florida Citrus Industry: Addressing Citrus
Greening Disease (Huanglongbing), National Research Council (2010) Strategic Planning for the Florida Citrus Industry: Addressing Citrus Greening (National Academies
Press, Washington, DC).
5. Bonani JP, et al. (2010) Characterization of electrical penetration graphs of the Asian citrus
psyllid, Diaphorina citri, in sweet orange seedlings. Entom Experim Applic 134:35–49.
6. Liu YH, Tsai JH (2000) Effects of temperature on biology and life table parameters of
the Asian citrus psyllid, Diaphorina citri Kuwayama (Homoptera: Psyllidae). Ann Appl
Biol 137:201–206.
7. Li W, Levy L, Hartung JS (2009) Quantitative distribution of ‘Candidatus Liberibacter
asiaticus’ in citrus plants with citrus huanglongbing. Phytopathology 99:139–144.
8. Pluke RHW, Qureshi JA, Stansly PA (2008) Citrus flushing patterns, Diaphorina citri
(Hemiptera:Psyllidae) populations and parasitism by Tamarixia radiate (Hymenoptera:
Eulophidae) in Puerto Rico. Fla Entomol 91(1):36–42.
9. Hall DG, Hentz MG (2011) Seasonal flight activity by the Asian citrus psyllid in east
central Florida. Entomol Experim Applic 139:75–85.
10. Chan M-S, Jeger MJ (1994) An analytical model of plant virus disease dynamics with
roguing and replanting. J Appl Ecol 31:413–427.
11. Jeger MJ, Madden LV, van den Bosch F (2009) The effect of transmission route on
plant virus epidemic development and disease control. J Theor Biol 258:198–207.
12. Sisterson MS (2008) Effects of insect-vector preference for healthy or infected plants
on pathogen spread: Insights from a model. J Econ Entomol 101:1–8.
13. Parnell S, Gottwald TR, Gilligan CA, Cunniffe NJ, van den Bosch F (2010) The effect of
landscape pattern on the optimal eradication zone of an invading epidemic. Phytopathology 100:638–644.
14. Mizell RF, 3rd, et al. (2008) Behavioral model for Homalodisca vitripennis (Hemiptera:
Cicadellidae): Optimization of host plant utilization and management implications.
Environ Entomol 37:1049–1062.
15. Jeger MJ, van den Bosch F, Madden LV, Holt J (1998) A model for analyzing plant-virus
transmission characteristics and epidemic development. IMA J Math Appl Med Biol 15:1–18.
16. Aurambout JP, Finlay KJ, Luck J, Beattie GAC (2009) A concept to estimate the potential
distribution of the Asiatic citrus psyllid (Diaphorina citri Kuwayama) in Australia under
climate change: A means for assessing biosecurity risk. Ecol Modell 220:2512–2524.
17. Hassel MP, Southwood TRR, Reader PM (1987b) The dynamics of the viburnun whitefly
(Aleurotrachelus jelinekii): A case study of population regulation. J Anim Ecol 56:283–300.
18. Inoue H, et al. (2009) Enhanced proliferation and efficient transmission of Candidatus
Liberibacter asiaticus by adult Diaphorina citri after acquisition feeding in the
nymphal stage. Ann Appl Biol 155:29–36.
19. Capoor SP, Rao DG, Viswanath SM (1974) Greening disease of citrus in the Deccan
Trap Country and its relationship with the vector, Diaphorina citri Kuwayama. Proceedings of the 6th Conference of the International Organisation on Citrus Virology,
eds Weathers LG, Cohen M (University of California, Riverside, CA), pp 43–49.
20. Pelz-Stelinski KS, Brlansky RH, Ebert TA, Rogers ME (2010) Transmission parameters
for Candidatus liberibacter asiaticus by Asian citrus psyllid (Hemiptera: Psyllidae).
J Econ Entomol 103:1531–1541.
21. Stansly P (2011) Living with citrus greening in Florida. Available at http://www.imok.ufl.
edu/docs/pdf/entomology/tja_cv_oct_2011.pdf. Accessed January 6, 2012.
22. Coletta-Filho HD, et al. (2010) In planta multiplication and graft transmission of ‘Candidatus Liberibacter asiaticus’ revealed by real-time PCR. Eur J Plant Pathol 126:53–60.
23. Folimonova SY, Achor DS (2010) Early events of citrus greening (Huanglongbing)
disease development at the ultrastructural level. Phytopathology 100:949–958.
24. Hall DG, Hentz MG, Ciomperlik MA (2007) A comparison of traps and stem tap
sampling for monitoring adult Asian citrus psyllid (Hemiptera: Psyllidae) in citrus. Fla
Entomol 90:327–334.
25. Capoor SP, Rao DG, Viswanath SM (1967) Diaphorin citri Kuway, a vector of the
greening disease of citrus in India. Ind J Agric Sci 37:572–576.
26. Hove-Musekwa SD, et al. (2011) Modelling and analysis of the effects of malnutrition
in the spread of cholera. Math Comput Model 53:1583–1595.
27. Gani R, Leach S (2004) Epidemiologic determinants for modeling pneumonic plague
outbreaks. Emerg Infect Dis 10:608–614.
28. Hung TH, Hung SC, Chen CN, Hsu MH, Su HJ (2004) Detection by PCR of Candidatus
Liberibacter asiaticus, the bacterium causing citrus huanglongbing in vector psyllids:
Application to the study of vector–pathogen relationships. Plant Pathol 53:96–102.
29. Patt JM, Sétamou M (2010) Responses of the Asian citrus psyllid to volatiles emitted
by the flushing shoots of its rutaceous host plants. Environ Entomol 39:618–624.
30. Costa MG, Barbosa JC, Yamamoto PT, Leal RM (2010) Spatial distribution of Diaphorina
citri Kuwayama (Hemiptera: Psyllidae) in citrus orchards. Scientia Agric 67:546–554.
31. Tiwari S, Lewis-Rosenblum H, Pelz-Stelinski K, Stelinski LL (2010) Incidence of Candidatus Liberibacter asiaticus infection in abandoned citrus occurring in proximity to
commercially managed groves. J Econ Entomol 103:1972–1978.
32. Shokrollah H, Abdullah TL, Sijam K, Abdullah SNA (2011) Potential use of selected citrus
rootstocks and interstocks against HLB disease in Malaysia. Crop Prot 30:521–525.
33. Lopes SA, Frare GF, Yamamoto PT, Ayres AJ, Barbosa JC (2007) Ineffectiveness of
pruning to control citrus huanglongbing caused by Candidatus Liberibacter americanus. Eur J Plant Pathol 119:463–468.
12218 | www.pnas.org/cgi/doi/10.1073/pnas.1208326109
ACKNOWLEDGMENTS. The authors thank David Smith who encouraged
C.C. to initiate this study and reviewed the manuscript. The authors also
thank Zindoga Mukandavire, James Keesling, Sergei Pilyugin, Phil Stansly, and
Ulisses Nunes da Rocha for interesting discussions and trustees of the
Smallwood Foundation for their commitment to provide funding for huanglongbing research at the Emerging Pathogens Institute. C.C. was supported
by the University of Florida Science for Life Program, funded by the Howard
Hughes Medical Institute.
Chiyaka et al.