Title: Citrus industry update
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Title: Citrus industry update
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Language: English
Publisher: Institute of Food and Agricultural Sciences
Place of Publication: Gainesville, Fla.
Publication Date: October/November 2009
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Volume ID: VID00009
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WU UNIVERSITY of
UF FLORIDA
IFAS


Citrus Industry Update


Working
To Keep You
Informed


Published by the University of Florida, Institute of Food and Agricultural Sciences, with the mission
of keeping the Florida Citrus Industry informed of current research concerning canker and greening.



What's in this issue:

Low-volume aerial applications for psyllid control Rogers and Avery.................................. ............ 1
Update on development of a repellent for Asian citrus psyllid Stelinski and Rouseff ........................3
Seasonality of psyllids carrying the HLB pathogen Rogers and Ebert.................................... ........... 4
Novel methods to treat new or existing HLB-infected citrus trees Dawson...................................... 5
Advanced citrus production systems: managing for productivity Schumann et al......................... 7
Can supplemental nutrient applications alleviate symptoms of HLB and improve
productivity? Spann ............................................................. ........................................ ......... 9
Research update on new tools being investigated for citrus leafminer control Stelinski ...............10
Five take-home messages for managing citrus canker on processing oranges
G raham and D ew dney ............................................................................. ............................ 11


Low-volume aerial applications for
psyllid control- Michael E. Rogers and
Pasco B. Avery

Using the best products available for psyllid
control, the length of time psyllid populations
remain at low levels without the need for
repeated insecticide applications is largely
dependent on movement of psyllids from
surrounding areas. Thus, coordinated grove
sprays may increase the effectiveness of psyllid
management programs and potentially reduce
the need for frequent reapplication of
insecticides. Where aerial applications can be
used, growers can work together to reduce the
time needed to treat groves across a large area,
thus minimizing the risk of psyllids moving
between treated and yet to be treated groves.
While aerial applications are usually much
faster (and cheaper) to apply than conventional
ground applications, weather usually dictates
how many acres can be treated per day with
applications usually halted by mid-day due to


wind conditions. By reducing the per acre
spray volume of aerial pesticide applications,
fewer refueling trips to the airstrip will be
needed, resulting in an increased number of
acres that can be treated in one day. If a grove
is a considerable distance to the airstrip, this
could also help reduce some of the application
costs as well.

Field trials were initiated in early 2009 to
evaluate the effectiveness of low-volume aerial
applications for psyllid control. Malathion 5
(EPA Reg. # 9779-5) was used in these trials
because the label permits use of this product in
a spray volume of 1 gallon of water per acre,
whereas other labeled products require much
higher spray volumes on a per acre basis.

The first trial was initiated on March 6 in the
Ft. Pierce vicinity. Blocks of citrus, 100 acres in

2009 Citrus Research and Education Center, University of Florida,
Institute of Food and Agricultural Sciences, 700 Experiment Station
Road, Lake Alfred, FL 33850, Phone: 863-956-1151.


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size, were treated by air with Malathion 5 at a
rate of 2 pints of product in 1 gallon of water
per acre with the spray equipment set to
produce droplets of approximately 300 microns
in size. Additional blocks, 100 acres in size,
were left untreated as controls for comparison
purposes. At the time of application, trees
were producing new flush so counts were made
of the number of psyllid eggs, nymphs, and
adults present before the application was made
and continued for about 3 weeks post
application. Results from sampling showed
that the low-volume aerial Malathion
application provided a significant reduction in
psyllid egg, nymph, and adult stages when
comparison was made of pre-spray and
post-spray counts in the treated plots. At 3 and
7 days after treatment (DAT), psyllid egg counts
were reduced 96 and 99% (respectively)
compared to the untreated plots. There was
no difference in egg counts at 18 DAT due to a
lack of suitable new flush for egg laying. At 3,
7, and 18 DAT, psyllid nymph counts were 92,
98, and 94% lower (respectively) compared to
the untreated blocks. No nymph counts were
made after 18 DAT since all new flush had
hardened off and no nymphs were present for
sampling. At 5, 17, and 24 DAT, adult psyllid
counts on sticky traps were 93, 97, and 94%
lower (respectively) in the treated plots when
compared to untreated plots.

A second low-volume aerial application trial
was initiated May 22, also in the Ft. Pierce
vicinity. In this trial, Malathion 5 was applied at
a rate of 2 pints per acre in 1 gallon of water
with an organosilicone surfactant, Silkin, added
to the spray solution at a rate of 0.25% v/v to
provide a more uniform spreading of the spray
solution on the plant foliage. Droplet size was
also reduced by adjusting the spray nozzle
deflectors to produce a droplet size of
approximately 150 microns, thereby increasing
the number of droplets deposited per square
inch of plant surface. Because of the difficulty
in locating areas with sufficient psyllid
populations for this trial, evaluations were
limited to a 160 acre area wherein we treated
plots of approximately 20 acres in size. No new


flush was present at this time so only adult
psyllid counts were made using sweep net
sampling. At 7, 14, and 24 DAT, adult psyllid
populations were 83, 85, and 67% lower
(respectively) when compared to the untreated
plots. Where psyllids were found in the treated
areas, they were usually collected within the
first 4-5 trees from the edge of the block
adjacent to untreated control blocks. Thus, it is
likely that these psyllids moved into the treated
blocks from the surrounding untreated plots.

In a third trial conducted in July near Fellsmere,
we compared the effectiveness of Malathion 5
and two pyrethroid insecticides when applied
for psyllid control as low-volume aerial
applications. None of the pyrethroid
insecticides currently registered for use in
Florida citrus can be applied low-volume by
plane. Thus, experimental use permits were
obtained for application of the pyrethroid
insecticides in this manner. The results of this
trial showed that low-volume application of
these two pyrethroids provided a level of
psyllid control similar to that provided by the
Malathion applications.

Of the three insecticides we have tested thus
far, only Malathion 5 (EPA Reg. # 9779-5) is
permitted for use as a low-volume aerial
application for citrus. Currently, the registrants
of the two pyrethroid insecticides tested are
working to obtain label changes that would
permit use of these products as low-volume
aerial applications in citrus. It is uncertain at
this time if/when those label changes might
occur. Additional work is planned for spring
2010 for continued testing of these and
additional products for use as low-volume
aerial applications for use in coordinated grove
spray programs.

For questions and further details, please
contact Dr. Michael Rogers: 863-956-1151,
mgrs@ufl.edu


Citrus Industry Update
Community Service Bulletin





UFW UNIVERSITY of
Ur FLORIDA
IFAS


Octe / N


Update on development of a repellent
for Asian citrus psyllid
Lukasz Stelinski and Russell Rouseff

Over the past year, we have been working to
develop an effective repellent for the Asian
citrus psyllid (ACP). Our work was initiated by
investigating the volatiles released by guava
plants and their effects on ACP behavior.
Following the discovery that synthetic dimethyl
disulfide (DMDS) was produced in large
quantities by wounded guava leaves, we
initiated an investigation of the effects of this
chemical on ACP behavior. DMDS is a known
plant defense chemical against insects that acts
as both a repellent and an insect neurotoxin. In
laboratory tests, we have confirmed that
volatiles from guava leaves significantly
inhibited ACP's response to normally attractive
citrus host-plant volatiles. A similar level of
inhibition was recorded when synthetic DMDS
was co-released with volatiles from citrus
leaves. In addition, the volatile mixture
emanating from a combination of intact citrus
and intact guava leaves induced a knock-down
effect on adult ACP suggesting toxicity of guava
volatiles to this insect. We quantified the
airborne concentration of DMDS that induced
the behavioral effect in the laboratory
behavioral tests and found it to be 107 pg/cc.
Compounds similar to DMDS, including dipropyl
disulfide, ethyl-1-propyl disulfide, and ethyl
disulfide, did not affect the behavioral response
of ACP to attractive citrus host plant volatiles in
laboratory behavioral tests. These data
suggested that the activity of DMDS on the
behavior of ACP is somewhat unique and not
shared by all disulfide compounds. However,
more recently we have found that certain other
sulfur compounds, including dimethyl trisulfide
and allyl methyl disulfide, are either slightly
more or equally active against the psyllid than
the originally identified DMDS. Determining
whether a blend of these chemicals will
increase the repellant effect further is currently
under investigation.

Field trials were conducted this past spring and
summer to test the effect of synthetic DMDS
released from polyethylene vials and other


devices on population densities of ACP. The
treatments compared were plots treated with
DMDS versus untreated control plots. In one
trial, 15 ml of synthetic DMDS was formulated
per polyethylene vial and approximately
200 vials were deployed per acre. This release
device was developed with one of our industry
partners (Alpha Scents). In this initial field
experiment, populations of ACP were
significantly reduced by deployment of
synthetic DMDS from the polyethylene vials
compared with untreated control plots. This
small plot field experiment confirmed the
results of our laboratory olfactometer assays.
Deployment of synthetic DMDS from
polyethylene vials reduced populations of ACP
in an unsprayed citrus grove for up to 3 weeks
following deployment. Given that population
densities were equivalent among plots prior to
the deployment of DMDS treatments, we
hypothesize that DMDS repelled adult ACP
from treated plots. By the fourth week, there
were no remaining DMDS in the polyethylene
vials, which likely explains why populations
were once again equivalent in treated and
control plots by the fourth week of the trial.
Given the volatility of DMDS, one of the main
obstacles to the development of a practical
DMDS formulation for ACP management will be
development of a slow-release device that
maintains the chemical above a behaviorally
active threshold for long periods. Ideally, a
slow-release device should be developed that
could achieve 150-200 days of behaviorally
efficacious release. We are working with ISCA
Technologies (Riverside, CA) to develop a
flowable formulation of the psyllid repellant
that also shows considerable promise.

In summary, our results indicate that synthetic
DMDS and certain related sulfur chemicals may
explain guava's behavioral activity against ACP.
DMDS appears to be a potential candidate
repellent for ACP. Other repellant compounds
similar to DMDS have been recently discovered
and they are being investigated further. Our
current on-going efforts include formulating
these repellant chemicals into controlled
release devices for extended release of the
chemical in the field. Control of ACP with
behavioral modification may be one potential


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UFW UNIVERSITY of
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IFAS


October / Noe br2


tool for management of this plant disease
vector.

For questions and further details, please
contact Dr. Lukasz Stelinski: 863-956-1151,
stelinski@ufl.edu

Seasonality ofpsyllids carrying the HLB
pathogen Michael E. Rogers and
Timothy A. Ebert

In January 2008, a study was initiated to
examine the seasonality of psyllids carrying the
HLB pathogen. The goal of this project was to
determine if there are periods of the year when
adult psyllids are less likely to be carrying the
HLB pathogen and, thus, insecticide use may be
decreased during these times. Through July
2009, more than 14,000 psyllids have been
collected and analyzed using real-time PCR to
determine (on a month by month basis) the
percentage of psyllids carrying the HLB
pathogen. Samples collected since July are still
in the process of being analyzed. Commercial
groves from which monthly samples have been
collected are located in the vicinities of Arcadia,
Lake Placid, Fort Meade, Lake Wales, and Lake
Alfred.

During 2008, there were two times of the year
when increases in the number of HLB positive
psyllids occurred at our study sites. The first
increased occurred in early spring with the
number of infected psyllids near 2% at one
location. Averaged across all sites, the
percentage infection rate was less than 1%.
A second more pronounced increase in the
number of HLB+ psyllids occurred during the
fall of 2008 at three of five study sites. The
maximum percentage of psyllid testing HLB+ at
these three locations during the fall months
ranged from 4% to more than 10%. When the
data were averaged across all study sites, the
percentage of infected psyllids did not exceed
5%.

During 2009, psyllid collections continued at
the same five locations sampled during 2008 to
determine if the trends observed in 2008 were
similar year to year. During 2009, a noticeable
increase in the overall percentage of HLB


infected psyllids was found. While there was
still a trend for periodic increases and
decreases in the percentage of HLB+ psyllids,
the periods when infection rates were highest
occurred in January, April, and July. While the
average psyllid infection rate was less than 5%
across all study sites, at one of these locations
(Lake Alfred) the percentage of HLB+ psyllids
was above 15% on each of these three months.

Because the overall percentage of HLB infected
trees at these five sites was relatively low
compared to disease incidence further south in
the state, we included a sixth study site located
in Homestead beginning in February 2009. At
this site, 100% of the citrus trees are showing
HLB symptoms and no insecticides are being
used for psyllid control. The reason for
including this site was to determine if all trees
are HLB infected and psyllid control was not
used, would we still see the rise and fall in HLB+
psyllids during the year, or in other words,
would the psyllid HLB infection rate remain
consistently high all season long. Data
collected thus far in 2009 from the Homestead
location have shown that there does appear to
be fluctuations in the number of HLB+ psyllids,
even where 100% of the citrus host plants are
HLB+. However, the numbers of infected
psyllids in such a situation is much greater with
up to 100% of the psyllid collected on one date
(February) testing HLB+ and a low of 20%
testing positive (March).

When the data are compared on a grove by
grove basis, one trend that is apparent from
our study is that there is a much lower rate of
HLB+ psyllids in groves where intensive HLB
management programs have been
implemented. For example, our study site near
Arcadia, which is on an intensive HLB
management program consisting of removal of
infected trees and insecticide applications for
psyllid management, the yearly average of
percent HLB+ psyllids is 0.5-0.6%. In the
coming season, we plan to modify our study to
more closely examine the potential grove
factors responsible for the fluctuations in the
percentage of HLB+ psyllids to better predict
when these increases are likely to occur.


Citrus Industry Update
Community Service Bulletin





UFW UNIVERSITY of
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IFAS


October / N


Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec


Cn
o)
.,
0D

_2


I
t-'
C

L


Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Seasonality of infected psyllids at locations where HLB infection rates in trees were low (15% or less,
upper graph) or high (~100%, lower graph). Data are still being collected for 2009.


For questions and further details, please
mrgrs@ufl.edu


Novel methods to treat new or existing
HLB-infected citrus trees
William 0. Dawson

Citrus greening (HLB) is causing a substantial
increase in the cost of producing citrus in
Florida, while also limiting production. For the
industry to be sustained, production costs need
to be reduced. Also, for the viability of the
industry, production levels need to be
maintained at levels sufficient to keep
processing plants busy. For trees in the field,
we need to develop solutions to reverse the
epidemic and allow infected trees to recover
health. New plantings need to be with trees


contact Dr. Michael Rogers: 863-956-1151,



that are resistant or tolerant to the disease.
The most economical and effective means of
controlling HLB would be through the
production of resistant or tolerant plants.
This would allow production levels and
economic inputs to be the same as that prior
to the introduction of HLB.

Traditionally, resistance or tolerance has been
produced in crops through breeding
programs. However, citrus breeding
programs require longer periods of time than
the industry can sustain. Thus, a faster
approach has to be found. Resistance or
tolerance has been produced in other crops
by inserting into the plants foreign genes that


Citrus Industry Update
Community Service Bulletin


Homestead 2009


4.0%
3.0%
2.0%
1.0%
0.0%



100.0%

80.0%

60.0%

40.0%

20.0%

0.0%


I I I I I I






October / November 2009


restrict the pathogen. This is possible with
citrus. From a scientific point of view, there are
two obstacles that must be overcome to
produce useful new citrus varieties:

1) Find methods to put new genes into citrus in
a timely manner; and,

2) Find genes that will protect citrus from HLB.

We need to find ways to produce new trees
and get them into the field producing as quickly
as possible. Actually, transforming citrus with
new genes already is being done routinely.
However, this also is a long-term process,
requiring years before new trees would be in
the field. The problem is that transformation
occurs at the single cell level, which then has to
develop into a new tree. This new tree will be
juvenile, which requires several years before
flowering and fruit production begin and longer
for other characteristics like thorniness to go
away. Since not all regenerated plants will be
identical to the parent, it is necessary for new
trees to be evaluated in the field for yield,
horticultural characteristics, and fruit and juice
quality. The time required for developing a
transgenic citrus tree has been reduced by the
ability to transform mature tissue. This cutting-
edge procedure has been developed in Spain
and is being transferred to Florida. This
procedure speeds up the process by a few
years. However, even with this improvement,
production by new trees will still be many years
away and these procedures are limited to new
plantings.

In our lab, we developed an alternative method
to express foreign genes in citrus trees using
citrus tristeza virus (CTV). We have developed
methods to manipulate the genetics of this
virus in the lab. This allows us to add or take
away genes from the virus. Thus, it is possible
to add genes that would be detrimental to
other pathogens. CTV and Candidatus
Liberibacter asiaticus (Las), the bacterium
thought to cause HLB, happen to reside in the
same place in citrus trees, which is the phloem.
Thus, this allows us to design CTV to combat
Las.


Each system for putting foreign genes into
citrus has advantages and disadvantages.
Transformation of citrus through tissue
culture should be stable and permanent in
new citrus lines, but takes a long time.
Expression of foreign genes inserted into the
virus will not be permanent. Eventually, the
antibacterial genes will be lost from the virus.
Our estimate, based on greenhouse
experiments, is that 80-90% of trees infected
with the virus vector would express the
antibacterial gene for 5-7 years and 50-70%
for 10-15 years. This would allow production
of trees without the increased use of
insecticides. The major advantage is that this
procedure can be used much sooner, as an
interim measure until transgenic trees are
available.

Advantages of the CTV vector compared to
transgenic citrus trees:

1. The major advantage is time. The vector
containing the antibacterial protein or peptide
could be introduced into nursery trees by
grafting vector-infected budwood, and the
resulting trees could be grown by methods
prior to the introduction of HLB. (This would
not require any extra manipulations in the
nursery.)

2. This approach avoids problems of juvenility
of transgenic trees and the need for
horticultural evaluation.

3. The vector-containing budwood could be
graft-inoculated to existing trees in the field.

4. The vector does not require transformation
technology for different varieties. The vector
can be graft transmitted to most commercial
varieties.

5. With an effective peptide or protein, it is
possible that the vector could be used to cure
HLB infected trees by graft inoculation into
trees already infected with HLB. However, we
emphasize that this experiment has not been
done yet.


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W UNIVERSITY of
UFLORIDA
IFAS


October / November2009


The second obstacle is finding a gene that can
be expressed that will control Las. The magic
bullet (gene) to control Las has not been found
yet. This is a major research objective in
several laboratories. Our laboratory and
several other laboratories are screening
antibacterial peptides against Las. These are
small proteins that are made by all higher
plants and animals as native defense
mechanisms against microorganisms. It is
possible that one of these will be a magic
bullet. We have already found some that work
against Las and allow trees to survive better,
but we think much more effective genes can be
found. Also, as a better understanding of Las is
developed, better proteins are likely to be
found based on blocking specific needs of the
bacterium.

For questions and further details, please
contact Dr. Bill Dawson: 863-956-1151,
wodtmv@ufl.edu

Advanced citrus production systems:
managing for productivity
-Arnold Schumann, Kevin Hostler,
Kirandeep Mann, and Laura Waldo

One of the important short-term strategies
used by citrus growers in South Africa to
survive their African strain of HLB disease is to
adopt the "open hydroponics system" (OHS) for
citrus production. The OHS technology was
developed by Professor Rafael Martinez Valero
Ph.D., a plant physiologist from the University
Miguel Hernandez in Alicante, Spain. Various
crop consultants have since adopted OHS and
introduced it to South Africa, Australia, and
California, USA. Unfortunately, OHS nutrient
technology has become part of the trade
secrets of consultants, and publicly available
research on OHS is almost non-existent, thus
prompting researchers at the University of
Florida to conduct new OHS experiments with
Florida citrus to develop guidelines for our
conditions. The OHS acronym was dropped in
favor of ACPS or "advanced citrus production
systems" because OHS implies indiscriminant
leaching of nutrients to the environment.
During the initial nine months of one 15-acre
ACPS experiment with 'Hamlin' orange on the


Ridge, we discovered that ACPS can be
extremely frugal with nutrient and water
resource utilization, which is good news in
times of reduced fruit prices, increased
production costs, and increasing vigilance by
environmental groups.

During the first nine months of growth under
ACPS drip fertigation, the 'Hamlin' block on
the Ridge at a planting density of 363 trees
per acre received 78% less fertilizer and 77%
less irrigation water than under a
conventional production practice of granular
fertilizer and microsprinkler irrigation.
Despite these significant savings of input
costs, the tree growth rates measured in ACPS
plots were approximately double the growth
rates measured in conventional plots, and
tree height after nine months was increased
by up to 20% in the ACPS plots.
Measurements of soil water, transpiration,
and photosynthesis suggest that the young
trees growing on an ACPS are subject to less
short-term water and nutrient stress than
conventionally grown trees. Leaf
transpiration rates were 45% higher in ACPS
plots than in conventional plots after several
rain-free days during which only the ACPS
plots received daily drip fertigation.
Photosynthesis rate measurements in the
leaves were 39% higher in ACPS plots than in
conventional plots at the end of the same
drying period. No wilting was visible at any
time, suggesting that these short-term
drought periods would be common in many
grower blocks but remain undetected.
Transpiration and photosynthesis measured
after a subsequent rainy period were the
same in ACPS and conventional plots,
suggesting that recovery from stress was
rapid. Short-term drought stress reduces
transpiration, and consequently uptake of
water and nutrients from the soil, leads to
premature stomatal closure, reduced carbon
dioxide absorption by the leaves, and
therefore lower rates of photosynthesis.
Because carbohydrates derived from
photosynthesis are the primary energy
sources for plants, transient reductions in
photosynthesis will slow growth and
productivity of the trees on average, as our


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UF FLORIDA
IFAS


October / Nv ember 2009


measurements have shown. Another possible
spinoff from better stress management may be
improved disease resistance by ACPS trees. We
have yet to prove this but faster growing, larger
trees with more energy and nutrient resources
to expend on fighting disease may be able to
mitigate some of the early symptoms of HLB
disease, perhaps by stimulating systemic
acquired resistance (SAR). Intensive foliar
nutrient spray programs adopted by some
growers are already suggesting similar effects
on HLB mitigation.

The three main components of an ACPS are
shown in the figure as 1) intensive fertigation,
facilitated by computer controlled pulse
irrigation and liquid fertilizer injection together
with monitoring equipment, 2) balanced
complete nutrition achieved with traditional
hydroponics nutrient formulation, and 3) high
density planting with suitable rootstocks to
achieve rapid bearing canopy development.
The main goals of ACPS are early, high
production, early return on investment, and
possibly disease avoidance and improved tree
longevity. Built-in redundancy from the high


planting density is designed to also
compensate for removal of HLB- or canker-
infected trees. Our projections from the first
nine months of growth in the Ridge
experiment suggest that the ACPS trees could
reach a productive size of about 5.5 feet tall in
as few as 2.25 years. Conventionally grown
trees would normally reach a similar size in
five years. Coupled with the rapid growth, the
high planting density of 363 trees per acre
(double the conventional density of about 150
trees per acre) should ensure that an
economically viable sustainable production
level can be reached sooner and, therefore,
avoid some of the early losses by HLB
infection. Adequate pest control is vital for
the success of ACPS, especially against Asian
citrus psyllids and citrus leafminers. The
frequent, vigorous leaf flushes stimulated by
the ACPS attract herbivorous insects, thus
requiring more intensive pest control
measures. Integrated pest management
which involves multiple control methods such
as pesticides (systemic and contact),
biocontrol or biopesticides, and new repellant
and pheromone chemicals may be the best


Main components of an ACPS


Monitoring equipment


s (1)
Intensive Jertigation


/ ACPS
/ ACPS


(2)
Balanced.' ',(3)
complete High density
nutrition planting



N,P K,Ca,Mg.S
Mn,Fe,Zn,Cu,B,Mo,CI
(100 ppm nitrogen)
\L


Computer control


Adapted


Three main components of an Advanced Citrus Production System strategy: 1) intensive
fertigation, 2) balanced, complete nutrition, and 3) high density planting.


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UFW UNIVERSITY of
Ur FLORIDA
IFAS


October / Noe br2


method for limiting insect populations in an
ACPS. The additional pest control required per
acre per year in an ACPS may seem
uneconomical but in reality due to the
compression in space (more trees per acre) and
time (higher growth rates) achieved, the costs
of pest control required to bring a new ACPS
grove into production in half the normal time
may even be lower than in a conventional
production system.

In summary, ACPS is used to grow citrus trees
quicker to "beat the disease cycle," and with
fewer nonrenewable resources than
conventional production methods.
Additionally, ACPS aims to minimize
environmental stresses to the trees,
particularly transient nutrient and water
stresses which are not normally noticeable but
which can reduce growth rates and may lower
a plant's resistance to disease.

For questions and further details, please
contact Dr. Arnold Schumann: 863-956-1151,
schumaw@ufl.edu

Can supplemental nutrient applications
alleviate symptoms of HLB and improve
productivity? Tim Spann

Everyone is well aware of the confusion that
can exist in trying to distinguish visible
symptoms of citrus greening disease
(Huanglongbing, HLB) from nutrient deficiency
symptoms. In fact, many papers describing the
leaf symptoms of greening will often describe
them as nutrient deficiency-like. In recent
reviews on greening, Jose Bove and John da
Graca both indicate that as the disease
progresses in a tree, symptoms of Zinc (Zn)
deficiency will develop. However, visible Zn
deficiency alone is not a good indicator of citrus
greening infection since Zn deficiency may
occur in uninfected trees and is quite
distinguishable from the typical asymmetrical
blotchy mottle of greening leaves associated
with high leaf starch. This article discusses our
current thinking on the citrus greening/leaf
nutrition connection and the research being
done to further our understanding of this topic.


Other micronutrient deficiencies, particularly
Boron (B), can also cause symptoms that are
frequently seen on greening infected trees. In
a paper from 1930, A.R.C. Haas described citrus
trees with corking and splitting of leaf veins,
abscission of leaves, and accumulation of
excessive amounts of carbohydrates in affected
leaves. One could easily believe he was
describing greening symptoms, but he was
actually describing B deficiency.

The visible connection between nutrient
deficiency and citrus greening is not new.
During the 1970s, two separate studies showed
that greening symptomatic leaves had lower
levels of Calcium (Ca), Magnesium (Mg), and Zn
compared to asymptomatic leaves, and
potassium (K) increased in symptomatic leaves.
Recent studies in Florida by a number of
UF/IFAS researchers have confirmed these
nutrient deficiencies in greening infected trees
in Florida. However, it has also been
determined that a number of these deficiencies
may be artifacts of the analysis because of the
high levels of starch that accumulate in the
leaves of HLB-infected trees.

When the analyses are corrected for the high
levels of starch, the changes in K, Ca, Mg, and B
associated with greening infection are real, and
show up consistently across HLB infected
groves. Changes in Zn are not consistent and
appear to be due to dilution caused by the high
starch content of HLB infection. It is likely that
these changes in K, Ca, Mg, and B are from
restrictions of nutrient uptake, transport, or
metabolism induced by HLB infection, based on
the role these nutrients play in plant
physiology. These consistent changes in
specific nutrients lead one to question whether
remedial foliar applications of these nutrients
can reduce the affects of HLB, and prolong tree
health and productivity.

Anecdotal evidence from one commercial citrus
grove in southwest Florida suggests that
remedial nutrient applications may sustain
symptoms of HLB-infected trees in the
short-term. IFAS is currently conducting at
least four field trials in various locations around
the state to attempt to replicate those results


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IFAS


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and determine exactly what nutrients/products
are effective, and how tree health, growth,
yield, bacterial titers, and disease spread are
affected. None of these trials is complete at
this time, so a definitive answer is still down
the road. In addition to field trials, a detailed
greenhouse trial is underway to determine the
effects of specific plant nutrients on HLB
infection under controlled conditions.

Yield data collected from the southwest Florida
grove Last year indicates that infected trees
produce smaller fruit compared with healthy
trees and that total yield on a per tree basis is
reduced. However, almost without exception,
the infected trees sampled were smaller than
the healthy trees, and when yield (total weight
of fruit) was expressed on a canopy volume
basis to correct for tree size, infected trees had
a similar yield to healthy trees. This indicates
that yield loss due to HLB infection may be a
result of poor tree growth and, thus, less fruit
producing wood. However, since we do not
have data from when the trees became
infected, it is possible that the infected trees
were smaller and weaker to begin with. This
leads to the question of whether particular
trees are more susceptible to infection, either
because they are more attractive to psyllids or
because they are weaker, compared to other
trees.

Questions that must be answered about this
strategy for dealing with HLB are numerous.
First and foremost, does HLB spread more
quickly in a grove where infected trees are
managed and not removed compared to a
grove with tree removal, assuming that psyllid
control is equivalent in the two situations? That
leads to questions about whether psyllids are
more attracted to infected trees compared to
healthy trees. Additionally, do remedial foliar
nutrient applications alter some aspect of tree
physiology that in turn affects psyllid feeding
preferences? These are just a few critical
questions that researchers must work towards
answering in order to determine if a plant
nutrition strategy is a viable option for
managing HLB in Florida citrus.


One thing that is for certain, regardless of the
management approach you take in your grove:
psyllid control is critical. All management
strategies for this disease are doomed to fail if
psyllid populations are not controlled to every
extent possible.

For questions and further details, please
contact Dr. Tim Spann: 863-956-1151,
spann@ufl.edu

Research update on new tools being
investigated for citrus leafminer control
Lukasz Stelinski

Research has continued on the development of
effective control tools for the citrus leafminer
(CLM). One of the main thrusts of this project
has been to develop pheromone based control
strategies for this pest that will serve as
alternatives to insecticides and that should be
comparable or better than insecticides in terms
of efficacy and cost. In this area, we have been
investigating a pheromone mating disruption
technology and a pheromone attract-and-kill
technology. Most recently, we developed and
evaluated the attract-and-kill formulation,
termed MalEx, for control of CLM. MalEx is a
viscous paste with UV-protective properties
that is dispensed as small (50 pIl) droplets using
custom-made calibrated pumps. MalEx is
manufactured by a company based in New York
State called Alpha Scents. The attract-and-kill
formulation is applied to tree foliage as small
droplets which release pheromone that is
highly attractive to males. Attract-and-kill
formulations work by attracting insects to small
droplets of the formulation. As the pest insects
touch the droplets, they obtain a lethal dose of
toxicant upon contact and die. A formulation
containing the CLM pheromone and 6%
permethrin was found to suppress CLM
populations in the field. Continuous treatment
of 1.2 acre blocks of citrus with MalEx over the
course of 112 days reduced larval infestation of
new leaf flush by 3.6-7.2 fold. We are currently
investigating whether other insecticides may be
more effective than permethrin and working to
extend the longevity of the formulation.
Control of CLM with MalEx should reduce the
number of required pesticide sprays for


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Ur FLORIDA
IFAS


October / Noe br2


leafminer management in both field and citrus
nursery settings. Alpha Scents is looking into
the process of registering the product for
Florida citrus.

For our ongoing mating disruption studies, we
have recently determined the most effective
dosage of pheromone and the exact blend of
pheromone components that are optimal for
CLM control. This work was completed with
USDA-ARS collaborators in Ft. Pierce. This has
effectively completed our research and
development work on optimizing this product.
This work has been conducted with an industry
collaborator (ISCA Technologies, Inc.) who is
developing and registering this pheromone
mating disruption product named SPLATTM for
CLM management in Florida. The SPLAT-CLM
formulation consistently reduces leafminer
infestation and population densities. Our
ongoing investigations will continue to develop
mechanized sprayers for optimizing
deployment of this product. ISCA Technologies
is in contact with the Florida citrus industry and
working to get their mating disruption product
registered for use in Florida citrus. Also, ISCA
continues work on decreasing the cost of
synthesizing the pheromone to make it more
economically accessible. Although it is difficult
to predict how long the registration process will
take, we are hopeful that this product will be
available for commercial use soon.

Finally, we have also been investigating new
pesticide tools for citrus leafminer control.
Over the past two years, we have tested a
product from Dow AgroSciences by the name
Intrepid 2F (methoxyfenozide), which is an
insect growth regulator that targets moth
pests. We have found that Intrepid 2F is
equally or more effective than the best current
insecticides that we have available for
leafminer control in citrus. Dow is in the
process of working to get this product
registered for use in Florida citrus. This
insecticide should be a valuable additional tool
for leafminer control. It represents a new
mode of action as compared with the current
leafminer pesticides we have available, so it will
help prevent development of resistance. Also,
given that we only have a couple of effective


leafminer pesticides available currently,
additional new tools are needed and will be
helpful.

For questions and further details, please
contact Dr. Lukasz Stelinski: 863-956-1151,
stelinski@ufl.edu

Five take-home messages for managing
citrus canker on processing oranges
-Jim Graham and Megan Dewdney

Canker-induced, premature fruit drop is of
great concern for processing orange varieties.
Infections that occur early in the season
(April-July) often escape growers' attention
until September-October when the lesions
range from the diameter of a pencil to the size
of a dime. Lesions at this time of year have
conspicuous yellow halos due to production of
ethylene which, more importantly, induces
premature fruit drop.

1) The first take-home message concerning
canker on oranges is that the earlier the
variety's maturity, the more susceptible the
fruit to infection. For example, Hamlin is much
more susceptible than Midsweet or Valencia
orange. Early varieties with better color, such
as Early Gold, are even more susceptible.

2) The second take-home message relates to
the most susceptible age class of the tree.
More new flush per tree canopy volume occurs
on younger fruiting trees. Repeated vigorous
flushes are extremely vulnerable to leafminer
damage. Infection of this susceptible, wounded
tissue promotes explosive increase in leaf
inoculum. The rapid buildup of canker on
leaves puts the developing fruit at great risk for
infection. Hence, leafminer control is essential
for canker control on younger trees, especially
more susceptible early oranges.

3) The third take-home message concerns the
period of greatest fruit susceptibility. For
oranges, this occurs when the fruit reaches % to
% inch in diameter and continues until fruit is
about 1% inches in size. Young Hamlin groves
experience the heaviest fruit drop. When
canker inoculum is well established from the


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October / N


previous season, rains in April, May, or June
promote early season infection. The rind is
susceptible throughout the entire period of
fruit growth but becomes more resistant with
time. Hamlin infections that occur after
mid-July do not appear to lead to premature
fruit drop.

4) The fourth take-home message follows from
the third. Initial timing for protective copper
sprays is related directly to fruit size and
thereafter can be related to the calendar. The
initial spray for Hamlin and other oranges
should be targeted to fruit % to % inch size and
continue every 21 days until the fruit reaches
12 inches in size, which usually occurs by mid-
to late July. Copper sprays of less susceptible
orange varieties may end in early to mid-July.
Hence, the number of sprays for Hamlin can


number 4-5, while those for mid- and late-
season varieties need not exceed 3.

5) The final take-home message relates to the
question of late season inoculum suppression -
Is it necessary? So far, it appears that much of
the leaf inoculum falls harmlessly to the ground
before the following spring fruit are present.
The concept of starting the new season with
inoculum reduced as much as possible is a good
one, but observations from groves with
well-established canker is that just a few
ill-timed early rains can cause very rapid
inoculum development on leaves and
devastating early infection of fruit. Well-
targeted early copper sprays are most critical
and beneficial for canker control the remainder
of the season.


TAW


Hamlin tree with fruit drop associated with canker infection in September 2009.

For questions and further details, please contact Dr. Jim Graham: 863-956-1151,
jhgraham@ufl.edu

Related publications:

Dewdney, M. and Graham, J. 2009. Cooperative producers keep greening disease under
control: A glimmer of hope on the HLB front lines. Citrus Industry Magazine 90(9):5-8.

Castle, B. and Rouse, B. 2009. Practical tips for establishing windbreaks. Citrus Industry
Magazine 90(10):19-21.


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Ur FLORIDA
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