Managing the health and
productivity of HLB-affected
groves
Tripti Vashisth
Assistant Professor and Citrus Extension Specialist
Citrus Research and Education Center
University of Florida
Contributors
• To all the contributors of HLB playbook
• Dr. Kelly Morgan
• Dr. Jim Graham
• Dr. Arnold Schumann
• Dr. Robert Rouse
• Dr. Gene Albrigo
• Dr. Jude Grosser
• Dr. Mongi Zekri
• Dr. Steve Futch
• Chris Oswalt
Outline
This presentation will focus on horticultural and cultural
practices that can be applied in existing groves:
• Nutrition
• Soil pH and bicarbonates
• Irrigation
• Pruning
• Fruit drop and quality
Proposed Working Model for HLB
Transmits
bacteria
Asian citrus psyllid
feeds on leaf
CLas thrives in
phloem
To fruits, flower
Phloem plugging
Tree growth
arrests and
decline
Fruit quality
lowers and fruit
drop occurs
X
Carbohydrates
To roots
To fruits, flower
Carbohydrates
X
To roots
Root loss occurs
Water +
Minerals uptake
reduces
Potential Strategies against HLB
Resistant Varieties
Transmits bacteria
Good horticultural
practices
Thermotherapy
and
Bactericide
X
Phloem plugging
X
Feeds on leaf
Psyllid control
Biotechnology
approach
Complete and
balanced plant
nutrition
To fruits, flower
To roots
Root loss and
water +minerals
uptake reduces
Well planned
irrigation
scheduling
Plant Nutrition
• Seventeen elements are essential
• Carbon (C), hydrogen (H) and oxygen (O), make up to 95% of
tree biomass
Photosynthesis
Carbohydrates
Proteins
Minerals elements
Organic compounds
Fatty acids
Mineral Nutrition
• There are 14 essential mineral nutrients
Macronutrients
Micronutrients
Nitrogen (N)
Phosphorus (P)
Potassium (K)
Calcium (Ca)
Magnesium (Mg)
Sulphur (S)
Iron (Fe)
Zinc (Zn)
Manganese (Mn)
Boron (B)
Copper (Cu)
Molybdenum (Mo)
Chlorine (Cl)
Nickel (Ni)
Liebig's law of the minimum
Mineral Nutrition
• Plant mineral nutrition is very important for a healthy and
productive tree
• When considering a new nutritional program. Keep in mind:
 A- All
 B- Balance
 C- Constant
Mineral Nutrition
 All- 17 essential elements
Example- Chlorophyll is critical for photosynthesis
Structure of chlorophyll
Magnesium
Nitrogen
 Balance is critical
Mineral Nutrition
• Foliar application of mineral nutrients is quick solution for
deficiency
• Young flush is more efficient in nutrient absorption
• Soil application is essential for long term solution
• Roots are most efficient in nutrients uptake and moving the
nutrients to other parts of plant
• Not every nutrient is mobile
Ways to Fertilize
• Conventional dry fertilizer: Not very expensive, readily available to
plant, subject to leaching, multiple applications
• Controlled release fertilizer: Relatively expensive, slowly released,
constant supply of nutrients, fewer applications
• Fertigation: Relatively inexpensive, flexibility in application, small doses
and constant supply, high maintenance, not suitable for all nutrients,
application during rainy season can be a concern
Mineral Nutrition and HLB
• HLB-affected trees have smaller and weaker root systems,
therefore, frequent small doses will be advantageous
• CRF and fertigation
• With CRF rate can be reduced by 10-20%
Mineral Nutrition and HLB
• HLB-affected sweet orange leaves showed lower concentrations
of K, Ca, Mg, Cu, Fe, Zn, Mn, and B
• Up to 3X the recommended rate of foliar application of Zn and
Mn can be beneficial
• HLB-affected trees require higher rates of some essential
mineral nutrients
Number of research trials are underway to evaluate required rate of
mineral nutrients for HLB-affected trees
Before making any changes to a fertilizer program
for HLB-affected or healthy trees, leaf and soil
nutrient analysis should be performed and taken into
consideration
Soil pH
Soil pH of the root zone
is a very critical factor
that affects nutrient
uptake especially
micronutrients
Optimum pH Range
Soil pH and Bicarbonates
• Bicarbonates increase soil pH
• High soil pH reduces availability of metal ions such as Ca,
Mg, Fe, Zn, Mn
• Rootstock sensitivity to high pH: Swingle > Carrizo > Sour
orange > Cleopatra
Soil pH, Water Quality and
Bicarbonates
• Irrigation water in Florida is often high in bicarbonates
• Irrigating with such water over long periods of time can
increase the soil pH
• Soil pH depends on irrigation water quality-the bicarbonate
concentration in the water, irrigation timing and quantity,
the buffering capacity of soil, and the rootstock variety
HLB, Soil pH and Bicarbonates
• Groves where soil pH is high and the irrigation water
contains bicarbonates - exhibit increased HLB symptoms and
severely declining trees
• Leaf and soil nutrient analysis of such groves exhibit a
reduced levels of Ca, Mg, K, and Fe
HLB, Soil pH and Bicarbonates
• Groves which have soil pH higher than 6.5 and greater than 100
ppm bicarbonates in irrigation water suffered from increased
fibrous root loss as well as reduced yields
• A clear correlation of yield loss was
and lower fibrous root density when compared
to low-bicarbonate stress trees
Fibrous root density (mg/cm3)
observed in trees under bicarbonate stress
Soil pH in the wetted zone
1.0
r2 = 0.25
0.8
0.6
0.4
0.2
0.0
4.5
5.0
5.5
6.0
Soil pH
6.5
7.0
7.5
Test soil pH and irrigation water quality (pH and
bicarbonates) regularly in wetted zone. If the soil and
irrigation water pH is greater than 6.5 and/or the
bicarbonates concentration is more than 100 ppm,
acidification should be done.
Managing Soil pH
• Water conditioning: Inject N-phuric acid or sulfuric acid
(40%) to reduce irrigation water to ≤ 100 ppm bicarbonate
• Fast acting
• Can be challenging in rainy season
• Soil conditioning: broadcast sulfur in wetted zone to reduce
soil pH
• Slow acting
• Works 24/7
Soil pH should be maintained close to 5.8 to 6.5
Irrigation
• Water is the primary contributor to the health and
productivity of citrus trees
• Water stress can result in physiological changes in tree
growth and fruit development
• Drought stress in the Fall can result in pre-mature fruit drop which can
result in reduced yields
• Drought stress can result in off season flowering. Off-season flowering
may contribute to a higher incidence of post bloom fruit drop
Irrigation and HLB
• Irrigation schedules with frequent irrigations of smaller amounts
of water benefited HLB-affected trees
• HLB-affected trees receiving small frequent doses of irrigation
displayed improvement in tree canopy density, greater leaf area,
and reduced leaf drop
• It is suggested to schedule frequent irrigation of
smaller amounts of water to manage HLB-affected
trees
Managing Root to Shoot Ratio
• HLB causes disruption of photosynthate translocation throughout the
tree as well as causes root loss
Root loss results in reduction of water and mineral nutrient uptake
Reduced roots cannot support plant growth requirements
In absence of mineral nutrients tree cannot function properly
Twig, shoot and root dieback, and tree declines
Imbalance
between root
to shoot ratio
Buckhorn pruning is not recommended as
substantial yield loss occurs in the first
four years which may not be overcome in
following years of production
Pruning Trial
• Sweet orange variety ‘Hamlin’ on
Swingle
• 15 year old trees, 100% infected
• Produced approximately 120-180 lbs
fruit/tree
• Original canopy measurement:
12 feet (H) X11 feet (W)
Treatments
• 4 Pruning treatments
1. No Pruning (12 feet)
• 2 Fertilizer treatments
1. Conventional fertilizer (CNV; 200
2. Reduced the top by 25% (9 feet)
lbs N/acre) + CREC Foliar Spray
3. Reduced the top by 50% (6 feet)
2. Controlled Release Fertilizer (CRF;
4. Severe pruning (SP) reduced by
80% (6 feet (H) X 4 feet (W))
130 lbs N/acre) + CREC Foliar
Spray
January 21, 2015
12 feet
0%
9 feet
25%
9 feet
25%
January 21, 2015
6 feet
50%
January 21, 2015
80%
Severe Pruning
Results Year 1- Yield
A substantial yield decrease in year 1
should be expected if trees are
significantly pruned
No difference in two fertilizer types was
observed
Results
Pre-harvest fruit drop
HLB symptomatic fruit
Brix
A clear relationship
Pruning resulted in more
between canopy and fruit
vegetative growth than
drop was observed
fruit growth
For Now…
Light pruning maybe beneficial to rejuvenate tree and
improve light interception as well as to remove any dead
wood
• Effective psyllid and leafminer control is very important!!
• Do not over fertilize the pruned tree, the tree cannot utilize
much fertilizer in absence of leaves
• Small and regular doses of fertilizer will be most beneficial
Fruit Quality and Pre-harvest Fruit
Drop
• The total soluble solids content (TSS) of dropped fruit ranged
from 6-8 degrees brix and the average diameter of dropped
fruit was less than 2.2 inch
• The TSS and size of the dropped fruit suggests that
carbohydrate stress to fruit is the cause
• Dropped fruit should not be picked from the ground
Pre-harvest Fruit Drop
• 2, 4-D, IAA, and GA have been shown to reduce fruit drop in
many fruit crops
• Current research shows that 2, 4-D and GA were not
consistent or effective in reducing HLB related pre-harvest
fruit drop
30.
• More research is needed
• In 2015-2016 there was less pre-harvest
fruit drop compared to previous seasons
Percent fruit drop
USDA
24.
CREC-IFAS
18.
12.
6.
0.
09-10
10-11
11-12
12-13
Year
13-14
14-15
15-16
Take home message….
Perform regular leaf
and soil nutrient
• Irrigate frequently with small doses of water
testing
• A complete balanced nutritional program should be applied
• Maintain optimum soil pH and regularly check irrigation water
quality
• Light pruning can be beneficial for tree, and should be followed
by good psyllid and leafminer control
• Minimize any abiotic and biotic stress on the trees