HUANGLONGBING (HLB) HOST
RANGE AND BACTERIAL
SPECIES DIVERSITY
Dr. Megan Dewdney
Assistant Professor and Extension Specialist
Outline
1. The bacteria that cause HLB
2. Bacterial diversity in groves
3. Citrus host range of HLB
Three Species of Bacteria Cause HLB
• Ca. Liberibacter asiaticus
– only known species in Florida
• Vectored by 2 psyllid species
– Diaphorina citri and Trioza erytreae (experimentally)
• Heat tolerant
• Found on the Indian Subcontinent, Southeast
Asia, Arabian peninsula, Brazil, Louisiana and
Florida
Ca. Liberibacter africanus
• Has not been found in Florida
• Vectored by 2 psyllid species
– Trioza erytreae and Diaphorina citri (experimentally)
• Bacterium and vector heat sensitive
• Found in many African countries, Madagascar
and Yemen at higher altitudes
Ca. Liberibacter americanus
• Newest species of bacteria that cause HLB
– Not found in Florida
• Vectored by Diaphorina citri
• Disease is not as severe as when caused by
Asian species
• Heat tolerant
• Only found in Brazil currently
• Is not found in citrus as often as in the past
New Disease of Potato –Zebra Chip
• Caused by a newly identified Ca. Liberibacter
– Identified in New Zealand
• Vectored by the potato/tomato psyllid
– Bactericera cockerelli
• Also damages tomatoes and peppers
• From South Texas to Colorado as well as
Mexico, Guatemala
Bacterial Diversity Study
186 samples
Asymptomatic grove
177 samples
HLB infected grove
Bacterial Diversity Study
Symptomatic Groves
Phylum
Class
Proteobacteria α
Proteobacteria
α –- proteobacteria
Order
Rhizobiales
Rhizobiales
Unclassified
γ - proteobacteria Enterobacteriales
Order %
91.5
91.5%
Ca. Liberibacter spp. are with in the
Bacteroidetes
Unclassified
RhizobialesUnclassified
Firmicutes
Bacilli
Bacilliales
4.5
1.1
5.6
0.5
Bacterial Diversity Study
Asymptomatic Groves
Phylum
Proteobacteria
Class
αα –- proteobacteria
Order
Rhizobiales
Rhizobiales
Unclassified
β - proteobacteria Burkholderiales
γ - proteobacteria Pseudomonodales
Xanthomonadales
Unclassified
Unclassified
Bacteroidetes
Unclassified
Unclassified
Actinobacteria
Actinobacteria
Actinomycetales
Deinococcusthermus
Deinococci
Deinococcales
Order %
29%
29
3.2
15.6
5.9
0.5
43.0
0.5
2.2
Future Directions
To look at bacterial
populations before
and after infection in:
Asymptomatic HLB infected
• Psyllids
grove
grove
• Dodder
Bacterial diversity of
roots from both
• Periwinckle
asymptomatic and
• Tobacco
HLB infected groves
HOST RANGE EXPERIMENTS
Questionable whether resistance occurs in Citrus
species and relatives
Lots of ‘tolerance’ in greenhouse, but plants
eventually die
Even if tolerant citrus species are found, tolerance
cannot be moved into commercial scions by
conventional breeding
Citrus varieties and relatives that have been
inoculated
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Citrus macrophylla (Alemow)
Valencia Sweet Orange
Flame Grapefruit
Nules clementine (mandarin)
Minneola Tangelo
Eureka lemon
Volkamer Lemon
Persian lime
Mexican lime
Citron
Carrizo
Poncirus trifoliata
Citrus halimii
Citrus indica
Citrus micanthra var. microcarpa
Citrus ambyllocarpa
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Sour orange
Meiwa kumquat
Khalily
Citrus hystrix
Swingle citrumelo
Sun Chu Sha (mandarin)
Palestine Sweet lime
Murraya paniculata
Citrus latipes
Cleopatra mandarin
Madam Vinous Sweet Orange
Duncan grapefruit
Ling Ping Yau pummelo
Hirado Buntan Pink pummelo
Siamese Sweet pummelo
Severinia buxifolia
• Research Results
– No citrus cultivar or relative found to be
resistant to HLB
– Some cultivars more tolerant to HLB
– Symptoms less severe
Cleopatra mandarin
Healthy
Infected
+++
Citrus macrophylla
Healthy
Infected
+++
Relative number of Ca. Liberibacter cells
Persian lime
Healthy
+++
Infected
Mexican Lime
+++
Citrus latipes
+
Volkamer Lemon
+++
Sun Chu Sha (mandarin)
+++
Valencia Sweet Orange
+++
Flame Grapefruit
++
Khalily/Carrizo
Conclusions
• 3 species of Ca. Liberibacter are thought to
cause HLB
• Only one species is found in Florida
• There is good association between symptoms
and Ca. Liberibacter spp.
• Citrus species vary in their symptoms of HLB
• The severity of symptoms is not always
associated with the number of Ca.
Liberibacter in a plant
Citrus Research and Education Center
Dr. Ron Brlansky
Dr. William Dawson
Dr. Nian Wang