HLB (Greening) Disease Research Update Steve Futch, Extension Agent, Multi-County, Lake Alfred Tim Gaver, Extension Agent, Ft. Pierce
History of HLB • 1919 -First reported in China • 1937 reported in South Africa • 2004 reported in Brazil * • 2005 reported in Florida * • 2008 reported in Louisiana * may have been present in location for 5-10 years after the introduction of pathogen
Several Forms of the Disease Bacterial type Location Asian Brazil, FL, China ‘ Candidatus Liberibacter asiaticus’ African South Africa ‘ Candidatus Liberibacter africanus’ American Brazil ‘ Candidatus Liberibacter americanus’ Why is the form of the bacteria important?
Vector of HLB (Greening) Form of HLB Vector Environmental (Greening) Factors Asian Diaphorina citri Heat tolerant (Brazil, FL, China) African Trioza erytreae Heat sensitive (South Africa) American Diaphorina citri Heat tolerant (Brazil) ← Adult Asian citrus psyllid and nymphs Trioza erytreae →
Alternative Hosts Orange jasmine – Murraya paniculata Box orange – Severinia buxifolia
About HLB (Greening) • Caused by a phloem-limited bacterium that has not been cultured. The phloem transports sugars bidirectionally through the plant. • Attacks the phloem of the citrus tree • In Florida, vectored by Diaphorina citri, Asian citrus psyllid • Can be spread with infected plant material • Can’t be spread by humans, animals, equipment, wind or rain
Difficult to Manage • HLB is difficult to manage due to: – Nonspecific nature of disease symptoms – Prolonged latency of the disease in field trees (9 months to 2 years or longer) – Younger trees show symptoms faster than mature trees (frequency of flushing and growth of tree?) – Probable irregular distribution of pathogen in tree – Effect of environment (esp. temperature) on symptom expression and possible bacterial multiplication
Difficult to Manage – Probable variations in tolerance to the bacterium by variety – Fastidious nature of the bacterium – Dependent on internal grove management as well as reduction of external inoculum sources (residential sources, adjacent groves, abandoned groves, i.e. the neighbor impact) – Trees may live for 5-8 years after infection and produce crops of limited economic value during noted period while serving as inoculum source
Diagnosis of HLB • Visual symptoms • Biological indexing • PCR based detection
Mason Mason Mottle Symptoms General Mottle on Top of Leaf Mason Strong Positive Mottle & Yellowing Citrus Greening Mason Symptoms Presented by: Dr. Phillip Mason, Plant Pathologist USDA, APHIS, PPQ Possible Symptoms Mottle Fades Through to Leaf Underside Mason Mason Mason Premature Fruit Drop Lopsided Fruit on Tree HLB Yellowing Symptoms on Pummelo in Decline
Genetic Resistance to HLB • No good source of genetic resistance currently known
Management of HLB • Dependent on prevention and reduction of inoculum in the field – Achieved through the use of disease-free planting material – Control psyllid population in groves – Timely removal of infected trees – Keep trees nutritionally healthy
HLB Symptoms • Develop in about 20% of grafted plants within 3-12 months of graft inoculation under greenhouse conditions • Latency period of the disease in field conditions is not clear • Information about the interval between psyllid inoculation of Ca. L. asiaticus in a field tree and the time when other psyllids can acquire the HLB bacteria from that tree is not known
Detection of Bacteria in Vector • PCR is used to indicate if vectors are positive for the bacteria • D. citri can retain the bacteria for up to 12 weeks suggesting the bacteria is replicating in the psyllid • Higher bacteria titer has been found in infected plants in the fall • Bacteria has been confirmed in the nymphs (3-5 th instar stages) • Transovarial transmission of Ca. L. asiaticus has not been detected in D. citri but has been in T. eryteae • Estimates indicate acquisition time ranges from 30 minutes to hours from feeding on infected plants • Estimates indicate adults able to transmit the pathogen after 8-12 days
Impact of Bacteria in the Tree • Various nutrients are deficient in greening infected trees (Zn, Mn, Fe) • Starch is significantly higher (3-4x) in greening infected leaves as compared to healthy leaves • Better plant nutrition will not cure greening, but may slow the disease progression • Will reduce tree life and fruit production
Greening Impacts on Fruit • Fruit from infected trees: – Smaller, misshapened, lopsided – Increase fruit drop prior to harvest – May reduce yields up to 25% in a single year study (due to smaller size and fruit drop (Oswalt, Spann) • Full impact on fruit quality is unknown, may have off flavor
Citrus Industry Opportunities?
Current Research Projects • Understanding the time between feeding on infected tree and the ability to transmit the disease • Understanding the latency period within the tree and vector • Know the bacteria is present in the root system, could assume it may be able to be transmitted to adjacent tree by root grafting • Conducting studies to determine psyllid transmission times in greenhouse • Determining if the adult psyllid can pass along the bacteria to offspring (transovarial) • Trying to understand the relationship between trees that exhibit greening symptoms but do not test positive with PCR tests
Current Research Projects • Understand the bacteria levels within the plant with time • Orange jasmine does not show symptoms of greening • Understanding on location where it is best to test the tree • Alternative hosts for psyllids are not all equally susceptible to greening • Working on culturing the bacteria – progress is slow • Gene sequencing is nearly completed • Progress on isolating compounds of guava that act as a repellent • 90% of new infections occur within 125 feet of infected tree (Source: UF, USDA and field observations)
Current Research Projects • It is know that sprouts from stumps of HLB positive trees can later be found to be contain HLB via PCR tests • Remedy has been approved for stump treatment and is effective when sprayed on the stump quickly after clipping • Studies will be developed to understand the impact of delaying application of Remedy by hours or days
Recommend
More recommend
