Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2016
Publication Date: 10/3/2016
Citation: Stover, E.W., Shatters, R.G., Gruber, B., Kumar, Moore, G.A. 2016. Influence of photoperiod duration and phloem disruption through scoring on growth, disease symptoms and bacterial titer in citrus graft-inoculated with Candidatus Liberibacter asiaticus. HortScience. 51:1215-1219.

Interpretive Summary: When citrus is inoculated with the pathogen responsible for huanglongbing (HLB, also known as citrus greening), it typically take 8-10 months to identify differences in susceptibility between citrus types. Previous observations indicated that differences in citrus susceptibility to HLB might be identified faster if trees were subjected to continuous lighting and/or if the bark of their trunks were scored. We found that continuous lighting enhanced disease progression in some types of citrus, but it made it more difficult to distinguish between HLB-susceptible and HLB-resistant citrus types. Bark-scoring enhanced early pathogen development in some citrus types but not others.

Technical Abstract: Plants inoculated with the huanglongbing (HLB)-associated bacterium, Candidatus Liberibacter asiaticus (CLas) are typically monitored for 8-10 months to identify differences in susceptibility between genotypes. A previous report indicated that continuous light accelerated development of HLB symptoms and field observations indicated that trees mechanically girdled by tags or tree ties showed greater symptoms. Therefore, an experiment was conducted assessing HLB development as influenced by light/dark periods of 12/12 and 24/0, in combination with scoring tree trunks to disrupt phloem. Sixty trees of each of three citrus genotypes (‘Kuharske’-previously shown to be HLB-resistant, rough lemon-previously shown to be HLB tolerant and ‘Valencia’-highly HLB susceptible) were bud-grafted using two CLas-infected buds (rough lemon and citron) per tree on 26 March 2012 and were placed in controlled growth chambers (one 12/12 light dark and one constant light, both chambers were 16 h at 30°C and 8 hr at 25°C) on 4 June 2012. Ten trees of each genotype in each growth room were scored 10 cm above the soil (cutting through the bark but not the wood) with a knife on 18 July 2012 and the scoring was repeated on the same trees at the same scoring wounds on 30 Aug. 2012. Trees were removed from growth rooms on 12 Dec. 2012 and subsequently maintained in a greenhouse. At two to three month intervals between June 2012 and May 2013, HLB symptoms and stem diameter at 5 cm above the soil were assessed, and three leaves per tree were collected for quantitative polymerase chain reaction (PCR) determination of CLas titer. On 29-31 Oct. 2012, chlorophyll levels were measured using a SPAD meter on all non-girdled trees. Six months after inoculation and three months following imposition of treatments, the ‘Valencia’ non-scored in the 12 hour light: 12 hour dark regime, the ‘Valencia’ non-scored trees in 24 hours of light and the ‘Kuharske’ scored trees in 24 hours of light displayed higher CLas titers than most other trees. After an additional two months, both scored and non-scored trees of all three genotypes in 24 hours of light had elevated CLas titers compared to trees in the 12 hour light: 12 hour dark regime. Development of mottle symptoms generally corresponded to CLas titers except that rough lemon displayed low mottling through eight months after inoculation. SPAD readings significantly correlated with CLas titer on the one date when tested, but r2 was very low. Growth of ‘Kuharske’ and rough lemon was enhanced, while ‘Valencia’ growth was reduced when graft-inoculated plants were maintained in continuous light. Scoring enhanced early Liberibacter development in ‘Kuharske’ when combined with continuous light, had no effect in rough lemon, and showed inconsistent effects in ‘Valencia’. While continuous lighting enhanced disease progression, it appeared to obscure differentiation between HLB-susceptible and HLB-resistant genotypes.