Albrecht, Ute

Candidatus Liberibacter asiaticus (Las) is a phloem-limited bacterium associated with huanglongbing (HLB), one of the most destructive diseases of citrus in Florida and other citrus-producing countries. Natural transmission of Las occurs by the psyllid vector Diaphorina citri, but transmission can also occur through grafting with diseased budwood. As a result of the difficulty of maintaining Las in culture, screening of citrus germplasm for HLB resistance often relies on graft inoculation as the mode of pathogen transmission. This study evaluates transmission efficiencies and HLB progression in graft-inoculated and psyllid-inoculated citrus under greenhouse and natural conditions in the field. Frequencies of transmission in graft-inoculated greenhouse-grown plants varied between experiments and were as high as 90% in susceptible sweet orange plants 6 to 12 months after inoculation. Transmission frequency in a tolerant Citrus × Poncirus genotype (US-802) was 31% to 75%. In contrast, transmission of Las after controlled psyllid inoculation did not exceed 38% in any of four experiments in this study. Whereas the time from inoculation to detection of Las by polymerase chain reaction (PCR) was faster in psyllid-inoculated US-802 plants compared with graft-inoculated US-802 plants, it was similar in graft- and psyllid-inoculated sweet orange plants. HLB symptom expression was indistinguishable in graft- and psyllid-inoculated plants but was not always associated with the number of bacteria in affected leaves. The highest number of Las genomes per gram leaf tissue measured in sweet orange plants was one to four × 107 in graft-inoculated plants and one to two × 107 in psyllid-inoculated plants. Highest numbers measured in tolerant US-802 plants were one to three × 106 and two to six × 106, respectively. Compared with artificial inoculation in a greenhouse setting, natural inoculation of field-grown sweet orange trees occurred at a much slower pace, requiring more than 1 year for infection incidence to reach 50% and a minimum of 3 years to reach 100%.

Huanglongbing (HLB) is a devastating disease of citrus and threatens the citrus industry worldwide. The suspected causal agent of the disease is a phloem-limited bacterium of the genus Candidatus Liberibacter transmitted through insect vector or grafting with diseased budwood. Currently, most seed source trees for citrus rootstock propagation are located outdoors and unprotected from disease transmission. In addition, fruit from HLB-affected scion varieties in Florida containing seeds enter the commercial trade and move into other citrus-growing areas. The objective of this study was to determine how Ca. L. asiaticus infection affects seed quality and seedling development and whether the disease appears in seedlings grown from infected fruit. Two experiments were conducted involving thousands of seedlings produced from seeds from infected rootstock seed source trees and ‘Valencia’ sweet orange trees, respectively. Infection of trees and fruit with Ca. L. asiaticus significantly reduced seed weight, seed germination, and seedling height. Seedlings did not develop symptoms typical of HLB throughout the experiment. Polymerase chain reaction (PCR) analysis initially identified two of 686 rootstock seedlings and three of 431 sweet orange seedlings positive for the pathogen when they were very young. Resampling and PCR analysis of these five seedlings at older ages consistently indicated they were negative for the pathogen and none of these plants ever developed symptoms of HLB. It is suggested that Ca. L. asiaticus may have been translocated into some part of the embryo during seed development but that it was not present in cells or tissue, which permitted replication or disease development as the seedling grew.