Graft Transmission Efficiencies and Multiplication of ‘Candidatus Liberibacter americanus’ and ‘Ca. Liberibacter asiaticus’ in Citrus Plants

In Brazil ‘Candidatus Liberibacter asiaticus’ and ‘Ca. L. americanus’ cause huanglongbing (also known as greening), the most destructive citrus disease. A shift in pathogen prevalence was observed over time, with a disproportional increase in ‘Ca. L. asiaticus’ occurrence. Graft transmission experiments were used for a comparative study of both species using budsticks from symptomatic branches of field-affected trees as inoculum. The plants were inoculated with ‘Ca. L. asiaticus’ or ‘Ca. L. americanus’ alone, or simultaneously with both species. Symptom manifestation and conventional and quantitative real-time polymerase chain reaction were used for plant evaluations. ‘Ca. L. americanus’ was detected mainly in symptomatic plants and ‘Ca. L. asiaticus’ was detected in symptomatic plants as well as in infected plants prior to symptom manifestation. Transmission percentages varied from 54.7 to 88.0% for ‘Ca. L. asiaticus’ and 10.0 to 45.2% for ‘Ca. L. americanus’ in two experiments. In co-inoculated plants, 12.9% contained ‘Ca. L. americanus’ only, 40.3% contained ‘Ca. L. asiaticus’ only, and 19.3% contained both species. Average bacterial titers for ‘Ca. L. asiaticus’ and ‘Ca. L. americanus’, in log cells per gram of leaf midrib, were 6.42 and 4.87 for the experimental plants and 6.67 and 5.74 for the field trees used as the source of inoculum. The higher bacterial populations of the ‘Ca. L. asiaticus’-infected plants provided an explanation for the disproportional increase in field prevalence of this species over time, based on the greater likelihood for pathogen transmission by the insect vector.

Cultivar and Seedling Susceptibility to Pecan Bacterial Leaf Scorch Caused by Xylella fastidiosa and Graft Transmission of the Pathogen

The xylem-limited bacterium Xylella fastidiosa was detected in 20 cultivars and 4 seedling trees with pecan bacterial leaf scorch (PBLS) symptoms during a 2-year survey of commercial pecan (Carya illinoinensis) cultivars. The orchard survey and inoculation trials indicated that several of the currently recommended cultivars, as well as older, popular cultivars, are susceptible to infection by the bacterium and develop PBLS. The severity of disease development was variable between cultivars and within cultivars. Eight of 9 cultivars growing in pots that were needle-puncture inoculated with X. fastidiosa became infected and developed the disease. Currently, no cultivars have been identified that are not susceptible to infection and symptom development. All 6 seedling types that were inoculated became infected and developed PBLS. The frequency of infection of the seedlings was greater than the cultivars; 88% of the inoculated seedlings and 44% of the inoculated cultivar trees were infected. It was demonstrated that the pathogen could be transmitted through scion wood. Because pecan cultivars are clonally propagated, graft-transmission may represent a major source of pecan infection.