Batuman, Ozgur

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is an invasive citrus pest that vectors the phloem-dwelling bacterium, Candidatus Liberibacter asiaticus (CLas), which is a putative causal agent of citrus greening disease known as Huanglongbing (HLB). In Florida, the D. citri–CLas pathosystem complex is at present considered the greatest threat the citrus industry must manage. Brassinosteroids (Brs), a relatively new class of plant hormones, are approved for commercial use in Florida citrus and are believed to delay HLB progression. Individual protective covers (IPCs), a type of psyllid exclusion mesh bags, are increasingly being adopted to efficiently protect newly planted citrus trees from HLB infection. However, IPCs typically must be removed after 2–3 years due to tree growth. With the aim of prolonging tree health after IPC removal, we are investigating the efficacy Brs in protecting and/or preventing trees from ACP and CLas infection once they are left exposed. Upon IPC removal from trees covered for 3 years, monthly Br treatments started in trees from three mandarin cultivars (Early Pride, SugarBelle and Tango) on two different rootstocks, sour orange and US-942. The IPCs effectively excluded psyllids, prevented CLas infection, and clearly improved most of the horticultural traits measured. Although this is an on going experiment, we have already found that treatment with Brs reduced CLas bacterial titer by 10-fold on average two months after IPC removal. The long-term efficacy of the treatments still needs to be assessed and will be discussed. 

‘Candidatus Liberibacter asiaticus’ (Las) is the prominent species of Liberibacter associated with huanglongbing, a devastating disease of citrus worldwide. In this study, we report the identification of an ∼8.3-kb DNA region of the Las genome containing eight putative open reading frames flanked by two inverted repeats, which was not present in the Las str. psy62 genome. Comparisons with other genome sequences established this region as a unique genetic element associated with genome plasticity/instability. Primers specific for both the presence (Las wild type) and absence (Las mutant) of this region were designed to study the population dynamics and host adaptation of the two strains. Las populations with and/or without the wild-type strain were detected and differentiated in >2,300 samples that included psyllids, periwinkle, and several species of citrus. In psyllids, although a mixed population of the wild type and mutant was observed in most samples (88%), the wild-type Las was detected alone at a rate of 11%. In contrast, none of the infected citrus plants were positive for the wild type alone, which harbored either the mutant strain alone (8%) or a mixed population of the mutant and wild type (92%). Furthermore, the dynamics of these two major Las populations varied with different citrus hosts, whereas an in-depth study on grapefruit that did not rapidly succumb to disease revealed that the population of mutant alone increased with time, indicating that the absence of this genetic element is associated with the fitness of Las in planta under the selection pressure of its host.