Alferez, Fernando

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. 

Huanglongbing (HLB), which is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), is a devastating disease that affects citrus trees worldwide. Because of the pervasiveness of the bacteria and psyllid vector, the disease is considered endemic in Florida. Although the effects of CLas on tree growth and physiology have been investigated for decades, most studies compared infected and noninfected trees under greenhouse conditions. This study used newly planted field-grown ‘Valencia’ sweet orange (Citrus sinensis) trees on two different rootstocks to monitor the distribution and accumulation of CLas in aboveground and belowground tissues following natural psyllid colonization and assess tree physiological responses and biomass reductions under HLB-endemic conditions. Trees were transplanted into the field with individual protective covers (IPCs), which are used to exclude psyllids and prevent infection. Openings were cut in the IPCs of half of the trees; to promote infection, these IPCs were temporarily removed during the main vegetative flushing period when psyllid populations were high. All trees that were exposed to psyllids became infected and displayed the symptoms typically associated with HLB. Throughout the study, higher levels of CLas were detected in the leaves compared with those in the fibrous roots. Trees that were not exposed to psyllids remained noninfected and healthy. After 18 months, a subset of trees was excavated to assess biomass differences between infected and noninfected trees. Infected trees had root system reductions of 37% and shoot system reductions of 20%, thereby significantly reducing the belowground-to-aboveground biomass ratio. Fibrous root loss was 49% and more severe than the loss of the rest of the root tissue. This study is the first to demonstrate the full extent of damage caused by CLas infection under natural HLB-endemic conditions. The results confirm previous observations that suggested fibrous root loss as one of the major consequences of infection and colonization with CLas. They also reinforce the benefits of using IPCs to prevent infection of young citrus trees during the first years of growth in the field.