Conventional Citrus of Some Scion/Rootstock Combinations Show Field Tolerance under High Huanglongbing Disease Pressure

Abstract

<jats:p>The citrus disease huanglongbing (HLB) has become endemic in Florida, with estimates that greater than 80% of citrus trees are currently infected. Although there are no commercial citrus varieties with strong HLB resistance, some field tolerance has been observed in trees exposed to the disease after they were mature. There is great urgency to identify citrus which may permit economic citrus production where HLB is endemic. Therefore, the objective of this study was to assess field tolerance to HLB. To expedite the trial due to urgency, nursery trees were purchased on rootstock varieties as available. The trial included the following unbalanced scion/rootstock combinations: ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, ‘Temple/Cleopatra’, ‘Fallglo/Kinkoji’, ‘Sugar Belle/Sour Orange’, ‘Tango/Kuharske’, and ‘Ruby Red/Kinkoji’, with most comparisons based solely on scion/rootstock combinations. A randomized complete block experiment was established at Fort Pierce, FL, in Sept. 2010. All trees exhibited symptoms of HLB and tested positive for the <jats:italic>Candidatus</jats:italic> Liberibacter asiaticus (CLas) bacterium by Oct. 2012, with similar titers [directly assessed as cycle threshold (Ct) using quantitative polymerase chain reaction (qPCR)] measured for all scion/rootstocks at most sample dates, but early titer development in ‘Ruby Red/Kinkoji’ was significantly lower than several other scion/rootstocks. Across all time-points, ‘Fallglo/Kinkoji’ had the lowest rating of distinctive HLB mottling and ‘Ruby Red/Kinkoji’ had the highest rating, but ‘SugarBelle/Sour Orange’ had the highest percentage of leaves affected. After 5 years, ‘SugarBelle/Sour Orange’ and ‘Tango/Kuharske’ had the greatest overall increase in trunk diameter, and were among the healthiest in overall appearance. In Oct. 2015, ‘SugarBelle/Sour Orange’ and ‘Tango/Kuharske’ trees had significantly greater fruit load (80–88 fruit/tree) followed by ‘Temple/Cleopatra’ and ‘Fallglo/Kinkoji’ (31–35 fruit/tree) while ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, and ‘Ruby Red/Kinkoji’ produced less than 20 fruit per tree. Despite becoming infected by CLas in less than 2 years after planting, the trees continued to grow and all scion/rootstocks displayed increasing fruit production, although very low in ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, and ‘Ruby Red/Kinkoji’. Growth and fruit production in the highest performing scion/rootstocks were likely less than would be expected for healthy trees, but these results are promising, with markedly better response of some scion/rootstocks with mandarin hybrid scions compared with trees with sweet orange or grapefruit scions. Larger fully replicated trials are underway. It is noteworthy that the most pronounced HLB symptoms and higher early pathogen titer, which are the two criteria most widely used in assessing HLB resistance, were not associated with the lowest growth and cropping, and focus on early symptomatic traits when screening for resistance may obscure important disease tolerance.</jats:p>

Leaf‐disc grafting for the transmission of Candidatus Liberibacter asiaticus in citrus (Citrus sinensis; Rutaceae) seedlings

“…The rapid spread of HLB across continents and the imminent threat it poses to the survival of the citrus industry worldwide has demanded accelerated efforts to develop resistant/tolerant germplasm (Stover et al, 2016). As the number of new plants of diverse genetic makeup are produced both sexually and asexually at increasing rates, the need for faster and more efficient HLB screening tests becomes crucial (Hilf and Lewis, 2016).…”

Influence of Citrus Source and Test Genotypes on Inoculations with Candidatus Liberibacter asiaticus

“…Several studies indicate differences in HLB susceptibility among citrus cultivars and/or species, which may be sufficient to be economically significant (Stover and McCollum, 2011;Stover et al, 2015aStover et al, , 2016. However, despite the rapid and devastating spread of HLB throughout Florida, it is remarkably difficult to quickly and efficiently screen for relative resistance and tolerance to HLB.…”

Produção de plantas transgênicas de citros expressando toxina de <i>Bacillus thuringiensis</i> visando ao controle de <i>Diaphorina citri</i>, inseto vetor do agente causal do <i>huanglongbing</i> (HLB)

“…Porém, são observadas diferenças de resposta de alguns genótipos (FOLIMONOVA et al, 2009;BOSCARIOL-CAMARGO et al, 2010). Particularmente os portaenxertos Poncirus trifoliata e alguns de seus híbridos apresentam algum nível de resistência ou tolerância ALBRECHT, 2015;STOVER et al, 2016; McCOLLUM; ALBRECHT, 2017; HATCHER, 2017; BOAVA; CRISTOFANI-YALI; MACHADO, 2017).…”

“…Outra linha de pesquisa para controle da doença é através de melhoramento genético, visando ao desenvolvimento de variedades resistentes/tolerantes, seja por meio de melhoramento convencional ou transgenia HALL, 2014;BOAVA et al, 2015;ALBRECHT, 2015;STOVER et al, 2016; BOAVA; CRISTOFANI-YALY; MACHADO, 2017). Na tentativa de controlar o HLB vários grupos têm usado a transgenia para o desenvolvimento de plantas que aumentem a resposta de defesa da planta contra Candidatus Liberibacter spp.…”

“…O HLB se caracteriza por induzir sintomas sistêmicos severos e ser de rápida disseminação, causando fortes impactos na produção mundial de citros. Por este motivo, associado à ausência de ferramentas disponíveis para o controle curativo da doença, ao tipo de interação entre patógeno e hospedeiro e à ausência de variedades comerciais resistentes (STOVER et al, 2016), a comunidade científica tem concentrado seus esforços na busca por estratégias de longo prazo, abordando diferentes linhas de pesquisa que visem ao controle da doença.…”