Long-Term Field Evaluation Reveals Huanglongbing Resistance inCitrusRelatives

Abstract

Citrus huanglongbing (HLB) is a destructive disease with no known cure. To identify sources of HLB resistance in the subfamily Aurantioideae to which citrus belongs, we conducted a six-year field trial under natural disease challenge conditions in an HLB endemic region. The study included 65 Citrus accessions and 33 accessions belonging to 20 other closely related genera. For each accession, eight seedling trees were evaluated. Based on quantitative polymerase chain reaction analysis of the pathogen titers and disease symptoms, eight disease-response categories were identified. We report two immune, six resistant, and 14 tolerant accessions. Resistance and tolerance observed in different accessions may be attributed to a multitude of factors, including psyllid colonization ability, absence of pathogen multiplication, transient replication of the bacterium, lack of pathogen establishment in the plant, delayed infection, or recovery from infection. Most citrus cultivars were considered susceptible: 15 citrons, lemons, and limes retained leaves in spite of the disease status. Resistance and high levels of field tolerance were observed in many noncitrus genera. Disease resistance/tolerance was observed in Australian citrus relative genera Eremocitrus and Microcitrus, which are sexually compatible with citrus and may be useful in future breeding trials to impart HLB resistance to cultivated citrus.

Construction of high-density genetic maps and detection of QTLs associated with Huanglongbing infection in citrus

“…C Las is a heat-tolerant gram-negative bacteria, resides only in the phloem of plant hosts, and is vectored by the sap-sucking Asian citrus psyllid (ACP, Diaphorina citri ) (Wang et al , 2017a). As an obligate and insect-transmitted plant pathogen, C Las attacks all species and hybrids in the genus of Citrus as well as some closely related genera (Ramadugu et al , 2016). Species infected include sweet orange ( Citrus sinensis ), mandarin ( Citrus reticulata ), pummelo ( Citrus maxima ), citron ( Citrus medica ), grapefruit ( Citrus paradisi ), lemon ( Citrus limon ), lime ( Citrus aurantifolia ), kumquat ( Citrus japonica ), and orange jasmine ( Murraya paniculata ).…”

“…Breeding and development of HLB resistant or tolerant cultivars is widely regarded to be the most practical strategy to support long-term control of this severe disease in the field. The reports of variability for resistance or tolerance to HLB within citrus and its relatives encourages breeding and selection for resistant or tolerant genotypes (Killiny et al , 2017; Miles et al , 2017; Ramadugu et al , 2016; Richardson and Hall, 2013). Traditional breeding through crossing elite cultivars with resistant materials can achieve this objective.…”

The enemy within: phloem‐limited pathogens

“…There are no known resistant citrus varieties or scion-rootstock combinations, although some are more susceptible than others (Fan et al, 2013;Folimonova et al, 2009). Some tested citrus relatives are resistant to HLB, but it remains unclear whether this is a result of plant processes or non-colonization by HLB vectors (Ramadugu et al, 2016).…”

Bacterial Citrus Diseases: Major Threats and Recent Progress

“…Regarding the search for alternatives using plant resistance, Ramadugu et al [119] found resistant accessions of Australian desert lime (Eremocitrus glauca), curry leaf (Bergera koenigii), Japanese prickly ash (Zanthoxylum ailanthoides), Hesperethusa (Naringi crenulata), orange jasmine (Murraya paniculata), and white sapote (Casimiroa edulis), in which the pathogen replicated transiently, but could not establish in the plant. The authors suggested that accessions of Eremocitrus and Microcitrus (ranked as tolerant), which are sexually compatible with Citrus, have the potential for the generation of HLB resistant cultivars.…”