Exogenous application of the plant signalers methyl jasmonate & salicylic acid induces changes in volatile emissions from citrus foliage & influences the aggregation behavior of ACP (Diaphorina citri), vector of Huanglongbing

Authors: Patt, Joseph - Joe; Robbins, Paul; Niedz, Randall; McCollum, Thomas; ALESSANDRO, ROCCO - Retired ARS Employee

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2018
Publication Date: 3/29/2018
Citation: Patt, J.M., Robbins, P.S., Niedz, R.P., McCollum, T.G., Alessandro, R. 2018. Exogenous application of the plant signalers methyl jasmonate & salicylic acid induces changes in volatile emissions from citrus foliage & influences the aggregation behavior of Asian citrus psyllid (Diaphorina citri), vector of Huanglongbing. PLoS One. 13(3):e0193724.

Interpretive Summary: Huanglongbing or citrus greening is a destructive citrus disease caused by a bacterium called Candidatus Liberibacter asiaticus (Las). We report on investigations into the use of two ‘signaling’ compounds to enhance plant resistance to psyllids and Las: methyl jasmonate (MJ) and salicylic acid (SA). The ‘signaling’ compounds are known to induce plant defenses against insects and pathogens. We examined whether spray applications of MJ and SA to healthy and Las-infected citrus would increase plant defenses and affect the psyllid’s behavior. Depending on several factors, we found that these compounds influenced changes in plant production of volatiles. MJ- and SA-treated trees emitted aromas at a higher rate than untreated trees, notably compounds known to attract natural enemies. Las-infected trees treated with MJ emitted lower proportions of a compound known to be a key attractant of the psyllid. Conversely, the aromas emitted from SA-treated trees were dominated by this attractant. Collectively these results suggest that MJ and SA may have potential in managing the psyllid.

Technical Abstract: Huanglongbing or citrus greening is a destructive disease that threatens citrus production worldwide; it is putatively caused by the phloem-limited bacterium Candidatus Liberibacter asiaticus (Las). Currently the disease is untreatable and control efforts focus on intensive insecticide use to control the vector, Asian citrus psyllid (Diaphorina citri). Emerging psyllid resistance to multiple insecticides has spurned investigations into the use of exogenously applied signaling compounds to enhance the citrus resistance to Diaphorina citri and Las. In the present study, we examined whether foliar applications of methyl jasmonate (MJ), a volatile signaling compound associated with the induced systemic resistance pathway, and salicylic acid, a constituent of the systemic acquired resistance pathway, would elicit the emission of defense-related foliar volatiles in citrus, and what effect this would have on the host plant searching behavior of Diaphorina citri. Comparisons were made of volatiles emitted from growing shoots of uninfected and Las-infected ‘Valencia’ sweet orange (Citrus sinensis) trees over two consecutive sampling days. A settling behavioral assay was used to compare psyllid attraction to MJ-treated v. untreated citrus sprigs. All three main effects, Las infection status, plant signaler application, and sampling day, influenced the proportions of individual volatile compounds emitted in different treatment groups. MJ- and SA-treated trees had higher emission rates than untreated trees. MJ application enhanced emission of E-ß-ocimene and indole, volatiles attractive to natural enemies, indicating that MJ application could be used in an ‘attract and reward’ conservation biological control strategy. Las-infected plus MJ-treated trees emitted lower proportions of methyl salicylate (MeSA), a key Diaphorina citri attractant, than did untreated + Las-infected trees. This suggests that MJ application could suppress MeSA emission from Las-infected trees, an approach that could help prevent psyllid colonization during shoot growth. Conversely, volatile emissions in SA-treated trees were dominated by MeSA. MJ application elicited aggregation behavior in Diaphorina citri. Similar numbers of psyllids settled on MJ-treated versus untreated sprigs, but a significantly greater percentage of the MJ-treated sprigs had aggregations of nine or more psyllids on them.