Exogenous application of methyl jasmonate and salicylic acid on citrus foliage: Effecs on foliar volatiles and aggregation behavior of Asian citrus psyllid (Diaphorina citri)

Authors: Patt, Joseph - Joe; ROBBINS, PAUL; MCCOLLUM, THOMAS

Submitted to: International Research Conference on Huanglongbing
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2016
Publication Date: 1/1/2017
Citation: Patt, J.M., Robbins, P.S., McCollum, T.G. 2017. Exogenous application of methyl jasmonate and salicylic acid on citrus foliage: Effecs on foliar volatiles and aggregation behavior of Asian citrus psyllid (Diaphorina citri). International Research Conference on Huanglongbing. https://escholarship.org/uc/item/2cr0f2kc.

Interpretive Summary: Methyl jasmonate (MeJA) and salicylic acid (SA) are of two plant hormones that are responsible for inducing the two main chemical defense systems in plants. The SA pathway is involved in citrus response to infection by the bacterium that is thought to cause Citrus Greening disease. Less is known about the role of jasmonates in citrus defense response. We examined the effects of spray applications of MeJA and SA on the production of aroma compounds from young, growing citrus foliage, the site of Asian citrus psyllid reproduction and development. MeJA and SA were mixed in a very weak detergent solution and sprayed onto potted Valencia sweet orange trees until they were drenched. Half of the trees were healthy and the other half had Citrus Greening disease. The leaf aroma of trees treated with SA and MeJA were significantly altered. Trees treated with SA produced large amounts of methyl salicylate, an aroma that is attractive to the psyllid. Aromas that attract the psyllid’s natural enemies were emitted by foliage treated with MeJA; these trees also produced more aroma compounds that are found in the essential oils produced by the oil glands in the leaves. Additionally, sick trees that were not treated with MeJA emitted greater amounts of methyl salicylate than sick trees treated with MeJA, suggesting that MeJA may interfere with the production of methyl salicylate in sick trees. Our results suggest that MeJA application may be useful for reducing production of methyl salicylate by infected trees. MeJA application also altered psyllid behavior. In laboratory assays, similar numbers of psyllids settled on untreated control and MeJA-treated citrus sprigs. However, the numbers of psyllid aggregations (> 10 psyllids) were significantly higher on MeJA-treated than on control sprigs. Identification of the volatiles responsible for eliciting aggregation behavior could lead to the development of highly effective scent attractants for use in psyllid detection and monitoring.

Technical Abstract: Methyl jasmonate (MeJA) and salicylic acid (SA) are well-known activators of chemical defenses in plants. The SA pathway is involved in citrus response to infection by Candidatus Liberibacter asiaticus (CLas); less is known about the role of jasmonates in citrus defense response. We examined the effects of spray applications of MeJA and SA on the volatile profile of young citrus foliage, the site of Asian citrus psyllid (ACP) reproduction and development. MeJA and SA were mixed in a 0.1% aqueous Tween solution and sprayed onto potted infected (CLas+) and healthy (CLas-)Valencia sweet orange trees to the point of runoff. Control trees received 0.1% Tween solution alone. A purge and trap headspace system was used to collect volatiles for two consecutive days from the following treatments: CLas-/control; CLas-/MeJA or SA; CLas+/control; CLas+/MeJA or SA. All CLas+ trees had Ct values in the low-20’s. The volatile profile of trees treated with SA was significantly altered; the primary effect being the emission of large amounts of methyl salicylate. Treatment with MeJA significantly altered foliar volatile emission in the following ways: 1) The total amounts of volatiles increased, CLas+ trees emitting the higher amounts than CLas- trees; 2) the proportions of limonene to (E)- ß-ocimene and that of of alkyl aldehydes (C8-C12) to monoterpenes were reversed relative to control trees; 3) natural enemies attractants, such as indole and (E)-jasmone, were emitted by treated foliage; and, 4) higher amounts and a greater variety of sesquiterpenes were emitted by treated foliage. Additionally, the CLas+/control trees emitted greater amounts of methyl salicylate than CLas+/MeJA-treated trees, suggesting that MeJA may antagonize the SA pathway and biosynthesis of methyl salicylate. Citrus foliage infected with CLas emits methyl salicylate, which is a potent ACP attractant and has been implicated as the means by which the pathogen promotes its dispersal by the psyllid. Our results suggest that MeJA application may be useful for retarding production of methyl salicylate by infected trees. MeJA application also altered psyllid behavior. In laboratory assays, similar numbers of psyllids settled on untreated control and MeJA-treated citrus sprigs. However, the numbers of psyllid aggregations (> 10 psyllids) were significantly higher on MeJA-treated than on control sprigs. Identification of the volatiles responsible for eliciting aggregation behavior could lead to the development of highly effective scent attractants for use in ACP detection and monitoring.