Presence of 16SrIV phytoplasmas of subgroups A, D and E in planthopper Haplaxius crudus Van Duzee insects in Yucatán, Mexico

Acquisition and Transmission of the Lethal Bronzing Phytoplasma by Haplaxius crudus Using Infected Palm Spear Leaves and Artificial Feeding Media

Lethal bronzing (LB) is a fatal palm disease caused by the phytoplasma 'Candidatus Phytoplasma aculeata'. This disease causes significant economic losses in palm industries and landscapes. The American palm cixiid, Haplaxius crudus, recently was identified as the vector of the phytoplasma. However, knowledge about LB phytoplasma transmission is limited due to the lack of a method to generate phytoplasma-infected insects in the laboratory. In this study, the acquisition and transmission of the LB phytoplasma by H. crudus were investigated. Successful acquisitions of the phytoplasma by H. crudus were observed at 2 days acquisition access period on LB-infected palm spear leaves. Analyses revealed increased phytoplasma infection rates of H. crudus with longer acquisition access periods and latent periods. A significantly higher phytoplasma infection rate was shown after various acquisition access periods and latent periods than the infection rate of the field-collected H. crudus population. Transmission of the phytoplasma from LB-infected spear leaves to sucrose media by H. crudus also was observed using digital PCR assays. These results further support the vector status of H. crudus and offer valuable information to understand LB phytoplasma transmission. Additionally, these results generate a critical baseline for future LB phytoplasma-vector research by providing a way to generate vectors with high phytoplasma infection rates in the laboratory setting.

Differentiation of Palm-Infecting Phytoplasmas in the Caribbean Basin Using High Resolution Melt Curve Analysis of the secA Gene

Palm lethal decline is a disease that is always fatal to infected palm hosts and is caused by three species of phytoplasma in the Caribbean basin: 'Candidatus Phytoplasma palmae', 'Ca. P. aculeata', and 'Ca. P. hispanola'. Movement of these pathogens throughout the Caribbean has been documented since their discovery in Jamaica. Over time, means of confirming infections in palms have improved. Current protocols utilize quantitative PCR (qPCR) for rapid amplification and distinction of these phytoplasmas using TaqMan probes and high-resolution melt-curve analysis (HRMA) of the 16S rRNA gene. These assays either do not detect all three phytoplasmas (HRMA) or do not distinguish between the three (TaqMan). In this study, a new qPCR-HRMA assay is developed that amplifies and distinguishes all three phytoplasmas currently known to kill palms in the Caribbean. Efficiency for the primer set secA614_F/secA759_R was shown to be consistent for all species at each concentration and yielded distinct melting temperature ranges for amplicons of 'Ca. P. palmae' (73.3 to 73.4°C), 'Ca. P. aculeata' (72.9 to 73.0°C), and 'Ca. P. hispanola' (73.5 to 73.6°C). This assay is a useful new tool not only for diagnostics that will contribute to monitoring and management programs, but it will also aid in basic research by allowing rapid screening of large samples in the context of vector surveys or identification of reservoir hosts.

Detection of 16SrIV-A phytoplasma DNA in Colpoptera sp. (Hemiptera: Nogodinidae) insects in Yucatan Peninsula, Mexico

Lethal yellowing (LY) is a disease that affects coconut and other palm species. It is associated to phytoplasmas of the group 16SrIV and the only reported insect vector for this pathogen so far is Haplaxius crudus. H. crudus is present in Mexico and has been associated to 16SrIV phytoplasmas, however, it was not detectable during a LY outbreak in the coast of Yucatan, Mexico, suggesting the existence of other vector species. To test this hypothesis a survey of insects was carried out and a total of 3074 insects were captured during a year of monthly sampling. Ten taxonomic orders were identified in this sample, Hemiptera being the most abundant (N=2094), and these were classified into nine families. The leafhopper Colpoptera sp. from to the Nogodinidae family was de most abundant representing 56% of the total number of insects sampled and 23% of these samples resulted positive for LY phytoplasma by PCR detection. The BLAST comparison, virtual RFLP and phylogenetic analyses of the sequenced amplicons relate the detected phytoplasma to the subgroup 16SrIV-A. The findings presented herein suggest that Colpoptera sp. could be considered as a new putative vector of the LY-causing phytoplasmas in Mexico and a candidate for further research.

In vitro transmission of 16SrIV phytoplasmas to coconut plants by Haplaxius crudus in Yucatan, Mexico

The coconut palm is an important crop worldwide. In America, it is affected by lethal yellowing (LY) disease, associated with the presence of 16SrIV ribosomal group phytoplasmas. Studies in Florida using insect-proof cages indicate Haplaxius crudus as a vector of LY phytoplasmas to palm species, including coconut. Here, an in vitro transmission system was used to verify that H. crudus collected in Yucatan, Mexico, transmit 16SrIV phytoplasmas to coconut. Three transmission trials were carried out using micropropagated coconut plants. In each trial, at least one plant was positive to 16SrIV phytoplasmas. In total, there were 4 positive plants out of 34 exposed to insects, and the phytoplasma presence was detected in root, stem, and leaf tissues. The phytoplasmas identified were 16SrIV-A and 16SrIV-D, both found in both plants and insects. In each assay where a plant was positive for either 16SrIV-A or 16SrIV-D, the same phytoplasma was present in the insect or insects used in this assay. This is the first demonstration of transmission of LY phytoplasmas to coconut plants by H. crudus in Mexico and with an in vitro system.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03069-z.

Genetic Variability of Palm Lethal Decline Phytoplasmas in the Caribbean Basin and Florida, U.S.A., Based on a Multilocus Analysis

Palm lethal decline phytoplasmas are an important group of plant pathogens that cause death in a variety of palm species throughout the Caribbean basin and the southeastern United States. The 16SrIV-D phytoplasma was introduced to the state of Florida, United States; it has since caused severe economic losses to the green industries of Florida and is threating natural ecosystems because of its ability to infect the native palm Sabal palmetto. In this study, the genetic variability of the 16SrIV-D phytoplasma was assessed over a 10-year period to determine if multiple introductions had occurred or if natural mutations were occurring. Furthermore, the genetic variability of the palm lethal decline phytoplasma group (16SrIV) was assessed with a multiple locus analysis using the 16S ribosomal RNA gene, the 16S-23S intergenic spacer region, and secA and groEL genes. Overall, no variability of the 16SrIV-D phytoplasma was documented in Florida over a 10-year period. The multilocus analysis showed support for three distinct species of the phytoplasma in the Caribbean basin that infect palms and further support that the 16SrIV-C from Tanzania is not closely related. Furthermore, 16SrIV-B and 16SrIV-D were found to be the same phytoplasma based on 100% identity between the two based on intergenic spacer region, secA, and groEL analysis. This study represents the first robust, multilocus analysis of palm-infecting phytoplasmas from the Caribbean and sheds light on the phylogeny and evolution of the group.

Haplaxius crudus (Hemiptera: Cixiidae) Transmits the Lethal Yellowing Phytoplasmas, 16SrIV, to Pritchardia pacifica Seem. & H.Wendl (Arecaceae) in Yucatan, Mexico

Lethal yellowing (LY) affects several palm species in the Americas. It is caused by 16SrIV group phytoplasmas. In Florida (USA), LY was shown to be transmitted by the planthopper Haplaxius crudus ( Van Duzee ) (Hemiptera, Cixiidae) to different palm species, including Pritchardia pacifica Seem . & H. Wendl . (Arecaceae) in insect-proof cage experiments in the 1980s, a result that had never been reproduced later. LY has destroyed many coconut plantations as well as other palm species in the Caribbean and Mexico. In order to evaluate if H. crudus is a vector of LY phytoplasmas in Mexico, experiments were carried out in Yucatan (Mexico). Several H. crudus from palms infected by LY in the field were introduced into cages containing young P. pacifica palms. These insects were able to transmit 16SrIV group phytoplasmas to P. pacifica palms. According to DNA sequences comparative analysis, virtual restriction fragment length polymorphism, and phylogenetic analysis, the phytoplasmas detected in these infected P. pacifica were of subgroups A and D. All of ten P. pacifica palms infected with the subgroup D phytoplasmas developed symptoms of LY and died, whereas only one of two palms infected with subgroup A developed LY symptoms and died. This is the first time, more than 30 years later, that the role of H. crudus as a vector of LY is confirmed.

Effects of Lethal Bronzing Disease, Palm Height, and Temperature on Abundance and Monitoring of Haplaxius crudus

Insect vector feeding preference and behavior play important roles in pathogen transmission, especially for pathogens that solely rely on insect vector transmission. This study aims to examine the effects of the 16SrIV-D phytoplasma, the causal agent of lethal bronzing (LB) disease of palms, on associated auchenorrhynchan insects. The numbers of auchenorrhynchans collected during weekly surveys during a yearlong study using yellow sticky traps were analyzed. The cumulative number of H. crudus was 4.5 times greater on phytoplasma-infected relative to non-infected palms. Other auchenorrhynchans showed no difference between phytoplasma-infected and non-infected palms or were greater on non-infected rather than on infected palms. Furthermore, we examined the effects of LB, palm height, temperature, and the interactive effects of these factors on H. crudus abundance. When the palms were infected with LB, at low temperature, H. crudus was more abundant on shorter than taller palms; however, H. crudus was more abundant on taller than shorter palms at the median and higher temperatures. These results may indicate that H. crudus prefers LB-infected palms over non-infected palms. The interactive effects of LB, palm heights, and temperature further suggest that vector monitoring and disease management should be optimized according to seasonal variation in temperature.