Psyllid Resistance Research Articles

A Transcriptome Analysis of Poncirus trifoliata, an Aurantioideae Species Tolerant to Asian Citrus Psyllid, Has Identified Potential Genes and Events Associated with Psyllid Resistance.

Citrus huanglongbing (HLB) is a devastating disease for citrus production, largely caused by the Asian citrus psyllid (ACP). Poncirus trifoliata exhibits high resistance to ACP; however, this resistance is weakened when C. sinensis is co-cultivated. This study aimed to identify the differentially expressed genes (DEGs) during ACP feeding and to uncover potential ACP resistance genes in P. trifoliata. In comparison to independent cultivation, 1247 and 205 DEGs were identified in P. trifoliata when co-cultivated with C. sinensis after 7 and 14 days, respectively. Analysis of enriched Gene Ontology categories revealed that DEGs were significantly associated with the cell wall, glucometabolic activities, and secondary metabolites. Additionally, these genes were found to be involved in phytohormone signaling, cell wall metabolism, redox state homeostasis, and secondary metabolites, as well as a number of transcription factor genes (TFs). Furthermore, we examined the impact of the ACP feeding factor on the gene expression patterns in P. trifoliata. Results showed an increase in the JA signaling pathway and various TFs. The RNA-seq results were verified using reverse transcription quantitative PCR. Our findings shed light on the molecular basis of ACP resistance in P. trifoliata and identified potential genes associated with this resistance.

Identification and Characterization of Potato Zebra Chip Resistance Among Wild Solanum Species

Potato zebra chip (ZC) disease, associated with the uncultured phloem-limited bacterium, Candidatus Liberibacter solanacearum (CLso), is transmitted by the potato psyllid Bactericera cockerelli. Potato ZC disease poses a significant threat to potato production worldwide. Current management practices mainly rely on the control of the psyllid to limit the spread of CLso. The present study investigated new sources of ZC resistance among wild Solanum species. A taxonomically diverse collection of tuber-bearing Solanum species was screened; one ZC-resistant accession and three ZC-tolerant accessions were identified among the 52 screened accessions. Further characterization of the resistant accession showed that the resistance was primarily associated with antibiosis effects due to differences in leaf trichome density and morphology of the wild accession, which could limit the psyllid feeding and oviposition. This germplasm offers a good resource for further understanding ZC and psyllid resistance mechanisms, contributing to potato breeding efforts to develop ZC resistance cultivars. Alternatively, it could be used as a potential trap crop to manage psyllid and control ZC disease.

Symbiotic Fungus Affected the Asian Citrus Psyllid (ACP) Resistance to Imidacloprid and Thiamethoxam.

The Asian citrus psyllid (ACP), Diaphorina citri (Kuwayama) (Hemiptera: Liviidae), is a notorious Rutaceae plant pest. Frequent and extensive use of pesticides has resulted in severe insecticide resistance in ACP populations. Fully understanding the mechanism of ACP resistance to pesticides is vital for us to control or delay the development of resistance. Therefore, we compared the difference in resistance to imidacloprid and thiamethoxam between Hunan (Yongzhou, Chenzhou) and Guangdong (Guangzhou) ACP populations and analyzed the correlations between the resistance level and genes and symbiotic fungi. The results showed that the resistance of the Guangdong ACP population to imidacloprid and thiamethoxam was lower than that of Hunan ACP population, and the relative expression of genes associated with P450 mono-oxygenase and acetylcholinesterase was significantly lower in the Guangdong ACP population than in Hunan ACP population. The differences of mean relative abundances of four symbiotic bacteria among three populations were marginally significant; however, the mean relative abundance of 16 fungi among three populations was significantly different, and positive linear correlations were observed between the resistance level and two fungi (Aspergillus niger and Aureobasidium pullulans) and two genes (CYP4C70 and CYP4DB1). Negative correlations were only observed between the resistance level and two fungi (Golubevia pallescens and Acremonium sclerotigenum). Moreover, four fungi were unique to the Chenzhou population which was the highest resistance to imidacloprid and thiamethoxam. These findings suggested the P450 mono-oxygenase and symbiotic fungi together affected ACP resistance to imidacloprid and thiamethoxam. In the future, we may use environmental G. pallescens and A. sclerotigenum to control or delay ACP resistance.

Leucaena feeding systems in India

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.Since its introduction to India in 1976 Leucaena leucocephala ssp. glabrata has spread rapidly, especially in the last couple of decades, mainly due to its use as either firewood or pulpwood. Use of its foliage for feeding livestock has been mainly a by-product of this activity. The foliage is highly nutritious because it has high protein concentration and good palatability, and the tree can withstand repeated defoliation. Research carried out by Indian scientists on leucaena has focused on mainly agronomic management, agroforestry studies of mixed cropping systems, mimosine toxicity, germplasm testing and economic evaluation. Feeding leucaena has had a positive impact on the dairy industry in particular. It is rarely purchased for fodder and is usually poached by smallholders from existing plantations, most of which are established by animal rearers for their own purposes. More widespread use of leucaena for fodder could be achieved in India by introducing varieties with either reduced seed production or sterile ones, which can be easily propagated vegetatively. In addition, psyllid resistance, suitable mechanized harvesting methods and training livestock owners in proper management of leucaena trees should help in making this high quality fodder more popular in India.

Environmental adaptation of leucaena in Western Australia – challenges and opportunities

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.There is considerable interest from Western Australian (WA) pastoralists on the potential role of leucaena (Leucaena leucocephala) in northern WA, where the potential area for dryland production of species of the genus Leucaena is high. Although it is highly regarded for animal production in other countries and in Queensland, leucaena is a contentious species since its status as an environmental weed precludes it from use on pastoral leases in the Kimberley and Pilbara regions of WA. Development of sterile/seedless forms would overcome risks of spread of the species as a weed. The key environmental constraints to growth of leucaena are likely to be the length of the dry season and low fertility of most soils other than the grey/black cracking clays (vertosols). Psyllid resistance and cool temperature tolerance are likely to be of secondary importance. Opportunities for irrigated production are also emerging and may allow leucaena species to be used in environments previously considered well outside their home-range. It is desirable now to re-examine the diversity of the wider leucaena genus for adaptation to WA conditions generally and for the purpose of selecting elite parent material for use in a sterile/seedless leucaena breeding program. These perennial species that can be under production for 30 to 40 years need to be evaluated in the target environments for at least 3‒5 years to fully understand their potential as adult plants.

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

Huanglongbing, also known as 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 efforts focus on intensive insecticide use to control the vector, Asian citrus psyllid (Diaphorina citri). Emerging psyllid resistance to multiple insecticides has generated investigations into the use of exogenously applied signaling compounds to enhance citrus resistance to D. 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 volatiles in citrus foliage, and what effect this might have on the host-plant searching behavior of D. 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 vs. Tween-treated 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 Tween-treated trees. Methyl salicylate (MeSA) and β-caryophyllene were present in higher proportions in the volatiles collected from Las-infected + trees. On the other hand, Las-infected + MJ-treated trees emitted lower proportions of MeSA than did Las-infected + Tween-treated trees. Because MeSA is a key D. citri attractant, this result suggests that MJ application could suppress MeSA emission from Las-infected trees, an approach that could be used to discourage psyllid colonization during shoot growth. MJ application enhanced emission of E-β-ocimene, indole, volatiles attractive to many of the psyllid’s natural enemies, indicating that MJ application could be used in an ‘attract and reward’ conservation biological control strategy. Volatile emissions in SA-treated trees were dominated by MeSA. MJ application elicited aggregation behavior in D. citri. Similar numbers of psyllids settled on MJ-treated versus Tween-treated sprigs, but a significantly greater percentage of the MJ-treated sprigs had aggregations of nine or more psyllids on them. Taken together, the results of this study indicate that exogenous applications of MJ or SA could be used to influence Asian citrus psyllid settling behavior and attract its natural enemies.

Efficacy of essential oil of Piper aduncum against nymphs and adults of Diaphorina citri.

Insecticide application is the main way to control Diaphorina citri. However, it causes environmental contamination, has a negative impact on beneficial organisms and leads to psyllid resistance. The essential oil of Piper aduncum has low toxicity towards the environment and contains dillapiol, which has proven to be effective against several crop pests. Here, we studied its efficacy against nymphs and adults of D. citri under laboratory conditions. Oils with three concentrations of dillapiol (69.3, 79.9 and 85.4%) at 0.5, 0.75 and 1.0% dilutions plus 0.025% adjuvant were tested. All treatments caused 90-100% mortality in nymphs. Topical treatments with oil containing 79.9 and 85.4% dillapiol at 0.75% and 1% dilutions were effective (mortality ≥80%) in adults. However, the essential oil showed no residual activity against adults (mortality ≤30%). Dillapiol-rich oil is a promising compound for D. citri control. © 2015 Society of Chemical Industry.

Growth performance of lesser-knownLeucaenaspecies/provenances at Gairo inland plateau, Morogoro, Tanzania

Growth performance and psyllid resistance was studied among nineteen lesser-known Leucaena species/provenances at Gairo inland plateau, Morogoro, Tanzania. Assessment was done at irregular intervals for survival, root collar diameter and diameter at 30 cm above the ground, height, diameter at breast height, multiple stems production, biomass and psyllid resistance. Final assessment of these tree attributes was done at 37 months after planting, while psyllid resistance was assessed at 9 and 37 months after planting. During the final assessment occasion, height ranged from 2.69 m for L. collinsii Ex. Chiapas to 4.87 m for L. diversifolia Ex. Veracruz. Diameter at breast height (Dbh) ranged from 2.26 cm for L. shannonnii Ex. Chiapas to 4.93 cm for L. diversifolia Ex. Veracruz, while multiple stems production ranged from 2709 stems ha−1 for L. pulverulenta Ex. Tamaulipas to 7135 stems ha−1 for L. leucocephala Ex. Morogoro and untransformed survival ranged from 43.75% for L. pulverulenta Ex. Tamaulipas to 100%...

Psyllid resistance in Leucaena. Part 1. Genetic resistance in subtropical Australia and humid-tropical Philippines

One hundred and sixteen accessions of Leucaena were assessed for resistance to the leucaena psyllid that has devastated stands of L. leucocephala pan-tropically since the mid-1980s. Damage to young leaf was assessed using a ratings scale, developed at the University of Hawaii, in replicated field trials at Brisbane, Australia and Los Banos, Philippines. The study identified a continuum of resistance among the accessions, from highly resistant to highly susceptible. Resistance levels were consistent between the two sites with few exceptions. L. collinsii subsp. collinsii, L. confertiflora, L. esculenta, L. pueblana, L. retusa, L. greggii and L. matudae were highly resistant at both sites, while L. leucocephala, L. lempirana, L. involucrata and L. multicapitula were highly susceptible at both sites. Other species were intermediate and were allocated to either resistant or susceptible categories. There was considerable variation in psyllid resistance within some species especially, L. trichandra, L. diversifolia and L. collinsii. Regression analysis was used to determine the influence of environmental variables on psyllid populations. Psyllid population outbreaks were restricted to a temperature range of 10degreesC minimum temperature and 33degreesC maximum temperature, but additional, unidentified factors influenced populations within this range. Leaf chemical characteristics were examined for their potential to confer psyllid resistance, but none were strongly correlated with accession damage ratings.

Psyllid resistance in Leucaena. Part 2. Quantification of production losses from psyllid damage

The psyllid insect (Heteropsylla cubana) is known to reduce biomass production in Leucaena species, but little information is available on the level of reduction, or whether a commonly used plant damage rating scale can be related to production losses. Biomass production losses due to predation by the psyllid were determined for 12 accessions of Leucaena in a randomised split-plot field experiment conducted at Brisbane, Australia. Regrowth from well-established Leucaena trees, cut back to bare stems 50 cm high, was measured over a 9-week period from plants subjected to psyllid damage and from plants with psyllids controlled by spraying with chlorpyrifos (1 mg ai/l water). Psyllid damage was scored using a ratings scale developed by the University of Hawaii. Loss of potential DM production due to psyllids ranged from 10% for L. trichandra OFI53/88 to 76% for L. collinsii ssp. zacapana OFI56/88. Production losses were significantly correlated with psyllid damage ratings for 10 of the 12 accessions, and also for a combined data set comprising these 10 accessions. From correlations for the combined data set, a 50% loss of potential DM production occurred at a psyllid damage rating of approximately 3.2. Results indicated that a "0" rating should be added to the scale to indicate the absence of psyllids.

Yield, Psyllid Resistance, and Phenolic Concentration of Leucaena in Two Environments in Hawaii

AbstractInterspecific hybridization in the genus Leucaena produces hybrid vigor, resulting in highly adaptable varieties that expand the growth range of the genus. This study was conducted to test the total biomass (combined stem and forage fraction), total forage dry matter (DM) yield, psyllid (Heteropsylla cubana Crawford) damage, and phenolic concentration of Leucaena F1 hybrids in comparison with species‐level selections in two distinctly different environments. A total of 19 Leucaena spp. and hybrid selections were tested for 2 yr at the Waimanalo and Mealani Research Stations in Hawaii. Thirteen species and hybrid selections were replicated three times and included species selections from L. pallida Britton & Rose; L. leucocephala (Lam.) de Wit; L. diversifolia (Schlechtend.) Benth. (2n = 4x); and L. trichandra (Zucc.) Urban [syn. L. diversifolia (Schlechtend.) Benth. subsp. trichandra (Urban) F.J. Pan] (2n = 2x). Additionally, five selections were interspecific F1 hybrids and two were advanced‐generation interspecific hybrids. Six other selections were augmented in the trials. Over 2 yr, L. pallida ✕ L. leucocephala F1 hybrids (K748 ✕ K636 and K748 ✕ K584) had the highest DM yield at both locations. At Waimanalo, K748 ✕ K636 produced 53.4 Mg ha−1 total DM biomass of which 24.6 Mg ha−1 was forage, while at Mealani, K748 ✕ K636 produced 29.4 Mg ha−1 total DM biomass and 9.6 Mg ha−1 forage. These yields were twice those of L. leucocephala at Waimanalo and eight times greater at Mealani. Leucaena leucocephala was the most susceptible to psyllid damage (P < 0.001), while the most resistant was L. trichandra (2n = 2x). Leucaena pallida hybrids had intermediate levels of psyllid resistance. Psyllid damage was negatively correlated with forage DM once at Waimanalo (r = −0.37, P < 0.05, n = 42). Forage phenolic concentration averaged 110 mg g−1 for all treatments at both locations and was correlated once to psyllid damage (r = 0.32, P < 0.05, n = 42) at Waimanalo. Superior yield, psyllid resistance and cold tolerance of L. pallida ✕ L. leucocephala hybrids was observed at low and high elevation sites.

Establishment, Forage Production, and Nutritive Value of Leucaena in Central Florida

AbstractTree and shrub legumes are increasingly being recommended for forage use in the tropics and subtropics. A forage production trial to evaluate members of the genus Leucaena varying in their level of resistance to the leucaena psyllid, Heteropsylla cubana Crawford, was conducted for 3 yr at the USDA‐ARS Subtropical Agricultural Research Station in central Florida. A total of eight selections (one or more of L. leucocephala, L. pallida, or L. esculenta, and hybrids L. diversifolia ✕ L. pallida, L. pallida ✕ L. leucocephala, and L. diversifolia ✕ L. leucocephala) were evaluated for establishment, winter survival, total dry matter (DM), edible DM (stems < 6 mm diam. + leaves), percentage of edible DM, and N and in vitro organic matter digestibility (IVOMD) concentration of the edible DM. Seedlings were transplanted in a double hedgerow pattern (two rows 1 m apart, with 0.25 m within‐row spacing) in Blichton loamy fine sand (Arenic Plinthic Paleaquult) in June 1989. Winter survival of the selections ranged from 48 to 62% and did not differ among selections. The three L. leucocephala selections had the highest edible DM yields averaging 14 Mg ha−1 yr−1 DM in yr 2 and 3. Nitrogen concentration ranged from 31 to 49 g kg−1 and IVOMD concentration from 350 to 560 g kg−1, with the L. leucocephala selections exhibiting the highest IVOMD. Leucaena esculenta had lower levels of both N and IVOMD concentration, averaging 37 and 380 g kg−1, respectively. Based on yield and nutritive value, L. leucocephala selections K584 and K636, although having less psyllid resistance than the hybrids or other species, appear to be better suited for central Florida.

Forage dry matter yields and psyllid resistance of thirty-one leucaena selections in Hawaii

Forage yields ofL. leucocephala (Lam). de Wit have been reduced as the result of psyllid damage sinceHeteropsylla cubana Crawford invaded the Hawaiian Islands in 1984. The forage productivity and psyllid resistance of 31Leucaena species and interspecific hybrids were assessed from 5 harvests in Hawaii during 1991 and 1992. The trial consisted of an augmented randomized complete block with 22Leucaena selections in all 4 replicates, 2 selections in 3 replicates and 7 selections in 1 or 2 replicates.

Condensed tannins in Leucaena and their relation to psyllid resistance

Leucaena species and hybrids were examined to determine the relative content of condensed tannins and its relationship to susceptibility to defoliation by the Leucaena psyllid (Heteropsylla cubana). The interspecific Leucaena hybrid KX2 (L. leucocephala × L. pallida) was examined for variability of condensed tannin levels. A broad range of condensed tannin values were found which indicate potential for improved forage value through selection for KX2 trees with lower condensed tannin while maintaining psyllid resistance. A significant correlation was not found linking condensed tannins and psyllid resistance.