Olufemi J. Alabi

Enhanced Acquisition Rates of ‘Candidatus Liberibacter asiaticus’ by the Asian Citrus Psyllid (Hemiptera: Liviidae) in the Presence of Vegetative Flush Growth in Citrus

Abstract The Asian citrus psyllid preferentially feeds and exclusively reproduces on young, newly emerged flush shoots of citrus. Asian citrus psyllid nymphs feed and complete their life stages on these flush shoots. Recent studies conducted under greenhouse conditions have shown that the transmission rates of ‘Candidatus Liberibacter asiaticus’ (CLas), the putative causal agent of huanglongbing disease of citrus, are enhanced when flush shoots are present. However, it is unclear if CLas acquisition by migrant adult Asian citrus psyllids is similarly enhanced. To address this knowledge gap, cohorts of Asian citrus psyllid adults were allowed 1-wk acquisition access period (AAP) on flushing and nonflushing shoots of qPCR-tested symptomatic (CLas+) and asymptomatic (CLas–) 10-yr-old sweet orange trees under field conditions. After the AAP, they were tested for CLas by qPCR. Progeny Asian citrus psyllid adults that emerged 4 wk post-AAP were similarly retrieved and tested. Eighty percent of flushing and 30% of nonflushing CLas+ trees produced infective Asian citrus psyllid adults, indicating that flush shoots have greater potential to be inoculum sources for CLas acquisition. Concomitantly, 21.1% and 6.0% infective adults were retrieved, respectively, from flushing and nonflushing CLas+ trees, indicating that Asian citrus psyllid adults acquire CLas more efficiently from flush shoots relative to mature shoots. In addition, 12.1% of infective Asian citrus psyllid adult progeny were obtained from 70% of flushing CLas+ trees. Significantly lower mean Ct values were also obtained from infective adults retrieved from flushing relative to nonflushing trees. The results underscore the role of flush shoots in CLas acquisition and the need to protect citrus trees from Asian citrus psyllid infestations during flush cycles.

Quality Matters: Influences of Citrus Flush Physicochemical Characteristics on Population Dynamics of the Asian Citrus Psyllid (Hemiptera: Liviidae)

Studies were conducted to relate the influence of the physical characteristics, leaf nutrient content and phloem sap amino acid concentration of citrus flush shoots on the densities of various Diaphorina citri life stages. Adult D. citri preferentially selected young shoots for feeding and numbers of D. citri immatures were positively correlated with flush shoot softness. Young flush shoots had higher concentrations of macro and micro nutrients relative to mature ones and this was associated with higher densities of all D. citri life stages. All D. citri life stages were positively correlated with higher nitrogen-carbon (N:C), nitrogen:sulfur (N:S) and nitrogen:calcium (N:Ca) ratios in leaf tissue, while densities of adults were negatively related to calcium, manganese and boron levels. Concentrations of total and essential amino acids were highest in phloem sap of young expanding flush shoots in both grapefruit and lemon, but dramatically declined as flush shoots matured. The sulfur-containing amino acids cystine, methionine and taurine occurred only in younger flush shoots. In contrast, cystathionine was only present in phloem sap of mature shoots. These results clearly indicate that young citrus flush shoots are a nutritionally richer diet relative to mature shoots, thus explaining their preference by D. citri for feeding and reproduction. Conversely, tissue hardness and the lower nutritional quality of mature flush shoots may limit oviposition and immature development. The data suggest that both physical characteristics and nutritional composition of flush shoots and their phloem sap are important factors regulating host colonization and behavior of D. citri, and this interaction can impact the dynamics and spread of HLB in citrus groves.

Near-Complete Genome Sequence of Grapevine Fabavirus, a Novel Putative Member of the Genus Fabavirus

ABSTRACT A novel virus-like sequence from grapevine was identified by Illumina sequencing. The genomic organization was most similar to that of members of the genus Fabavirus. Polyproteins RNA-1 and RNA-2 of the virus tentatively named grapevine fabavirus (GFabV) shared 34 to 23% sequence identities with Broad bean wilt virus 2 (BBWV2), respectively. GFabV was successfully graft transmitted to Vitis vinifera cv. Cabernet Franc.

Identification and molecular characterization of a novel sugarcane streak mastrevirus and an isolate of the A-strain of maize streak virus from sugarcane in Nigeria

Sugarcane and maize plants showing symptoms typical of those described for the so-called “African streak viruses” (AfSVs) were encountered during field surveys conducted from February to July 2015 to document viruses infecting both crops across the northern Guinea savannah region of Nigeria. As part of this study, two categories of complete mastrevirus-like genome sequences were obtained from nine samples (maize = 2; sugarcane = 7). In pairwise comparisons, the full-length genomes of the first sequence category (2,687 nt each; maize = 2; sugarcane = 2) shared 96 to 99% identity with global isolates of the A-strain of maize streak virus (MSV-A), indicating that sugarcane may also serve as a reservoir host to MSV-A. Analysis of the complete genomes belonging to the second sequence category (2,757 nt each; sugarcane = 5) showed that they shared 42 to 67% identity with their closest AfSV relatives, thus indicating that they represent sequences of a novel mastrevirus. Both sequence categories shared 61-62% sequence identity with each other. Further analysis revealed that the novel sugarcane-infecting virus, tentatively named as sugarcane chlorotic streak virus (SCSV), arose from a putative interspecific recombination event involving two grass-infecting mastreviruses, eragrostis streak virus and urochloa streak virus, as putative parental sequences. The results of this study add to the repertoire of diverse AfSVs present in cereal and sugarcane mixed cropping landscapes in the northern Guinea savannah region of Nigeria, with implications for disease epidemiology.

A Mixed Infection of Lettuce chlorosis virus, Papaya ringspot virus, and Tomato yellow leaf curl virus-IL Detected in a Texas Papaya Orchard Affected by a Virus-Like Disease Outbreak

Severe virus-like symptoms consisting of mosaic, distortion, yellowing, and brittleness were observed on papaya plants in a 20-ha orchard in South Texas during the 2014-15 growing season. Incidence of symptomatic plants increased from ∼40 to 100% within 6 months of the outbreak; the most severely affected plants were stunted, and fruit yield and quality were reduced compared with asymptomatic plants. The orchard papaya plant virome was explored using the Illumina NextSeq 500 platform and results were validated by Sanger DNA sequencing of complete viral genomes obtained by PCR amplification. The combined results revealed the presence of Papaya ringspot virus (PRSV; Potyvirus), Lettuce chlorosis virus (LCV; Crinivirus), and Tomato yellow leaf curl virus-IL (TYLCV-IL; Begomovirus). The RT-PCR analyses of leaves from 51 randomly sampled papaya plants indicated the presence of PRSV, LCV, and TYLCV-IL in 100, 39.2, and 15.7% of the samples, respectively. Plants infected with PRSV, in combination with LCV and/or TYLCV-IL, exhibited more severe symptoms compared with plants infected with PRSV alone. Furthermore, successful whitefly-mediated transmission of TYLCV-IL and LCV was accomplished by exposing virus-free papaya seedlings to viruliferous Bemisia tabaci (Genn.) under greenhouse conditions. The results of this study document a new host record for LCV and the first successful whitefly-mediated transmission of TYLCV-IL and LCV to papaya. As a perennial crop, infected papaya serving as an over-seasoning reservoir for TYLCV-IL and LCV, presents a new challenge to viral disease management in papaya orchards.

Prunus geminivirus A: a novel Grablovirus infecting Prunus spp.

Increased use of metagenomics for routine virus diagnosis has led to the characterization of several genus level geminiviruses from tree fruit long thought to exclusively host RNA viruses. In this study, the identification and molecular characterization of a novel geminivirus is reported for the first time in Prunus spp. The virus, provisionally named Prunus geminivirus A (PrGVA), was identified by Illumina sequencing from an asymptomatic plum tree. PrGVA was subsequently confirmed by rolling cycle amplification, cloning, and Sanger sequencing of its complete genome (3,174 to 3,176 nucleotides) from an additional 18 (9 apricot and 9 plum) field isolates. Apart from the nonanucleotide motif TAATATT↓AC present in its virion strand origin of replication, other conserved motifs of PrGVA support its geminiviral origin. PrGVA shared highest complete genome (73 to 74%), coat protein amino acid (83 to 85%) and rep-associated amino acid (74%) identities with Grapevine red blotch virus (GRBV). PrGVA was graft but not mechanically transmissible. Quantitative polymerase chain reaction screening of Prunus spp. in the National Clonal Germplasm Repository collection using newly designed primers and probes revealed 69.4% (apricot), 55.8% (plum), and 8.3% (cherry) incidences of PrGVA. PrGVA is proposed as a novel member of the genus Grablovirus based on its close genome and phylogenetic relationship with GRBV.

First Report of Grapevine yellow speckle viroid 1, Grapevine yellow speckle viroid 2, and Hop stunt viroid Infecting Grapevines (Vitis spp.) in Nigeria

Grapevine yellow speckle viroid 1 (GYSVd-1), Grapevine yellow speckle viroid 2 (GYSVd-2; genus Apscaviroid), and Hop stunt viroid (HSVd; genus Hostuviroid) are three of six viroid species documented globally from grapevines (Di Serio et al. 2017). These small plant-pathogenic, non-protein coding, circular RNAs may or may not elicit disease symptoms in grapevines depending on vagaries of the environment and their presence in mixed infection with other grapevine-infecting viruses. During 2016, surveys were conducted in the Northern Guinea Savannah agroecological zone of Nigeria to document the occurrence of grapevine viruses and viroids. A total of 318 leaf tissue samples belonging to five cultivars were collected during the survey across 28 vineyards. The samples were dried under CaCl₂ at room temperature, and then shipped under USDA-APHIS-PPQ permit to Texas AM MF576417–428) shared 96 to 100% identity with several HSVd GenBank isolates. The complete GYSVd-1 RNA derived in this study (365 to 367 nt; MF576399–407) shared 96 to 99% identity with several GYSVd-1 GenBank isolates. The complete GYSVd-2 RNA derived in this study (361 nt; MF576408–416) shared 97 to 99% identity with several GYSVD-2 GenBank isolates. No discernible symptoms were associated with the presence of all three viroid species in their source grapevines. To our knowledge, this is the first report of viroids infecting grapevine in Nigeria. Studies are ongoing to further elucidate the prevalence of all three viroids in Nigerian vineyards and their occurrences in mixed infections with other graft-transmissible agents of grapevine.

COPF: Citrus orchard perimeter fencing as a strategy for reducing Asian citrus psyllid (Hemiptera:Liviidae) infestation

Citrus grove colonization by the Asian citrus psyllid (Diaphorina citri) starts from border trees. This behaviour can be exploited by placing barriers along grove borders to prevent psyllid incursion and thwart population establishment and growth. We tested the effects of psyllid‐resistant mesh fencing along grove borders for season‐long control of D. citri populations. Deployment of sticky cards at different heights revealed that most D. citri (>99%) were collected on traps placed at or below 3.7 m along grove border. Thus, psyllid‐resistant mesh fencing (3.7 m in height) was installed along the eastern and southern borders of a 3‐year‐old grapefruit grove to protect against D. citri infestations, while the west and north edges were left unprotected. A similar adjacent grove with no border fencing served as untreated control. A reduction of 55%–98% of D. citri numbers was observed in the grove with border fences relative to the untreated control. D. citri density reductions were more pronounced on the sides with border fences suggesting that the observed effects were due to the mesh fencing. Thus, border mesh fencing as a standalone approach or coupled with other management strategies, such as border sprays, could be an effective psyllid management strategy. The season‐long benefits of border fences are not limited to D. citri population control, but also significant reductions of insecticide input and nontarget effects, thus substantially mitigating the risks of secondary pest outbreaks and ensuring the sustainability of D. citri management in citrus groves.