Lawrence Kenyon

Emergence and diversity of begomoviruses infecting solanaceous crops in East and Southeast Asia.

Over the past three decades diseases caused by whitefly-transmitted geminiviruses (begomoviruses) have emerged to be important constraints to the production of solanaceous crops, particularly tomato (Solanum lycopersicum) and peppers (Capsicum spp.), in many tropical and subtropical regions of the world. The most studied of these is Tomato yellow leaf curl virus (TYLCV), which has spread to many other areas from its likely origin in the Mediterranean basin region. The virus is usually associated with the polyphagous and virus-vectoring-efficient B-biotype of its vector whitefly (Bemisia tabaci). However, in Southeast and East Asia, a wide variety of distinct local begomovirus species have been identified from tomato and pepper crops over this period, and TYLCV was detected in Japan only in about 1996, China in 2006 and Korea in 2008, despite B-biotype whiteflies being present in several of the countries of the region since at least the early 1990s. Continental Southeast Asia appears to be a major center of diversity for begomoviruses and some species may have spread across the region; Tomato yellow leaf curl Thailand virus (TYLCTHV) appears to have spread from the Thailand-Myanmar region into southern China and is now displacing the local tomato-infecting species in Taiwan, and Tomato yellow leaf curl Kanchanaburi virus (TYLCKaV) appears to have spread from the Thailand-Vietnam region to Java, Indonesia. Since many of the native tomato- or pepper-infecting begomoviruses and associated satellite DNAs have also been detected in local weed species, it seems likely that their ancestors originated in these weed hosts, but with the expansion and intensification of tomato and pepper production in the region, there was selection for recombinant or mutant forms with greater virulence on tomato and/or pepper. Expansion and intensification of these crops may also have resulted in increased populations of local, and if present, B- or Q-biotype whiteflies, aiding the increase and spread of local begomovirus species.

Molecular diversity of poleroviruses infecting cucurbit crops in four countries reveals the presence of members of six distinct species

When 66 cucurbit samples with yellowing symptoms from fields in Mali, the Philippines, Thailand and Uzbekistan were screened by RT-PCR using universal polerovirus primers, 21 were identified as harboring polerovirus RNA. When these 21 samples were screened with specific primers for the known cucurbit-infecting poleroviruses, suakwa aphid-borne yellows virus and a recombinant strain of cucurbit aphid-borne yellows virus were detected for the first time in the Philippines and Thailand. However, seven polerovirus-positive samples did not react with any of the known species-specific primers. Sequencing of 1.4-kb universal polerovirus RT-PCR products revealed the presence of two poleroviruses that had not been described previously. These viruses, from Mali and Thailand, were provisionally named pepo aphid-borne yellows virus and luffa aphid-borne yellows virus, respectively.

Virus Diseases of Peppers (Capsicum spp.) and Their Control

The number of virus species infecting pepper (Capsicum spp.) crops and their incidences has increased considerably over the past 30 years, particularly in tropical and subtropical pepper production systems. This is probably due to a combination of factors, including the expansion and intensification of pepper cultivation in these regions, the increased volume and speed of global trade of fresh produce (including peppers) carrying viruses and vectors to new locations, and perhaps climate change expanding the geographic range suitable for the viruses and vectors. With the increased incidences of diverse virus species comes increased incidences of coinfection with two or more virus species in the same plant. There is then greater chance of synergistic interactions between virus species, increasing symptom severity and weakening host resistance, as well as the opportunity for genetic recombination and component exchange and a possible increase in aggressiveness, virulence, and transmissibility. The main virus groups infecting peppers are transmitted by aphids, whiteflies, or thrips, and a feature of many populations of these vector groups is that they can develop resistance to some of the commonly used insecticides relatively quickly. This, coupled with the increasing concern over the impact of over- or misuse of insecticides on the environment, growers, and consumers, means that there should be less reliance on insecticides to control the vectors of viruses infecting pepper crops. To improve the durability of pepper crop protection measures, there should be a shift away from the broadscale use of insecticides and the use of single, major gene resistance to viruses. Instead, integrated and pragmatic virus control measures should be sought that combine (1) cultural practices that reduce sources of virus inoculum and decrease the rate of spread of viruliferous vectors into the pepper crop, (2) synthetic insecticides, which should be used judiciously and only when the plants are young and most susceptible to infection, (3) appropriate natural products and biocontrol agents to induce resistance in the plants, affect the behavior of the vector insects, or augment the local populations of parasites or predators of the virus vectors, and (4) polygenic resistances against viruses and vector insects with pyramided single-gene virus resistances to improve resistance durability.

Differential effect of hot water treatment on whole tubers versus cut setts of yam (Dioscorea spp.)

BACKGROUND The use of thermotherapy or hot water treatment (HWT) is recommended for the management of plant-parasitic nematodes and other pathogens for a range of planting material, especially vegetatively propagated crops including yams, Dioscorea spp. The sprouting (germination) and consequent viability of yam following HWT, however, appear to be influenced by the post-treatment method of planting (whole or cut setts) and cultivar. The present study was established to evaluate the sensitivity of the most popular yam cultivars in Benin and Nigeria, West Africa, to HWT at 50-53 degrees C for 20 min. RESULTS Sprouting of both setts and whole tubers of most cultivars was affected by HWT. Across experiments, 47% of HWT material, compared with 61% of non-HWT material, sprouted over 8 weeks. When cut into setts, 41% of HWT or untreated tubers sprouted, compared with 72% of whole tubers. Whole, untreated tubers had highest sprouting rates (84%), and setts following HWT had the lowest (38%). Yam planting material was also not completely free of parasitic nematodes following HWT. The reaction to HWT or cutting was highly cultivar specific. CONCLUSION Yam cultivars vary in their sensitivity to hot water therapy. Care is therefore advised in selecting yam cultivars for HWT, especially when using cut setts.

First full-length genome sequence of the polerovirus luffa aphid-borne yellows virus (LABYV) reveals the presence of at least two consensus sequences in an isolate from Thailand

Luffa aphid-borne yellows virus (LABYV) was proposed as the name for a previously undescribed polerovirus based on partial genome sequences obtained from samples of cucurbit plants collected in Thailand between 2008 and 2013. In this study, we determined the first full-length genome sequence of LABYV. Based on phylogenetic analysis and genome properties, it is clear that this virus represents a distinct species in the genus Polerovirus. Analysis of sequences from sample TH24, which was collected in 2010 from a luffa plant in Thailand, reveals the presence of two different full-length genome consensus sequences.

Virus surveys of Capsicum spp. in the Republic of Benin reveal the prevalence of pepper vein yellows virus and the identification of a previously uncharacterised polerovirus species

Surveys were conducted in 2014 and 2015 in Southern and Northern Benin, respectively, to identify the viruses infecting peppers (Capsicum spp.). The samples were screened by ELISA for cucumber mosaic virus (CMV), pepper veinal mottle virus (PVMV), potato virus Y (PVY) and tomato yellow leaf curl virus (TYLCV). A generic reverse transcription PCR (RT-PCR) was used to test for the presence of poleroviruses. ELISA tests confirmed the prevalence of all viruses, while the RT-PCR detected pepper vein yellows virus (PeVYV) which is reported for the first time in Benin. A further, divergent polerovirus isolate was detected from a single pepper sample originating from southern Benin. Screening of samples collected from solanaceous plants during virus surveys in Mali (conducted in 2009) also detected this divergent polerovirus isolate in two samples from African eggplants. The complete genome sequence was obtained from the Mali isolate using transcriptome sequencing and by conventional Sanger sequencing of overlapping RT-PCR products. Based on the sequence characteristics of this isolate we propose a new polerovirus species, African eggplant yellowing virus (AeYV).