Tomato yellow leaf curl virus in Tomato in Texas, Originating from Transplant Facilities

Bemisia tabaci on Vegetables in the Southern United States: Incidence, Impact, and Management

Simple Summary

The sweetpotato whitefly, Bemisia tabaci, was initially discovered in the United States in 1894 but was not considered an economic insect pest on various agricultural crops across the southern and western states. After the introduction of B. tabaci Middle East-Asia Minor 1 (MEAM1) into the United States around 1985, the insect rapidly spread throughout the Southern United States to Texas, Arizona, and California. Extreme field outbreaks occurred on vegetable and other crops in those areas. The sweetpotato whitefly is now regarded as one of the most destructive insect pests in vegetable production systems in the Southern United States. The direct and indirect plant damage caused by B. tabaci has led to substantial economic losses in vegetable crops. Bemisia tabaci outbreaks on vegetables in Georgia resulted in significant economic losses of 132.3 and 161.2 million US dollars (USD) in 2016 and 2017, respectively. Therefore, integrated pest management (IPM) tactics are warranted, including cultural control by manipulation of production practices, resistant vegetable varieties, biological control using various natural enemies, and the judicious use of insecticides.

Abstract

Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is among the most economically important insect pests of various vegetable crops in the Southern United States. This insect is considered a complex of at least 40 morphologically indistinguishable cryptic species. Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) was initially introduced in the United States around 1985 and has since rapidly spread across the Southern United States to Texas, Arizona, and California, where extreme field outbreaks have occurred on vegetable and other crops. This pest creates extensive plant damage through direct feeding on vegetables, secreting honeydew, causing plant physiological disorders, and vectoring plant viruses. The direct and indirect plant damage in vegetable crops has resulted in enormous economic losses in the Southern United States, especially in Florida, Georgia, and Texas. Effective management of B. tabaci on vegetables relies mainly on the utilization of chemical insecticides, particularly neonicotinoids. However, B. tabaci has developed considerable resistance to most insecticides. Therefore, alternative integrated pest management (IPM) strategies are required, such as cultural control by manipulation of production practices, resistant vegetable varieties, and biological control using a suite of natural enemies for the management of the pest.

Integrated Management of Tomato Yellow Leaf Curl Virus and its Whitefly Vector in Tomato

Whitefly-transmitted Tomato yellow leaf curl virus (Family Geminiviridae; Genus Begomovirus) severely restricts tomato production in the Southeastern United States. Whitefly and tomato yellow leaf curl virus management studies typically investigate control tactics individually, but successful management of this pest complex more often relies on a combination of tactics. This study examined the individual and combined effects of tomato yellow leaf curl virus-resistant cultivars, insecticides, and metallic reflective mulch on whiteflies, tomato yellow leaf curl virus disease incidence, and marketable tomato yields using split-split plot trials over 3 yr. Reflective mulch significantly reduced whitefly adults and nymphs and tomato yellow leaf curl virus symptom severity in all 3 yr of the study. Reflective mulch treatments also provided greater marketable tomato yield in 2 out of 3 yr. Imidacloprid and cyantraniliprole treatments reduced whitefly adults and nymphs' establishment and marginally increased yields, but there was no significant insecticide effect on tomato yellow leaf curl virus incidence/symptom severity compared with the non-treated check. Virus-resistant tomato cultivars did not influence whitefly populations, but provided consistent reduction in virus disease incidence. Interactions between host plant resistance and insecticide treatments ranged from strongly additive in the standard white plastic mulch treatment to only marginally additive in the reflective mulch treatments in terms of enhancing tomato yields. tomato yellow leaf curl virus-resistant tomato cultivars and reflective mulch provided the bulk of the protection against tomato yellow leaf curl virus disease incidence. However, it was the combination of all the best tactics (reflective mulch, cyantraniliprole, 'Security' hyb. tomato yellow leaf curl virus-resistant) that provided the maximum increase in marketable tomato yield (2.8-fold) over the least effective combination (white mulch, no whitefly insecticide, 'FL47' hyb. tomato yellow leaf curl virus-susceptible).

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.

Ongoing geographical spread of Tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV) seriously impacts tomato production throughout tropical and sub-tropical regions of the world. It has a broad geographical distribution and continues to spread to new regions in the Indian and Pacific Oceans including Australia, New Caledonia and Mauritius. We undertook a temporally-scaled, phylogeographic analysis of all publicly available, full genome sequences of TYLCV, together with 70 new genome sequences from Australia, Iran and Mauritius. This revealed that whereas epidemics in Australia and China likely originated through multiple independent viral introductions from the East-Asian region around Japan and Korea, the New Caledonian epidemic was seeded by a variant from the Western Mediterranean region and the Mauritian epidemic by a variant from the neighbouring island of Reunion. Finally, we show that inter-continental scale movements of TYLCV to East Asia have, at least temporarily, ceased, whereas long-distance movements to the Americas and Australia are probably still ongoing.

Phylogenetic relationships, recombination analysis, and genetic variability among diverse variants of tomato yellow leaf curl virus in Iran and the Arabian Peninsula: further support for a TYLCV center of diversity

The discovery of five strains of TYLCV in Iran, including the most well-known and widespread, TYLCV-IL, spurred a detailed study of the full-length genomes of additional TYLCV field isolates and an in-depth analysis of phylogenetic relationships, extent of recombination, and genetic variability of TYLCV isolates within Iran and throughout the Arabian Peninsula. Phylogenetic analysis of complete genome sequences of TYLCV isolates from Iran and other countries revealed four monophyletic clusters could be differentiated based on geographical origin, indicating that recent dispersal of these populations (by the vector or by humans) from these four regions has occurred minimally, or not at all. Genetic analysis revealed that TYLCV-IL isolates from southern Iran possessed greater genetic variability than the northeastern isolates, a pattern that may be reflective of evolution driven by geographically dependent isolation. Similarly, isolates of TYLCV-OM originating from Oman showed greater genetic variability than TYLCV-OM variants from Iran. Major recombination events, which were detected in all strains of TYLCV had breakpoints initiating in the C1, C1/C4, C2/C3 and V1 open reading frames (ORFs) and ending at the non-coding region and the C1, C1/C2 and C3 ORFs. Hence, these regions have consistently served as hot spots for recombination worldwide during the evolution of all currently recognized isolates and strains of TYLCV.

98% identical, 100% wrong: per cent nucleotide identity can lead plant virus epidemiology astray

Short-form publications such as Plant Disease reports serve essential functions: the rapid dissemination of information on the geography of established plant pathogens, incidence and symptomology of pathogens in new hosts, and the discovery of novel pathogens. Many of these sentinel publications include viral sequence data, but most use that information only to confirm the virus' species. When researchers use the standard technique of per cent nucleotide identity to determine that the new sequence is closely related to another sequence, potentially erroneous conclusions can be drawn from the results. Multiple introductions of the same pathogen into a country are being ignored because researchers know fast-evolving plant viruses can accumulate substantial sequence divergence over time, even from a single introduction. An increased use of phylogenetic methods in short-form publications could speed our understanding of these cryptic second introductions and aid in control of epidemics.

Validation of high rates of nucleotide substitution in geminiviruses: phylogenetic evidence from East African cassava mosaic viruses

Whitefly-transmitted geminiviruses are major pathogens of the important crop cassava in Africa. The intensive sampling and sequencing of cassava mosaic disease-causing viruses that occurred in the wake of a severe outbreak in Central Africa (1997-2002) allowed us to estimate the rate of evolution of this virus. East African cassava mosaic virus and related species are obligately bipartite (DNA-A and DNA-B segments), and these two genome segments have different evolutionary histories. Despite these phylogenetic differences, we inferred high rates of nucleotide substitution in both segments: mean rates of 1.60x10(-3) and 1.33x10(-4) substitutions site(-1) year(-1) for DNA-A and DNA-B, respectively. While similarly high substitution rates were found in datasets free of detectable recombination, only that estimated for the coat protein gene (AV1), for which an additional DNA-A sequence isolated in 1995 was available, was statistically robust. These high substitution rates also confirm that those previously estimated for the monopartite tomato yellow leaf curl virus (TYLCV) are representative of multiple begomoviruses. We also validated our rate estimates by comparing them with those depicting the emergence of TYLCV in North America. These results further support the notion that geminiviruses evolve as rapidly as many RNA viruses.

Functional analysis of bipartite begomovirus coat protein promoter sequences

We demonstrate that the AL2 gene of Cabbage leaf curl virus (CaLCuV) activates the CP promoter in mesophyll and acts to derepress the promoter in vascular tissue, similar to that observed for Tomato golden mosaic virus (TGMV). Binding studies indicate that sequences mediating repression and activation of the TGMV and CaLCuV CP promoter specifically bind different nuclear factors common to Nicotiana benthamiana, spinach and tomato. However, chromatin immunoprecipitation demonstrates that TGMV AL2 can interact with both sequences independently. Binding of nuclear protein(s) from different crop species to viral sequences conserved in both bipartite and monopartite begomoviruses, including TGMV, CaLCuV, Pepper golden mosaic virus and Tomato yellow leaf curl virus suggests that bipartite begomoviruses bind common host factors to regulate the CP promoter. This is consistent with a model in which AL2 interacts with different components of the cellular transcription machinery that bind viral sequences important for repression and activation of begomovirus CP promoters.