Seed Transmission of Tomato Leaf Curl New Delhi Virus from Zucchini Squash in Italy

Abstract

Tomato leaf curl New Delhi virus (ToLCNDV) is a bipartite begomovirus affecting tomato cultivation on the Indian subcontinent. Recently, however, a new strain of the virus, named ToLCNDV-ES, has spread to Mediterranean countries such as Spain, Italy, and Tunisia, and occurred in Cucurbita crops, causing economic damage. Although ToLCNDV is spread by the sweet potato whitefly (Bemisia tabaci), like other begomoviruses, it has not been clear how ToLCNDV suddenly spread from the Indian subcontinent to the Mediterranean region. In 2017, ToLCNDV was diagnosed in young seedlings germinated naturally from fruits fallen in the prior year on a farm located in Giugliano in Campania, Naples, Italy, suggesting a possible role of the seeds in vertical transmission of the virus. Because sweet potato whiteflies were widespread naturally in that region, it was necessary to verify that in an artificial insect vector-free condition. Seeds were harvested from two ToLCNDV-infected zucchini squash cultivars in Naples in 2017 and 2018 to examine whether ToLCNDV can be transmitted from zucchini squash seeds to young plants. Viral DNA was amplified from these seeds and 1- to 3-week-old seedlings germinated from them with a ToLCNDV-specific primer set. According to PCR results, viral contamination was confirmed from all harvested seeds and dissemination was proven from 61.36% of tested seedling samples. Mechanical transmission from seed-borne virus-infected seedlings to healthy zucchini squash plants was also succesful, demonstrating that seedlings from ToLCNDV-infected seeds did act as inoculum. This is the first report demonstrating that ToLCNDV is a seed-transmissible virus in zucchini squash plants in Italy.

Large-Scale Seedling Grow-Out Experiments Do Not Support Seed Transmission of Sweet Potato Leaf Curl Virus in Sweet Potato

“…Seedlings tested in that study were initially germinated in a greenhouse, thereby allowing the possibility of whitefly presence and potential vector transmission. Based on our lack of evidence for seed transmission of SPLCV and that of TYLCV seed transmission studies [ 33 , 34 ], we suggest that those reports of seed transmission of other begomoviruses [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ] be re-examined. Biological assays through the large-scale cultivation of contaminated seedlings using strict conditions excluding any possible introduction of whiteflies present in the environment (not merely testing seeds or embryos for the presence of the targeted begomovirus) will provide more robust evidence for or against seed transmission.…”

“…After this first report of begomovirus seed transmissibility, several other reports of seed transmitted begomoviruses were published, including tomato yellow leaf curl virus type strain Israel (TYLCV-IL) in tomato, pepper, and soybean by the same research group [ 23 , 24 , 25 ]. In addition, several other begomoviruses reported to be seed transmissible are bitter gourd yellow mosaic virus in bitter gourd ( Momordica charantia ) [ 26 ], tomato leaf curl New Delhi virus in chayote ( Sechium edule ) [ 27 ] and zucchini squash ( Cucurbita pepo ) [ 28 ], dolichos yellow mosaic virus in lablab-bean ( Lablab purpureus ) [ 29 ], and pepper yellow leaf curl Indonesia virus in pepper ( Capsicum annuum ) in Indonesia [ 30 ]. While those reports concluded that seed transmission of various begomoviruses occurred, the studies were typically performed using a very small number of seedlings in grow-out experiments.…”

Metagenomic analyses and genetic diversity of Tomato leaf curl virus complex affecting tomato plants in Kenya

“…Since our analyses reveal clustering of isolates from geographically proximal countries, the dissemination of the ToLCArV-like isolates is likely to have occurred via virus-infected planting material or spread by cross-border spread of viruliferous white ies, leading to genetic similarity among these isolates. Although, our study did not investigate mode of virus transmission, evidence of seed transmission has recently been reported in other closely related begomovirus species from tomato [65] and other hosts [66][67][68][69]. Thus, further research is required to understand how speci c begomovirus species are spread across various borders in East Africa and to determine the epidemiological and ecological implications.…”

Pest categorisation of tomato leaf curl New Delhi virus

“…While seed transmissibility of begomoviruses is a recurring debate (Kil et al., ; Kothandaraman et al., ; Pérez‐Padilla et al., ), seed transmission was reported for a distinct isolate of ToLCNDV found in chayote ( Sechium edule L.) in Tamil Nadu, India (Sangeetha et al., ), and was recently reported for ToLCNDV in zucchini squash from Italy (Kil et al., ). Seeds germinating from leftover fruits that had fallen in the previous year developed into virus‐infected seedlings and provided evidence that seed transmission can occur.…”