While the wild cucurbit Ecballium Elaterium (E. Elaterium) is frequently infected by the ToLCNDV Spain strain, it is not a relevant reservoir for viral disease spread, as VIRTIGATION partners University of Catania (UNICT) and CSIC prove in a new paper published in Insects. UNICT and CSIC’s new publication also clarifies key aspects of the whitefly transmission of ToLCNDV Spain strain in zucchini crops.
Current research on the ToLCNDV Spain strain
One of the most damaging and widespread begomoviruses is the Tomato leaf curl New Delhi virus (ToLCNDV), usually transmitted by the Bemisia tabaci (i.e. Silverleaf whitefly). However, Indian researchers reported in 2018 ToLCNDV infections in the cucurbit chayote caused by the Trialeurodes vaporariorum (i.e. greenhouse whitefly), which usually transmits plant viruses other than begomoviruses like ToLCNDV. Upon their discovery of ToLCNDV in the 1990s, Indian scientists initially found that this plant virus was mainly infecting tomato crops in India. ToLCNDV was limited to the Indian subcontinent and other Asian countries until the 2012/2013 season, when Spanish scientists, including VIRTIGATION CSIC principal investigators Jesús Navas-Castillo and Elvira Fiallo-Olivé, discovered zucchini crops being infected by ToLCNDV in the southeastern provinces of continental Spain, Murcia and Almería. Soon thereafter, researchers found more ToLCNDV-infected zucchini crops in other countries of the Mediterranean basin, including Italy, Algeria, Tunisia, Morocco and Greece.
In 2016, the Spanish team including CSIC scientists Navas-Castillo and Fiallo-Olivé performed a genetic analysis of the viral isolates present in Spain, and discovered that these were closely related to those characterised in the other Mediterranean countries. They all belonged to a novel strain, named the “Spain strain”, or ToLCNDV-ES. Although investigators reported that ToLCNDV Spain strain isolates have been infecting tomato plants in Spain in the field, these infections are ultimately quite rare. The ToLCNDV Spain strain is however much more efficient at infecting zucchini and other cucurbit crops, as well as a wide range of wild plants, including the perennial wild cucurbit E. Elaterium (also known as the “squirting cucumber”). In fact, E. Elaterium is the wild plant most commonly infected by ToLCNDV Spain strain, which could suggest that it is a key host and reservoir in the epidemiology of this plant virus.
Agroinoculation and whitefly transmission experiments
In a new paper published in the Insects journal, CSIC researchers Navas-Castillo and Fiallo-Olivé joined forces with entomologists Alessia Farina and Carmelo Rapisarda from the University of Catania (UNICT) to clarify key aspects of whitefly transmission of ToLCNDV Spain strain in zucchini crops, and verify whether the wild cucurbit E. Elaterium really is a relevant reservoir for its viral disease spread. For this purpose, CSIC and UNICT researchers conducted a series of agroinoculation, whitefly transmission and virus detection experiments. Agroinoculation experiments with the ToLCNDV Spain strain used different source plant–target plant–whitefly species combinations. They detected ToLCNDV by molecular hybridization after tissue printing on nylon membranes, using specific digoxigenin-labelled DNA probes for each viral component. The transmission experiments of ToLCNDV Spain strain between the cultivated zucchini crop cv. Milenio and E. elaterium plants focused on whitefly transmission by the whiteflies Trialeurodes vaporariorum and Bemisia tabaci MED. CSIC and UNICT scientists determined virus infection by tissue-printing and molecular hybridization with digoxigenin-labelled ToLCNDV DNA probes.
Trialeurodes vaporariorum whitefly does not transmit ToLCNDV Spain strain between zucchini & E. Elaterium
In the transmission assays conducted with Trialeurodes vaporariorum using ToLCNDV Spain strain agroinfected zucchini plants, both as the source of inoculum and target plants, none of the 40 inoculated zucchini plants (i.e. two independent experiments with 20 plants each) were infected by the virus. However, when UNICT and CSIC researchers used the Bemisia tabaci MED, 13 and 11 out of 20 target zucchini plants were infected in two independent experiments, respectively. Infection was revealed by the expression of typical viral symptoms and confirmed by molecular hybridization using ToLCNDV-ES DNA probes.
Bemisia tabaci MED does not transmit TOLCNDV Spain strain efficiently between zucchini & E. Elaterium
In the transmission assays implemented with Bemisia tabaci MED using ToLCNDV Spain strain agroinfected zucchini plants as the source of inoculum and E. elaterium as target plants, only 1 out of the 20 E. elaterium plants used in two independent experiments were infected, as revealed by the absence of symptoms and negative molecular hybridization results. On the other hand, the CSIC and UNICT scientists found none of the 20 zucchini plants inoculated with the same whitefly vector by starting from infected E. elaterium plants in two independent experiments to be infected. While CSIC and UNICT investigators cannot rule out completely that E. elaterium could potentially serve as a low-efficiency reservoir when Bemisia tabaci MED vector populations are extremely high, they still deem it unlikely that E. Elaterium plays a key role in viral disease spread of ToLCNDV Spain strain.
Ultimately, the findings of VIRTIGATION UNICT and CSIC partners in their new paper in Insects can be seen as a good starting point to further advance the understanding of ToLCNDV epidemiology. Furthermore, the actors and stakeholders of the horticultural value chain can build on these new research findings in developing better control strategies to fight the devastation that the ToLCNDV Spain strain causes in cucurbit crops.
More info about UNICT and CSIC's research paper
The full version of UNICT and CSIC’s scientific publication in the Insects journal titled “Tomato Leaf Curl New Delhi Virus Spain Strain Is Not Transmitted by Trialeurodes vaporariorum and Is Inefficiently Transmitted by Bemisia tabaci Mediterranean between Zucchini and the Wild Cucurbit Ecballium elaterium” is available online here since 15 April 2023. The dataset underlying UNICT and CSIC’s peer-reviewed, open access publication is available in the VIRTIGATION Zenodo community. Find out more about VIRTIGATION’s scientific publications on our website here.
© Title image: 2016, Robert Flogaus-Faust, Wikimedia Commons, CC BY 4.0
