First Report of the Presence of Tomato apical stunt viroid on Tomato in Sénégal

Rapid diagnostic detection of tomato apical stunt viroid based on isothermal reverse transcription-recombinase polymerase amplification

Tomato apical stunt viroid (TASVd) is a serious threat to tomato plants that can cause a considerable yield loss. In the present study, two isothermal molecular diagnostic assays based on reverse transcription-recombinase polymerase amplification (RT-RPA) utilizing the AmplifyRP® platform for plant pathogen detection were developed. The results of this research demonstrated distinct specificity of both developed assays, AmplifyRP® Acceler8™ and AmplifyRP® XRT, expressed in the absence of any cross-reaction activity to all total RNA extracts obtained from plants infected with other pospiviroids. The RT-RPA assays detected viroid RNA in 81- and 27-fold dilutions of the original TASVd-infected crude extract for AmplifyRP® Acceler8™ and AmplifyRP® XRT, respectively. The sensitivity tests in serial water dilutions showed the ability of AmplifyRP® Acceler8™ and AmplifyRP® XRT to detect 8 and 80 fg of pure TASVd RNA transcript, respectively. The influence of crude extract on viroid RNA transcript detection was also examined and a decrease of sensitivity of approximately 100-fold for both RT-RPA assays was revealed. To our knowledge, this is the first report describing development of RT-RPA assays to detect TASVd in plants using the AmplifyRP® platform that can be further employed both in laboratory conditions and in the field for on-site diagnosis.

Possible Overestimation of Seed Transmission in the Spread of Pospiviroids in Commercial Pepper and Tomato Crops Based on Large-Scale Grow-Out Trials and Systematic Literature Review

Several outbreaks of pospiviroids have been reported in pepper and tomato crops worldwide. Tracing back the origin of the infections has led to different sources. In some cases, the infections were considered to result from seed transmission. Other outbreaks were related to transmission from ornamental crops and weeds. Pospiviroids, in particular potato spindle tuber viroid, are regulated by many countries because they can be harmful to potatoes and tomatoes. Seed transmission has been considered an important pathway of introduction and spread. However, the importance of this pathway can be questioned. This paper presents data on seed transmission from large-scale grow-out trials of infested pepper and tomato seed lots produced under standard seed-industry conditions. In addition, it presents the results of a systematic review of published data on seed transmission and outbreaks in commercial pepper and tomato crops. Based on the results of the grow-out trials and review of the literature, it was concluded that the role of seed transmission in the spread of pospiviroids in practice is possibly overestimated.

Rasta Disease of Tomato in Ghana is Caused by the Pospiviroids Potato spindle tuber viroid and Tomato apical stunt viroid

Rasta is a virus-like disease of unknown etiology affecting tomato (Solanum lycopersicum) plants in Ghana. Symptoms include stunting; epinasty, crumpling, and chlorosis of leaves; and necrosis of leaf veins, petioles, and stems. Leaf samples with rasta symptoms were collected from commercial tomato fields in Ghana in October 2012 and applied to FTA cards, and RNA extracts were prepared. Reverse-transcription polymerase chain reaction (RT-PCR) tests with primers for Columnea latent viroid, which causes rasta-like symptoms in tomato plants in Mali, were negative, whereas tests with degenerate viroid primer pairs were inconclusive. However, tomato seedlings (Early Pak 7) mechanically inoculated with RNA extracts of 10 of 13 samples developed rasta-like symptoms. In RT-PCR tests with RNA from leaves of the 10 symptomatic seedlings and primers for Potato spindle tuber viroid (PSTVd) or Tomato apical stunt viroid (TASVd), the expected size (approximately 360 bp) of DNA fragment was amplified from eight and two seedlings, respectively. Sequence analyses confirmed that these fragments were from PSTVd and TASVd isolates, and revealed a single PSTVd haplotype and two TASVd haplotypes. The PSTVd and TASVd isolates from Ghana had high nucleotide identities (>94%) with isolates from other geographic regions. In a host range study, PSTVd and TASVd isolates from Ghana induced rasta symptoms in the highly susceptible tomato cultivar Early Pak 7 and mild or no symptoms in Glamour, and symptomless infections in a number of other solanaceous species. PSTVd and TASVd isolates were seed associated and possibly seed transmitted.

Viroid-infected Tomato and Capsicum Seed Shipments to Australia

Pospiviroid species are transmitted through capsicum and tomato seeds. Trade in these seeds represents a route for the viroids to invade new regions, but the magnitude of this hazard has not been adequately investigated. Since 2012, tomato seed lots sent to Australia have been tested for pospiviroids before they are released from border quarantine, and capsicum seed lots have been similarly tested in quarantine since 2013. Altogether, more than 2000 seed lots have been tested. Pospiviroids were detected in more than 10% of the seed lots in the first years of mandatory testing, but the proportion of lots that were infected declined in subsequent years to less than 5%. Six pospiviroid species were detected: Citrus exocortis viroid, Columnea latent viroid, Pepper chat fruit viroid, Potato spindle tuber viroid, Tomato chlorotic dwarf viroid and Tomato apical stunt viroid. They were detected in seed lots exported from 18 countries from every production region. In many seed lots, the detectable fraction (prevalence) of infected seeds was estimated to be very small, as low as 6 × 10⁻⁵ (~1 in 16,000; CI 5 × 10⁻⁶ to 2.5 × 10⁻⁴) for some lots. These findings raise questions about seed production practices, and the study indicates the geographic distributions of these pathogens are uncertain, and there is a continuing threat of invasion.