Viroid-infected Tomato and Capsicum Seed Shipments to Australia

Abstract

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.

Keywords: PSTVd; capsicum; detection; pospiviroid; seed; tomato; trade; viroid.

Figure 1

Variation in the numbers of infected seed lots detected each year. (a) The numbers of infected capsicum seed lots and (b) the numbers of capsicum seed lots tested each year by Australian laboratories, and (c,d) equivalent counts of infected tomato seed lots and numbers of tomato seed lots tested. Testing of tomato seed for Potato spindle tuber viroid (PSTVd) started in 2008 (S) but only a small fraction of imports was tested. Testing of tomato seed imports became mandatory in 2012 (M), and the full suite of tests for the viroids was introduced early in 2013, but data was not compiled for all tested seed lots until 2013. Mandatory testing of capsicum (chilli, pepper and capsicum) seed was introduced in 2013 (M).

Figure 2

Proportions of tested capsicum and tomato lots (blue and yellow symbols respectively) found to be infected each year. Testing of tomato seed for PSTVd started in 2008 (S tom). Testing of tomato seed imports became mandatory in 2012 (M tom). Testing of capsicum seed was introduced in 2013 and was mandatory from the start (M cap).

Figure 3

Detections of viroid species in capsicum and tomato seed lots by year. Numbers of detections of viroid species in capsicum (a) and tomato (b) seed lots. PSTVd (blue) and Tomato apical stunt viroid (TASVd) (red) were both detected six times in tomato seed lots in 2012, and Pepper chat fruit viroid (PCFVd) (purple) and PSTVd (blue) were both detected twice in tomato seed lots in 2016.

Figure 4

Exports of infected seed lots from major seed production regions. Numbers of infected seed lots (red bars) and the proportions of infected seed lots (pie charts), as calculated from the number of infected lots and the total known number of lots tested. Countries were identified from certification documentation that arrived with the seed lots. Seed lots were only included in this dataset when the production country data was collected.

Figure 5

Estimates of the fraction of infected seeds (red points) in PSTVd-infected seed lots detected by the EMAI (a) and CHS (b) laboratories. The size of the seed lots (x-axis) is represented by the numbers of subsamples tested from the lot. Credible intervals (blue bars) and the minimum possible best estimates given the number of subsamples tested (green dotted line) are shown. Seed lots (x-axis) were bi-modally distributed in terms of sizes (grey bars): many lots were comprised of either fewer than 4000 seeds (<=2 subsamples) or 100,000 seeds or more (50 or more subsamples).