Seed Transmission of Turnip Mosaic Virus Demonstrated Unequivocally in a <i>Brassica</i> Species

Turnip mosaic virus (TuMV) causes important diseases in Brassicaceae crops worldwide. In 2023, <i>Brassica rapa</i> ssp. <i>perviridis</i> cv. Tendergreen seedlings with virus-like symptoms were found growing within an insect-proof glasshouse. The affected seed lot (A), three others of 'Tendergreen,' and five belonging to other <i>B. rapa</i> subspecies, <i>B. juncea</i>, or <i>Wasabia japonica</i> were sown in trays within controlled environment rooms (CERs) from which other plants were absent. TuMV was detected in 10% of seed lot A seedlings but none from other seed lots. Next, seed lot A (experiment 1) or seed from 'Tendergreen' mother plants infected with <i>B. napus</i> TuMV resistance-breaking strain isolate 12.1 (experiment 2) were sown in trays. In each experiment, these trays were subdivided into two batches, one being placed inside transparent plastic boxes, each batch then being placed in different CERs. TuMV was detected in 10 and 9% of seedlings inside or outside the boxes (experiment 1), or 1% of seedlings from both situations (experiment 2). Because virus contamination by aphid vectors or contact was excluded, TuMV seed transmission was demonstrated unequivocally. A complete TuMV genome obtained from an infected seedling (isolate BRSB1, accession PQ160044) was compared with 44 other genomic sequences from TuMV phylogroup World-B. It belonged to the same subclade as Australian resistance-breaking strain isolates 12.1 and 12.5 (99.9% nucleotide identities) and 10 New Zealand sequences (99.2 to 99.5% nucleotide identities). Our findings have important implications concerning sowing crops with TuMV-infected seed and spreading readily seedborne or resistance-breaking TuMV strains nationally or internationally.