In vitro virus elimination in three Andean root crops: Oca (Oxalis tuberosa), ulluco (Ullucus tuberosus), and arracacha (Arracacia xanthorrhiza)

Abstract

Accessions of three Andean root crops - oca (Oxalis tuberosa Molina), ulluco (Ullucus tuberosus Caldas), and arracacha (Arracacia xanthorrhiza Bancr.) were imported into quarantine in New Zealand. Using nitrocellulose membrane-ELISA, herbaceous hosts, and electron microscopy, the accessions were found to be infected with the following viruses. In oca: Arracacha virus B (AVB), Papaya mosaic virus (PapMV), and Ullucus mild mottle virus (UMMV); in ulluco: Ullucus virus C (UVC), Ullucus mosaic virus (UMV), PapMV, and UMMV; and in arracacha: Arracacha A virus (AVA). To comply with quarantine requirements the viruses had to be eliminated. Except with one accession of oca that was infected with TMV/U, this was successfully achieved using a protocol of in vitro virus elimination that combined heat and chemical therapy. Stem nodal sections of oca and ulluco and shoot tips of arracacha were established in tissue culture on a growth medium containing 50 mg/litre ribavirin. These explants underwent 10 days of continuous heat therapy of alternating 4 h light at 35°C: 4 h dark at 31°C, after which time the resultant shoots were 1 cm long and ready to be excised and grown up on growth medium without ribavirin. They were grown under normal tissue culture conditions of 24°C under fluorescent lights with a photoperiod of 16 h. The plantlets were then introduced to glasshouse conditions in preparation for field trials, analyses, and evaluations.