Alternative hosts and the epidemiology of potato leafroll virus in Queensland.

Potato leafroll virus (PLRV) was the most frequently detected virus in potato crops grown at a trial site in the Lockyer Valley. Queensland. Levels of infection with PLRV were significantly higher in autumn crops grown from one-off certified (saved) seed (maximum 51%) than in crops grown directly from certified seed (maximum 6%). Potato viruses S and X were very uncommon (maximum incidence each of 1%) and potato virus Y was not detected in these crops. Tomato, Nicandra physalodes and Physalis sp. were identified as field hosts of PLRV in Queensland and, additionally, Stachys aruensis (Labiatae), Nicotiana benthamiana, Nicotiana tabacum and Solanum americanum (Solanaceae) were shown to be experimental hosts. In field trials in the Lockyer Valley and the Redlands district, PLRV was detected in 45-65% of symptomless tomato plants growing within 300 m of the nearest source of PLRV-infected potatoes. Virus levels were lower in the leaves of potato cv. Pontiac than in those of cvv. Bison and Sebago 5 weeks after primary (aphid-borne) infection, but these differences were no longer apparent after 11 weeks. Four weeks after emergence, virus levels in cv. Bison were markedly higher than in cvv. Pontiac and Sebago in plants with secondary (tuber-borne) infection. After 9 weeks, virus levels in all cultivars were similar and were greater in lower than in upper leaves.