EPIDEMIOLOGICAL STUDIES ON VIRUSES INFECTING NARCISSUS

Of the 13 viruses that infect narcissus in the UK, six are transmitted by aphids, probably all in the non-persistent manner, five are transmitted by nematodes, and the mode of spread of two is unknown.

In a field trial with four narcissus cultivars, two nematode-transmitted viruses, tomato black ring and raspberry ringspot, infected 10–65% and 0–10% respectively of the plants during two years' growth on infective land. In 13 field trials in 1975–1979 in which the initial incidence of the aphid-transmitted narcissus yellow stripe virus was 7% or 10%, the factors of increase in incidence in one year ranged from 1.0 to 3.0-fold and the median was 1.2-fold. The modes of spread of narcissus tip necrosis virus and narcissus mosaic virus were not discovered; no spread occurred in a 2-year field trial. These results suggest that prospects for maintaining the health of virus-tested stocks of narcissus are good, provided that effective measures to prevent re-infection by nematode-transmitted viruses are adopted.