Strains of <i>Prunus</i> necrotic ringspot virus in hop (<i>Humulus lupulus</i> L.)

SUMMARY Purified preparations of Prunus necrotic ringspot virus (NRSV) from hop plants formed two light‐scattering zones when centrifuged in sucrose density gradients; the upper and lower zones contained particles 25 mμ and 31 mμ in diameter respectively whose sedimentation coefficients were 79 S and 107 S. NSRV isolates from hop were of two distinct serological types: ‘A’ strains, serologically very closely related to NRSV isolates from apple; and ‘C’ strains more nearly related to NRSV from cherry. The variety Fuggle is tolerant to hop mosaic (not related to NRSV) and different selections of apparently healthy female plants usually contained A strains; but C strains were usually isolated from nettlehead‐diseased plants. Either A or C strains occurred in male plants grown with the hop‐mosaic tolerant varieties. In mosaic‐sensitive varieties (Goldings and Bramlings) apparently healthy female plants tested were usually infected with C strains; either A or C types occurred in mosaic‐sensitive male plants. NRSV was not detected in the seventy‐four hop seedlings obtained from virus‐infected plants. Some varieties developed nettlehead when infected with NRSV (A) or (C) + the hop form of arabis mosaic virus, but not with NRSV (A) or (C) alone. Others developed nettlehead when infected with arabis mosaic virus + NRSV (C) but not with arabis mosaic + NRSV (A). A and C strains can multiply together in the same hop plant. There is evidence of partial antagonism, however, and the fluctuating behaviour of the nettlehead syndrome probably reflects changes in the relative concentration of the two serotypes.