The distribution of hop latent viroid within plants of Humulus lupulus and attempts to obtain viroid-free plants

The detectability of hop latent viroid (HLVd) was investigated in field-grown hop (Humulus lupulus L.; an herbaceous perennial in which all the aerial parts die at the onset of winter) plants, using dot-blot hybridisation. The viroid was readily detected in all aerial tissues in the second half of the growing season but it could not be detected very early in the season. Between early- and mid-season, HLVd was first detected at the base of the new stems and then apparently spread up them as they grew but only became detectable near the tips of the shoots at mid-season, approximately at the time most elongation growth ended and flowering began. Petioles were the most convenient tissues to test, being easy to collect and, relative to leaf lamina tissue, low in inhibitors. Both dot-blot and in situ hybridisation failed to detect HLVd in shoot tips taken from plants grown at two ‘low’ temperatures (10°C and 15°C). Failure to produce any viroid-free plants by in vitro culture from such tips suggested that they did contain viroid but at levels too low to detect by either method. Lower temperatures and smaller explants are now being investigated as means of producing viroid-free plants.