Use of continuous light to improve diagnosis of potato leaf roll virus in tuber indexing tests

SUMMARY Controlled environment cabinets were used to determine light.and temperature conditions needed for full expression of potato leaf roll virus (PERV) symptoms in potato plants grown from eyeplugs taken from tubers of cv. Désirée. PLRV symptom development was poor under conditions in which growth was etiolated. In general, typical rolling of lower leaves developed faster and was more distinct at 18 than at 22°C, and when light intensity was increased, but the two continuous light regimes used gave better results than any of those involving 10‐or 18‐h photoperiods. At 18°C, distinct symptoms suitable for scoring during tuber indexing developed in all infected plants after four to five weeks or five to six weeks with continuous light at 7000 or 4000 lux respectively, but only after six to eight weeks with 18 h/day at 7000 lux, which was the best of the 10‐ and 18‐h regimes. Yellowing and shedding of lower leaves developed faster in healthy plants kept under either of the continuous light regimes than in healthy plants kept for 18 h/day at 7000 lux or in infected plants kept in any of these three regimes, and was very obvious after only six weeks. In terms of symptom appearance in infected plants and defoliation m. healthy ones, a regime of 23 h/day at 7000 lux gave similar results to those obtained with continuous light at 7000 lux. In trials done in winter in a heated glasshouse illuminatcd by mercury vapour lights, continuous illumination had the same effects: distinct PLRV symptoms and obvious loss of lower leaves devefoped after only five weeks in all the infected and healthy plants respectively.