The Cause of Black Blotch Disease of the Raspberry

Summary

Actively growing, single-stemmed plants of three black currant cultivars, each with 15 – 16 nodes, were exposed to photoperiods of 10, 14, 15, 16, 17, 18, 20, and 24 h at 18°C for 8 weeks for determination of the critical photoperiods for growth cessation and floral initiation. In all three cultivars, growth cessation was induced by short day (SD) conditions, with a critical photoperiod of 16 h, and the response was advanced by decreasing the photoperiod. The critical photoperiod for 50% flowering was 16 h in the cultivars ‘Öjebyn’ and ‘Ben Tron’, and 17 h in ‘Kristin’. Unexpectedly, however, not all plants flowered after exposure to a 10 h photoperiod, and the number of flowers per plant increased several-fold as the photoperiod was increased from 10 h to 15 h in all cultivars. Apparently, this unexpected result was due to the fact that all plants had only 15 – 16 nodes at the start of the experiments, which is marginal for “ripeness to flower” in black currant. While growth cessation was almost immediate in a 10 h photoperiod, causing only a few additional leaves to be formed during the experiment, the slower response to longer photoperiods apparently enabled the plants to reach the critical size at an earlier stage of the treatment period. However, although plants with 15 – 16 nodes were only marginally responsive to SD induction, buds situated as far down the shoot as the fifth or sixth node were competent to flower. It is therefore suggested that the inability of small black currant plants to flower resides in limitations of the leaves to respond to SD and to produce a florigenic signal, while their buds are fully competent to respond to such a signal.