Tumor Incidence and Shoot Morphology Differences in Rooted Cuttings from Rhododendron With and Without “Tissue Proliferation”

Abstract

Tissue proliferation (TP) occurs in Rhododendron and is characterized by basal tumors that often develop numerous dwarf shoots. Growers need to know if the TP condition will persist in plants grown from normal-appearing cuttings collected from plants with TP tumors. Cuttings of seven cultivars were collected from plants with TP [TP(+)] and without TP [TP(-)] and rooted. Plants were grown in containers outdoors for a minimum of 2 years. For all cultivars, plants grown from TP(+) cuttings had more leaves per growth flush than plants grown from TP(-) cuttings. Shoots of TP(+) plants were either similar in length to shoots of TP(-) plants, or were shorter, as was the case for `Boule de Neige', `Catawbiense Album', and `Montego'. `Montego' and `Scintillation' TP(+) leaves were shorter, narrower, had smaller leaf areas and larger length to width rations than leaves from TP(-) shoots. No difference in leaf size and shape were detected for other cultivars. TP tumors were not observed on any plants. Another study examined the effect of stock plant age from tissue culture on development of TP symptoms in rooted cuttings. `Montego' plants grown from cuttings collected from microplants, 3-year-old TP(+) plants, 6-year-old TP(+) plants and TP(-) plants were compared. Plants grown from 6-year-old TP(+) and TP(-) cuttings did not develop tumors. 83% of plants from microplant cuttings and 74% of plants from 3-year-old TP(+) cuttings formed tumors. Plants from microplant cuttings were more likely to develop large tumors that surround half or more of the stem than plants from 3-year-old TP(+) cuttings.