Redevelopment of Tissue Proliferation Symptoms in Rooted Rhododendron Cuttings

Abstract

Tissue proliferation (TP) of Rhododendron sp. is characterized by basal tumors that often develop into numerous dwarf shoots. Growers need to know if the TP condition will recur in plants grown from normal-appearing cuttings collected from plants with TP tumors. Stem cuttings of seven cultivars were collected from stock plants with TP [TP(+)] and without TP [TP(–)] and rooted. Plants were grown in containers outdoors for 2 years and were then evaluated for tumor formation and other TP-related morphological symptoms. 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'. Plants grown from TP(+) cuttings of all cultivars had more leaves per growth flush than did plants grown from TP(–) cuttings. `Holden', `Montego', and `Scintillation' TP(+) leaves were narrower than leaves from TP(–) shoots and had greater length: width ratios. Leaves of TP(+) `Montego' and `Scintillation' plants were shorter and smaller than leaves from their TP(–) counterparts. Tumors were not observed on any propagated plants, regardless of the TP status of cutting stock plants. To further test the influence of age and TP status of source plants used for cutting propagation, `Montego' plants were grown from cuttings collected from the following sources: 1) in vitro shoot cultures; 2) 3-year-old plants with TP; 3) 6-year-old plants with TP; and 4) TP(–) plants. Cuttings from TP(+) micropropagated plants less than 3 years old were more likely to develop tumors than were cuttings from older plants. Eighty-three percent of plants from microcuttings and 74% of plants from cuttings of 3-year-old TP(+) plants formed tumors, whereas no plants grown from 6-year-old TP(+) or TP(–) cuttings did so. Large tumors that surrounded half or more of the stem were more likely to develop on plants grown from microcuttings than on plants grown from the next youngest, 3-year-old TP(+), stock plants. Growers must be aware that cuttings from TP(+) plants may produce plants that exhibit morphological and growth abnormalities, possibly even including tumor redevelopment.