Some factors affecting the establishment and growth of bud grafts of roses : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Botany at Massey University
A brief review of abnormal growth of rose scion buds and the high proportion of buds which fail to produce normal growth in the production of rose plants by bud-grafting in New Zealand, introduces the subject. The history of investigations, prior to 1968, into this problem is outlined. A description of normal growth patterns of shoots on roses, of axillary buds and of production methods in New Zealand is followed by a detailed description of the abnormal teratomatous growth, known as proliferation, which is characteristic of a proportion of affected buds. The proposal is put forward that this condition is caused by infection with an organism that initiates galling similar in appearance to crown gall and that subsequent symptoms develop consequent to such a tumorous transformation but not necessarily due to the continued presence of the causal organism. In this study no causal organism was successfully isolated. The claims of a number of possible other causal agents are examined including development of the graft union, chemical factors, non-transforming bacteria, mites, fungi and viruses. Reasons are advanced why none of these provide a satisfactory explanation in agreement with experimental and observational evidence. This evidence is discussed in relation to the etiology and development of the disease syndrome. The evidence presented is strongly in support of a tumorous transformation, initiated by a soil-borne pathogen, occurring at the proximal end of the original stock cutting at the time of its insertion in the ground. Root initiation may be restricted by the position of the gall and the first shoot growth at the top of the stock may show slight symptoms. The infected plant is predisposed to show subsequent symptoms but these are dependent on subsequent operations and the time and conditions when they are carried out. These symptoms are the development of excessive callus at the point of excision of the stock top and at the incision of the bud-graft, the production of teratomatous shoots, known as proliferation, by the scion bud or failure of the scion bud to grow despite a successful graft union. These symptoms may be reduced or prevented by successful normal growth of the scion. A comparison of the etiology and development of the syndrome of proliferation disease is made with the classical characteristics of the crown gall syndrome. Recent research publications on crown gall are reviewed to establish that recent findings are not contrary to the proposal that rose proliferation disease is caused in a manner directly analagous to crown gall. It is proposed that the evidence supports the assumption that a particular strain of Agrobacterium tumefaciens or some bacterium closely related to it is the etiological agent causing rose proliferation disease.