Graft Incompatibility in Radiata Pine in New Zealand

It has been generally considered that the incidence of graft incompatibility in Pinus is low compared with other genera. This paper cites an example of a sudden increase to more than 50% incompatibility in a 14-year-old seed orchard of Pinus radiata D. Don. The types of incompatibility present are examined and the implications for New Zealand's clonal seed orchard programme are discussed. INTRODUCTION The use of grafts has been important for the production of genetically improved seed of radiata pine {Pinus radiata D. Don) in New Zealand. Suitable tip-cleft grafting techniques were developed for this species some 20 yr ago (Thulin, 1957) and over the last 10 yr there has been an average of 10 000 grafts made per year at the Forest Research Institute. A large proportion of these have been used for the establishment of seed orchards of which there are now close to 200 ha in New Zealand. The problem of incompatibility between root stock and scion has been known and studied by horticulturists for a long time {see e.g. Mosse, 1962) and foresters too are well aware of it. In species such as Douglas fir it is particularly serious, and largely because of this tree-breeders in one major Douglas fir area of Canada have changed from clonal to seedling seed orchards (Heaman, 1967). In Pinus the problem is generally regarded as less severe but it has been observed in Europe, North America, and Australasia and recorded in the literature by a number of authors, e.g. Perry, I960; North Carolina State University, 1963, 1970; Brown, 1971. Probably the majority of the world's tree improvement programmes in Pinus currently utilise grafted seed orchards, and are therefore potentially liable to problems of incompatibility. To date the emphasis has been on the word potentially, and as recently as last year Brown (1971) in Australia reported ". . . it appears that graft incompatibility in Pinus radiata is of little practical significance. . . ." Such a statement could correctly have been made in New Zealand in 1970, but not in 1971. THE NATURE OF GRAFT INCOMPATIBILITY Perhaps one of the major characteristics of incompatibility is its variability, both in occurrence and in symptoms. It may occur almost immediately after grafting, but alternatively there may be no symptoms for many years; and while incompatibility may lead to rapid death of the plant, it does not necessarily do so. Great variation in the incidence of incompatibility may occur between clones of the one species, and even N.Z. JI For. Sci. 3 (1): 82-90 No. 1 Sweet and Thulin — Graft Incompatibility 83 within a very incompatible clone it is rare for all ramets to show symptoms. Examples have been published of site and mineral nutrient levels affecting incompatibility, and Mosse (1962) has suggested that there may in fact be no clear-cut demarcation between compatible and incompatible graft combinations: rather external factors may often control the balance. A number of horticultural workers have attempted to classify different types of incompatibility. Of these probably the simplest classification is that of Mosse (1962) who recognised only two types, viz. localised incompatibility and translocated incompatibility. In the former the incompatibility reaction appears to depend on contact, because it can be overcome by a mutually-compatible intermediate stock. With localised incompatibility either the cambium and vascular cylinder of the stock and scion do not join, or at some stage after joining disorganisation develops and continuity is lost: as a result stem breakage at the union occurs very readily. With translocated incompatibility in contrast, vascular and cambial continuity is normal at the union, and the major indication of incompatibility is phloem degeneration. Grafts with this sort of incompatibility rarely die from stem breakage at the union. INCOMPATIBILITY IN RADIATA PINE For radiata pine in New Zealand, the earliest signs of incompatibility occur during the first growing season after grafting. Such incompatibility is not widespread and is essentially restricted to a few clones. At the Forest Research Institute it has been particularly bad in only one clone where approximately half of all grafts made die within the first two years. By leaving foliage on the rootstock death is prevented, indicating that mortality in that clone is caused by the inability of carbohydrates to reach the root system. There is evidence from radioactive-tracer studies that in the grafts which do die, the secondary phloem of scion and rootstock fails to form a functional union (Cameron and Thomson, 1971). The establishment of Gwavas seed orchard in Hawke's Bay has taken place over a number of years, and from counts made in 1971 it has been possible to note for that site changes in the incidence of graft incompatibility with time. Sampling in 1971 showed that the incidence of trees dead or with clear signs of ill-health was 11% in 13-yr-old plantings, 5% in 7-yr-old plantings and 3% and 1% in 5and 3-yr-old plantings respectively. There was a strong clonal effect, with quite a high proportion of the dead and dying trees belonging to five or six clones only: many of these trees displayed symptoms of incompatibility, of a type which will be described later. Until 1971 this level of mortality was regarded as typical of that found in radiata pine seed orchards in New Zealand, and it compares with the 7% mortality at age nine recorded from New South Wales by Brown (1971). However, in 1971 at the 1957-58-established Kaingaroa seed orchard (ca. 90 miles from Gwavas) the incidence of mortality was seen to increase very rapidly. This is illustrated by Table 1 which compares by clones the percentage of trees dead or with clear signs of incompatibility in 1971 at Kaingaroa and Gwavas. At age 14 more than 50% of the Kaingaroa trees were either dead or dying with the values ranging from 23% in the most-compatible to 81% in the least-compatible clone. New Zealand Journal of Forestry Science Vol. 3 TABLE 1—Percentage of trees dead or with clear signs of ill-health in 1971