ISHS Acta Horticulturae 57: I International Symposium on Tropical and Subtropical Fruits
AVOCADO ROOTSTOCK BREEDING, ESPECIALLY IN RELATION TO PHYTOPHTHORA |
| Authors: | B.O. Bergh, G.A. Zentmyer, R.H. Whitsell, S.B. Boswell, W.B. Storey |
| Abstract:
Desired traits of avocado rootstocks are listed, and some genetically superior sources of these traits noted. The chief avocado root-stock problem in most regions is Phytophthora cinnamomi root rot. Suggestions for avoiding root rot, and for coping with infestations of it, are given. The most promising method of control appears to be resistant rootstocks. These are being sought in California and in Central and South America. Several compatible lines with moderate resistance have been identified. These are being hybridized inter se to combine the individual sources of resistance. The genetic possibilities are analyzed, and eventual procedures suggested. Another approach is irradiation of a moderately resistant line in hopes of obtaining mutations for enhanced resistance. In fruit trees generally, rootstock desiderata may be grouped under the three headings: availability and graftability of the rootstock itself, self, scion productivity, and scion fruit quality. In the avocado (Persea americana, Miller), no physiological rootstock effects on fruit quality are as yet known, and so only the first two groups of desiderata are involved in breeding programs. The parent or source tree should be hardy to cold and other adverse environmental factors. For the seedling rootstocks that continue to be used almost exclusively around the avocado world, the parent tree should be fruitful and as genetically uniform as possible. Development of a superior rootstock may make asexual propagation highly desirable (Bergh 1975a). The rootstock should be vigorous, and in particular should attain graftable diameter quickly. It should be conducive to good take by the scions grafted on it. To facilitate heavy fruit production over a long period, the rootstock should impart reasonable precocity, high average productivity, longevity, and adequate vigor, but dwarfing or semi-dwarfing effects are desirable since avocado trees tend to excessive vigor at the expense of fruitfulness (and dwarf trees lower picking costs). The rootstock should be free from sunblotch virus, and should be resistant to soil salinity, to chlorosis, to other adverse factors, to Verticillium wilt, and above all to Phytophthora cinnamomi root rot. Genetic differences among available stocks are known for many of these desiderata. Among the three horticultural races or botanical varieties (Bergh, 1975bb), the Mexican is most resistant to cold, and, because its fruits are usually smaller, it usually produces more seeds | |
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