ISHS Acta Horticulturae 166: Propagation of Ornamental Plants
THE CLONAL PROPAGATION OF PHYTOPHTHORA RESISTANT AVOCADO ROOTSTOCKS |
| Author: | D. Phillips |
| DOI: | 10.17660/ActaHortic.1985.166.8 |
| Abstract:
Avocado rootstocks, currently in use commercially, are highly susceptible to a soil borne plant pathogen, Phytophthora cinnamomi Rands. P. cinnamomi is an Oomycete which usually reproduces asexually by the production of either motile sporangiospores, zoospores or resistant resting spores, chlamydospores. These spores are present in the soil and attack plant roots causing tissue necrosis and preventing the take up of water, eventually resulting in plant death. The disease is a major threat to the viability of the avocado industry in Queensland and Northern New South Wales, where every existing orchard is believed to be infected by the pathogen (Dr. Ken Pegg*, pers. comm.) Because of this I am investigating the relation between certain environmental factors and the resistance of two avocado rootstocks, Duke 7 and Topa Topa, to the pathogen. In this study I needed large numbers of uniform rootstocks of the cultivars concerned. Preliminary experiments using open-pollinated seed of Duke 6 and Topa Topa gave a high percentage of successful germination in seedling trays incubated at 25°C in the dark. Germination usually occurred within 1–2 weeks. However, seedlings showed great variability in growth form and response and did not store longer than 4–6 months at 4°C. More uniform, comparable results could be obtained using clonally propagated rootstocks. There are four main methods of propagating avocados asexually: (1) Striking cuttings, (2) Marcottage or air layering, (3) Tissue culture, (4) Etiolation technique. However, the first three techniques are inadequate at the commercial level of propagation (Tony Whiley, pers. comm.). Striking cuttings is successful, but takes 7–11 months for rooted cuttings to reach graftable size. The rooting percentage of mature wood cuttings is unacceptably low and unreliable. Air layering is a successful technique, but has two disadvantages, (1) large trees are required to obtain sufficient marcots, (2) it can take 15 months to produce a saleable grafted nursery tree. Tissue culture is still in the developmental stage. Callus masses can be cultured in vitro from nearly all plant parts, but it is still not possible to regenerate an entire and normal plant from a callus mass. The fourth technique, the "Etiolated Cutting or Nurse Seedling" technique is apparently the best process currently available, whereby clonal saleable nursery trees can be produced in 9–10 months. This technique was first developed by Frolich and Platt (1972) at Riverside, California, modified by Brokaw Nurseries, California, in 1978 (Overby 1978) and then adapted to Australian conditions by Dr. Tony Whiley and Alistair Inch in 1980 at Maroochy Horticultural Institute and Birdswood Nurseries, Nambour, Queensland, respectively. (Dr. Tony Whiley kindly provided me | |
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