Avocado is primarily propagated commercially by budding or grafting upon seedling rootstocks. However, the variability of seedling populations with respect to certain desirable characteristics, such as resistance to Phytophthora root rot and tolerance to salinity and calcareous soils, has posed problems (Ben-Ya’acov and Michelson, 1995). Seedling production has largely changed
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from grafting nursery-grown trees to grafting container-grown trees. Seeds are taken from fruit picked from trees free of sun-blotch and are treated in a water bath at 49–50°C for 30 min, cooled and surface-dried in a partially shaded area. Seeds are planted (broad side down) in polyethylene bags, with a well-draining potting mix. Seeds germinate in about a month. The papery seed coats should be removed. If seeds are not very fresh, cutting off a piece of seed tip or doing some vertical cuts at the side of the seed will improve germination speed and percentage; sometimes a slice of bottom can also be cut off .
Seedlings may be cleft or side-wedge grafted 2–4 weeks after germination.
Scion wood from terminal growth of sun-blotch-free cultivars should be used.
Propagation is usually done in shade houses, preferably with temperature control if the environment has a wide temperature range. Grafted plants must be hardened for approximately 2 weeks under full sunlight before fi eld transplanting. Under open-fi eld conditions in the central coast of Peru, with 13–15°C lowest temperatures in winter, Duarte et al. (1975) reduced by 2 months the time to obtain a grafted plant with seeds sown at the start of autumn and by 1 month with seeds sown at the end of autumn, by spraying the seedlings three times with 250 or 500 ppm GA at 2-week intervals, starting when they had reached 15 cm.
Leafy avocado cuttings of some genotypes under mist consistently rooted nearly 100% under practically any conditions, while others did not root at all or rooted with diffi culty (Kadman and Ben-Ya’acov, 1970b). Generally, West Indian cultivars with strong resistance to salinity are diffi cult to root.
Cuttings from mature trees were diffi cult to root and, in those that rooted well, they took 4–10 months to root. Cuttings from 1-year-old seedlings show a higher percentage of rooting in 4–12 weeks. A 50% light intensity in the intermittent-mist system during the summer is better for rooting than full sunlight. The anatomy of the avocado stem provides a reason for the diffi culty in rooting as the fi bre bundles and the sclereid ring are thicker in the West Indian types, intermediate for Guatemalan and hybrid types, and least for the Mexican cultivars.
Air layers and cuttings have been successfully rooted. Variability exists in ease of rooting air layers between races and even among cultivars of a race, and the diffi culty of large-scale production has discouraged commercial development. Generally, Mexican cultivars root most easily, followed by Guatemalan and West Indian, whether by air layering or by cuttings. Time required for rooting of air layers ranged from 146 to 518 days, depending upon cultivar and the time of the year. Etiolated stems show few or no sclereid connections between fi bre bundles, suggesting that the sclerenchyma ring may be acting as a barrier to root emergence (Gomez et al., 1973). Hence, shoots of most avocado cultivars produced in light do not root nearly as well as shoots produced in darkness.
Seedlings produced upon scions of the desirable rootstock cultivar are grafted by tip grafting as close to the base as possible. Shoots of the scion
are allowed to grow and then cut back to near the base. When new buds show signs of growth, the entire plant is placed in a dark room, with the temperatures maintained at 21–23°C. When the new shoots reach about 8–10 cm in the dark, the plants are again placed in light, with a tar-paper collar placed around each etiolated stem and fi lled with vermiculite to continue exclusion of light from the base of the shoots. Only the tips of the shoots are exposed to light, in order to produce green leaves. This procedure is done under shade to prevent sunburn. Shoots are then allowed to grow until several leaves have matured. The collar is then removed and shoots are detached for rooting in propagation frames. Rooting hormones have shown no benefi cial eff ects, although some reports from Australia indicate that a 360o scrape of the etiolated shoot prior to the application of the potassium salt of indole butyric acid consistently gave a crown of roots. Rooted shoots are transplanted into 10 cm peat pots and grown in an enclosed area for further root and top growth and gradually hardened. These rooted plants are then transplanted into larger polyethylene bags for more growth and hardening, then grafted when they attain appropriate size. The side-wedge technique is usually used, so some terminal foliage on the rootstock is retained until scion growth begins.
In California, South Africa and to a lesser degree in Spain and Chile, clonal rootstocks are used and propagated based on the double-grafting–etiolation method. A large avocado seed such as ‘Lula’ is grown in a container to serve as a nurse stock. When the nurse seedling that was sown into a planting sleeve is large enough for grafting, a splice or cleft graft with a tolerant rootstock is made. A metal girdle is loosely put above the bud or graft union so that it will eventually girdle the stem; the grafted rootstock is forced to root above the girdle (Brokaw, 1975). The nurse seed with the grafted tolerant rootstock is then grown in the dark till the graft is 20–40 cm tall. The container is then removed from the dark, and sterile rooting medium is supplied to cover the base of the bud union. Adventitious roots develop from the tolerant rootstock.
Once the rootstock growth hardens off , it is grafted with the desirable scion variety and grown for 2–3 months. The double-grafted plant is then grown for several months before planting. Rooting hormone or minor cuts near the bud union between the tolerant rootstock and the nurse seed can enhance the rooting success of the rootstock. Once in the fi eld it is assumed that the rootstock will extend its roots and the nurse seed will be excluded from the system due to the girdle. In South Africa, an improvement to this method was developed (de Villiers and Ernst, 2007). This consists of positioning a 55 ml micro-container on the etiolated shoots coming from the grafted rootstock. Roots will form in these containers, and after grafting the desired cultivar on to these shoots, they will be separated from the nurse seedling just above the nurse graft and taken to the nursery for hardening; after this they will be transplanted into nursery containers or bags. The advantages of this improvement are that more than one plant can be obtained from a nurse seed;
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reduced plant cost because of the effi ciency of this technique; the root system of the rootstock is separated from that of the seedling, which assures better sanitary conditions, and it can be inspected at transplant time; plants with a better root system and with a more uniform growth in the fi eld are obtained.
Field preparation
Land preparation for avocado does not diff er from that for other tree crops.
Development of a drainage system is a prime consideration. Subsoiling or ripping down to at least 0.5 m or more, preferably running diagonally across the slope to allow subsurface movement of water, aids drainage. If soil pH needs adjusting, this could be done during the fi nal stages of land preparation.
Cover crops, such as legumes or grain, can be preplanted a year before orchard planting to increase organic matter and minimize erosion and root rot.