The avocado can tolerate neither water stress nor excess moisture, especially when drainage is inadequate. Water stress reduces yields, fruit size and tree vigour. The soil around the trees should be moist but not wet. Irrigation frequency depends upon fi eld condition, soil drainage and tree density, as well as canopy size and prevailing weather conditions and past irrigation records.
Evapotranspiration data and tensiometer readings averaged over a period of days provide a more accurate means of determining irrigation timing. The application rate is calculated, taking into account the evapotranspiration rate since the last irrigation, rainfall, percentage canopy coverage of the ground and effi ciency of water use. Young, non-bearing trees require light, more frequent irrigation. Older trees can withstand longer intervals between irrigations but never to a point of water stress. Only 50% of the tree’s requirements should be given in the middle of the cool season and spring, in order to favour fl owering rather than vegetative growth. When fruit set is completed, irrigation reverts to normal amounts during fruit development.
High irrigation rates are necessary during fl owering and may be necessary as the fruits approach maturity and if the weather is hot and dry (Fig. 7.3). Care has to be taken not to let the soil dry too much at the time of fl owering and fruit set, since a marked increase of water in the soil can produce a signifi cant drop of small fruit. Irrigation is necessary down to at least 60 cm. Total water applied per year is estimated at about 35–50 ha-cm for mature trees (Gustafson, 1976).
On level land, less-effi cient furrow irrigation or sprinklers can be used, with micro-irrigation systems (drip or micro-sprinklers) being more effi cient in
Avocado 175
water use. Mature trees require eight or more drip emitters around the tree, providing greater control and higher effi ciency of water use (up to 80%). Later plantings of avocado in California are on the sides of steep hills, which can be irrigated only by drip irrigation or mini-sprinklers, as in Chile.
Pruning
The growing tips should be pinched off young trees, allowing the development of a more compact tree. This pinching is continued until the tree is too tall.
Lower limbs are removed only if they interfere with irrigation and fertilization.
Management should be keyed to avoiding vegetative and reproductive growth occurring at the same time. Since avocado is polyaxial, it must continue to increase in size to remain productive. Shoot growth in the warm season occurs in spurts of up to 1 m, which is needed in order to put on leaves required for fruit growth (Fig. 7.3). Tree trimming aims at a canopy that has fruit at all heights and reduces the competition for light. Pruning at the bud ring (several closely spaced buds without subtending leaves), formed at the conclusion of a shoot growth fl ush, releases more buds and increases shoot complexity, and hence bearing sites. Cutting below this ring depresses tree vigour and releases only one bud (Cutting et al., 1994). Bud-ring pruning can extend the period before trimming is necessary. Pruning should be timed to the end of the autumn period. Tree growth can also be reduced by spraying with paclobutrazol, an inhibitor of gibberellin synthesis, though its use is not approved in some countries. Tree thinning is recommended in South Africa when 90% of the orchard fl oor is shaded. Topping (stag-horning) is also practised on healthy trees to rejuvenate a crowded orchard, where the trees are cut back to 1–3 m from the ground. Tree thinning, topping or stumping is essential to avoid canopy crowding and the loss of bearing volume in the lower third or more of the tree. Severe topping to ~3 m means the loss of production for 2–3 years, while less severe topping to ~5 m can increase fruit yield and the amount of fruit set in the lower half of the tree (Crane et al., 1992). Cultivars with upright vigorous growth habits may be severely aff ected by topping and orchards may have to be rejuvenated in sections over several years, by topping to 5 m. For very high density, the plants should be cylindrically shaped, while for not so high densities a pyramidal shape is more appropriate.
Fertilization
Fertilizer practices diff er in avocado-producing areas, due to diff erences in climate, soil, cultivars and management practices. Numerous attempts have been made to determine critical foliar nutrient levels, in order to regulate the application of macro- and microelements. The following ranges of foliar
levels for macronutrients are established: nitrogen (N) 1.6–2.0%; phosphorus (P) 0.07–0.20%; potassium (K) 0.75–2.0%; calcium (Ca) 1.0–3.0%; and magnesium (Mg) 0.25–0.50% (Malo, 1976). The ranges for micronutrients are: iron (Fe) 50–200 ppm; zinc (Zn) 50–150 ppm; and manganese (Mn) 30–70 ppm.
Critical levels have been diffi cult to establish, due to the highly variable yields of avocado. Nitrogen seems to be one of the controlling factors in avocado yields, as this is the only element that has shown a curvilinear relationship to yields. Maximum production of ‘Fuerte’ is found at a moderate level of N in the leaves, with reduced yields occurring at levels below and above the moderate level. In ‘Fuerte’, a range from 1.6 to 2.0% N in the leaves in late summer is a desirable level in order to maintain high production. Nitrogen fertilization is not recommended during the cool season, with application delayed to the summer leaf and root fl ush (Fig. 7.3). This application should include P and K. A later application of P and K should also occur near the peak of fruit set (Whiley et al., 1988a).
As for many tree crops, including avocado, a soil pH of 7.0 and above creates problems with Fe and Zn defi ciencies, refered to as lime-induced chlorosis. Iron chelate is used to correct Fe defi ciencies. Soil application of Zn is more eff ective in acid soils than in alkaline soils, and foliar application has not proved successful. In Florida, where defi ciencies of Fe, Zn and Mn are common, good results are obtained by combining the chelates of these elements and applying them through the drip-irrigation system. Boron (B) defi ciency occurs in some soil types, and ‘Sharwil’ appears to be more sensitive to B defi ciency.
Pest management
Diseases
A number of avocado diseases have been reported from producing areas around the world (Table 7.5), the most serious being root rot caused by P.
cinnamomi Rands. This is suspected when trees show a gradual decline, with leaves becoming smaller, yellow-green in colour and shedding. In severe cases, twig dieback occurs. The destruction of the unsuberized feeder roots is associated with high soil moisture in poorly drained areas of the fi eld, with P.
cinnamomi thriving under wet soil conditions, especially when the temperature ranges from 21 to 30°C and with a soil pH of 6.5 (Zentmyer, 1976). Soil fumigants, fungicides and sanitation have been used, with a research emphasis on the development of resistant rootstocks, such as ‘Martín Grande’,
‘Thomas’, ‘Barr-Duke’ and ‘D9’ (Gabor et al., 1990). The use of resistant rootstocks is integrated with hygiene, sanitation and cultural methods (Coff ey, 1987). Trunk-injected phosphonate fungicide (Aliette-Fosetyl-Al) is timed to coincide with the shoot maturation (Fig. 7.3), the phosphonate being carried
Avocado 177
to the mature shoots and then translocated to the roots, peaking in the root in about 30 days (Whiley et al., 1995). In California, the root-ball of replant trees should be drenched with a solution of phosphonate at the time of planting, followed by two to three foliar sprays with the same chemical. This chemical can also be sprayed on to the foliage of diseased trees as long as functional leaves are present. If too few functional leaves remain on the tree, heavy pruning is performed to induce new shoots and leaves before spraying phosphonate. Phosphonate applications are repeated three or four times a year. However, the chemical does not eliminate the fungus from the soil and must be used with management practices.
In Mexico, Phytophthora root rot control is based upon integrated crop management (ICM). The ICM approach is based upon improving plant vigour, restoring the equilibrium between the root and foliar systems, increasing the soil fl ora benefi cial to the avocado and harmful to disease organisms, improved irrigation and fertilization, reduced fungal and insect attacks and avoiding management practices that weaken the tree and favour the disease attack.
Table 7.5. Some important diseases of avocado.
Common name Organism Parts affected,
symptoms Region or
country Root rot Phytophthora
cinnamomi
Feeder roots, tree decline
Worldwide Verticillium wilt Verticillium dahlia
(Verticillium albo atrum)
Wilting of branches, death of trees
California, Australia, Florida Armillaria root rot
and crown rot
Armillaria mellea Large roots, gradual
death of trees California, Mexico Sun blotch Sun-blotch viroid Stunted, decumbent
growth, distorted leaves, yellow or red streaks on fruit
Many areas
Anthracnose Colletotrichum gloeosporioides
Leaves, most serious on fruit, especially after harvest
Worldwide
Stem-end rot Dothiorella aromatica (Dothiorella gregaria) Diplodia natalensis
Purplish-brown spots on fruit surface, fl esh discoloration, offensive odour
USA, South Africa, Israel, Australia, parts of South America, Caribbean Cercospora spot Cercospora purpurea Leaves, young stems,
fruit
Florida, other areas Scab Sphaceloma perseae Foliage, fruit Humid tropics
and subtropics
The main measures consist of: (i) periodic incorporation of cattle manure to a depth of 30 cm to keep the organic matter content of the soil around 3–3.5%, checking this periodically; (ii) chemical fertilization through soil and foliage to keep adequate foliar levels of elements; (iii) rejuvenation pruning of aff ected trees with more than 70% defoliation to restore root/foliage ratios;
(iv) proper irrigation to avoid plant stress, by not allowing soil moisture to drop below 70% of fi eld capacity; and (v) adequate sanitary controls to reduce damage by other enemies (Mora et al., 2000). This approach has provided results comparable to or better than the use of chemicals. Chemicals are still recommended when tree decline is serious. In Australia, root rot has been minimized by building up heavy mulch with bagasse, grass or cereal straw.
This reinforces the importance of using organic matter to create a better environment for plant-friendly organisms and fungus antagonists.
Many of the diseases reported are not necessarily serious, although the potential for causing heavy losses exists. Avocado scab is considered to be an important disease of avocado fruit and foliage in Florida (McMillan, 1976).
Among fruit diseases, anthracnose, stem-end rot and avocado scab can cause serious problems. Anthracnose requires control practices in the fi eld during fruit development.
Sun blotch, caused by a viroid, is of concern. Trees are stunted, with cracked bark, necrotic streaks on branches, and white or light green areas on the fruit. There are no known vectors, and what makes it a potentially serious problem is the presence of many symptomless carrier trees. The disease is transmitted by use of seedling rootstocks from such trees, as well as grafting with scion wood from infected plants. Scion wood from a healthy tree grafted upon a ‘carrier’ rootstock becomes infected. In the major avocado-growing countries, indexing techniques have been developed and used to identify healthy cultivars for scion wood, as well as those needed for rootstocks (Broadley, 1991).
INSECTS
There are many insect pests reported in avocado orchards, although they do not usually pose any serious problems (Table 7.6). Occasionally, a sudden increase of a specifi c insect can cause severe damage. This increase in an insect pest is often associated with a sudden change in weather conditions.
Avocado red mite can cause signifi cant leaf damage and a reduction in photosynthesis and transpiration (Fig. 7.7), possibly leading to a reduction in yield. Insects such as scales, aphids, mealy bugs and various mite species are also commonly found in orchards, but natural enemies have been shown to provide satisfactory control.
In some producing areas, fruit fl ies may require some form of disin- festation procedure for fruit to be exported to some markets. Studies in Hawaii with ‘Sharwil’ avocado have shown that it is not normally a host to the Mediterranean fruit fl y (C. capitata), melon fl y (Dacus cucurbitae) and the
Avocado 179
Oriental fruit fl y (D. dorsalis) at the mature green harvest stage. Under dry conditions, eggs are deposited between the pedicel and the fruit, making it a host.