AVOCADO - Tropical Fruits - page 178

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
af 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