AVOCADO - Tropical Fruits - page 176

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 dii 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 ef 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 (Cof ey,
1987). Trunk-injected phosphonate fungicide (Aliette-Fosetyl-Al) is timed to
coincide with the shoot maturation (Fig. 7.3), the phosphonate being carried