Agriculture Reference
In-Depth Information
Fig 4.20 Avocado feeder roots infected with Phytophthora showing
dead roots (dark) and regenerating, white feeder roots.
Plant resistant rootstocks. Because the avocado
originated in Central America and
P. cinnamomi
in
New Guinea, there is no evolutionary history between
the host and pathogen; this is known as a new encounter
disease, for which conventional resistance is not
generally available. However, some rootstocks are more
tolerant than others, but are not immune to the disease.
They can only be grown in infested soils if remedial
treatments are applied. Clones of recommended
rootstocks are more resistant than seedlings.
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Fig 4.21 Symptoms of Phytophthora root rot in nursery shock.
calcium and boron are particularly important for root
growth, and satisfactory levels of these elements must be
maintained. Avoid large amounts of fertiliser or animal
manures, which may be high in ammonia and salts -
these are toxic to feeder roots.
Irrigate carefully. Soil moisture monitoring devices, such
as tensiometers, should be used to guard against both
under- and over-irrigation. Use high-quality water
because roots are more susceptible to root rot when
stressed by salinity.
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Systemic chemicals must be used with the other
recommended practices. These can be applied to the soil,
injected into trees or used as foliar and bark sprays.
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Increase the organic matter content of the soil. Grow
cover crops between young trees. Mulch under trees with
coarse materials, such as avocado prunings, wheat straw,
sorghum stubble or hardwood chips. Keep the mulch
away from the trunk. Mulching promotes rooting and
the development of benefi cial microorganisms and soil
fauna antagonistic to
P. cinnamomi
. Apply additional
mulch, as required, to supplement natural leaf fall.
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SOOTY BLOTCH
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Cause
The fungus
Stomiopeltis
sp.
Symptoms
Sooty blotch is a black, superficial blemish on branches,
stems, leaf veins and fruit.
Apply gypsum under the canopy of each tree at
0.5-1.0 kg/m
−2
. Gypsum supplies calcium, which
suppresses the formation of spores. Use lime if pH
correction is required.
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Source of infection and spread
Spores are produced abundantly after rain from superficial
fungal growth on affected avocado tissues. They germinate
readily in the film of water on leaves and fruit.
Provide adequate nutrition. Ideally, tree nutrition should
be monitored closely with leaf analysis. Phosphorus,
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