Agriculture Reference
In-Depth Information
Importance
White root rot is only recorded from the Granite Belt
district of Queensland, where it continues to cause serious
tree losses. The disease is most severe in replant orchards,
where the previous apple trees were affected. In the past,
serious losses occurred in new orchards established in land
cleared of native vegetation susceptible to the fungus. This
disease will become more important as the numbers of
medium to high density orchards increase.
Management
Thoroughly remove infected roots from affected trees.
Remove affected trees from the orchard as soon as
possible, preferably while the tree is still alive, to make
it easier to remove all the roots.
Fig 3.35 White root rot in a root. Note the hair-like fungal structures
(synnemata) on the root surface.
Remove at least two, and preferably three, healthy-
appearing trees on either side of the affected trees. In high-
density orchards, remove trees within 2 m from the root
system of affected trees.
Symptoms
Both apple and pear are hosts.
Trees develop an unthrifty appearance with leaf yellowing,
cessation of shoot growth, small leaves, premature leaf fall
and small, shrivelled fruit. These symptoms are not
distinctly different from those caused by other soil-borne
pathogens (e.g. Armillaria root rot, crown rot and
Phytophthora root rot) and root-lesion nematodes.
Crown: the bark of the crown roots and the base of the
trunk can show a dark, wet rot. A distinct sharp margin is
evident between healthy and infected bark. A thin layer of
white fungal growth occurs under the bark, and is more
prominent after wet weather.
Treat mature trees on either side of the vacant space, as
well as the vacated soil, with a registered soil sterilant
fungicide.
Take care before replanting young trees into spaces left
by trees killed by white root rot in mature orchards.
Planting young trees in the midst of a mature orchard
Roots: this disease causes significant root damage in a
relatively short period, with both fine and major roots
equally affected. Infected roots appear to have a dark, wet
surface rot, etching into healthy internal wood. Affected
roots can be covered with white strands of fungal growth,
which can also grow into soil and leaf litter in wet
conditions. Hair-like growths of the fungus called
synnemata can appear on the surfaces of the lower trunk
at soil level and on the roots on undisturbed trees that
have been infected for some time.
Source of infection and spread
The fungus survives in the soil on old rotted roots and root
debris left in the ground and on the roots of various native
trees and weeds such as f fleabane ( Conyza spp.) and
stinking roger ( Tage te s minut a ). Apple trees planted into
infested soils become infected when their roots contact
infected root material. The interconnected root zones of
medium and high-density orchard plantings will greatly
increase the distribution and rapidity of spread of this
disease within orchards.
Fig 3.36 Dark, wet rot associated with white root rot. Note the sharp
margin between healthy and affected areas.
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