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showing damage, may be an important reservoir for these nematodes (Taylor
et al
., 1978). Nematodes cause shortening and malformation of roots and
subsequent death of
Sclerocarya birrea
trees (Hall
et al
., 2002). Nematode
problems are well known in most exotic fruit trees. For instance, apple, peach,
citrus and mango are respectively affected by 40, 41, 35 and 20 different
species of nematodes in southern Africa (Kleynhans
et al
., 1996). On the other
hand, the plant-parasitic nematodes affecting fruit trees of the miombo are little
known.
20.3.2 Trunk- and leaf-damaging pests
Fungal diseases
Trunk-damaging fungi largely cause diseases of three kinds: dieback and canker
diseases; vascular wilt diseases; and wood rot diseases. Bark-infecting fungi are
seen to cause fluxing (oozing), necrosis, and canker of small branches and
shoots. In Zambia such damage has been reported to contribute to dieback of
Uapaca
spp. (Parker, 1978). The bark-infecting genus
Botryosphaeria
includes
many species, some of which are parasitic and cause dieback and canker
diseases on woody hosts. Some are host-specific and others have a wider host
range. The dominating species,
Botryosphaera dothidea
, is known to attack
over 100 species of importance in the region, among them apple, eucalypts,
grevillea, guava, loquat, macadamia, mango, maple, mulberry, papaya, peach
and pear (Sinclair
et al
., 1989).
Botryosphaeria ribis
attacks avocado, guava
and pear in Malawi (Lee, 1970).
Botryosphaeria
species infect trees which are
weakened by drought. Although there are many other dieback and canker
fungi,
Botryosphaeria
spp. are obvious candidates for attacking miombo fruit
trees.
Cryphonectria cubensis
is the causal agent of eucalyptus canker in many
countries throughout the world. The miombo species
Syzygium cordatum
is
host of
Cryphonectria cubensis
in South Africa and a source of infection for
eucalyptus canker there.
Cryphonectria cubensis
has been shown to be a
potential pathogen of guava (
Psidium guajava
) and other exotic and
indigenous members of the family Myrtaceae (Swart
et al
., 1991).
A well-known vascular wilt disease of fruit trees such as avocado is caused
by
Verticillium
. The causal organism,
Verticillium dahliae
, is soil-borne and
enters the roots and invades the tree's vascular system, causing sudden wilt of
branches or whole trees.
Ceratocystis
causes wilt disease but mostly infects the
trunk with the help of a vector, as in the case of Dutch elm disease, transmitted
by a bark beetle in Europe and North America. Also, pruning wounds can be
infection points. Fungal decay of the wood is a common feature where the
trunk or limbs of a tree have been damaged by other agents. A wound is the
necessary entry point into the wood for these fungi.
Phellinus
spp. and
Daedalea
spp. are examples of pathogenic decay fungi, evident as brackets on
trunks and branches. When the fruit bodies appear on the trunk a considerable
rot has already developed in the trunk. Saprophytic wood decay fungi such as
Schizophyllum commune
frequently fruit on dead wood on trunks and can be
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