Pest Management in Miombo Fruit Trees - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 371

number of fungi, which can be either soil-borne or seed-borne. Seed-borne fungi
are carried with, on or in the seed. The symptoms of seedling rot caused by
damping-off fungi vary with the age and stage of development. Root rot of older
seedlings is a late seedling rot that manifests itself as partial or complete
destruction of the root system. Stunted growth can be a prolonged effect of a
partially destroyed root system. This late-stage seedling rot is often not recognized,
being taken for drought or nematode damage. At least 30 different species of fungi
are known to cause damping-off in tree seedlings, mainly representatives of the
genera Fusarium , Pythium , Phytophthora and Rhizoctonia. Several different
species of Fusarium can cause root rot of tree seedlings. Among the different
Fusarium fungi, F. oxysporum is the most common seedling fungus and it usually
causes post-emergence damping-off. One of the main sources of Fusarium
inoculum is the seed, although other sources are also important. Some species are
more aggressive than others. Less aggressive fungi, although living on the plant,
may not develop disease until the seedling becomes stressed by moisture, drought
or heat. Therefore, the cultural practice of moisture-stressing seedlings to harden
them to drier conditions has to be applied gradually so as not to cause fungal
attack.
The water mould fungi, species of Pythium and Phytophthora , are
commonly occurring pathogens that cause damping-off root rot in nurseries,
and tree seedlings (like other plant seedlings) are susceptible. For instance,
damping-off and root collar restriction are common in Sclerocarya birrea
nurseries (Hall et al ., 2002). The most important of the factors that increase
damping-off in container nurseries is dirty seed. Fungal contamination is more
common in dirty seed lots, which produce weak germinants that are
particularly susceptible to damping-off. Because many of the fungi responsible
for damping-off are seed-borne, seeds can be sanitized prior to sowing.
20.3 Field Pests of Miombo Fruit Trees
Pests attack every part of mature fruit trees, causing damage of varying degree.
Many ecologists have found it useful to group pests into feeding guilds in order
to study the ecological interactions between pests, their hosts, their natural
enemies, and climate (Speight et al ., 1999). Accordingly, pests of miombo fruit
trees have been conveniently divided into three major guilds: root-damaging;
stem- and leaf-damaging; and flower-, fruit- and seed-damaging. Tentative
numbers of insect pest species recorded as causing damage to various parts of
the major miombo fruit trees are presented in Table 20.1. These records were
generated from the Agroforestry Pest Database (G. Sileshi, unpublished results).
20.3.1 Root-damaging pests
Fungal diseases
Among the root-infecting fungi, the honey fungus Armillaria is probably the
most important in miombo fruit trees. Parinari curatellifolia subsp. curatellifolia ,