Agriculture Reference
In-Depth Information
Trees may die quickly after infection or slowly decline over
several years. Sudden death is more likely when plants are
stressed during f flowering, fruit development or hot
weather.
For further information refer to the chapter on Common
diseases of perennial fruit crops.
RHIZOCTONIA ROOT ROT
Cause
The fungus Rhizoctonia spp.
Fig 13.10 Rhizoctonia root rot on a nursery tree.
Symptoms
Young nursery plants are most likely to be affected.
Roots turn brown and the outer bark may slough off. Black
sclerotia of the fungus may form on the roots. The above-
ground symptoms are similar to those caused by other
pathogens and factors that adversely affect root function.
The symptoms include death of leaf tips, defoliation,
stunting and plant death.
indirectly because soot cover reduces light penetration and
photosynthesis by the plant.
Source of infection and spread
The fungi are air-borne and attracted to honeydew
produced by insects, particularly scale.
Importance
Severe infections of sooty mould can cause unthriftiness
and stunting, as the soot cover inhibits normal leaf
functions (respiration and photosynthesis).
Source of infection and spread
The fungus is present in most soils and, once introduced,
remains there indefinitely. It survives in the soil as sclerotia,
fungal mycelium and in infected plant parts. The fungus is
spread by water, soil movement and contaminated tools and
planting material. Disease development is favoured in soils
that are moderately wet rather than waterlogged or dry.
Management
Control the insects producing the honeydew.
Severe infections may be washed off with soapy water,
but should usually disappear during dry weather once
the cause has been removed.
Importance
Rhizoctonia is a common cause of death in young plants.
It does not appear to be a problem in more mature trees.
Management
Grow nursery plants in soil-less potting mix, or sterilised
or pasteurised soil mixes.
Maintain optimum plant growth and avoid injuring
plants, because wounds provide a means of entry for the
fungus.
SOOTY MOULD
Cause
Several fungi, most commonly Capnodium sp.
Symptoms
A black, soot-like growth grows over the surface of plant
parts. Severe sooty mould may affect plant growth
Fig 13.11 Sooty mould gives leaves a sooty appearance and is often
associated with scale insects.
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