Agriculture Reference
In-Depth Information
Plant resistant varieties where appropriate, and where
available.
Races or strains may also occur within a species. This
sometimes develops in response to a host-resistance gene
being used for management.
•
Further details are given in the sections on specifi c crops.
•
Symptoms
Powdery mildew appears as spots or patches of white to
greyish, powdery growth (mycelium) on the surface of
leaves and other plant parts. The mycelium is most visible
on the upper leaf surface, often covering it completely as
the disease progresses.
TRANSIT ROT (RHIZOPUS SOFT ROT)
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Cause
The fungus
Rhizopus stolonifer
.
Importance and hosts
Transit rot or Rhizopus soft rot occurs throughout the
world on a wide range of fruit, vegetable and f flower crops.
As the name 'transit rot' implies, the disease is usually only
important during storage, transport and marketing of
produce. The fruit crops most commonly affected are berry
fruits, grape, kiwifruit, mango, papaya and stone fruit,
particularly peach, nectarine and cherry. Rhizopus can
spread rapidly in produce held under warm, moist
conditions, causing severe losses within a short period.
Source of infection and spread
Fungi causing powdery mildew grow largely on the surface
of plants. They are obligate parasites and obtain nutrients
by sending feeding organs (haustoria) into the epidermal
cells of plants.
The superficial fungal mycelium produces chains of spores
(conidia), which are widely dispersed by air currents. The
spores do not require free water for germination and
germinate freely in relatively low humidities including
moisture from morning dews, fog or condensate. Disease
development is favoured by warm, dry and especially,
cloudy conditions, which limit damage to the fungus by
ultraviolet radiation. Humid, wet weather slows disease
progress. Overwintering fruiting bodies called
cleistothecia are produced late in the season, or when
conditions become unfavourable. These appear as tiny,
pinhead size, yellow-gold and later brown to black bodies
within the mycelium. These fruiting bodies survive in leaf
litter and crevices of plants or on alternative host species
until spring, when ascospores are released to begin new
infections. In warmer areas, the fungi may survive on
alternative hosts or as mycelium or conidia in buds and
other plant parts.
Symptoms
Infected fruit first show circular, water-soaked areas and
the skin is easily displaced from the f flesh. The disease
develops rapidly into a soft, wet rot covered by a cottony
white fungal growth, which produce small, white, then
black, stalked fruiting bodies (sporangia or spores). The
bushy growth of the fungus extends to the surface of
healthy parts of the fruit and sometimes to the surface
of the packing container, forming a nest of mycelium. Soft
rotting develops when enzymes, produced by Rhizopus,
dissolve cell walls, causing the cells to collapse and leak,
producing a soft decay. Juices released by soft rot often
have a fermented or acidic odour, giving the disease the
alternative name 'leak' in strawberries.
Importance
Damage from powdery mildew may take some time to
develop. Efficiency is reduced in affected leaves and fruit
can be scarred and damaged, causing produce to be
downgraded. Severe outbreaks can cause defoliation,
exposing fruit to sunburn and predisposing them to
secondary rots.
Source of infection and spread
Rhizopus is an extremely common fungus on dead or
decaying plant material. Spores of the fungus are abundant
in the atmosphere but infect mature fruit, usually only
through wounds or damaged tissue. Warm, humid-moist
Management
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Use appropriate cultural-management procedures,
including removal of diseased twigs and crop debris,
to reduce inoculum levels.
Apply pre-infection (protectant and post-infection,
eradicant/curative fungicides.
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Fig 2.22 Transit rot (Rhizopus soft rot) on peach and papaya.
