Agriculture Reference
In-Depth Information
POWDERY MILDEW
■
Cause
The fungus
Sphaerotheca humuli
.
Symptoms
Light yellow to green patches develop on young crown
leaves, becoming water-soaked and showing a white,
powdery growth. Mature leaves are not susceptible.
Spreading, circular white patches occur on the surface of
small developing fruit. These patches may coalesce to
cover large areas but remain largely superficial. As the
fruit develops, the white mould disappears leaving light
grey, scarred areas. Malformed fruit may result from
restricted growth of tissues underlying affected areas.
Fig 14.25 Phytophthora root rot on field papaya plants after cyclone
Larry. Compare the row with blown over, susceptible trees with the
row of tolerant trees, still standing. Inset: root rot damage.
Source of infection and spread
The white mildew produced on leaves and fruit contains
large numbers of spores that are spread to adjacent trees by
wind. The fungus is most active during the winter months.
Disease development is favoured by moderate temperatures
and low-to-moderate relative humidity.
Management
•
Remove and destroy all fallen fruit and trunks of trees.
Apply recommended fungicides during wet weather.
•
Choose only well-drained sites for planting papaya.
•
Plant only disease-free trees.
•
Importance
This is a common field disease causing losses through fruit
scarring and malformation. The disease disappears with
the new spring growth, leaving scarred areas on fruit.
Ensure adequate surface drainage by constructing
diversion and interception drains.
•
Plant on mounds (0.5-1 m), positioned to allow rapid
removal of surface water.
•
Management
•
Do not replant areas where phytophthora diseases have
previously occurred.
•
Spray with recommended fungicides when powdery mildew
infection begins or when conditions favour the disease.
Avoid introducing the fungus into clean areas.
•
Fig 14.26 Phytophthora root rot showing collapsed older leaves.
Inset: symptoms on young tree.
Fig 14.27 Powdery mildew on a green papaya fruit.
