Agriculture Reference
In-Depth Information
Most diseases produce characteristic symptoms that often
allow an accurate diagnosis or, alternatively, narrowed
down to a few possibilities. Sometimes a definite diagnosis
can be made only by using laboratory tests that allow the
pathogen to be isolated from diseased tissue and identified.
These tests should always be done if there is any doubt
about the cause of a particular disease or if problems are
being encountered during its control. Information that
helps identify the cause of a disease includes the cultivar
affected, location of affected plants in a field, weather
conditions, crop sequences, and fertiliser and chemical
treatments applied to the crop.
Disease development and management
All diseases caused by pathogens are the result of an
interaction between the host plant, a pathogen, and
environmental factors such as light, temperature and
moisture. Environmental factors affect the development of
both the host and the pathogen. This interaction is known
as the 'disease triangle' and all components must be
compatible for a disease to develop (Fig 1.13).
Fig 1.12 The head of plant parasitic nematode showing the stylet, a
spear-like structure. Below: root-knot nematode symptoms.
Symptoms of disease
The first step in the diagnosis of a disease is recognising
the visible signs or symptoms in the plant. Symptoms are
the results of disturbing one or more of the vital functions
of the plant, such as:
Disease-management strategies aim to favour the host
plant's growth and development while attacking
vulnerable stages in the life cycle of the pathogen to
prevent or restrict its development. The three key means
of disease management are: exclude the pathogen; reduce
inoculum levels of the pathogen; and protect the host
plant.
uptake of water and minerals by roots (e.g. root rots)
•
translocation of carbohydrates, water and minerals (e.g.
vascular wilts and cankers)
•
photosynthesis and respiration (e.g. leaf blights, leaf
spots, mosaics)
•
reproduction (e.g. fruit rots, smuts)
•
Exclusion or eradication
•
Use of pathogen-tested seed and vegetative propagation
material (e.g. budwood, cuttings and nursery trees
produced under strict hygiene procedures).
Humans
Quarantine, including international, national and
State quarantine zones, prevents movement of infected
plant material. Illegal movement of material is a
major threat to several of Australia's horticultural
industries.
•
The eradication of a pathogen before it becomes
widespread. This is more likely to succeed if an incursion
is detected soon after it has occurred and the pathogen
cannot be dispersed by air-borne spores or insect
v e c t o r s .
•
Reduction in inoculum levels
•
Crop rotation reduces pathogen populations during the
growth of nonhost crops.
Incorporating organic manures into the soil increases
the activity of microorganisms antagonistic to soil-borne
plant pathogens.
•
Fig 1.13 The disease triangle.
