Plant pests, diseases and disorders

1. Plant pests, diseases and
disorders

2. Crops need to be protected from loss or damage from pests
and diseases. It is important to correctly identify the pest,
disease or disorder so that the problem can be effectively
treated.
Common plant pests
• Mites
• Scale
• Aphids
• Moths
• Others – including mealybug, fruit fly, weevil, locust

3. Plant pests
Mites
Belong to the spider family and live in a wide range of habitats
Scale
Most scale insects are parasites of plants, feeding on sap drawn directly from the
plant's vascular system. Adult female scales are almost always immobile and
permanently attached to the plant they have parasitised. They secrete a waxy
coating for defence and it is this waxy covering that makes them quite resistant to
pesticides.
Aphids
Aphids are small plant-feeding insects. Like scale, some species of ants farm
aphids, protecting them on the plant they eat, and eating the honeydew that the
aphids release from their anus
Moths
Moths, and their caterpillars, are a major agricultural pest in many parts of the
world

4. Plant diseases and disorders
Pathogens are micro-organisms that can cause disease and these can be
classified into 4 groups
• Fungi
• Bacteria
• Viruses
• nematodes
nematodes
fungi

5. Plant diseases and disorders
Fungal diseases
Fungi are simple plant-like bodies, which do not produce their own food,
and therefore live parasitically on living plants, or saprophytically on
dead rotting material. Examples of fungal diseases attacking the green
tissue of plants, i.e. leaves and unripe fruit include Powdery mildew,
Downy mildew, Black spot, Botrytis and Stripe, Stem and Leaf Rust.
Examples of fungal diseases attacking roots and woody stems include,
crown and root rots, damping-off diseases, fusarium root rot, Verticillium
wilt or Black heart and rhizoctonia root rot

6. Bacteria
Bacteria are microscopic in size cells with simple nucleus and
no chlorophyll. As plant diseases they are not as important as
fungi. Bacteria also play an important part in the breaking down
of plant material and the fixation of nitrogen from the
atmosphere.
Viruses
These organisms are not cellular, but are ribonucleic acid (RNA)
particles which can multiply only within living cells, by distracting
the cells from their normal function to produce more virus
infected cells, and to produce substances required by the virus.
Viruses are usually spread by vectors, such as insects. Infected
pollen is another method whereby viruses can spread.

7. Nematodes
Nematodes are long eel-like organisms ranging in length from 0.5 to 3.0mm.
They move through soil via the moisture films between and around the soil
particles. All nematodes spend time in the soil but some also attack the stems
and leaves. They enter the leaves via the stomata. The disease damage
caused by nematodes is a result of the saliva secreted while the organism
feeds. The above ground symptoms of nematodes are not specific, plants
simply lack vigour. Field diagnosis of nematode damage is based on digging
up plant roots and inspecting the nature of damage. Root knot nematode
results in small bead or gall style swellings on the roots. Lesion nematode
results in open wounds on the root system.

8. Recording and reporting plant disease
A comprehensive report will include:
• Geographic location (i.e. mapped site)
• Photographs of the problem, either the observed pest, or the observed
symptoms
• A sample placed in a sealed jar or bag
• Plant affected
• Growth stage of the affected plant
• Approximate number of affected plants and the size of the area affected
• Possible source of introduction
• The date and time of the observation of the problem
It is preferable to use as many of these methods as necessary to make
sure you can find it again

9. Selecting treatment methods
Agricultural methods of controlling pests, diseases and disorders include:
• Quarantine
• use of disease free or resistant (or tolerant) planting material
• cultural - for example sanitation and disposing of crop residues
• soil sterilisation
• crop rotation
• provision of alternative hosts
• avoidance or amelioration of unfavourable conditions, for example careful
timing of weed, soil and water management operations
• chemical agents such as protectants and eradicants

10. Horticultural methods of controlling pests, diseases and disorders include:
• Quarantine
• use of disease free or resistant (or tolerant) planting material
• cultural - for example sanitation, the establishment and maintenance of
standards of hygiene such as removing diseased roots, limbs or whole plants;
cleaning pruning and budding and grafting equipment; disposing of crop residues
• soil sterilisation
• provision of alternative hosts
• avoidance or amelioration of unfavourable conditions, for example careful
timing of weed, soil and water management operations
• biological agents - use of natural enemies to attack disease causing organisms,
for example, No Gall® for Crown Gall in stone fruit
• chemical agents such as protectants and eradicants

11. Pest control
Chemical Control
The most frequently used pesticides are fungicides, insecticides,
herbicides and miticides but there are also nematicides,
molluscicides and petroleum oils. The way in which a pesticide works
is known as its mode of action. Understanding the modes of action of
pesticides may help in deciding the best pesticide to use.
Understanding the life cycle of pests and the developmental stages
of crops is vital in terms of targeting vulnerable stages of pest
organisms with control measures.
The decision to use pesticides should take into account the level of
disease or pest in the plantings, the stage of development of the
plants and weather and other environmental conditions.

12. Physical and Mechanical Control
This is one of the oldest methods that has been in use. It includes
measures like the collection of egg masses and other inactive stages and
the removal of infested parts or whole plants. This method can prove to be
effective during the initial stage of the pest incidence and when practised
as a concerted effort by a large number of farmers in a particular area.
Biological Control
The biological control involves a large scale multiplication of natural
enemies and release of such organisms, or creating conditions under
which the naturally occurring organisms can act effectively.
Cultural Control
This is a preventive method which is inexpensive and may prove more effective and
efficient after acquiring a thorough knowledge of the life history and habits of a pest.
Sanitation and hygiene provide the basis for cultural control of diseases. Crops in
good growing condition are best able to withstand the effects of disease.

13. Cultural control in agriculture
Deep ploughing after harvesting the crop (to expose the hiding and resting
insects), adjusting the time of sowing (to avoid the peak incidence period), clean
cultivation, the removal of alternative wild hosts, catch crops and suitable rotations
are some of the important measures included under this method of control. Oats
are an important element of crop rotation, suppressing the wheat disease ‘take-all’.
Cultural control in horticulture
Proven disease-free planting material should always be used. Many diseases
“carry over” in roots, leaves, unharvested fruit and infected wood. Diseased wood
should be pruned out and burnt together with normal pruning's. Pruning and fruit
thinning practices that improve aeration, light penetration and drying of the canopy
and fruit will help reduce disease and also help control pests such as mealybug.

14. Selecting Equipment for Treatment
Methods of applying pesticides
Most agricultural/horticultural pesticides are applied as:
• sprays or
• fumigants
Other application methods tend to be confined to specialised or limited
operations and include:
• dust application
• granule application
• seed and grain treatments, and
• pesticide baits
The equipment that can be used includes:
• granule applicators
• sprayers
• fumigant injection rigs