Plant pests, diseases and disorders

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

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

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

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

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. 

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.

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

 

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

 

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

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.

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.

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.

Selecting Equipment for Treatment

Methods of applying pesticidesMost 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