CROP RESISTANCE TO PESTS

Mechanisms of resistance to insect pests have been defined as involving antixenosis (non-

preference), antibiosis, and tolerance (Smith, 1989). In some cases, it may not be possible to

separate antixenotic effects from antibiotic effects. Antixenosis as a resistance mechanism is

where the pest prefers other cultivars or species as hosts.

Interaction of crops with plant pathogens

• Causes of plant diseases can be divided into two major categories: BIOTIC and ABIOTIC.

A) Biotic Cause:

• There are five major biotic causes of plant diseases.
• These are the fungi, bacteria, viruses, nematodes and phytoplasmas.

a) Fungi:

• belong to the Ascomycetes and the Basidiomycetes.
• reproduce both sexually and asexually via the production of spores and other structures.
• Spores may spread long distances by air or water, or they may be soil borne.
• Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells.
• Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells.

b) Bacteria:

• are actually saprotrophic, and do no harm to the plant itself. However, a small number, around 100 known species, are able to cause disease.
• Most plant pathogenic bacteria are rod-shaped (bacilli).
• Five main types of bacterial pathogenicity factors are known: uses of Cell wall-degrading enzymes, Toxins, Effector proteins, Phytohormones and Exopolysaccharides

c) Viruses, viroids and virus-like organisms:

• Under normal circumstances, plant viruses cause only a loss of crop yield.
• Plant viruses must be transmitted from plant to plant by a vector. This is often by an insect (for example, aphids), but some fungi, nematodes, and protozoa have been shown to be viral vectors.

d) Nematodes:

• parasitize plant roots.
• Root knot nematodes have quite a large host range, whereas cyst nematodes tend to be able to infect only a few species.
• Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle.

e) Protozoa:

• transmitted as zoospores that are very durable, and may be able to survive in a resting state in the soil for many years.
• They have also been shown to transmit plant viruses.
• When the motile zoospores come into contact with a root hair they produce a plasmodium and invade the roots.

g) Parasitic plants:

• Dodder transmit virus or virus-like agents from a host plant to healthy plant.

B) ABIOTIC Cause:

• Often plant abnormalities caused by abiotic factors are called INJURIES.
• Common abiotic factors that can influence plant health include nutritional problems (including nutrient deficiencies or toxicities), pesticide exposure, environmental pollutants, and adverse weather.e.g. mango tip rot or necrosis.
• Abiotic damage often occurs on many plant species.