4. Nature/Concept of Plant Disease Objective: Introduce students to the

meaning of plant disease. Introduction

It is not known whether plant feel pain or discomfort, because plants do not communicate to us.

It is thus, difficult to pin-point exactly when a plant is diseased.

However, it is generally accepted that a plant is healthy or normal, when it can carry out its physiological functions to the best of its genetic potential.

Nature/concept of disease cont

Disease in plants can then be defined as a series of harmful physiological processes

caused by continuous irritation by the primary agent.

Or a series of invisible or visible responses of plant

cells and tissues to a pathogenic organism or environmental factor that result in adverse changes in form, function or integrity of the plant and may lead to partial impairment or death of the plant parts or the entire plant.

A series of physiological processes means that disease is not static but dynamic.

Nature/concept of disease cont

Normally, the kind of cells and tissues affected determine the type of physiological function of the plant that will be disrupted first.

For example: Root infection may cause them to rot and

make plants unable to absorb water and nutrients from the soil.

Infection of the foliage interferes with photosynthesis.

Nature/concept of disease cont

Plant physiological processes affected normally include:

Growth Photosynthesis Respiration Reproduction Translocation of photosynthetes, etc.

The primary agent is essential for occurrence of disease.

This primary agent is called the pathogen or causal agent of disease.

5. Symptomatology Learning objectives:

Students to understand and categorise various disease symptoms and signs for purposes of disease diagnosis.

Familiarise with plant disease symptom terms. Introduction: Symptoms are:

The detectable expressions of disease or the detectable responses of the plant to disease are called symptoms.

The study of disease symptoms is called symptomatology.

Symptomatology cont

Symptoms are a result of complicated physiological disturbances.

For a particular disease, these disturbances are commonly combined to form a distinctive symptom complex.

Symptom complexes can be utilised to assist in disease diagnosis.

Symptomatology cont

Classification of symptoms. Plant disease symptoms can be classified in a

number of ways, and the terminologies used to define symptoms is extensive.

However, for the purposes of this class, a simplified classification of symptoms will be used.

1. Symptoms may be: (a) Localised in form of lesions such as – leaf

spots, galls, canker, rots, etc. Such symptoms are confined to a specific area of infection.

Localised symptoms

Localised symptoms

Symptomatology cont

Plant disease symptoms may also be: (b) Generalised or systemic.

Generalised symptoms involve or include changes in habits, e.g.

Wilt Chlorosis Etiolation, etc

Generalised symptoms

Generalised symptoms

Symptomatology cont

2. In addition, symptoms may also be classified as: (i) Primary symptoms – These are symptoms

which cause direct changes in tissues attacked by the primary agent or the pathogen.

(ii) Secondary symptoms – These are symptoms which are caused by the indirect effect of the pathogen on distant or un-invaded plant tissues. E.g. – Wilting as a result of root infection.

Usually, but not always, localised symptoms are primary symptoms and generalised symptoms are secondary symptoms.

Primary symptoms

Primary symptoms

Secondary symptoms

Symptomatology cont

3. Symptoms may also be classified as: (a) Morphological symptoms –

Symptoms which can be externally detectable, therefore, seen by the naked eye.

(b) Histological symptoms – those symptoms which can be detectable only by the use of the microscope through microscopic examination of the diseased tissue.

Symptomatology cont

General Categories of symptoms Plant disease symptoms may be divided into

three general categories based on the effect t on plants:

1. Necrotic symptoms: Those symptoms resulting from the ceasation of function, thus leading to death.

Examples include: Blight – rapid dealth of foliage. Die back – progressive death of twigs and branches

from their tips toward the trunk. Spot – Lesion, usually defined circular or oval in shape

with a central necrotic area surrounded by various coloured zones.

Decay – Disintegration of dead tissue, etc.

Necrotic symptoms

Necrotic symptoms

Symptomatology cont

2. Hypoplastic symptoms: These are symptoms which result from

underdevelopment or retardation of functions, e.g.

Chlorosis – failure of chlorophyll development Dwarfing – Sub-normal size in an entire plant

or some of its parts. Rossetting – crowded condition of foliage due

to lack of internode elongation. Etiolation – yellowing due to lack of light, etc.

Hypoplastic symptoms

Symptomatology cont

3. Hyperplastic symptoms (hypertrophy): These are symptoms which result from

over-development or acceleration of function, e.g.

Gall/tumour formation – swelling or overgrowth on a plant or plant part produced as a result of infection.

Scab – roughened crust-like lesions.

Hyperplastic symptoms

Symptomatology cont

Signs: Physical presence of the pathogen

structures on the an infected area, e.g.

Presence of fungal mycelium or spores Bacterial ooze, etc

Signs caused by rust fungi

Signs of rust fungi