1. Chapter 3: Major economically important Plant diseases in
Ethiopia and their management
Definition of some important terminology
 Plant Disease?
 A physiological disorder or structural abnormality that is harmful to the plant or only it’s parts or products that reduced
the economic value.
• A plant disease is an abnormality in the structure and/or function of the host plant cells and/or tissue as a result of a
continuous irritation caused by a pathogenic agent or an environmental factor.
• A disease is not static; it is a series of changes in the plant.
• Plant disease is the result of an infectious, or biotic (a living component of an ecosystem) agent or a noninfectious, or
abiotic (nonliving, physical and/or chemical component) factor.
 Plant injury is an abrupt alteration of form or function caused by a discontinuous irritant. Plant injury includes insect,
animal, physical, chemical or environmental agents.

2.  A causal agent is a general term used to describe an animate or inanimate factor which incites and governs disease and
injury.
 A causal organism is a pathogen of biotic origin.
 The signs and symptoms
 Signs are the appearance and/or physical evidence of the causal factor of the plants abnormality.
 Signs are the physical evidence of damage caused by biotic or abiotic agents such as the pathogen itself, pests,
spores, fruiting bodies, chemical residue, bacterial ooze and so forth.
 Symptoms are the visible response of a plant to biotic and/or abiotic factors that result in a change or abnormality in the
plant.
 Symptoms can take form as galls, chlorosis, ring-spots, wilt, rotand so on.
 The life cycle of an infectious disease is the sequence of distinct events, such as sexual reproduction, that occur between
the appearance and reappearance of the causal organism.
 The stages of the disease cycle are the appearance, development and perpetuation of a pathogen and the effect of the
disease on the host.
Definition of some important terminology….

3. 3
Principles of Plant Disease Management
Principles of Plant Disease Management
i) Avoidance: It is preventing the contact between the host and the pathogen. It is done by identifying geographical area,
selection of a proper field, planting time, disease escaping varieties and avoidance of insect vectors and weed hosts.
ii) Exclusion: It means preventing the entrance and establishment of pathogens in uninfected crops in a particular area.
• It can be achieved by
 quarantine,
 inspection and certification,
 seed treatment and
 also by using certified seed or plants,
 sorting bulbs before planting,
 discarding any that are doubtful, possibly treating seeds, tubers or corms before they are planted and
 most importantly refusing obviously diseased specimens from dealers.

4. 4
iii) Eradication: It involves elimination of a pathogen once it is established on a plant or in a field.
• It can be done by crop rotation, sanitation, rouging, soil treatment, use of antagonists and heat and chemical
treatment to diseased plant material.
iv) Protection: It is the use of some protective barrier between the susceptible host and the pathogen.
• In most cases, a protective spray or dust is applied to the plant in advance of the occurrence of pathogen.
• Sometimes, it is achieved by killing insects or other inoculating agents.
v) Host resistance/Immunization: It utilizes in-built mechanism to resist various activities of pathogen.
• The infection or subsequent damage by pathogen can be rendered ineffective through genetic manipulation
or by chemotherapy.
vi) Therapy: It is achieved by treating the plant with that will inactivate the pathogen. Chemotherapy,
thermotherapy are effective.

5. 5
Integrated Disease Management
• Integrated plant disease management (IDM) can be defined as a
• decision-based process involving coordinated use of multiple tactics for optimizing the control of pathogen
ecologically and economically.
• In most cases IDM consists of scouting with timely application of combination of strategies and tactics.
The basic objectives of any IDM program should be to achieve at least the following:
• Reduce the possibility of introducing diseases into the crop.
• Avoid creating conditions suitable for disease establishment and spread
• Simultaneous management of multiple pathogens
• Regular monitoring of pathogen effects, and their natural enemies and antagonists.
• Use of economic or treatment thresholds when applying chemicals.
• Integrated use of multiple, suppressive tactics.
Integrated Disease Management