Fundamentals of Plant Pathology

CLASSIFICATION OF PLANT DISEASES

Based on plant part affected

• Localized, if they affect only specific organs or parts of the plants.
• Systemic, if entire plant is affected. or

They can be classified as root diseases, stem diseases, foliage/foliar diseases, etc.

Based on perpetuation and spread

• Soil borne -when the pathogen perpetuates through the agency of soil.
• Seed borne -when the pathogen perpetuates through seed (or any propagation material).
• Air borne -when they are disseminated by wind e.g. rusts and powdery mildews.

Based on the signs and symptoms produced by the pathogens

• Diseases are classified as rusts, smuts, powdery mildews, downy mildews, root rots, wilts, blights, cankers, fruit rots, leaf spots, etc. In all these examples, the disease are named after the most conspicuous symptom of the disease appearing on the host surface.

Based on the host plants affected

They can be classified as cereal crop diseases, forage crop diseases, flax diseases, millet diseases, plantation crop diseases, fruit crop diseases, vegetable crop diseases, flowering plant diseases, etc.

Based on major Causes

They can be classified as fungal diseases, bacterial diseases, viral diseases, mycoplasmal diseases, etc.

Based on Infection Process

• Infectious -All the diseases caused by animate causes, viruses and viroids can be transmitted from infected host plants to the healthy plants and are called infectious.
• Non-infectious- Non-infectious diseases can not be transmitted to a healthy plant. Also referred as non-parasitic disorders or simply physiological disorders, and are incited by abiotic or inanimate causes like nutrient deficiency or excess or unfavorable weather conditions of soil and air or injurious mechanical influences.

Classification of Animate Diseases in Relation to Their Occurrence

• Endemic diseases -which are more or less constantly present from year to year in a moderate to severe form in a particular geographical region, i.e. country, district or location.
• Epidemic or epiphytotic diseases - which occur widely but periodically particularly in a severe form. They might be occurring in the locality every year but assume severe form only on occasions due to the favourable environmental conditions occurring in some years.
• Sporadic diseases occur at irregular intervals and locations and in relatively few instances.
• Pandemic diseases: A disease may be endemic in one region and epidemic in another. When epiphytotics become prevalent through out a country, continent or the world, the disease may be termed as pandemic.

Disease triangle

• The interaction of the host, the pathogen and the environment results in disease development. It is generally illustrated by a triangle, also called a disease triangle.

Disease Development in Plant Population
This is determined by:

• Host: All conditions in host that favour susceptibility.
• Pathogen: Total of virulence, abundance etc.
• Environment: Total of conditions that favour the pathogen and predispose the host plants to pathogen attack.
• Time: Specific point of time at which a particular event in disease development occurs and the duration or length of time during which the event takes place.

‘Effective disease control or measures aim at breaking this E-H-P triangle’.