Epidemiology of plant diseases

Epidemiology of plant diseases. Collected by Syed Zahid

1. Epidemiology of Plant diseases Epidemic When a pathogen spreads to and affects many individual; Within a certain area and within a relatively short time, this type of phenomenon is called an epidemic. An epidemic has been defined as any increase of disease in a plant population.

2. Epidemiology : The study of epidemics and of the factors that influence them is called epidemiology. Epidemiology deals with the interaction of host, pathogen and environmental factors that leads to an epidemic disease. Plant disease epidemics; sometimes called epiphytotics , occur annually on most crops in many parts of the world. Most epidemics are more or less localized and cause minor to moderate losses.

3. Examples Irish potato famine of 1845-1846 was caused b\ the Phytophthora late blight epidemic of potato.

4. Examples The Bengal famine of 1943 was caused by Bipolaris oryzae ( Syn.Name ; Cochiobolus / Helminthosporum ) brown spot epidemic of rice.

5. The Bengal Famine

6. The elements/component of an epidemic: Plant disease epidemics develop as a result of the timely combination of the elements that result in plant disease: 1. Susceptible host plants 2. A virulent pathogen and 3. Favourable environmental conditions over a fairly long period of time 4. Human may unwittingly help to initiate and develop epidemic through some of their activities ( eg . By topping or pruning plants in wet weather).

7. Interaction among the component Interaction of the four components can be visualized as a tetrahedron, or pyramid, in each plane represents one of the components. This figure is referred to as the disease tetrahedron or disease pyramid. So, the components/elements of epidemic: 1. Susceptible host 2. Virulent pathogen .. 3. Favourable environment 4. Time and 5. Human

8. Disease Triangle The interaction of the three components like host, pathogen and environment may be visualized as a triangle where each arm of the triangle represents one of the three components.

9. Disease tetrahedron: The interaction of the four components viz. host, pathogen, environment and time can be visualized as a tetrahedron or disease pyramid where each plane represents one of the components. If the four components of the disease tetrahedron could be quantified, the volume of the tetrahedron would be proportional to the amount of the disease on a host population.

10. Managed Pathosystem : Disease development in cultivated plants is greatly influenced by a fifth component: human . Human can play a role on the other components/factors, such as selection of the host, planting materials, changing planting/sowing time, irrigation, fertilization, spraying of pesticides etc.

11. Managed Pathosystem : They also modify the effect of environment on disease development by delaying or speeding up planting or harvesting, by planting in raised beds or in more widely spaced beds, by protecting plant surfaces with chemicals before rains, by regulating the humidity in produce storage areas, etc. Thus proper interactions of human with the other four factors could be the way of proper management of plant pathogen. This phenomenon is known as managed pathosystem .

12. Forms of epidemic: Based on occurrence, mode of spread and severity of infection, the epidemic may be defined in various ways: Local : When an epidemic occurs in a particular area. Regional : When an epidemic occurs over a wide areas or regions. Endemic : An epidemic which has established in a particular area for a long time and more or less appears constantly from year to year. Example: Gray spot of mustard in Bangladesh caused by Alternaria brassicicola and A . brassicae .

13. Forms of epidemic: Pandemic : When an epidemic occurs over most of a continent and causes mass mortally; Example: Late blight of potato caused by Phytophthora infestans ( lrish famine of 1845). Sporadic : An epidemic that occurs over a large area with irregular intervals and location, and in relatively fewer populations. Cyclic : An epidemic which appears periodically after certain intervals of time in a particular area.