3. Plant disease epidemiology : Concepts of monocyclic, polycyclic and polyetic diseases withone example of each, disease triangle and forecasting of plant diseases.

 3. Plant disease epidemiology

Concepts of monocyclic, polycyclic and polyetic diseases with one example of each, disease triangle and forecasting of plant diseases.

Studying plant diseases in a particular plant population is known as plant disease epidemiology (PDE) .

 Plant diseases are caused by the vast circle of diversified pathogens which continuously tend to undergo mutations and recombination, thus generating their new strains efficiently for their survival in the ecosystem.

PDE involves the striving efforts of the researchers to intervene(take part in something so as to prevent or alter a result or course of events)  the favorable conditions of plant disease development thus considerably lowering the economic losses .

 Firstly in 1963, J.E van der Plank introduced about the tools that could have been possibly used for predicting epidemic break down of some diseases caused by foliar pathogens . 

Visually, PDE involves integrated strategy via incorporation of agronomical, biological, ecological and statistical tools . 

Explaining these tools, biological studies are necessary for better understanding of pathogen reproductive cycle and its developmental stages. Similarly, disease incidence is calculated via incorporation of agronomic values .

While plant species, pathogen races, environmental fluctuations, insect vectors are broadly a part of ecological studies. All the above mentioned practices are calculated via statistical tools to better understand PDE.

Types of PDE :

Various types of PDE can be stated depending upon the virulence of pathogen strains and cultivation plans. These may include monocyclic, polycyclic, bimodal polycyclic and polyetic PDEs .

Monocyclic:

Slowly reproducing pathogens having one infection cycle in a single year can result in Monocyclic PDE. 

For example, Fusarium wilt of flex caused by soil born Fusarium oxysporumf.sp. lini

 Polycyclic :

the pathogens embedding the multiple infection cycles in a single season lead towards the Polycyclic PDE. 

e.g. airborne popular fungal disease powdery mildew.

 Polyetic diseases : 

If a single crop is being grown every year in that vicinity again and again, this leads towards the accumulation of the pathogen strains and building up of the inoculums for a particular disease. This type of PDE is referred as polyetic PDE and may be caused via both monocyclic and polycyclic pathogens . 

For example, powdery mildew of apple epidemics caused by polycyclic pathogen i.e. Podosphaera leucotricha and Dutch elm disease epidemics  due to monocyclic Ophiostoma novoulmi.

The disease triangle :

The disease triangle embeds three common elements i.e. host, pathogen, environment . 

The disease triangle is a conceptual model that shows the interactions between the environment, the host and an infectious (or abiotic) agent.

Exclusively for PDE, the triangle completes when the host is susceptible, pathogen is virulent and environment is favorable for infection establishment .

 In simple words, the conjugation of above three guarantee the disease development. 

The other important thing that can play its role in all this process is time. 

So to rule out the chances of disease, any of the three factors can be modified or fluctuated.

 For example, plants susceptibility can be minimized via incorporation of disease resistant or tolerant plant cultivars.

Pathogen can be managed via application of chemicals, cultural methods or other practices . 

The environmental conditions are natural and cannot be manipulated via physical efforts by human beings . But the forth factor i.e. time allows us to quickly manage the pathogen before occurrence of favorable pathogenic environmental conditions. 

The time is very crucial as it is variable for every pathogen regarding infection establishment .

Plant disease forecasting:

 PDE is predicted via a management system through complete understanding of disease severity known as plant disease forecasting (PDF) .

 PDF is utilized by the state departments and farmers for making the economic decisions for the better management of plant diseases at field levels. 

The PDF involves various questions which are answered by the growers. 

These questions include information about susceptibility of cultivars, weather forecasts and existing disease symptoms.

 Intelligently answering these questions by growers can help them in taking correct decisions through PDF systems .

 PDF system relies upon plant-pathogen-environment interaction studies to predict the PDE.

 Here the most important factor that is uncertain and crucial is environment as the cultivars and pathogen existence stands stable. 

Environmental conditions directly affect the pathogen and cultivar performance as reviewed earlier with examples. 

A better PDF system embeds reliability, simplicity, cost effectiveness, and wide applicability to a number of diseases . 

The PDF are normally designed for irregular and more damaging diseases rather than the regular diseases as growers well understood about the diseases that prevail every year and they can efficiently manage them using various methods. 

The first PDF was emerged to warn the farmers regarding Stewart’s wilt and it was basically designed via indexing winter temperature with a theme that lower temperature will cause the death of vector thus leading towards the reduction in PDE .

 But talking about the multiple PDF system, a successful example is EPIPRE (Epidemiology, prediction and prevention) system developed by Netherlands to forecast about PDE of winter wheat against multiple pathogens.

Reference:

file:///C:/Users/KKW%20PC212/Downloads/1522791098HV_5_1_7-11.pdf