Plant disease trinity and epidemics

Ngangbam Punita *

Isn’t it infuriating when we take tremendous efforts to nurture our plant and they succumb to diseases and all our efforts go away in vain ? On a global scale 36.5% of agricultural products are lost annually due to diseases, insect-pests and weeds. On the other hand, by 2050 the world population is expected to hit 10 billion. We need to provide sustenance for this huge population.

But the challenge here is that every year the area under agriculture is reported to be diminishing due to several reasons. Among many solutions, the two may be discussed as probable solutions to this problem.

Firstly, to reduce the annual losses due to diseasesor/and pests. Secondly, to explore genome editing technology. However, the latter does come with certain ethical issues. This article attempts to focus on ways to reduce annual crop losses on account of diseases.

Plant disease has always been a challenge for humans with regard to food security. History has laid down several examples of catastrophic losses caused by plant pathogens. The Great Bengal Famine of 1943 led to the death of three million people. A Nation is considered to have a strong foundation when its population attains food security as well as nutritional security.

To prevent the occurrence of catastrophic losses, we need a better understanding of how plant disease actually develops. The disease triangle drawing most likely was first published by Stevens in 1960, although earlier plant pathologists certainly recognized the interaction among plant, pathogen, and environment. The three factors include a virulent pathogen, a susceptible host, and a conducive environment.

These factors form the trinity of plant disease develop- ment popularly called as the disease triangle. The triangular relationship is unique to phytopathology as compared to veterinary or medi- cal science because plants possess little thermal storage capacity and their im- mobility precludes escape from inhospitable environment.

For a successful disease outbreak, the prevalence of the above three factors is mandatory. Absence of any of the three components leads of failure in disease development. Even if the pathogen is virulent and the host is susceptible, if the environmental factors are not favourable then there will be no disease.

In case of resistant cultivars, even if the pathogen is virulent and the environment is conducive, there will be no disease outbreak. The three components are represented on the vertices of the triangle and the area under the triangle represents the amount of disease development.

Quantitatively spea- king, if all the factors are favourable then an equilateral triangle will be formed as all factors contribute equally. If any one of the factors is null, then there will not be a triangle formed thus indicating no disease development.

Recent approaches include time and human as additional factors to the original three components forming a disease tetrahedron. Indeed, the amount of each of the three components of plant disease and their interactions in the development of disease are affected by a fourth component : time.

Both the specific point in time at which a particular event in disease development occurs and the length of time during which the event takes place affect the amount of disease. If the four components of the disease tetrahedron could be quantified, the volume of the tetrahedron would be proportional to the amount of disease on a plant or in a plant population.

Disease development in cultivated plants is also influenced greatly by a fifth component : humans. Humans affect the kind of plants grown in a given area, the degree of plant resistance, the numbers planted, time of planting, and density of the plants. By the resistance of the particular plants they cultivate, humans also determine which pathogens and pathogen races will predominate.

By their cultural practices, and by the chemical and biological controls they may use, humans affect the amount of primary and secondary inoculum available to attack plants. 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, and so on.

The timing of human activities in growing and protecting plants may affect various combinations of these components to a considerable degree, thereby affecting the amount of disease in individual plants and in plant populations greatly.

Frequently, there is tendency to resort directly to apply higher dosage of chemicalsto control a disease as it often provides immediate results compromising ill impacts to the environment. A proper understanding of the disease trinity helps to escape disease and provides an outline to plan Integrated Disease Management (IDM) strategies rather than directly resorting to chemical interventions.

For instance, planting of early maturing varieties of pea helps to escape powdery mildew infection in pea plants. This is an example of manipulation of the environment component. Every disease causing pathogen has its own favourable environment and age of plant for infection.

Example: Late blight of potato requires a temperature of 17–20! and intermittent rain for 2–3 days. Therefore, it is important to have a clearer under- standing of the pathogen and plant growth behaviour.

Several other IDM strategies that favour plant growth and development in general and prevent pathogen proliferation may be listed below :

o Crop rotation to prevent pathogen population build up in the soil
o Follow recommended spacing to avoid overcrowding which is favou- rable for foliar diseases
o Sowing of seeds at 2–3 cm depth. Increase in planting depth increases pathogen infection
o Destroying off season volunteer crops, weeds, and crop debris
o Avoid excessive application of nitrogenous fertilizers
o Application of organic amendments to enhance the soil microbiome diversity
o Adoption of appropriate irrigation system (drip irrigation is more advisable as sprinkler irrigation creates a favourable environment for foliar pathogens)