Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Monocropping of genetically identical crop varieties would enable the pathogen to evolve.
Reason R: Pathogen evolution is rapid in biotrophic interactions.

In light of the above statements, choose the correct answer from the options given below:

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Monoculture = Uniform Selection Pressure = Rapid Pathogen Adaptation.
This is why genetic diversification (using multiline varieties or cultivar mixtures) is recommended to control epidemics.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Agricultural ecosystems differ from natural ecosystems because they often consist of large expanses of genetically uniform crop varieties (monocultures).
This setup has profound consequences on pathogen population genetics and evolutionary dynamics.
Detailed Explanation:
Let us analyze both statements:
Assertion A:
Monocropping of a single genetically uniform variety exerts immense, uniform directional selection pressure on the pathogen population.
Any rare mutant pathogen that can overcome host resistance will multiply unchecked across the entire crop area, leading to rapid selection and evolution of virulent strains.
Thus, Assertion A is true.
Reason R:
Biotrophic pathogens depend entirely on living host cells to complete their life cycle.
Because of this intimate biological dependence, host-parasite interactions are governed by a gene-for-gene co-evolutionary arms race.
This makes pathogen evolution exceptionally rapid in biotrophic interactions compared to necrotrophs.
Thus, Reason R is true.
Analyzing the connection:
The reason why monocropping enables rapid pathogen evolution is the uniform, widespread selection pressure created by genetically identical host plants across thousands of hectares.
While biotrophic evolution is indeed rapid, this biotrophic interaction is not the primary cause of evolutionary pressure in a monocropping system; even necrotrophic pathogens adapt quickly under uniform host landscapes.
Therefore, both statements are true, but R is not the correct explanation of A.

Step 2: Final Answer:
Both A and R are true, but R is not the correct explanation of A.
Thus, the correct option is (B).