Question:
Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Pear cultivars when grafted on Pyrus serotina or P. ussuriensis shows quick decline
Reason (R) : Mycoplasma damages the xylem tissue at graft union depriving the roots of food material
In light of the above statements, choose the most appropriate answer from the options given below.
Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Pear cultivars when grafted on Pyrus serotina or P. ussuriensis shows quick decline
Reason (R) : Mycoplasma damages the xylem tissue at graft union depriving the roots of food material
In light of the above statements, choose the most appropriate answer from the options given below.
Assertion (A) : Pear cultivars when grafted on Pyrus serotina or P. ussuriensis shows quick decline
Reason (R) : Mycoplasma damages the xylem tissue at graft union depriving the roots of food material
In light of the above statements, choose the most appropriate answer from the options given below.
Show Hint
Phytoplasmas and mycoplasma-like organisms are strictly phloem-limited pathogens. Any statement claiming they damage the xylem is physiologically incorrect.
Updated On: Jul 31, 2026
- Both (A) and (R) are correct and (R) is the correct explanation of (A).
- Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
- (A) is correct but (R) is not correct.
- (A) is not correct but (R) is correct.
Show Solution
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The Correct Option is C
Solution and Explanation
Step 1: Understanding the Concept:
Pear decline is a devastating disease affecting pear orchards, particularly when European pear (Pyrus communis) cultivars are grafted onto certain rootstocks.
The disease is caused by Candidatus Phytoplasma pyri, a specialized, cell wall-less prokaryote (formerly referred to as a mycoplasma-like organism or MLO).
The pathogen is transmitted by the pear psylla vector (Cacopsylla pyricola) and resides exclusively within the food-conducting tissues of the host plant.
Step 2: Detailed Explanation:
Assertion (A) is correct:
European pear cultivars grafted onto Asian rootstock species, specifically Pyrus serotina (Japanese sand pear) or Pyrus ussuriensis (Ussurian pear), are highly susceptible to pear decline.
When these trees are infected, they can undergo a rapid collapse known as “quick decline,” where the leaves wilt suddenly, dry up, and the tree dies within a few weeks.
Reason (R) is incorrect:
The physiological explanation provided in the Reason contains a critical scientific error.
Phytoplasmas (mycoplasmas) do not inhabit or damage the water-conducting xylem tissue.
Instead, they are phloem-limited pathogens that reside inside the sieve tube elements of the phloem tissue.
The quick decline occurs because the phytoplasma induces necrosis and callose deposition in the sieve tubes of susceptible rootstocks just below the graft union.
This phloem damage blocks the downward transport of photosynthates (organic food material/carbohydrates) from the leaves to the root system.
The roots are starved of food, die, and can no longer absorb water to support the tree, leading to sudden wilt and death.
Since the Reason claims the damage occurs in the xylem tissue, it is biologically incorrect.
Step 3: Final Answer:
Therefore, Assertion (A) is correct but Reason (R) is false.
Pear decline is a devastating disease affecting pear orchards, particularly when European pear (Pyrus communis) cultivars are grafted onto certain rootstocks.
The disease is caused by Candidatus Phytoplasma pyri, a specialized, cell wall-less prokaryote (formerly referred to as a mycoplasma-like organism or MLO).
The pathogen is transmitted by the pear psylla vector (Cacopsylla pyricola) and resides exclusively within the food-conducting tissues of the host plant.
Step 2: Detailed Explanation:
Assertion (A) is correct:
European pear cultivars grafted onto Asian rootstock species, specifically Pyrus serotina (Japanese sand pear) or Pyrus ussuriensis (Ussurian pear), are highly susceptible to pear decline.
When these trees are infected, they can undergo a rapid collapse known as “quick decline,” where the leaves wilt suddenly, dry up, and the tree dies within a few weeks.
Reason (R) is incorrect:
The physiological explanation provided in the Reason contains a critical scientific error.
Phytoplasmas (mycoplasmas) do not inhabit or damage the water-conducting xylem tissue.
Instead, they are phloem-limited pathogens that reside inside the sieve tube elements of the phloem tissue.
The quick decline occurs because the phytoplasma induces necrosis and callose deposition in the sieve tubes of susceptible rootstocks just below the graft union.
This phloem damage blocks the downward transport of photosynthates (organic food material/carbohydrates) from the leaves to the root system.
The roots are starved of food, die, and can no longer absorb water to support the tree, leading to sudden wilt and death.
Since the Reason claims the damage occurs in the xylem tissue, it is biologically incorrect.
Step 3: Final Answer:
Therefore, Assertion (A) is correct but Reason (R) is false.
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