Question

Sequence the Cross combinations of the apple rootstock
(A) Malling 2 $\times$ Northern Spy
(B) Malling 1 $\times$ Northern Spy
(C) Northern Spy $\times$ Merton 793
(D) Northern Spy $\times$ Malling 2
Choose the correct answer from the options given below:

• A. M.25 - M.111 - M.104 - M.106
• B. M.111 - M.25 - M.106 - M.104
• C. M.104 - M.106 - M.111 - M.25
• D. M.104 - M.111 - M.106 - M.25

Hint:

All Malling-Merton (MM) series rootstocks carry genes from 'Northern Spy' because it was used as the donor parent to introduce resistance to woolly apple aphid.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
This question focuses on the genetic parentage and cross combinations of the Malling-Merton (MM) series and related rootstocks of apple (*Malus domestica*).
These rootstocks were bred by crossing various Malling (M) clones with 'Northern Spy' to introduce resistance to woolly apple aphid (*Eriosoma lanigerum*).

Step 2: Detailed Explanation:
Let us analyze each cross combination and identify its resulting rootstock:
(A) Malling 2 $\times$ Northern Spy:
This cross was made to produce semi-vigorous, well-anchored rootstocks.
Depending on the specific selection retained, this cross combination yielded both M.104 (MM.104) and M.111 (MM.111).
Both are highly resistant to woolly apple aphid, with MM.111 being widely used for its drought tolerance.
(B) Malling 1 $\times$ Northern Spy:
This cross yielded M.106 (commonly known as MM.106), a highly popular semi-dwarfing rootstock that is sensitive to collar rot but offers excellent production and early bearing.
(C) Northern Spy $\times$ Malling 2 (represented as D in the question):
This reciprocal cross yielded M.25, a highly vigorous rootstock that induces early and heavy cropping compared to other vigorous stocks.
Let us match this with the sequential options:
- (A) Malling 2 $\times$ Northern Spy $\rightarrow$ M.104
- (B) Malling 2 $\times$ Northern Spy (selection variant) $\rightarrow$ M.111
- (C) Malling 1 $\times$ Northern Spy $\rightarrow$ M.106
- (D) Northern Spy $\times$ Malling 2 $\rightarrow$ M.25
This sequence matches the order: M.104 -- M.111 -- M.106 -- M.25.

Step 3: Final Answer:
The correct sequential representation of these cross combinations is M.104 - M.111 - M.106 - M.25.
This corresponds to Option (D).