Cross-protection in plant virology: How can cross protection against a virulent virus strain be achieved in a susceptible crop?

Introduction / Context:
Cross protection is a classical plant virology phenomenon where infection by a mild virus strain prevents or reduces disease caused by a more virulent, closely related strain. It has practical value in certain cropping systems when reliable mild strains exist and can be managed safely.

Given Data / Assumptions:

• The aim is resistance to a severe strain of the same virus.
• Cross protection leverages viral interference rather than transgenic methods.

Concept / Approach:
Establishing a persistent, mild infection can occupy replication niches, saturate movement pathways, and trigger host responses that collectively limit superinfection by aggressive strains. This is biologically distinct from coat protein–mediated resistance or RNAi strategies stemming from transgenes.

Step-by-Step Solution:

Verification / Alternative check:
Historical use includes citrus tristeza virus and other pathosystems where approved mild strains provided field-level disease management.

Why Other Options Are Wrong:

• Coat proteins alone or nucleocapsid genes do not constitute cross protection; those relate to CPMR or genetic engineering.
• “Any of the above” is incorrect because cross protection has a specific definition.
• SAR inducers can help but are not cross protection per se.

Common Pitfalls:
Confusing cross protection with transgenic resistance; ignoring the regulatory and epidemiological considerations of intentionally spreading mild virus strains.

Final Answer:
Inoculate with a mild strain of the same virus to induce protection against the severe strain