In plant virology and crop protection, what are the key disadvantages and risks associated with using cross-protection (pre-inoculation with a mild strain to protect against a severe strain)?

Introduction / Context:
Cross-protection is a classical plant virology strategy in which a plant is deliberately inoculated with a mild strain of a virus to protect it against subsequent infection by a severe strain. Although the approach can work, it carries notable disadvantages that agronomists and growers must weigh before field deployment.

Given Data / Assumptions:

• The method uses a live, mild (attenuated) virus strain.
• Plants may be exposed to other endemic viruses in the environment.
• Field conditions can favor viral evolution and spread via vectors.

Concept / Approach:
The disadvantages cluster around evolutionary risk (mutation/reversion), epidemiological risk (spread), and ecological/virological interactions (synergism). Any of these can undermine yield or biosecurity. Therefore, a comprehensive answer should acknowledge all categories of risk rather than a single issue.

Step-by-Step Solution:

Verification / Alternative check:
Case reports document breakdowns of cross-protection due to strain evolution and mixed infections. Regulatory frameworks often require containment or monitoring precisely for these reasons.

Why Other Options Are Wrong:

• A, B, C each capture only one facet; a complete statement must include all.
• E is incorrect because these risks are well recognized in the literature and practice.

Common Pitfalls:
Assuming a mild strain is genetically stable; RNA viruses, in particular, mutate rapidly. Also, confusing cross-protection with vaccination in animals leads to underestimating plant-to-plant spread.

Final Answer:
All of the above