What are viruses in the context of plant pathology?

In the context of plant pathology, viruses are indeed classified as infectious intracellular pathogens composed of protein and nucleic acid. They are much smaller than bacteria or fungi and consist of a core of genetic material, either DNA or RNA, surrounded by a protective protein coat. This structure allows them to invade host plant cells, where they replicate and often disrupt normal cellular functions, leading to disease symptoms in the plant.

Viruses reproduce inside living cells by hijacking the host's cellular machinery, which distinguishes them significantly from living organisms. This infectious nature and the reliance on host cells to propagate highlight their unique role in the ecosystem, particularly as they can cause significant agricultural damage and affect crop yields.

The other options do not accurately describe viruses. Single-celled organisms that perform photosynthesis refer to microorganisms such as algae or bacteria, which do not fit the definition of viruses. Multi-cellular parasites refer to higher-order life forms, which are also not related to the single-celled nature of viruses. Lastly, pathogenic fungi are a separate group that includes complex organisms; while they can also damage plants, they are fundamentally different from viruses in structure and reproduction.