General Characteristics of Orchid Viral Diseases
Orchid viral diseases are the number one killer affecting orchid cultivation, and orchid enthusiasts all turn pale at the mere mention of viruses. Orchid viral diseases are also ubiquitous. They not only infect cultivated orchid varieties but also infect wild orchids growing under natural conditions. Once an orchid is infected by a virus, its ornamental and cultivation value will...
A virus is an obligate parasite, much smaller in size than bacteria, generally about 10-30μm in size, and cannot be seen under an ordinary optical microscope. Viruses have no cell wall or nucleus structure, and appear fibrous, rod-shaped, etc. Under general circumstances, some viruses can be extracted from plant tissues to form crystals. Under an electron microscope, the virus crystals contain extremely neatly and tightly arranged virus particles.
A virus is composed of two parts: nucleic acid and protein. The nucleic acid contained in viruses parasitic on Chinese orchid plants is overwhelmingly ribonucleic acid (RNA). The viral nucleic acid is enclosed within a protein coat. When a virus infects a cell, only the nucleic acid enters the plant cell, while the protein coat is left outside. Therefore, only the nucleoprotein portion is infectious, and its pathogenicity is mainly determined by its nucleic acid portion. A single virus may have two different forms. Particles of different forms may have different genetic roles. Only when particles of different forms enter the host together can the virus express its full infectivity, pathogenicity and other genetic characteristics.
Orchid viruses have a very high capacity for multiplication. Their mode of multiplication differs from the reproduction of cellular organisms. They do not absorb nutrients within the parasitic cell to form a large virus and then split into small viruses; instead, they replicate in the same manner as ribonucleic acid (RNA). Under the direction of the viral nucleic acid, the host cell synthesizes substances identical to the viral nucleoprotein within the cell, thereby forming new viruses. The harmful effects of viruses also lie roughly in altering the metabolic pathways of the parasitic cell and disrupting normal physiological processes. Within the orchid body, the virus appears to destroy the chloroplasts within the cell, causing the cell to lose its green color, creating obstacles to the cell's assimilation of nutrients, and causing leaf tissue to become yellowed or necrotic. Whether it is the virus particles directly acting on the cells or the toxins released by the virus that have an effect must be further studied.