Plant Diseases – Viruses

Viruses are an interesting classification of plant diseases, though they differ from the kingdoms we have looked into so far (bacteria, plant parasites, and water molds).  Viruses are comprised of bits of nucleic acid (genetic code) encased in protein. They cannot live or reproduce outside of a host cell, but they can orchestrate their reproduction in host cells and can cause disease. Viruses can infect bacteria, algae, fungi, plants, and animals. Viruses are typically host- specific, and are very small (20 – 2,000 nanometers).  Virus shapes can be rod-like, spherical, or icosahedral (20 facets). Nearly 1,000 virus species infect plants.

According the Essentials of Plant Pathology (by Dr. Gail L Schumann and Dr. Cleora J. D’Arcy), virus origin theories includes:

• Descendants of genes that escaped from host cells. (Interestingly, Lynn Margulis and Karlene V. Schwartz observe in Five Kingdoms An Illustrated Guide to the Phyla of Life on Earth that viruses seem more closely related to their hosts than to other members of their species).
• Evolved as independent life forms, likely predating other forms of life.

Viruses are typically named after the first economically important host in which they were found, and the symptom they induce. The initials of the name are used to identify it. For example, the first well-studied plant virus was the Tobacco Mosaic Virus, or TMV.  Mosaic refers to the yellow and green mosaic pattern the virus exhibits in the tobacco plant leaves (the symptom). In the San Francisco Bay Area, several viruses are a threat, including the Tomato Yellow Leaf Curl Virus (TYLCV), and the Grape Leafroll Virus (GLRaV).

Fig Mosaic Virus (FMV) - in residential garden in Berkeley, California

Since viruses are not mobile, they are typically spread via insect vector or nematodes, especially those with piercing-sucking mouth pieces (stylettes). When an insect  or nematode pierces a plant for the sap, viruses may be sucked up and stored in the vector's body, and then deposited in the sap of other plants as it feeds. When deposited into a plant cell, the virus is replicated. Variability may arise from mutation. Viruses rarely kill the plant, but may cause symptoms such as stunting and dwarfing, chlorosis (yellowing), mosaic patterns, ringspot, flower break, leaf roll, and fruit deformation. Viruses do not produce survival structures, but many can exist as inanimate molecules for years in the soil or leaf debris without a living host (others can exist only for a few hours or minutes). Since we have no cure for viruses, prevention is the best course of action, including managing insect vectors that transport the virus.