Grape Leafroll Associated Virus (GLRaV)

The Grape Leafroll Associated Virus (GLRaV) colonizes the phloem in the plant’s vascular system, cutting off distribution of nutrients to leaves, shoots and fruit. This delays maturity of the grapes, reduces fruit yield, and lowers accumulation of sugars (the Brix), affecting wine quality. Symptoms include reduced vigor, yellowing of the leaf margin and veins, and the characteristic under curl of the leaves. The symptoms are more noticeable in red varieties than white varieties. Symptoms are typically most evident in mid-August, but, in some cases, there are no visible symptoms. The virus is a group of ten strains, identified as GLRaV- followed by a number. Take a look at the symptoms on a variety of grape types:

http://youtu.be/Es41Tdgd9L0

According to the brochure, "Grapevine Leafroll – an Increasing Problem in the Finger Lakes, the US and the World", published by the Cornell University Cooperative Extension, this is a worldwide problem threatening many grape-growing regions. The virus has been found in both eastern and western United States grape growing regions, including California. Even the San Francisco Bay Area backyard gardener may encounter the virus.

The virus spreads through infected bud wood cuttings and graft unions. If the grafts are not clean, infected starter plants can be distributed unwittingly around the globe. European researchers discovered the virus is also spread through insect vector - Pseudococcus mealy bugs (grape, obscure, and longtailed) and soft scales (Parthenolecanium corni and Pulvinaria innumerabilis). The vectors hitch hike on farm workers, equipment, and plant debris, and can quickly spread from an infected field to a neighboring clean field.

Since vines cannot be cured once they are infected, the best strategy is preventing infection. Plant only certified clean root stock. Inspect the plants routinely, looking for symptoms and signs. Manage mealy bugs and soft scales using organic methods. Once infection is detected, replace the infected vines with certified clean stock at once. Testing is available for several strains of the Grape Leafroll associated viruses; see the U.C. Davis Integrated Pest Management (IPM) site for information.

Tomato Leaf Curl Virus (TYLCV)

The Tomato Leaf Curl Virus (TYLCV) infects tomato plants, and sometimes other hosts in the Solanaceace family (nightshade and jimsonweed). Symptoms include small leaves that curl upward, crumpling, and yellowing on the margins and veins. Flowers fall off the plant, reducing the crop. The plants themselves look stunted—due to shortened internodes—giving the plant a bushy appearance. Other hosts may exhibit minimal symptoms, or no symptoms.

According to the Tomato Yellow Leaf Curl brochure, published by the University of California Agriculture and Natural Resources Statewide IPM Program, the disease was first observed in Israel in the 1940s; spread to the Caribbean islands and southeastern United States in the 1990s; moved to northern Mexico, Texas, and Arizona in the 2005-2006; and was found in a greenhouse in Imperial County in 2007. The disease thrives in tropical and subtropical environments, and in Mediterranean climates.

Upward leaf curling and reduction in internode length in tomato infected with TYLCV.
Copyright, ASPnet

Bemisia whiteflies and some leafhoppers are the main vectors for the disease. After feeding on an infected plant, the insect is infected for its lifetime and spreads the virus by feeding on uninfected plants. When tomato plant hosts are not available, it is thought that the virus can exist in weed hosts. The virus is not transmitted in seeds or mechanically (by touch), but can be transmitted if infected plants are transported to uninfected areas. If the virus spreads to California, it could have a significant negative impact for commercial tomato growers, and for backyard tomato enthusiast

For management tips, see the Integrated Pest Management (IPM) site. In general, recommendations include planting TYLCV-resistant varieties, maintaining “tomato-free” periods when there is no host, eliminating weeds that could be hosts during tomato-free periods, removing diseased plants, avoiding transport of infected plants to uninfected areas, and managing whiteflies. TYLCV symptoms resemble symptoms for other diseases, but accurate tests are available to identify the virus.

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.