- What viruses are, and how they damage plants
- Symptoms of virus diseases
- Potential exotic disease threats to Alaska
- How to report or submit samples for diagnosis
- Vocabulary associated with virus diseases
Viruses are incredibly small pathogens, consisting of nucleic acid (DNA or RNA) with a protein coat. Their genetic material only codes for a handful of genes related to replication, structure and movement in their host. They are obligate parasites, only replicating inside living cells. All organisms (animal, plant, fungi, bacteria, etc.) are susceptible to viruses. Some viruses have huge host ranges, for example Cucumber mosaic virus can infect over 1200 species of plants. Other plant viruses have very narrow host ranges, and only infect a few related species. Plant viruses are often named for the host they are first found in.
Plant viruses can harm plants in multiple ways. Replication of the virus saps host resources, as well as damaging host cells and organs. Infection can kill plants, reduce growth, abort flowers, lower yield, lower quality, as well as incur losses due to trade restrictions. Viruses contribute to food insecurity worldwide, and billions of dollars are lost every year.
Symptoms of virus infection can vary widely, dependent on host, virus isolate, and environmental conditions. Viruses may cause stunting, chlorosis (yellowing), mosaic (pattern of light and dark tissue), necrosis (death of tissue), ringspots (rings of light-colored tissue, sometimes concentric), line patterns (similar to ringspots, but not complete circles), rugosity (blisters or uneven surface), leaf rolling or curling, and other deformations.
Viruses move from plant to plant by several mechanisms. Vectors such as aphids, beetles, whiteflies, fungi, and nematodes transmit many viruses. Pollen transmission may occur, as well as seed transmission. Many viruses are mechanically transmissible, by sap contact. Grafting and other propagation techniques are also routes of transmission.
Exotic Virus Pathogens
Cucumber green mottle mosaic virus
Cucumber green mottle mosaic virus (CGMMV) is a member of the Tobamovirus Family, a group of 35 devastating viruses. The virus primarily infects cucurbits like cucumber and squash. CGMMV was discovered in the 1930’s in the UK, and is now found in Africa, Asia, Europe, Australia, and even Antarctica (at a research station). Losses of 40-80% may occur in greenhouse production. In 2013, the virus was discovered in Canada, as well as a California cucumber field and neighboring melon field. CGMMV was discovered again in California in 2014, in a different introduction. Particles are very stable, and can remain infectious for at least 8 months at room temperature. The virus may persist in plant debris, soil, and seed surfaces. The virus is easily transmitted mechanically (by rubbing and sap transmission), as well as through pollen, seed, and water. There are no known animal vectors. Infected seed is the most likely source of CGMMV introduction to an area. CGMMV is carried on the seed coat, as well as within the seed. Typically, only 1-5% of infected seed give rise to infected seedlings, but due to the ease of transmission, epidemics still occur. Routine plant handling in greenhouses and fields provides ample opportunities for spread.
Hosts and Symptoms
Hosts of CGMMV are generally limited to the Cucurbitaceae. Cucumber, squash, and melons are the main hosts, along with common weeds such as pigweed (Amaranthus spp.), lambsquarters (Chenopodium spp.), and purslane (Portulaca spp.). Tomato has also been infected experimentally.
In general, symptoms are more severe in spring, when temperatures and light levels are lower. Seedlings often don’t display strong symptoms, though cotyledons may yellow. Leaves may show vein-clearing (loss of green on veins and surrounding veins). Young leaves may be crumpled. Mature leaves may be chlorotic or bleached, mottled (light and dark areas), blistered or stunted.
Fruit may display green mottling, spotting, or streaking. Fruit may show no symptoms, or fruit with no external symptoms may be internally discolored or necrotic.
Roots may be underdeveloped. These symptoms are not definitive for CGMMV, testing is required for identification. Several testing methods are available: antibody-based (ex. “dipsticks”, lateral flow devices, or ELISA), and nucleic acid-based (ex. PCR).
Management of CGMMV
The International Seed Health Initiative for Vegetable Crops utilizes stringent seed testing to exclude CGMMV. Growers should only buy seed from reputable dealers. Incoming cucurbit seed can be treated by heat (160°F for 3 days), chemical (trisodium phosphate 10% (w/v) solution for 20 minutes, followed by rinse), or both. CGMMV infection sometimes extends beyond the seed coat into the seed where heat and chemical treatment are not completely effective. Surfaces should be cleaned of debris before sanitizing, to ensure adequate contact, and minimize chemical degradation. Planting trays and tools should be sanitized with 10% bleach, or 2% potassium peroxymonosulfate. Quaternary ammonium compounds are not as effective for CGMMV. Sanitize grafting tools and surfaces. Hands and boots should be sanitized before entering and leaving greenhouses. Sanitize floors, walls, and equipment. Soil mixes should be pasteurized or sterilized. Inspect seedlings regularly after the two-leaf stage. Symptomatic plants should be sampled and tested. Destroy weeds in and around greenhouses.
Groundnut bud necrosis virus
Groundnut bud necrosis virus (GBNV) is a member of the Tospoviridae Family. These viruses are spread by thrips, in a circulative, propagative manner (they circulate through the insect body, they also replicate in the insect). The virus must be acquired by thrips larvae, adults cannot acquire it. Once acquired, both larval and adult thrips can transmit the virus for life. Several of the major thrips vectors are present in Alaska. GBNV is not seedborne. At present, it is distributed throughout Asia: Bangladesh, China, India, Indonesia, Iran, Nepal, Pakistan, Sri Lanka, and Vietnam. Losses have been estimated at over $89 million annually.
Hosts and Symptoms
The main hosts are in the Families Fabaceae and Solanaceae. In potato, leaves may develop brown necrotic ringspots, veinal necrosis, downward curling, and foliage may collapse (resembling late blight). Potato stems and petioles may become necrotic. In India, up to 30% losses have occurred in infected potatoes.
Tomato leaves may develop necrotic rings, and foliage may collapse (resembling late blight). Tomato stems may develop necrosis. Tomato fruit may have concentric rings and patchy color.
In legumes (beans and peas), symptoms include leaf chlorosis and necrosis, vein browning, stem necrosis, and pods with ringspots.
Other hosts include onions, cucumber, pumpkin, carrot, petunia, beet, pigweed, and lambsquarters. Other viruses may cause similar symptoms (ex. Tomato spotted wilt virus), requiring laboratory identification and confirmation.
Management of GBNV
Introduction of GBNV to an area is often from movement of infected planting stock. Inspect all incoming materials, reject those with disease symptoms. Transport of infected thrips may also occur, as there are many interceptions of thrips at ports and airports.
Plum pox disease, or sharka (caused by Plum pox virus)
Plum pox virus (PPV) is the cause of plum pox disease, also known as sharka. PPV is a member of the Potyviridae Family, the largest plant virus Family (with over 200 virus species discovered). PPV is considered one of the most important virus diseases of stone fruit in Europe and the Mediterranean region. The estimated impact on the peach, plum and apricot industries is $600 million per year. Some susceptible plum cultivars (varieties) may lose 80-100% of their crop. Presence of PPV in a country can result in loss of exports, as well as increased domestic prices due to loss of fruit.
Plum pox disease (sharka) was first reported on plums in Bulgaria in 1915. It initially spread slowly in Europe, but is now found throughout Asia (including Japan), the Mediterranean region, Russia, Argentina, Chile, Canada, and the U.S. In 1999, PPV was found in Pennsylvania, and later in Michigan and New York. In 2000, PPV was found in Ontario and Nova Scotia. It has been eradicated in Pennsylvania, Michigan and Nova Scotia. Eradication efforts continue in New York and Ontario. There are 8 strains currently recognized worldwide, the strain currently found in the U.S. is less aggressive than many. Some strains appear to have a host dependency, and are not easily transmitted to other hosts.
Long distance movement of PPV is through movement of plant materials. Some strains may be seed transmitted. PPV is aphid transmitted, in a non-persistent manner (virus is acquired and lost quickly by the aphid). Virus is acquired and transmitted when the aphid performs short test probes of potential food sources. The virus does not replicate in the insect. Infective aphids can carry virus on their mouthparts for 1-3 hours, but lose it quickly after probing a new leaf. Aphids may also acquire PPV from harvested fruit. Approximately 20 aphid species can transmit PPV, at least four of the major vectors are present in Alaska.
Aphids can spread PPV distances over one mile, but more often within 100 yards of the virus source. Infections in an orchard often skip trees, giving a widely scattered pattern. In many areas, PPV spreads the most in spring and fall. PPV is also mechanically transmissible by sap to sap contact, as well as by grafting.
Hosts and Symptoms
All cultivated stone fruit species (apricot, peach, plum, etc.) are susceptible to PPV infection. There are over 60 host species in 16 plant families. Symptoms may be conspicuous or subtle, depending on host cultivar, virus isolate, time of infection, and environmental conditions. In general, leaf symptoms include vein yellowing and light green or yellow rings. Symptoms develop in cooler weather, fading during warm periods. Symptoms may be sporadic, and may only appear on isolated leaves or branches. Symptoms may take years to develop, or may remain symptomless.
In plums, leaves may develop pale green or yellow chlorotic spots, blotches, bands, rings, or line patterns. Symptoms are most easily seen after leaves are fully expanded. Fruit may develop rings or blotches, and may be deformed or display uneven ripening. Fruit may drop prematurely, and stones may show necrotic rings.
In peach, new leaves may be distorted when first unfolding, often with wavy edges and a slight twist. Veins may develop pale green or bright yellow flecks or line patterns. Leaf symptoms may disappear with time. Fruit may develop lightly pigmented rings or line patterns. Flowers may exhibit color break (streaks or stripes of different colors).
Apricot leaves display lighter symptoms than plum or peach. Fruit may be misshapen, brown, or necrotic. The stone may display rings.
Cherry (multiple species) leaves may show pale green patterns or rings. Fruit may show deformities, including notched marks, as well as chlorotic/necrotic rings. Fruit may drop prematurely.
Fruit yield and quality are reduced for all hosts; sugar content is reduced, and fruit becomes unmarketable. Even symptomless trees suffer yield losses. Productive lifespans of orchards are also reduced.
PPV also infects wild and ornamental Prunus, including European bird cherry. Non-Prunus hosts include tomato, nightshades, petunia, peas, clover, sweet clover, celosia, chrysanthemum, veronica, vetches, and zinnia. Weedy hosts include buttercup, chickweed, dandelion, lambsquarters, and shepherd’s purse.
Symptoms of PPV infection may be confused with other viruses, as well as fungal diseases like rusty spot (powdery mildew, Podosphaera spp.), bacterial canker (Pseudomonas spp.), nutritional deficiencies, mechanical damage, or insect damage caused by thrips, leafhoppers, or scale.
Management of PPV
PPV is managed by exclusion of potentially infected materials, through the use of quarantines and certification. Only certified virus-free material should be imported into Alaska. Symptomatic plants need to be sampled and tested. Infected plants must be removed and destroyed. Contact insecticides are used in states where PPV occurs, these kill faster than systemics. Remember that aphids acquire PPV quickly, and transmit it quickly. Control weeds and other hosts in and near nurseries and greenhouses.
For a printable version of information on plum pox disease view and download our factsheet here.
Reporting and submitting samples
Early Detection and Rapid Response (EDRR) is a framework used by federal and state agencies to manage exotic or invasive pests. The goals are detection of pests before they can establish, assessment of management options, and quick response to prevent spread and minimize damage and expense. If you find a disease you don’t recognize as native, submit a sample to a laboratory for diagnosis.
- Call the lab to ensure they can perform the tests needed. This also gives them lead time to prepare for your samples and expect their arrival. Call your local UAF Cooperative Extension Office, and fill out an identification form; or call the Alaska Division of Agriculture, Plant Materials Center, Plant Pathology Laboratory: (907) 745-8138, http://plants.alaska.gov/PathologyForms.html.
- Examine the plant carefully; damage further down may cause symptoms higher on the plant.
- If possible, include a healthy example, as well as various stages of disease development from early to more severe.
- Keep samples cool, not frozen.
- Send entire plants if possible, including roots. Place roots in a plastic bag, and rubber band to keep soil from damaging or obscuring other symptoms on plant.
- Fleshy specimens (such as potatoes or fruit) should be as firm as possible, with early to intermediate symptoms. Place in paper bags (they will rot in plastic bags). Place wrapped samples in plastic zipper-top bags. Place the plastic bag inside another plastic zipper-top bag (this gives a double-bagged sample for security). Pack box with sufficient newspaper or paper towels to absorb any leaks, and to protect samples.
- Wrap other sample types in dry paper towels or newspaper. Do not add moisture, as samples can rot in transit. Place wrapped samples in plastic zipper-top bags. Place the plastic bag inside another plastic zipper-top bag (this gives a double-bagged sample for security). Pack remainder of box with newspaper to protect samples.
- Collect specimens as close to mailing as possible, keep cool, and ship by fastest method available. Ship early in the week (Mon.-Wed.) to ensure it arrives before the weekend. Delays can result in samples rotting, or being overgrown with other organisms; this may make accurate diagnosis difficult or impossible.
- If sending to the Alaska Division of Agriculture, the address is:
Plant Pathology Laboratory
Alaska Plant Materials Center
5310 S. Bodenburg Spur Rd.
Palmer, AK 99645
- Digital photos may also be submitted to aid in diagnosis: firstname.lastname@example.org
Sources and More Information
Gergerich, R.C., and Dolja, V.V. 2006. Introduction to Plant Viruses, the Invisible Foe. The Plant Health Instructor. DOI: 10.1094/PHI-I-2006-0414-01
U.S. Code of Federal Regulations for PPV: https://www.ecfr.gov/cgi-bin/retrieveECFR?gp=1&SID=36beb9336d8b90a8225b7196b3567b10&h=L&mc=true&r=SUBPART&n=sp7.5.301.xx11
Mackesy, D.Z., and Sullivan, M. 2016. CPHST Pest Datasheet for Tobamovirus Cucumber green mottle mosaic virus. USDA-APHISPPQ-CPHST.
Mackesy, D.Z., and Sullivan, M. 2016. CPHST Pest Datasheet for Tospovirus Groundnut bud necrosis virus. USDA-APHISPPQ-CPHST. Revised August 2017 by L. R. Morales.
Pundhir, V.S., Akram, M., Ansar, M., and Rajshekhara, H. 2012. Occurrence of stem necrosis disease in potato caused by Groundnut bud necrosis virus in Uttarakhand. Potato Journal 39(1): 81-83.
Akram, M., and Naimuddin. K. 2010. First report of Groundnut bud necrosis virus infecting pea (Pisum sativum) in India. New Disease Reports 21: 10. doi:10.5197/j.2044-0588.2010.021.010.
Akram, M., Naimuddin, K., Pratap, A., Malviya, N., and Yadav, P. 2013. First report of Groundnut bud necrosis virus infecting wild species of Vigna, based on NP gene sequence characteristics. Phytopathologia Mediterranea, 52(3): 532-540.
Sullivan, M., and Mackesy, D. 2011. CPHST Pest Datasheet for Plum pox virus. USDA-APHIS-PPQ-CPHST. Revised April, 2014.
Damsteegt, V. D., Scorza, R., Stone, A. L., Schneider, W. L., Webb, K., Demuth, M., and Gildow, E. 2007. Prunus host range of Plum pox virus (PPV) in the United States by aphid and graft inoculation. Plant Disease 91:18-23. DOI:10.1094/PD-91-0018