Is there an area in your garden where you are battling noxious weeds or other invasive plants? If so, soil solarization is a solution that avoids the use of herbicides and saves you from the on-going work of pulling up the invaders. Our hot summer months are the ideal time to employ this method.

Soil solarization is a simple, non-pesticidal method for controlling diseases, nematodes and weeds that is ideal for the Central Valley of California. Placing clear plastic sheets over wet soil during hot summer temperatures for 4 to 6 weeks will kill most diseases, pests and weeds. In addition, research has shown that this method speeds up the breakdown of organic material in the soil and aids in the release of soluble nutrients such as nitrogen. Plants grown in solarized soil often grow faster and produce greater, better-quality yields.

Solarization most effectively controls soil-borne fungal and bacterial plant pathogens such as those causing Phytophthora root rot, Southern blight, Verticillium wilt, Fusarium wilt, damping off, crown gall, tomato canker and potato scab.

Nematodes can be controlled but not completely eliminated by solarization, because they are mobile and can move through the soil. Control is most effective in the top 12 inches of soil.

Most annual and perennial weeds can be controlled by solarization. It is particularly effectively in controlling winter and summer annuals. Perennial weeds are more difficult to control because roots and rhizomes can be buried deep in the soil. Solarization is not effective in controlling field bindweed, or yellow and purple nutsedge.

While solarization kills many soil pests, beneficial soil organisms either survive or recolonize the soil rather quickly. Earthworms can retreat to lower depths. Beneficial fungi such as mycorrhizal fungi and bacteria that parasitize plant pathogens quickly reestablish themselves.

Solarization is most effective when days are long, temperatures are hot, skies are clear, and winds are mild. June to August is therefore the most effective time in the Central Valley. Prepare the soil by removing weeds, rocks or other debris. Break up dirt clods and create a smooth soil surface.

Saturate the top 12 to 24 inches of soil. Wet soil conducts heat the best and is more effective in killing soil organisms. Cover the soil immediately after irrigation. Usually irrigation is not needed again, but very light or sandy soils may require a second irrigation. If needed, drip lines can be installed under the plastic.  

Use clear plastic to snugly cover the smooth soil surface. Lay the plastic as close as possible to the soil surface so there are few air pockets. Thin clear plastic (1 millimeter) provides the greatest heating potential but is more susceptible to tearing. Slightly heavier plastic (1.5 to 4 millimeters) may be best. The type of drop cloths used in painting work well. The plastic sheeting should be slightly bigger than the area to be solarized. Dig a trench around the area. Cover one edge of the plastic with soil to hold it down. Pull the plastic tight across the soil and bury the opposite edge. Do the same with all sides. Keep the plastic as close to the soil surface as possible.

After 4-6 weeks the plastic can be removed. Treated soil can be immediately planted or left fallow until the next growing season. Additional information can be found in the UC IMP Pestnote, Soil Solarization for Gardens & Landscapes.

The UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, call the Hotline at (530) 538-7201 or email mgbutte@ucanr.edu.

If you seek a peaceful place to explore nature and local history on foot or by bicycle, it may be time to explore Verbena Fields. This 20-acre site was historically a floodplain for Lindo Channel and later became used as a gravel mine for road construction.

It is now the home of a visionary restoration project within the City of Chico Lindo Channel Greenway. You can find Verbena Fields by following East First Avenue to its terminus in a quiet residential neighborhood. In addition to walking and biking opportunities, the site offers visitors a chance to explore the history of local wetlands, salmon lifecycles, gravel mining, and the culture of the Mechoopda Indians. It's a pocket of urban wilderness, designed to delight and inform.

The area was restored in partnership with state and local agencies, including the California Water Resources Board (a major funder), the CSU, Chico Research Foundation, Big Chico Creek Watershed Alliance, Streaminders, and the Mechoopda tribe. The restoration plan included input from neighbors, along with experts in wetland restoration, urban runoff mitigation, native plants, and local fauna. Together, these entities transformed a pile of rocks, broken asphalt, and noxious weeds into a natural refuge for humans, animals, and plants.

A combination of native and non-native plants reflects the site's mixed-use history. As part of the restoration, the site was planted in natives, including western redbud, California poppy, buckwheat, cottonwood, willow, and yellow and purple lupine (to name a few), and they exist side by side with walnut, mimosa, datura, and even a clump of bearded iris. Unfortunately, yellow star thistle is also becoming more abundant; early in the project's history, goats were introduced to control and eradicate this nuisance weed, but it is a tenacious and formidable intruder.

An attractive sign along the southern border of the site greets visitors by detailing the site's history and offers a project map and overview. You can expect to see information about native vegetation, the basics of wetland expansion and floodplain restoration (including the function of stormwater bioswales), and artful drawings of wildlife. A portion of the sign is devoted to the history of the Native American uses of the area, as well as the purpose of the on-site Mechoopda Maidu Interpretive Garden – so named because the project includes plants traditionally used by the tribe. The sign explains that the Interpretive Garden “will serve the community as a model of collaboration on the combined strengths needed to preserve, sustain, and protect the land. Together we embrace this mutual obligation to honor, protect, and steward the land with gratitude.” These words are, like Verbena Fields itself, inspiring and moving.

Let's take a walking tour of this site, exploring what it has to offer along the way,

A wide, level path of crushed gravel (handicap accessible and suitable for small children and strollers) meanders along the perimeter of Verbena Fields. A second path bisects the site approximately at its midsection. We enter at the southwest corner, where Verbena Lane and East First Avenue intersect. Signage lets you know that your dog is welcome, on a leash, and at this corner there is a drinking fountain and a dog bag dispenser. To the left, a level strip of small trees, shrubs, and wildflowers borders the backyard fences that make up the western boundary of the site. To the right, desire lines (informal paths) lead from level ground to the Mechoopda Gathering Circle, a lowland area that features a beautiful tile mosaic depicting aspects of tribal life.

Back on the upland path, and moving north towards Lindo Channel, a large grove of diverse trees is clustered on the right. An old walnut, an almond, a couple of oak species, and other mature trees create a dense wall that is growing up against an artifact from the past: a huge mound of river rock. Continuing east to follow the path as it runs parallel to Lindo Channel, a footbridge spans a side channel created to link the Channel to the low-lying wetland areas. An open, rocky floodplain flanks the Channel, and here yerba santa and naked buckwheat are interspersed with poppies and lupine. A short trail leads to an interpretive sign about floodplain restoration, complete with cross sections and a definition of floodplains and their important functions in urban and natural settings.

To the right is the open grassy area that comprises the bulk of the site and surrounds the wetland expansion portion of the project. The bisecting trail runs through the lupine and poppy (and also star thistle) that grow among the grasses. This middle trail affords access to a heavily shaded grove in which someone has fashioned a 10 to 12-foot- tall teepee from lumber scraps. An old rug has been placed inside, and it looks like a peaceful place to meditate, or the perfect place to play a game of fort.

The northeast corner of the project site is at the intersection of Lindo Channel and the backyard fences of the neighbors. As it heads back towards East First Avenue, the path is bordered by redbuds and oaks.

Springtime at Verbena Fields is particularly beautiful: grasses are green and supple, redbuds, poppies, and lupines combine to make a palette of bright color, and fresh leaves bud from the limbs of the deciduous trees. Summer is dryer and crisper, but the shady groves of large trees and the low-lying cool spots offer respite from the heat. Fall brings the changing of colors when the grasses mellow to shades of heather, and in winter (if we are lucky with the rains) one can appreciate the value of the bioswales and wetlands features, as well as the view of the foothills to the east.

Whatever the season, Verbena Fields is a delightful place to visit. Just follow East First Avenue eastward, away from the busy bustle of traffic, to find it.

 A virus can reduce a plant's growth, lower its yield, and result in inferior fruit, vegetables and flowers. Viruses are often very host specific. Recognizing symptoms can help you take appropriate action, where possible, to manage a virus and hopefully prevent its spread. The following are several viruses that can plague the home garden.

Mosaic Viruses

Viruses of this type can be identified by green and yellow mottling (“mosaic”) symptoms. In addition to the common Tomato Mosaic Virus (TMV) discussed in the previous article, mosaic viruses can affect cucumber, cauliflowers and lettuces. Choose resistant plants when available for your garden. Mosaic viruses are mostly spread by insects, especially aphids and leafhoppers. Try covering your plants with a floating row cover or use aluminum foil mulches to prevent these insects from infecting your plants. Some types of weeds may serve as hosts of the virus, so controlling weeds may be a deterrent.

Cucumber Mosaic Virus (CMV) is one of the most common mosaic viruses. It is usually spread by aphids. It was first found on cucumbers, hence its name; but it can affect many other woody and herbaceous plants, including tomato, melon, and squash. Symptoms include leaves mottled with yellow and/or light and dark green spots or streaks. Young plants may have twisted leaves and become stunted.

Tomato plants infected with CMV are stunted and exhibit “shoestring syndrome,” in which the edges of the leaves fail to develop. Leaf veins appear as long, narrow strips. Tomatoes are small and misshapen, and yield is reduced.

Once plants are infected with CMV, there is no control. Remove and destroy all the infected plants. Disinfect gardening tools that have come into contact with infected plants.

Cauliflower Mosaic Virus (CaMV) primarily infects plants such as cabbage, brussels sprouts, cauliflower, broccoli, rape seed and turnip. It is transmitted in a non-circulatory manner by aphids, which can acquire this virus within one minute of feeding on an infected plant and can transmit the virus to a healthy plant just as quickly. Systemic symptoms include vein clearing – a condition in which the veins on a plant's leaves become translucent. Later, a striking mosaic pattern may develop. Plants can be stunted, and infected cabbage heads develop internal brown flecks and spots when stored in the refrigerator.

Temperatures between 61 and 68 degrees favor symptom expression in plants. CaMV is often found as a mixed infection along with Turnip Mosaic Virus, resulting in more severe symptoms than when either virus is present alone.

Eradicate cruciferous weed hosts (such as wild mustard, wild radish and wintercress) and incorporate crop debris immediately after harvest.

Lettuce Mosaic Virus (LMV) is spread by aphids and infected seeds. It is seedborne in lettuce and can infect numerous other crops and weeds. With this virus, the leaves of young plants become stunted and deformed and leaves may show a mosaic pattern.

Infected plants may reach full size, but the older outer leaves will be yellow, twisted, and otherwise deformed. On head lettuce, the plant may not form heads or the wrapper leaves may curve away from the head. Developing heads may be deformed or brown, necrotic black or brown flecks occur on the wrapper leaves.

Plant resistant lettuce varieties when available and use certified seed that has been tested for the virus. Control weeds to remove potential virus sources and destroy post-harvest residue from crucifers, broccoli, cabbage, cauliflower, Chinese cabbage, and mustard.

Practice good aphid management. Control aphid-protecting ants, knock off aphids with a stream of water, or use a silver-reflective mulch to repel the aphids. Apply fertilizer only as necessary.

Beet Curly Top Virus (BCTV) is spread not by aphids, but by leafhoppers. This virus causes leaf curling and distortion, rigidity, stunting and eventual death in beets, Swiss chard, spinach, tomatoes, beans, peppers, cucumbers, squash and many other vegetables. Leaves may turn yellow with purple veins. Fruit may be deformed or fail to develop. The disease spreads by leafhoppers in a random fashion.

To determine the difference between wilting from lack of water and wilting from curly top virus, soak the soil around plants in the early evening. If the plant doesn't revive overnight, it's likely the plant has Curly Top Virus.

There is no cure or chemical control for curly top. Shade cloth can be placed over plants using stakes to discourage leafhoppers and to help tomato and pepper plants grow better and produce more fruit during the hottest summer months. Remove and destroy diseased plants.

Viruses that affect ornamental plants

Some viruses have actually been encouraged for their aesthetic effect on flowers or leaves. The most famous example of this is the Tulip Breaking Virus which can create flame patterns on tulip flowers and led to the speculative market in tulip bulbs that gripped much of Dutch society in the early 17^th century. Today, “broken” tulips with patterns on the flowers are the result of breeding, not viral infection.

There is a virus that affects Abutilon spp. (flowering “maple” or Indian mallow) and is considered desirable. Abutilon Mosaic Virus (AMV)is transmitted by infected sap moved on cutting tools and through grafting. This virus causes yellow to whitish blotches on leaves. It causes no apparent harm and propagators select infected plants to sell.

Much less attractive and desirable is the Rose Mosaic Virus Complex (RMVc), which is one of the most widespread rose diseases in the United States. This virus may be a complex of more than one pathogen. Symptoms in leaves include yellow line banding in a zigzag or “oak leaf” pattern, mosaic, and watermarking (characterized by faint yellow swirls or squiggles). In vein banding, the veins are yellow, and the leaf is green. This virus may also cause distorted or stunted blooms.

Symptoms of Rose Mosaic Virus are more pronounced in spring and fall. In summer heat, growth appears normal and the virus seems to disappear. A plant may be several years old before showing any symptoms. Damage can range from mild and easily overlooked to seriously stunting. Virus-infected plants may grow more slowly, produce delayed or fewer flowers, and become more susceptible to frost damage.

No rose is immune to these viruses. They spread through grafts and circulate through the plant. They are not spread by insects or pruning tools, but infect through budding, grafting, or rooting cuttings from infected plants. A rose can be symptomless until growing in the landscape; once infected it remains infected throughout its lifespan. Pruning is ineffective because the virus is spread throughout the plant. Replace infected roses if their performance is unsatisfactory. Purchase roses that have been tested and certified to be virus free.

The UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, call the Hotline at (530) 538-7201 or email mgbutte@ucanr.edu.

Most gardeners are aware of plant diseases that can wreak havoc in their gardens. If you grow fruit trees, you may be confronted with curled, reddened peach leaves (peach leaf curl fungus). If you grow apples or pears, your trees may sometimes look like they were scorched with a blow torch (fire blight bacterium). But did you know that viruses can also lurk in the leaves, stems, and fruit of your garden vegetables and flowers? A virus infection can reduce a plant's growth, lower its yield, and/or result in inferior fruit, vegetables, or flowers.

Viruses are often very host specific. No plant viruses are known to affect humans, so feel at ease in your garden when dealing with them. They consist of genetic material (DNA or RNA) surrounded by a coat of proteins. They require a host in order to replicate and have a high rate of mutation and are so small they cannot be seen with a light microscope.

Viruses attach to cells, penetrate, uncoat, replicate, assemble, and then are released. The time between initial introduction and systemic plant infection varies from a few days to a few weeks. Transmission to other plants completes a virus's life cycle.

History

The first plant virus to be discovered was found on the leaves of tobacco plants in the late 19^th century. Researchers found that sap from infected tobacco leaves caused green and yellow mottling (“mosaic”) symptoms in healthy plants. Unlike bacteria, this virus could not be filtered out, but boiling the sap destroyed the infectiousness. Tobacco Mosaic Virus (TMV), the first plant virus discovered, remains one of the most common plant viruses. It attacks not just tobacco but also other members of the nightshade family (Solanaceae), including eggplant, cucumber, pepper, potato, tomato, petunia, snapdragon, delphinium, and marigold.

Virus Transmission

Transfer of virus-infested sap may occur during agricultural practices (through damage caused by tools or hands) or naturally (by an animal feeding on the plant). Plant viruses can also be transmitted by insects, nematodes, fungi, and through pollen, seed, or vegetative propagation from infected parent plants.

Insect vectored viruses can only be transmitted when the insect vector feeds on an infected plant and then feeds on a healthy plant. Insect transmission occurs in one of three ways: non-persistent, semi-persistent or persistent. In non-persistent transmission, a virus attaches to the distal tip of insect mouth parts and inoculates the next plant the insect feeds on. In semi-persistent viral transmission, the virus enters the foregut of the insect and can be transmitted to many plants. Viruses that pass into insect blood and then to the salivary glands are known as persistent, and these may be propagative or circulative. Propagative viruses replicate in both the plant and insect, whereas circulative viruses cannot propagate in the insect. Most viruses are actively transmitted to healthy plants in a matter of seconds, hours, or days by their vectors. Vectors (most often insects) can be very specific, often only transmitting one type of virus.

There are many, many plant viruses that collectively have the potential to infect virtually all species of plants. Fortunately, host ranges of individual viruses vary from very narrow to very broad. Plants can prevent viral infection through either passive or active defenses. In passive defenses, some plants simply don't produce one or more of the factors required for a particular virus to reproduce and spread within that species. Active defenses detect and destroy or wall off virus-infected cells by the action of resistant genes in the plant.

Virus Symptoms

Viruses typically spread throughout the whole plant and cause a systemic infection. A plant's response to infection can range from completely symptomless to severe disease and plant death, depending on the host and the particular virus.

Typical leaf symptoms of a plant virus include mosaic patterns, chlorotic (yellow) or necrotic (brown) lesions, yellowing, stripes or streaks, vein clearing (becoming translucent), vein banding (where bands of a lighter or darker color occur alongside veins), and leaf rolling and curling. Flower symptoms include deformation and changes in the color and pattern of the flowers. Fruit and vegetable symptoms may include mosaic patterns, stunting, discoloration or malformation, and chlorotic ringspots. Stems may develop stem pitting and grooving or tumors in response to virus infection.

Disease Management

Understanding a virus disease is the first step. Consult the UC IPM website to help identify virus disease possibilities that might infect the plant species of concern. There is no cure for a virus disease once a plant is infected. Prevention or reducing spread between plants is the only possibility. Some viruses are spread by aphids and other insects that suck plant juices so controlling them on vegetables can be helpful.

Herbaceous ornamentals and certain vegetables are generally more susceptible to serious injury or death from viruses, especially when plants are young. Prevention includes using certified virus-free seed or plant material and elimination of virus reservoirs in surrounding vegetation. Grow virus resistant cultivars when available. Provide proper cultural care to improve plant vigor. Management depends on how a virus enters the plant, how it is transmitted between plants, and where it survives when susceptible vegetables are not present. Dig out and destroy virus-infected plants.

Tobacco mosaic virus (TMV) is also known as tomato mosaic virus. It commonly appears at this time of year. TMV infects a plant by entering a break in the cell walls caused by insects or other physical damage. Leaves infected with this virus become mottled with yellow and green spots. The dark green spots often appear thicker and somewhat raised, giving the leaves a blister-like appearance. Mottled areas may turn brown and wither. Plants infected with TMV may be stunted and produce very little fruit.

To manage TMV, remove and destroy any symptomatic plants. Avoid use of tobacco around tomato plants. Use only certified virus-free seed. Sanitation is the single most recommended practice to control TMV, because transmission of the virus from plant to plant usually occurs from contaminated tools or hands. The most common sources of infection are the debris of infected plants that remains in the soil. After removing diseased plants, wash your hands and decontaminate tools. Surface sterilize seeds with bleach before planting. There are no chemical sprays that will prevent, control or eradicate viruses.

Viruses rarely kill woody plants but may alter their appearance with line patterns or ring spots on their leaves. When available, planting certified virus-free stock is the best approach. Rose mosaic virus, is a common example that often has clear visual symptoms in the leaves, especially in cool spring weather. Controlling insects is not an effective method of preventing virus infection in woody ornamentals since low densities of pests can spread viruses throughout the life of perennial shrubs and trees. Virus infected ornamentals should be removed if severe stunting compromises the value of the plant in the landscape. Locally, many very productive almond orchards continue to be hosts of a mild plant virus. There is some debate in the commercial nursery industry as to whether or not this needs to be addressed with certified virus clean stock when trees are propagated for new orchards.

Several other viruses that affect garden plants and vegetables will be discussed in the next Real Dirt column.

The UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, call the Hotline at (530) 538-7201 or email mgbutte@ucanr.edu.