Larry Bettiga, Viticulture Farm Advisor, UC Cooperative Extension, Monterey, San Benito and Santa Cruz Counties

Grapevine leafroll and red blotch disease are two virus-associated diseases that should be on the radar of all grape growers. The following article will hopefully provide you an update on these virus diseases base on our current knowledge. Surveying vineyards during harvest is a great time to assess vineyard blocks for the presence of disease symptoms.

 Grapevine Leafroll Disease

Leafroll is one of the more important virus diseases of grapevines. It occurs in every major grape growing area of the world. There are five grapevine leafroll associated viruses (GLRaVs) that are serologically distinct. These single stranded RNA viruses are placed in a family called Closteroviridae. The majority of these are grouped in the genus Ampelovirus (GLRaV-1, -3, and -4), most of the viruses in this genus have been demonstrated to be vectored by mealybugs and scale insects in vineyards. GLRav-2 is in the genus Closterovirus, and GLRaV-7 is in the genus Velarivirus, there is no known vector of these two genera.

 These viruses can cause similar symptoms in infected grapevines. All the GLRaVs can be transmitted by vegetative propagation and grafting; GLRaVs in Ampelovirus can also be transmitted by the mealybugs and soft-scale insects in vineyards. GLRaV-3 is the predominant species found in most vineyards worldwide. Recent surveys in the north coast have shown 80% of symptomatic vines sampled were infected with GLRaV-3.

 To further complicate matters there are variants that have been identified for given GLRaV species. For GLRaV-3 there are several distinct variants known to exist. What needs to be better understood is the significance of these GLRaV-3 variants and their interactions with other viruses when multiple infections exist in a vine. For GLRaV-2 the “Red Globe” variant is known to cause graft incompatibility when grafted onto certain rootstocks (5BB, 5C, 3309C and 1103P) resulting in the decline and death of vines.

 In the post-phylloxera infestation plantings that have occurred on the central coast during the past 20 years there has been an increased incidence of grapevine leafroll disease. The use of non-certified scion material has been a major contributor to this disease increase. The other issue has been the spread of leafroll (primarily GLRaV-3) from infected vineyards to adjacent vineyards planted with California-certified stock. UC research documented the rapid spread of leafroll into a certified planting from an adjacent infected block. During the 5 years of observation the annual rate of increase in leafroll symptomatic vines was more than 10% in a Napa Valley site.

 Recognizing Leafroll

Leaf symptoms become visually apparent by early summer and generally intensify into midsummer and fall. Physical stresses to the vine may increase symptom severity and there are similar symptoms caused by other abiotic and biotic injuries. On affected vines, the margins of the leaf blades roll downward, starting with the basal leaf on the cane. Areas between the major veins turn yellow or red, depending on whether the cultivar produces white or red fruit. In some cultivars, the area adjacent to the major veins remains green until late fall.

 The most important effect of leafroll disease is a reduction in the yield and quality of fruit from infected vines. Yield losses of 10 to 20% are fairly typical. Because leafroll viruses damage the phloem of infected vines, sugar accumulation is delayed and color pigment production is reduced. Fruit from infected vines can be low in sugar, poorly colored, and late in ripening.

 It is important to remember that the lack of symptoms in a grapevine does not guarantee freedom from infection by the viruses that are the causal agents of leafroll disease.

Leafroll disease on Pinot noir (top) showing burgundy red between green main leaf veins accompanied by downward rolling of the leaf margins; on Chardonnay (bottom) leaves show a more generalized chlorosis and downward rolling of the leaf margins in late fall.

 Lab Testing

Leafroll viruses may be diagnosed using ELISA and RT-PCR tests. Virus titer levels are variable not only within the year, but also within the vine. Collect petioles in late summer and fall, or shoots/canes for cambium scrapings in fall and winter. PCR and ELISA tests are not available for all GLRaVs. Check with the commercial lab for their preferred sampling method and collection time prior to taking samples.

 Mealybug Vectors

The most common mealybug found in California vineyards is the grape mealybug (Pseudococcus maritimus). Obscure mealybug (P. viburni) is present in central coast vineyards but less common than the grape mealybug. The vine mealybug (Planococcus ficus) was introduced into California in 1994 and has now been found in most production area of the state. Less common is the long-tailed mealybug (P. longispinus) found primarily in the cooler areas of the south central coast. The Gill's mealybug (Ferrisia gilli) is the fifth species found in California but is currently very limited in distribution with populations found in the Sierra foothills, in the northern coast (Lake County) and in the southern San Joaquin Valley.

 All the above species are capable of being a vector for leafroll disease. Research has shown that mealybugs can become infective after one hour of feeding on a leafroll virus infected vine and can transmit the virus to a clean host after one hour of feeding. Although all female instars can transmit the virus once infected, the first instar is the most effective vectors of the disease. The first instar or “crawler” moves to find a feeding spot and is considered to be the most common dispersal stage of a mealybug population. Wind, equipment, workers and infested nursery stock can also move mealybugs.

 Movement of leafroll disease into a recently planted vineyard of certified planting stock from an infected block on the opposite side of the avenue. Note the vines showing symptoms are nearest the avenue and are not present on vines further down the rows.

 Management of Grapevine Leafroll Disease

1. Plant Material.The first management strategy should be to plant certified vines that have been grown and produced by a nursery participating in the California Grapevine Registration and Certification Program. Once virus infected a vine will remain infected, there is no cure. Commercial nurseries that produce certified grapevines and participate in the California Grapevine R&C Program obtain their clean stock from the Foundation Plant Services at the University of California, Davis. UC Davis has a foundation vineyard for major grape cultivars and clones. Before being planted in the foundation vineyard, all vines are tested across biological indicators, and by ELISA and RT-PCR. The foundation vineyard is monitored by visual inspections in spring and fall, and a portion of it is retested every year by ELISA and RT-PCR for viruses known to spread naturally. This provides the highest level of confidence about the virus status of the selections.

Both the fruiting scion and the rootstock need to come from certified mother plants. A very common spread of leafroll is the use of infected bud wood from commercial vineyards. The lack of symptoms in the source vineyard cannot be relied upon as a guarantee that there is no virus; many of the major grapevine viruses show no symptoms during some or all of the season. Particularly if wood is collected during the dormant season, it is unlikely that the source vines will show distinct symptoms of virus infection. Selected grapevines should also be pre-tested for virus by a competent diagnostic laboratory if this type of material is going to be used. Even with vine testing sourcing bud wood from established vineyards carries a risk of introducing virus into a new planting.

 2. Learn to recognize leafroll symptoms.Leafrollsymptomsbecome visually apparent by early summer and generally intensify into midsummer and fall as noted above. Symptoms can vary by leafroll species, multiple virus infections, and by cultivar and rootstock combination. Symptoms are generally more apparent in cultivars producing red or black fruit than in white fruiting cultivars. Remember that the lack of symptoms in a grapevine does not guarantee freedom from infection by the viruses that are the causal agents of leafroll disease.

 3. Recognize and be aware of potential leafroll vectors.As discussed above mealybugs and scale insects are known vectors of some species of GLRaVs. Monitor and be aware of which insect vectors may be in your vineyards. More information on these insects is available in Grape Pest Management UCANR publication 3343 or in the online UC IPM guideline for grapes, http://www.ipm.ucdavis.edu. Know which species of mealybugs are present in your vineyards, their population dynamics are different and will influence the timing of any needed control practices. European fruit lecanium scale (Parthenolecanium corni) is a common insect found in California vineyards, it and other scale insects has been shown to transmit some GLRaV species.

 4. Be aware of potential spread from leafroll infected blocks.Leafroll infected blocks can be a source for vector and disease spread into adjacent clean plantings. Consider if plant removal is a viable option to reduce further spread for both the source and clean blocks. Vector control may be a management decision to consider. Recent research suggests the rate of disease spread of GLRaV-3 is greater when higher mealybug population levels are present. Treatment of virus source blocks should minimize the infective vectors leaving the block; the treatment of clean blocks should be targeted to kill infective vectors quickly upon entering the block and to reduce secondary spread to adjacent vines.

 5. Area-wide management.When both mealybug populations and the virus causing leafroll disease are present in an area, cooperation between neighboring vineyard owners will be necessary to improve on reducing the spread of disease from infected source blocks to non-infected vineyards.

 Grapevine leafroll disease is actively being studied both here in the US and internationally. Improvements in identification techniques and better understanding of disease epidemiology in vineyards will hopefully improve our ability to develop management practices to reduce economic impacts.

  Red Blotch Disease

Grapevine red blotch disease was suspected as a concern on vines growing in the Napa Valley in 2008. In 2011 a DNA virus was identified in independent studies in California and New York and shown to be associated with the symptoms on infected vines. Since the initial identification of Grapevine red blotch-associated virus (GRBaV), it has been found to be widespread in vineyard producing areas of the United States and Canada. A recent survey of a grape herbarium collection at UC Davis has shown one plant specimen collected by Harold Olmo in 1940 from a Sonoma County vineyard to be positive for GRBaV.

 Recognizing Red Blotch

In red wine cultivars, irregular red blotches form on the leaf blades on the basal parts of shoots. Veins on affected leaves can turn pink to red in color. Symptoms can vary between cultivars and the severity may also vary between years. In white cultivars the symptoms are not as dramatic. Interveinal chlorosis is most common followed by irregular chlorotic blotches. These symptoms can begin to appear as early as July and as late as September. In comparing red blotch to leafroll disease, leafroll symptoms are generally more uniform across the leaf blade, the veins remain green, and the there can be a downward rolling of the leaf margin. For more pictures of red blotch symptoms on different cultivars got to: GRBaV symptoms

Late summer symptoms on Chardonnay, red blotch on the upper photo and leafroll on the bottom.

 Lab Testing

The identification of GRBaV can be difficult to determine by visual observation due to the similarity of the symptoms to leafroll disease and other nutrient deficiencies. This is especially true in the case of white cultivars. Co-infections with other viruses can also affect symptom expression. Suspected infections should be confirmed by having samples assayed by a PCR test by a commercial diagnostic lab. Check with the commercial lab for their preferred sampling method and collection time prior to taking samples.

Disease Spread

GRBaV is spread by the propagation of planting stockor grafting non-infected vines using infected budwood. The widespread occurrence of red blotch disease would suggest this type of spread has occurred. Since the identification of the virus and the availability of the PCR testing in 2012, grapevine nurseries have been testing their increase blocks and removing infected vines to eliminate this type of spread. The recently established Russell Ranch Foundation Vineyard at UC Davis has been tested and all vines are free from GRBaV. Each vine planted at the Russell Ranch has undergone extensive virus testing following a process known as Protocol 2010. For nurseries participating in the CDFA R&C Program this will provide a source for future increase blocks to supply certified vines for vineyard plantings.

 GRBaV is a DNA virus and is closely related to a family of viruses called Geminiviridae. Insect such as leafhoppers and whiteflies can vector other virus diseases within this family. Researchers are currently testing potential insect vectors of GRBaV. Although there was a report of Virginia creeper leafhopper being a vector in a greenhouse study other researchers have been unable to duplicate that study. The evaluations of other potential insect vectors have not yet identified one that can successfully acquire and transmit the virus in the field. Although there is anecdotal information that there is spread within some vineyards to date there is insufficient data to support that claim.

 Vine Effects

Research has shown that when comparing GRBaV infected vines to ones that have no known GRBaV, leafroll-associated viruses, vitiviruses, or Nepoviruses that Brix were lower and malic acid in the juice were higher at harvest for Cabernet Sauvignon and Chardonnay but not Zinfandel. For Chardonnay, yield was also reduced for infected vines. A study looking at the effect of dropping crop to improve quality on infected vines saw little beneficial effect from that practice. For most cultivars, there is a decrease in total phenols, tannins, and anthocyanins (for red wine cultivars) for vines infected with GRBaV.

 Management of Red Blotch Disease

As with leafroll disease the first management strategy should be to use propagation material that is free from known viruses when establishing new vineyards or grafting existing sites. Meetings of the Grapevine Regulations Working Group have been recently conducted to discuss proposed changes to the Grapevine Registration & Certification Program with regards to red blotch disease. Until budwood increased from the Russell Ranch Foundation vineyard is available for use it is important that propagation material is tested to avoid virus disease.

 If you have blocks that have leaf symptoms and have had delayed maturity or low crop yield have a virus panel run by a commercial lab to confirm which viruses are present. Remember symptoms are going to be more noticeable in red wine cultivars and less so with white cultivars. For confirmed GRBaV infected vineyards the management response may vary depending on the vine effects that are being observed. The difference in sugar accumulation between infected and non-infected vines in some vineyards has been as much as 5 Brix. In vineyards with a combination of infected and non-infected vines this wide variation in maturity has resulted in selective harvests to improve fruit uniformity. For infected sites that fail to meet yield and quality expectations vineyard removal is the best solution. If only a low percent of vines in a block are infected, then rogueing and replanting is an option. If vine removal and replanting is an option there is currently assistance available (see below).

 Financial Assistance Available to Replant Red blotch-affected Grapevines:

The Agricultural Act of 2014 (the 2014 Farm Bill) authorized the Tree Assistance Program (TAP) to provide financial assistance to qualifying orchardists and nursery tree growers to replant or rehabilitate eligible trees, bushes and vines damaged by natural disasters.

 The 2014 Farm Bill makes TAP a permanent disaster program and provides retroactive authority to cover eligible losses back to Oct. 1, 2011. In California, producers who are replanting vines affected by grapevine red blotch disease may be eligible for assistance and should contact their local Farm Service Agency Office to schedule an appointment for the required visual inspection. For more information go to: Fact Sheet

 References

Al Rwahnih, M., et al. 2013. Association of a DNA virus with Grapevines affected by Red Blotch disease in California. Phytopathology 103:1069-1076.

Daane, K. M. et al. 2012. Biology and management of mealybugs in vineyards. p. 271-307. In: N. J. Bostanian et al. (eds.), Arthropod Management in Vineyards: Pests, Approaches and Future Directions. Springer. 505p

Golino, D. A., et al. 2002. Grapevine leafroll disease can be spread by California mealybugs. California Agriculture 56:196-201.

Golino, D. A., et al. 1992. Grapevine virus diseases. In: Bettiga, L, (ed.), Grape Pest Management, 3rd ed. Oakland: University of California Division of Agriculture and Natural Resources, Publication 3343, 157-173.

Golino, D. A., et al. 2008. Leafroll disease is spreading rapidly in a Napa Valley vineyard. Calif. Agric. 62:156-160.

Krenz, B., Thompson, J., et al. 2012. Complete Genome Sequence of a New Circular DNA Virus from Grapevine. J. Virol. 86:7715.

Krenz, B., et al. 2014. Grapevine red blotch-associated virus is widespread in the United States. Phytopathology. First Look.

Oberholster, A., et al. 2015. Impact of red blotch disease on grape and wine composition and quality. American Society of Enology and Viticulture National Conference Technical Abstracts, p. 75. (2015).

Poojari, S., et al. 2013. A leafhopper transmissible DNA virus with novel evolutionary lineage in the family Geminiviridae implicated in grapevine redleaf disease by nextgeneration sequencing. Plos One 8:e64194.

Sharma, A. M. et al. 2011. Occurrence of grapevine leafroll-associated virus complex in Napa Valley. PLoS One 6(10): e26227.

Smith, R., et al. 2015. Effect of crop reduction of vines infected with grapevine red blotch-associated virus on fruit maturity. American Society of Enology and Viticulture National Conference Technical Abstracts, p. 136-137.

Sudarshana, M. and M. Fuchs. 2015. Grapevine red blotch In: Wilcox, W., et al, (eds.), Compendium of Grape Diseases, Disorders and Pests, 2^nd ed. The American Phytopathological Society.122-123.

Tsai, C. W., et al. 2010. Mealybug transmission of grapevine leafroll viruses: Analysis of virus–vector specificity. Phytopathology 100:830-834.

Authors: Richard Smith¹, Michael Cahn¹, Tamara Voss², Toby O'Geen³, Eric Brennan⁴, Karen Lowell⁵ and Mark Bolda⁶

1 – UC Cooperative Extension, Monterey County; 2 – Monterey County Water Resources Agency; 3 – Dept of Land Air and Water Resources, UC Davis; 4 – USDA Agricultural Research Service; 5 – Natural Resources Conservation Service; 6 – UC Cooperative Extension, Santa Cruz County.

For access to full report please vist http://cemonterey.ucanr.edu/files/219694.pdf

Summary: After four years of drought, groundwater levels in the Salinas Valley are at historically low levels which threaten to adversely affect farming in the Salinas Valley. Given the prospect of a strong El Niño this coming winter, it seems prudent to plan to capture as much of the rainfall as possible to maximize infiltration into the soil and recharge groundwater. The east side of the Salinas Valley is particularly hard hit because it receives a lower proportion of recharge from the Salinas River than other hydrological subunits in the Salinas Valley and its water levels have fallen to lower levels. Low residue cover crops have been shown to effectively increase rainwater infiltration on Chualar loam soils and they provide a practice that we can employ to increase groundwater recharge. Given the low water status of the aquifers and the forecast for significant rainfall, there is an opportunity to proactively implement practices that can maximize water capture this winter. In this article we discuss practices growers can employ in production fields and on farm edges to maximize rainwater infiltration and restore groundwater resources.

Introduction: The four years of drought have left groundwater levels in much of the Salinas Valley are at historically low levels (Figure 1). Due to the types of sediments in Salinas Valley, it is unlikely that the low groundwater levels will cause land to subside; however other deleterious effects may result. The most immediate effect is that shallower wells may become unreliable (i.e., either produce less water or suck in air or sand).   This is an immediate expense for growers, who will need to service these wells or drill deeper. As groundwater levels drop below sea level, the salty ocean water will move into coastal aquifers worsening current seawater intrusion issues. Although tremendous efforts have been made in the Salinas Valley to stop seawater intrusion, the prolonged drought increases the potential for seawater to move inland. Groundwater levels on the east side of the Salinas Valley are particularly concerning because they are declining the fastest, which has set up a gradient for saline groundwater to flow toward this part of the valley.

It is encouraging that El Niño conditions continue to persist in the eastern Pacific, and may lead to significant precipitation on the Central Coast this winter.   The challenge is to infiltrate as much of this rain as possible to help recharge the groundwater, rather than allow it to escape as run-off into the ocean. Much of the east side agricultural land has moderate to excellent ability to infiltrate rainfall which will directly recharge the underlying aquifer. For example, infiltrating an additional 4 inches of rainfall per acre across 100,000 acres could potentially add 33,000 acre-ft of recharge during the winter.   Enhancing recharge in areas with declining groundwater levels would be especially beneficial in reducing seawater intrusion. Increasing the infiltration during severe storm events would also lower flood risk and erosion damage. The objective of this article is to briefly review potential strategies to increase infiltration during the winter storms.  

East side Hydrology: Inflows to the Salinas Valley groundwater basin are estimated at 504,000 acre feet/year during an average rainfall year, with about 50 percent from stream recharge (including Nacimiento and San Antonio reservoir releases), 44 percent from deep percolation from precipitation and agricultural return flows, and 6 percent from subsurface inflow from adjacent groundwater basins (MW, 1998). Groundwater recharge in Forebay, Pressure, and Upper Valley subareas of the Salinas Valley Groundwater Basin are primarily from infiltration from the Salinas River. Releases from San Antonio and Nacimiento reservoirs flow down the river recharging the aquifer of these hydrological regions.

Inflow to the East Side Subarea results from a combination of infiltration along small streams on the west side of the Gabilan Range, direct recharge by precipitation on the valley floor, and subsurface inflow from the Pressure and Forebay Subareas (Brown and Caldwell, 2015). In the East Side Subarea, Shallow Aquifer, the seasonal pattern of groundwater head elevation changes are correlated most strongly to annual precipitation (Brown and Caldwell, 2015). In wells with perforations in both the east side shallow and deep aquifers, fall groundwater head generally follows the pattern of cumulative precipitation surplus, with head declining during relatively dry periods and rising during relatively wet periods. There is, however an overall long-term decline in groundwater head over the period of record (1953-2013) for the East Side subarea (Figure 1) (Brown and Caldwell, 2015). Groundwater levels on the east side of the Salinas Valley are particularly concerning because they have set up a gradient for saline groundwater to flow toward this area of the Valley (Figure 2). The cumulative storage change for the East Side subarea has also been negative for the entire period of record (1944-2013), investigated in “State of the Salinas River Groundwater Basin” report, ending at about -332,600 acre-feet in 2013. (Brown and Caldwell, 2015)

Figure 2. Groundwater head elevation in the Pressure 180-foot and East Side shallow aquifers. Source: Monterey County Water Resources Agency; Map date August 20, 2015.

Soil Types and Groundwater Recharge Potential: Common soil series on the east side of the Salinas Valley include Chualar, Antioch, Arroyo Seco, Danville, Elder and Placentia. The Chualar series is the most common soil type and has moderately good potential for infiltrating water from winter storms (Table 1, Figure 3). However, it has a tendency to form a crust and shed water during high intensity rainfall events. Other common soils such as Arroyo Seco, and Elder are highly permeable and have excellent potential to recharge groundwater. Soils such as Antioch are poorly suited, but deep tillage increases the permeability of subsoil horizons and greatly improve its recharge capability. Soils such as Placentia, Danville and Salinas have slow percolation rates. These soils have fine texture throughout the profile yet improving soil structure could increase their ability to accept water.

Table 1. Summary of groundwater recharge potential ratings of common soils on the east side of the Salinas Valley based the Soil Agricultural Groundwater Banking Index (O'Geen et al., 2015).

Figure 3. Suitability of east side soils (modified by deep tillage) for groundwater recharge (O'Geen et al., 2015).

Practices to Increase Rain Water Infiltration:  The overall approach to increase infiltration on agricultural land during winter storms is to implement practices that maximize infield infiltration in conjunction with practices on field edges which slow and retain run-off so that it has chance to infiltrate.  

IN FIELD PRACTICES

Full-Season Cover Crops: Cover crops can improve the infiltration of water into the soil by protecting the soil surface, eliminating surface crusting caused by the impact of raindrops and maintaining aggregate stability and creating soil macropores. Full-season cover crops are those that are planted in the fall and incorporated into the soil in late winter or early spring. They increase infiltration by creating root pathways that facilitates downward movement of water. In addition, they slow the movement of water over the soil surface giving it more time to infiltrate. Downward movement of rainwater through the soil leaches salts that have accumulated over the growing season and contributes to groundwater recharge. However, full-season cover crops remove water from the soil by their transpiration later in the growth cycle and, in low rainfall years, they may dry the soil and little recharge will occur. For instance in a trial conducted from 2010-11, we observed 5.5 inches of water percolated into the soil in the bare fallow treatment, but only 3.0 inches in the full-term cover crop treatment (incorporated into the soil in March). The difference in the two treatments was due to evapotranspiration by the cover crop later in February and March when day length increases. This is good for reducing nitrate leaching, but less useful for groundwater recharge. Given the current drought situation and a pressing need to facilitate groundwater recharge, understanding the impact of cover crops on infiltration and options to manage them in Salinas Valley settings is critical. Low residue cover crops offer a unique approach to address operational constraints that may make full season cover crops impractical in the vegetable cropping operations that cover much of the land in the Salinas Valley.

Low Residue Cover Crops in Vegetables: Low residue cover crops are planted in the fall and killed 60 days later (e.g. mid-January) when they have produced about 0.5 tons/acre of dry biomass. This is typically when they've produced about 10 to 20% of the potential biomass of a full-season cover crop. Typical varieties used in vegetable production systems for low residue cover crops include cereals like rye (Merced and AGS104), as well as winter-dormant triticales (Trios 888). Both are typically sown in the fall following listing; earlier planting dates can provide protection to the soil for early storms in November and December.

Seeding can be done by dribbling the seed onto the furrow bottom, followed by shallow harrowing. It is important to use sufficient seed (e.g. 80-100 lbs rye seed/acre) to get rapid growth that can quickly protect the soil from early storms. Planting seed in the furrow is tricky because soil can fall from the edge of the bed back into the furrow (after the harrow passes) and bury the seed too deeply. The ideal seeding depth is 1-2 inches; four inches is too deep and will greatly reduce seed emergence. Under ideal conditions the soil may be moist enough to germinate the seeds without irrigation, or an early rain provides the moisture for germination.

Winter-dormant triticales (e.g. Trios 888) grow more slowly in the winter, which may reduce the risk of producing too much biomass that could be a problem in preparing the soil for the subsequent cash crop (see photos below). Low residue cereal cover crops are typically killed with an herbicide such as glyphosate or a grass selective material such as clethodim or sethoxydim when 60 days old; cereal cover crops on the furrow bottom are difficult to kill mechanically. At this stage the cover crop residue has a low C:N ratio (9-12) and decomposes rapidly in the presence of favorable moisture and temperatures. In studies conducted from 2009 to 2011, we observed that sufficient killed residue remained on the soil surface for 4-6 weeks to continue to protect the surface from raindrop impact and soil crusting. The dead roots of the killed cover crop retain their function of providing channels for rapid infiltration as well. Therefore though the cover crop only grew until mid-January, the soil was protected by the cover crop from about December to mid-March. The cover crop and its residue reduced runoff on a Chualar loam soil where our studies were conducted. We observed that 47.2% of the rainfall ran off of the field in the bare fallow treatment, compared with only 2.3% runoff in the rye and 9.2% in the winter dormant triticale (Figure 5). The rye cover crop increased the amount of water that infiltrated into the soil by 119,827 gallons/A (1/3 of an acre-foot) over the bare fallow treatment. The increased infiltration in the low residue rye cover crop treatment increased the quantity of chloride and sodium leached from the soil by >80% over the bare fallow treatment. This salt leaching provides considerable agronomic benefit, as low rainfall years allow salts to reach levels that may damage crops.

Figure 4. Winter dormant triticale (Trios 102) planted on the furrow bottom. Photo on right is 3 weeks after being treated with glyphosate. Note dead residue covers furrow bottoms.

Figure 5. 2009-2010 Trial. Total runoff from cover crop and bare treatments between mid January and March 7, 2010.

Weed control in the low residue cover crops can be carried out by lillistoning the bed tops and sides, but the furrow sweeps must be lifted to avoid disturbing the protective cover crop residue. You can see an example field preparation following a low residue cover crop in this video https://www.youtube.com/watch?v=k0oVVJ_BA7s . Initial studies with this technique were done with vegetables. There is a delicate balance of covering the soil and obtaining the benefits of increased water infiltration, but having the residue decompose quick enough to allow for ease of bed preparation for planting the subsequent vegetable crop. The quantity of cover crop residue at the time of killing and the spacing of the rain events that allow for cover crop decomposition determines how successfully this technique works. To be on the safe side, in your first efforts using this technique, it is prudent to only use this technique on fields that are scheduled for planting later in the spring to make sure the residue does not create issues for the subsequent cash crop (contact Richard for specific questions: 831-759-7357).

Low Residue Cover crops in Strawberries: Low residue cover crops can also be used in strawberry production systems during the winter. This technique has been used to some extent by growers on hills, and can greatly reduce erosion and improve the water quality of run-off (Table 2). However, cover crops in furrow bottoms were less effective in reducing the quantity of runoff from strawberry fields than in listed vegetable beds. The lack of increased infiltration is due to the volume of runoff generated by beds covered with plastic; soils on the furrow bottom quickly become thoroughly saturated and water quickly runs off. This may be particularly problematic on steeper ground where runoff from individual furrow drainage joins at the end of the rows and becomes an extremely erosive concentrated flow. In this setting a particularly dense seeded cover crop, particularly at the end of rows, may help dissipate the energy of the concentrated flow moving downslope. Often it is necessary to address the lack of improved in-field infiltration by use of underground outlets and sediment basins where runoff carries high sediment loads. Where sediment loads are minimal, vegetated ditches may be helpful to slow the water and allow for infiltration in the ditch (see below).

The use of furrow-bottom cover crops in strawberry systems is still very useful on the east side of the Salinas Valley and other locations on slopes because of reductions in sediment loss. Cover crop choice is important in strawberry systems. For example, fast growing cover crops like barley require more management early in the winter. In our studies, we found that by late December, barley was tall enough to begin shading the strawberry plants. At that point, it can be managed by weed wacking or treatment with a grass selective herbicide like sethoxydim or clethodim (selective for killing grass and do not damage strawberries). In contrast to barley, winter-dormant triticale (Trios 102) grows more slowly and therefore doesn't require management until about late January.

Table 2. Run-off and sediment loss during rain events in a strawberry trial, 2006-07. Data were collected from 11 storm events totaling 4.28 inches.

Low Residue Cover crops: Organic Systems: Low residue cover crops can be used in organic fields as long as they can be killed before they produce too much biomass. Grass cover crops (i.e. cereals) are not recommended, because they are difficult to kill with tillage. Organic herbicides work better on young plants (i.e., cotyledon stage) and are weak on grasses.  Growing mustard in the strawberry furrows is one exciting approach that seems to have lots of potential in this region. We've evaluated this over several years at the USDA-ARS in Salinas and have been impressed because mustard establishes quickly with relatively little moisture and is easy to kill with a single cut with a weed whacker. We used hand pushed planters like the ‘Clean Seeder AP' to plant a single line of mustard in the furrow bottom in early December, and typically weed wacked it in late January to early February when it was about the height of the strawberry bed top. Unlike grass cover crops, mustard does regrow after its cut down. After weed whacking, the high-nitrogen mustard residue decomposes quickly and creates relatively few challenges when the furrows are shallow-cultivated to prepare them for strawberry harvesting. Planting a single line in the furrow center helps to keep the base of the mustard plants away from the plastic, which minimizes damage to the plastic during weed wacking. Several mustard cover crop varieties (i.e. Kodiak, Ida Gold, and Caliente) can work, although Ida Gold seems especially well-suited to this system. It grows tall and fills in the furrow quickly, and its relatively large seeds seem more tolerant to deeper planting depths. An appropriate seeding rate for Ida Gold mustard in strawberry furrows is about 10 pounds/acre (approximately $30/acre for seed) which will result in about 30 to 40 plants per foot of furrow bottom. To reduce labor costs with planting mustard, the USDA-ARS has developed a simple planter that plants two strawberry furrows at a time, and will be available for growers to borrow to try. The planter will be demonstrated during a field day at the USDA-ARS in September, and about 1000 pounds of free mustard seed will be available for interested growers to try. Although we have not measured runoff from mustard furrows, we have observed that it dries down furrows and therefore will likely reduce run-off and sediment loss. More research is needed to document the effect of mustard on infiltration and ground water recharge when planted in strawberry furrows, but observation in work done thus far suggests such plantings may be beneficial.

Figure 6. Ida Gold mustard ready for weed whacking, January 30, 2014.

Other In-Field Practices to Increase Infiltration:

• Surface application of gypsum can improve aggregation of soil particles, which improves soil structure, reduces crusting and thereby increases water infiltration into the soil. Gypsum provides calcium which changes the manner in which soil particles are able to flocculate (come together) as aggregates. This approach can be particularly effective for soils that crust, such as those found on the east side of the Salinas Valley. To be effective, gypsum needs to be applied on the surface of the soil (not incorporated) before the first rainfall. The effect of the gypsum declines over time as the rainfall solubilizes the gypsum and carries it away from the soil surface where it is needed.
• Leaving fields unlisted: For fields that will be planted in the late spring, leaving them flat (unlisted) through the winter may also help infiltrate more of the rainfall, as unlisted fields will reduce concentrated flow in furrows where soil may become saturated. If it is possible to leave fields unlisted, leaving a rough soil surface will also help infiltrate a greater portion of the rainfall by creating less runoff potential.
• Tillage can improve water infiltration by breaking the soil crust and slowing runoff by creating a torturous path for the water to follow. Obviously, obtaining access to the field with tillage equipment can be difficult to impossible in wet years.
• 80-inch wide beds appear to have less runoff than 40-inch wide beds in moderate rain storms. However, it is unclear how well 80-inch beds will reduce runoff in a large El Niño rain event of 3 or more inches.  

To view Field-Edge Practices please view Part 2 of Groundwater Recharge on East Side Soils of Salinas Valley Blog

Authors: Richard Smith¹, Michael Cahn¹, Tamara Voss², Toby O'Geen³, Eric Brennan⁴, Karen Lowell⁵ and Mark Bolda⁶

1 – UC Cooperative Extension, Monterey County; 2 – Monterey County Water Resources Agency; 3 – Dept of Land Air and Water Resources, UC Davis; 4 – USDA Agricultural Research Service; 5 –USDA Natural Resources Conservation Service; 6 – UC Cooperative Extension, Santa Cruz County.

For access to full report please visit  http://cemonterey.ucanr.edu/files/219694.pdf

FIELD-EDGE PRACTICES

 Rainfall that cannot be infiltrated within a field will run-off to surrounding areas and eventually flow off-site. Several strategies can capture, slow, and facilitate infiltration of such run-off. In general, strategies become more costly and harder to implement the further downslope they occur. Row arrangement that slows run-off is more cost effective than building large recharge basins that routinely fill with sediment. Full control of run-off almost always requires suites of practices rather than a single approach. Many of the strategies described below are described in more detail in the Resource Conservation of Monterey County and Monterey County Agricultural Commissioner's 2014 publication, Hillslope Farming Runoff Management Practices Guide. This 52 page guide is available as a free download here: http://tinyurl.com/Runoff-Management-Practices.

Permanent Vegetative Cover. In areas of the ranch that routinely receive runoff, establishing permanent vegetative cover is very helpful. While this removes ground from production, if it is a part of a ranch that is routinely damaged by winter rains it may allow management strategies that are worth the sacrificed ground. For example, a grassy area that receives relatively sediment free runoff could serve to defuse energy and infiltrate water and avoid need for regular sediment basin maintenance. Where such an area overlies a soil that allows deep percolation this will lead to groundwater recharge. If permanent cover is not practical or acceptable from an operational perspective, setting aside an area for this purpose that can be planted later in the spring (to allow for more dense vegetation to be disked in) may still be beneficial. Grassed waterways may serve this function, and are typically planted to perennial grasses. Some ranches with wash facilities on site may have areas that receive waste water. Where practical, if water can be carried to this area during rainy winter months when production (and wash activities) are not underway extra benefit of the area may be possible. Vegetated filter strips placed strategically along the contour of a slope may be feasible in some operations, or narrower areas strategically placed to diffuse the energy of water sheeting off a plastic hoop house. For example, seeding this splash impact area to a low grass may keep the soil surface more open and able to infiltrate water than one with poor soil structure resulting from drainage onto bare ground.

Sediment traps: Because significant erosion can occur during major storm events, structures are needed that can minimize clogging of downstream run-off control practices such as vegetated ditches, weirs, and retention basins. Sediment traps can intercept and settle sand and large silt particles suspended in run-off from fields during storm events. These structures are usually shallow basins (2 to 3 feet deep) located at the lower corner of a field. They intercept run-off before it flows into major ditches that convey it across a ranch.   Trapped sediment needs to be removed after major storm events for these structures to function efficiently during the winter. A check at the outlet of the trap can be used to adjust the height of water by adding and removing slats of wood. Changing the height of the check dam allows more time to allow for sediment in a heavy flow with resultant high water level, to settle out before overflowing into a culvert or other conveyance.         

Enhance ditches for infiltrating run-off: Permanent ditches that convey field run-off can be enhanced to minimize bank erosion during storm events and increase infiltration.   Many farm ditches are narrow with steep banks that are prone to erosion and blow-outs during large storm events. Wider ditches with a U-shape instead of a V-shape reduce erosion by spreading the water and reducing the erosive energy as it flows. Water in a wider ditch may also flow more slowly, allowing more opportunity for infiltration and recharge to groundwater. Providing some armor to soil (e.g. rocks) to dissipate the energy of run-off entering from culverts and smaller tributaries can also protect against erosion, although it is important to place such protection carefully to avoid creating paths of preferential flow that may be even more damaging. Weirs can be spaced at regular distances within the ditch to slow the flow of water during moderate run-off events.   These weirs can be designed to be removable or so that the cross section of the passage way can be adjusted to handle high flow rates without overflowing the banks of the ditch. During small and medium storm events, weirs can retain and infiltrate a large portion of the run-off.

Vegetating ditches   Vegetation in permanent ditches helps protect the banks to prevent erosion, and avoid blow outs with massive volumes of sediment during large, intense storms. Because infiltration is better when there is good surface soil structure, typically the case when there is vegetation, vegetated ditches may also improve infiltration. Key design features will influence the ability of a ditch to retain its function during large storms. For example, as noted above U-shaped ditches are better than V-shaped ditches.   A 1:3 to 1:4 slope (1 foot of depth to 4 feet of width) would be a good target to optimize ditch stability and enhance infiltration. Ditches can be seeded with fast growing grasses such as barley or rye if the objective is to have vegetation only during the winter months.   Grasses planted in ditches may be killed with an herbicide before they produce seed, to reduce the potential to attract rodents.   Also the ditches can be returned to an unvegetated condition before spring crops are planted. Red fescue provides a dense permanent vegetation that has very small seeds that are less attractive to rodents (Figure 7). Studies conducted at the USDA-ARS research station in Salinas demonstrated that these ditches were effective in infiltrating run-off and mitigating transported sediment and pesticides.    

Figure 7. Permanent ditch planted with red fescue can infiltrate run-off and protect the sides of the ditch from eroding during large storm events. This ditch is located at the USDA-ARS along Spence Road in Salinas.

Lined Waterway. If vegetation is not sufficient cover for a conveyance channel use of concrete or rock riprap may be necessary. Some growers use plastic. While this reduces recharge potential as the water does not infiltrate from a plastic lined ditch, if it can safely convey the runoff to a suitable basin where infiltration is possible. The reduced sediment load resulting from the lined ditch will be beneficial. Significant recharge depends on placing the basin on a suitable soil and delivery of relatively sediment free water to ensure that the bottom of the basin retains high infiltration rates.

Retention basins: A basin that can retain run-off reaching the lower end of a ranch can provide an additional opportunity to infiltrate storm water (Figure 8). Retention basins designed for infiltrating run-off can be relatively shallow (2 to 4 feet deep), and can be located in areas of the ranch that are undesirable for farming, such as on an irregularly shaped section of a field. For optimum benefit, it is important to consider soil properties underlying the basin. For example, a soil with a hardpan at 3 feet depth will be less effective than one with no impeding layer. A ditch conveying run-off might be widened to create some of the function of a shallow retention basin, or a berm constructed between a field and roadway can create a retention basin. To avoid blow outs, basins must be sized appropriately, based on expected intensity of storm events and size and slope of the area that will drain to them. The outflow structure of the basin should be engineered to allow controlled overflowing during large storm events and to ensure that outflow is channeled to minimize erosion of the basin and any conveyances that receive overflow.   Even dead vegetation on the bottom or sides of a ditch can enhance recharge by creating an organic matter layer that protects surface soil structure and facilitates infiltration.

Figure 8. Shallow retention basins can infiltrate run-off from agricultural fields before it flows offsite

Road Protection. Many of the strategies described above will work on roads as well. A few others may also be useful. For example, use of cross ripping or waterbars on roads that do not need to be driven during winter months may be helpful as supplemental protection when roads are seeded for erosion control. A temporary slope drain may also be used when cost, labor or time constraints make construction of underground outlets and permanent sediment basins impractical. These temporary systems use a flexible pipe to capture concentrated runoff at the top of the slope and convey it downslope to a stable outlet where it is released in a sediment basin or similar.

Citations and Other Resources:

Brown and Caldwell, State of the Salinas River Groundwater Basin, prepared for Monterey County Resource Management Agency, January 16, 2015.

Montgomery Watson, Salinas Valley Historical Benefits Analysis, prepared for Monterey County Water Resources Agency, April 1998.

Soil suitability index identifies potential areas for groundwater banking on agricultural lands. O'Geen, T. et al. 2015. California Agriculture. Online:http://californiaagriculture.ucanr.edu/landingpage.cfm?article=ca.v069n02p75&fulltext=yes

Low residue winter cover crops for winter vegetable production. Smith, R., M. Cahn, A. Heinrich and B. Farrara. YouTube video: https://www.youtube.com/watch?v=k0oVVJ_BA7s

Hillslope Farming Runoff Management Practices Guide. This 52 page guide is available as a free download here: http://tinyurl.com/Runoff-Management-Practices.

Local USDA Natural Resource Conservation Service and Resource Conservation Districts have resources to help growers with farm edge practices: Salinas NRCS office: 831-424-1036 x101, Resource Conservation District of Monterey County: 831-424-1036 x124 0r 126.

The garden symphylan (Scutigerella immaculata) (Figure 1), a white, highly mobile, centipede-like, 1/4 inch long soil arthropod, is a serious soil pest of several high-value crops in the Central Coast of California such as lettuce, strawberry, broccoli, cauliflower, artichoke and celery. The garden symphylan feeds on roots of both direct-seeded and transplanted crops alike causing severe stunting and plant mortality. Besides feeding on the roots, they also survive feeding on organic matter, and other soil dwelling fungi. The garden symphylans use the channels created by other soil organisms such as earthworms for vertical and lateral movement through the soil profile. Their seasonal movement in the soil is also influenced by soil moisture, and temperature. Incidence of garden symphylan infestation is mostly reported in heavier or clay soils with higher organic matter content than lighter or sandy soils. The garden symphylans spend their entire life in the soil and are well adapted to the subterranean habits. They lack eyes but have long antennae and thousands of sensory hairs on the body, which possibly help taste and feel the surroundings.  

The garden symphylans are primarily managed using preventative insecticide application, although other tactics such as crop rotation, planting less susceptible crops, flooding the field, reduced tillage, and conservation of beneficial organisms have been suggested. Success and effectiveness of these non-chemical tactics were constrained by several factors such as lack of fit to the current production practices, susceptible crops being grown, varied topography, and enormous population size.

It is important to remember that garden symphylans are very difficult to manage because of their behaviors such as high mobility, and their adaptations to the soil conditions. They can move to deeper soil layers when conditions are not favorable in the upper soil layers (such as high temperature or low water content). Often, garden symphylans aggregate in high densities in certain spots in the field and the damage is concentrated in those spots (Figure 2). Thus, it is very difficult to predict their incidence and plant damage in the field.

One strategy to manage garden symphylans is to determine which insecticides would repel garden symphylans in the soil. This will at least provide some control until the seedling establish in the soil. Studies were conducted to establish relative efficacy of insecticides against garden symphylan based on repellency behavior and how many died. Based on the studies, Belay, Vydate, Mustang, Lorsban, Mocap, Aza-direct Leverage and Torac showed signs of repellency to garden symphylans. Aza-direct is the only organically approved insecticide that elicited repellency. The insecticides, Capture, Vydate, Belay, and Mustang caused 44 to 95% garden symphylan dead. 100% of the garden symphylans were killed when Torac was used. Torac is not registered on any crops at this moment. These studies provide guidelines on efficacy of insecticides against garden symphylans. For further reading please click the link below to access the published article.

http://cemonterey.ucanr.edu/files/217611.pdf

Figure 1. Garden symphylan

Figure 2. Garden symphylan damage