Romaine lettuce, spinach, and carrots were chosen as the subjects of this study and will be discussed in detail. Treatments were steam and steam + hydrogen peroxide applied by 3 different steam applicators. Pests monitored were lettuce drop and pythium wilt, crop yields, and soil temperature, weed control and hand weeding times, were conducted in plots in the Salinas Valley. Steam application and the cost were evaluated.Many soilborne pathogens affect vegetables; among them are Pythium spp., which weakens the roots of lettuce and Sclerotinia minor, which affects the lettuce crown and stems in contact with the soil causing them to breakdown, collapse, and die . There is little new research regarding the efficacy of steam on pathogens in leafy crops. Some studies have stressed that S. minor and Pythium spp. are important pathogens with the potential to affect crop health in lettuce, spinach and carrot production . Pythium wilt is caused by a fungal water mold that is most abundant in moist to wet field situations and can persist in the soil without a host and infect the roots of lettuce . Spinach and lettuce are susceptible to the fungus under moist field conditions, which can result in total crop collapse . The pathogen develops in soil temperatures of 5 °C to 43 °C, with optimal temperatures of 20-24 °C that are common in a Mediterranean climate, such as California . Depending on the environment, Pythium spp. can remain dormant in the soil for up to 8 years . Pythium zoospores travel through the soil via water and causing infection, drying marijuana whereas oospores are survival spores that allow the pathogen to persist in the soil . Lettuce drop is a fungal disease known as white mold and lettuce drop . The pathogen is most abundant in moist field settings and has a broad host range including weeds and other vegetables .

Lettuce drop affects the crown and stem of lettuce and persists in the soil as sclerotia that can last up to 8 years under dry conditions . The sclerotia are hard, black angular structures that can range in size depending on their developmental state but are smaller compared with S. sclerotiorum, which produce large smooth sclerotia . Under environments favorable to S. minor, the lettuce plants will start showing white mycelium on the crowns of the lettuce . The pathogen grows in temperatures of 6 °C to 30 °C and 18 °C is the optimal temperature for this pathogen . Lettucedrop is a serious problem in all lettuce production areas of the world and can cause crop losses up to 70% . Pinel et al., evaluated the control of S. minor and Pythium spp. with a self-propelled steam applicator in a leafy vegetable trial in Italy. They found a significant reduction of both pathogens to a soil depth of 10 cm. A similar study done by Triolo et al., which was treated once and followed for 5 years, observed the steam treatment reduced S. minor by 68.6% after treatment. Organic vegetable crops are valuable and weeds present a major risk to profitability, given the total value of high value crop production can cost up to $19,000 ha-1 . Some annual weed species can produce seeds more rapidly than others. For example, burning nettle can set seed in 45 days and thrive in a 65-day lettuce field . It is essential for organic growers to know the type of weeds that persist in their weed seed bank to know when to plant certain crops during the year. A worst-case scenario would be planting a high value organic crop in a severely infested weed field. For example, common purslane can remain viable in the soil for up to 20 years and a single plant can produce 52,300 seeds . In addition, perennial weeds such as yellow nutsedge can live short or long lives and white clover seed can persist 80 years or more .Over the years, horticultural studies and technology have been advancing. Olericulture is a branch of agriculture that relies on science to maintain profitable production in vegetable crops.

Many of the 400 commodities produced in California consist of high value crops like almonds and lettuce . In addition, these commodities are the products that make California one of the leading states in producing quality and high yielding specialty crops that boost California’s economy . Profitable high value vegetable crops, such as lettuce and spinach, are not only a cool season and a quick maturing crop but are an example of some important crops that are grown nearly year-round on the Coast of California. Lettuce People discovered in the early 1900s, that the Coastal areas of California have favorable soils and a Mediterranean climate which is well suited for growing high value crops throughout the year . Currently, iceberg, romaine, and leaf lettuces are commonly grown . Lettuce demand in the Western US increased during the 1930s, and the advent of refrigerated rail cars enabled the shipment of lettuce across the continent . While the industry benefited primarily from improved cooling facilities in the Salinas Valley and an accessible railroad, romaine and leaf lettuce became a popular crop by the 1950s that continues to be . Lettuce is the 8th most important commodity produced in California and together with Arizona, these states produce 95% of lettuce in the United States, with a $3.1 billion farm value . According to the 2020-2021 USDA/NASS overview, there was a 22% increase in firesh market romaine lettuce production from 2019 to 2020, with Monterey County continuing to be the leader accounting for 61.9% of the state gross value total. Lettuce producers use transplants for some of the lettuce plantings, but most plantings are direct seeded into 40-inch beds with two seedlines or 80-inch beds that can have 5 to 6 seedlines . With higher density plantings, 2.5 inches of space are established between rows to ensure good spacing and to maximize yield . After the crop is established, it is thinned at the two to four leaf stage by a thinning machine to ensure crops are not too crowded . Consequently, thinning too much or too little canpotentially affect the market quality of the product due to crop overlap or not having enough marketable plants to harvest. .The most common spinach types grown in California are firesh bunched, bagged spinach for salad mixes, and processed . Four states grow 98% of spinach in the United States: California, New Jersey, Texas, and Arizona . Most spinach produced in California is for the clipped and bagged spinach market, with California being the largest producer . Moreover, spinach is currently ranked the 23rd top commodity in California with a value of $281.8 million . The majority is grown in Monterey County, with 25% produced in Southern California and the San Joaquin Valley . All commercial spinach producers grow hybrid spinach that is direct seeded at a high density on 80-inch-wide beds with up to 42 seedlines . Spinach growers early on worked to ensure that there are no weeds present on bed tops because they use mechanical harvesters and have a very low tolerance for weeds, as they can contaminate packaged spinach .Carrots grown in California have a firesh market value of $643 million greater than other carrot-producing states like Michigan and Texas . As stated by the Crop Protection Research Institute, carrots are primarily grown in the Cuyama Valley, Kern, and Santa Barbara County areas and California production accounts for 85% of firesh market carrots in the United States . A disease that causes significant damage is carrot blight caused by Alternaria dauci, which can weaken the tops of the crop which are needed for mechanized harvesting. Therefore, farmers rely heavily on fungicide sprays and is the reason why only 10% of farmers grow carrots organically . Carrots are grown at high densities just like spinach, which makes cultivation difficult. Weeds can also be a problem by clogging machine harvesters .

Carlesi argues it is important that weeds are controlled during the early stages of carrot growth. Competition from weeds can reduce yield and because the crop canopy is not as dense, it can allow light penetration through the canopy, cannabis drying rack which can encourage additional weeds to emerge . Organic growers have few pesticides to control weeds and diseases. In addition, treated carrot seeds are used and are always seeded into the soil on 40-inch beds each with a total of six to eight rows . In addition, carrots are mostly started by sprinklers and then switch to furrow irrigation to prevent the spread of carrot blight and bacterial blight . Other diseases like Pythium spp. can also affect the crop in the early seedling stages causing early die off due to wet soils where overhead irrigation is used and later by causing cavity spot, which can result in major economic losses .Cover crops are used to not only trap soil nutrients and prevent it from leaching, but also to control weeds . In addition, using cover crops to control Pythium spp. in a 3-year interval can be effective because of improved soil fertility and its ability to suppress pathogens . Rotations with broccoli are very effective because once the crop residue is broken down in the soil it produces glucosinolates that can suppress pathogens . In addition, grain crops can also be effective, but legumes have poor performance in suppressing pathogens . Pathogen distribution can be influenced by irrigation and poor water drainage . There needs to be enough water for the zoospores of Pythium for successful infection and spread to other parts of the field . Pythium wilt primarily infects below the ground, causing roots to decay and turn gray and brown, causing damping-off in early plant development . A study by Pinel et al., was able to control Pythium spp. with steam 90-99% of the time in the first 5-10 cm of the soil and observed it did not recolonize during the 3-month trial duration. Prevention of weed seed maturity is necessary to reduce replenishment of the seedbank and cultural management of some weeds after crop harvest is crucial since weeds have already dispersed their seeds into the soil . Tillage immediately following harvest will minimize weed seed set . In addition, inadequate rotation programs, and continuous planting of lettuce can cause a reoccurrence of common purslane in the soil, especially because it can reroot in the ground and can continue to grow . Because crop rotations allow use of different weed control means, broccoli would allow use of oxyfluorfen post emergence herbicide in a integrated pest management program to control purslane, but growers do not make money on broccoli which is why they stick to growing lettuce and grow broccoli when they have to.Crop and weed competition are always at play and all control measures must be used to help the crop to out compete the weeds. The key to weed control is to act early to prevent weed establishment and promote crop competition with weeds. Integrated weed management takes advantage of the crops larger size, such as transplants and cover crops that can rapidly cover the soil surface. The transplants have the ability to establish earlier than the weeds . High density cover crops suppress weeds, and high density lettuce help suppress weeds, although cultivation and weed removal is difficult . Understanding weed seedbanks and integrating an ecological-based approach to weed management will allow growers to develop better weed control programs . These weed control programs can be improved in the future by looking at seed dispersal and emergence in terms of species and creating models of competition to better plan a weed field site program . Using living mulches can also aid in suppressing weed germination due to its ability to prevent light transmission into the ground and its ability to control the temperature of the soil . In addition, the cover crop can serve as an added allelopathic affect, which can also aid in impeding weed development and germination of annual weeds, but allelopathy is not a dependable tool since it is variable . Another pest management practice that is used is intercropping. This involves rotating crops and controlling weeds at the same time, resulting in yield increases . While using crop rotations to control weeds is effective, allowing fields to be fallow can also be of great benefit .

A shank fumigation trial was conducted in 2007 at the UC Kearney Agricultural Center , near Parlier, to determine the effect of two fumigation shank types and five soil surface treatments on 1,3-D emissions and control of representative soilborne pests following removal of a plum orchard. Soil texture at the site was a Hanford fine, sandy loam with pH 7.2, 0.7% organic matter, and a composition of 70% sand, 24% silt and 6% clay. The experiment included 10 treatments with 1,3-D in a split plot design with surface treatments as the main plots and two application shank types as the subplots, as well as an unfumigated control and a methyl bromide plus chloropicrin standard for comparison . Individual plots were 12 feet by 100 feet, and each treatment was replicated three times. Fumigant application. Fumigants were applied using commercial equipment on Oct. 2, 2007. Methyl bromide with chloropicrin was applied at 350 pounds per acre with a Noble plow rig set up to inject fumigants 10 inches deep through emitters spaced 12 inches apart while simultaneously installing 1-mil high-density polyethylene film. The 1,3-D treatments, at 332 pounds per acre, were applied using either a standard Telone rig with shanks spaced 20 inches apart and an injection depth of 18 inches or a Buessing shank rig with shanks spaced 24 inches apart and the fumigant injection split at 16- and 26-inch injection depths. The Buessing shank also had wings above each injection nozzle to scrape soil into the shank trace and minimize rapid upward movement of the fumigant . Following 1,3-D application, a disk and ring roller was used to level and compact the surface soil before surface seals were applied over the fumigated plots. Average soil temperature at 20 inches during fumigation was 70°F, grow rack with lights and soil moisture was 8.2% to 10.5% weight per weight in the top 3 feet. Surface treatments included HDPE film; virtually impermeable film, VIF ; and a series of intermittent water applications .

The intermittent water seals treatment was applied using a temporary sprinkler system installed in the plots following fumigation and the post fumigation tillage operation; water was applied four times in the first 2 days after fumigation: 0.5 inch after 3 hours, 0.2 inch after 12 hours, 0.2 inch after 24 hours and 0.2 inch after 48 hours. All plastic films were removed 10 days after fumigation. Fourteen days after the initial 1,3-D fumigation, the metam sodium treatment was applied through sprinklers at 160 pounds per acre in 2.75 inches of water. For the dual application treatment, 21 days after the initial treatment, soil was inverted with a moldboard plow and an additional 1,3-D treatment was applied with the previously described Telone rig and rolling operation. Emissions data collection. Fumigant emissions from eight 1,3-D treatments — two application shank types times four surface seal methods — were monitored in three replicate plots for 10 days following the initial application. Emission of 1,3-D from the soil surface was monitored using previously described dynamic flux chamber techniques . Briefly, a flow-through flux chamber with a 10-inch-by-20-inch opening was installed on the surface following fumigant injection and installation of the films or after the initial water seal treatment . These chambers allow semi-automated, continuous sampling of fumigant concentrations in the air above the surfaces. The cis– and trans-isomers of 1,3-D were trapped in charcoal sampling tubes . The two 1,3-D isomers were summed as total 1,3-D for data analysis and reporting. Individual tubes were removed from the flux chambers every 3 to 6 hours and stored frozen until laboratory processing. Emission flux and cumulative emission during the 10-day monitoring period were calculated based on surface area and air flow rates through the flux chambers, and treatment differences were compared using analysis of variance . The concentration of 1,3-D in the soil-gas phase was determined 6, 12, 24, 48, 120 and 240 hours after treatment. At each time point, samples were collected using a multiport sampling probe and a system of gas-tight syringes to draw air from eight depths through charcoal sampling tubes. Samples were stored frozen until analysis. In the laboratory, all samples were processed using procedures described by Gao et al. . Briefly, sample tubes were broken and trapped fumigants were extracted from the trapping matrix with ethyl acetate and analyzed using a gas chromatograph equipped with a micro electron capture detector .

Pest control data collection. Pest control efficacy was evaluated using citrus nematode bioassay counts, fungal dilution plating, and weed emergence counts and biomass collections from each replicated plot. The pest control data from this research station emission flux experiment were reported in Jhala et al. . Rose and tree nursery trials In addition to the emission flux and efficacy study conducted at KAC, two field trials were conducted in commercial nurseries to evaluate pest control efficacy and nursery stock productivity. Fumigation and surface treatments in the nursery experiments were the same as in the flux study with minor exceptions . The commercial nursery trials were arranged as randomized complete block experiments with a split plot arrangement of 1,3-D treatments. The whole plot factor was surface treatment, and the split plot factor was the shank type. Individual plots in these experiments were 22 feet by 90 feet, and each treatment was replicated four times. Fumigant application. In 2007, the experiment was established in a garden rose nursery near Wasco. The soil at the rose nursery site was a McFarland loam with pH 6.2, 0.9% organic matter and 74% sand, 13% silt and 13% clay. Treatments were applied on Nov. 7, 2007, when the soil temperature was 64ºF and soil moisture averaged 9.2% w/w from 2 to 5 feet. The experiment was repeated in 2008 in a deciduous tree nursery near Hickman, in a Whitney and Rocklin sandy loam soil with pH 6.5, 0.8% organic matter, and 66% sand, 23% silt and 11% clay. Treatments in the tree nursery trial were applied on Aug. 13, 2008, when the soil was 80ºF and soil moisture ranged from 5.0% to 12.6% w/w in the top 5 feet. Immediately following 1,3-D application, a disk and roller were used to compact the soil and disrupt shank traces and HDPE and VIF were installed using the Noble plow rig. For the water seal main plots, a temporary sprinkler system was installed after the post fumigation tillage operation and intermittent water seals were applied: 0.5 inch after 3 hours, and 0.2 inch each after 12, 24 and 48 hours.

The dual application 1,3-D treatments were applied in the garden rose experiment on Nov. 28, 2007, but were not included in the 2008 tree nursery experiment. Metam sodium was applied in 2.75 inches of irrigation water through sprinklers 14 to 30 days after the initial 1,3-D treatment in both experiments. All plastic films were removed 2 to 3 weeks after fumigation at both sites. Crop production and data collection. Both nursery trials were managed by the cooperating growers using their standard practices for planting, fertilization, in-season tillage and budding and harvest operations. In the 2007 rose experiment, two rows each of the rose rootstock ‘Dr. Huey’ and the own-rooted garden rose variety ‘Home Run’ were planted as hardwood cuttings in December 2007. Rose nursery stock was planted 7 inches apart in furrows spaced 3 feet apart, and the field was furrow irrigated during the 2008 and 2009 growing seasons. The own-rooted cultivar was harvested after one growing season in January 2009, and the unbudded ‘Dr. Huey’ rootstock was harvested in February 2010 after an additional growing season. At both harvest dates, all plants in one 90-foot row were lifted using a singlerow undercutting digger, plants were bundled and tagged by plot, and graded in a commercial packinghouse. In the 2008 tree nursery trial, two rows each of the peach rootstock ‘Nemaguard’ and the plum rootstock ‘Myro 29C’ were planted with 8 inches between plants and 5 feet between rows in December 2008. The tree nursery plots were sprinkler irrigated during the 2009 growing season. Due to the market needs of the cooperating nursery, the rootstocks in the tree trial were not available for harvest and grading as a part of the experiment. Pest control efficacy and crop productivity were evaluated during the 12- or 26-month nursery production cycle. Nematode control was determined using a citrus nematode bio-assay in which two sets of muslin bags containing 100 grams of soil infested with citrus nematode were buried at 6, 12, 24 and 36 inches below the soil surface in each plot prior to fumigation. The initial population of citrus nematodes in infested soil was 4,086 and 3,876 nematodes per 100 cubic centimeters of soil in 2007 and 2008, respectively. The bags were recovered 1 month after fumigation, nematodes were extracted from 100 cubic centimeters of soil using the Baermann funnel protocol, indoor grow rack and surviving nematodes were identified and counted. To evaluate the effect of fumigation treatments on soil fungal populations, ten 1-inch-by-12-inch soil cores were collected from each subplot 2 weeks after fumigation. Soils were homogenized, and a subsample was assayed for Fusarium oxysporum Schlecht. and Pythium species using dilution plating techniques on selective media. Pythium species samples were plated on P5ARP medium for 48 hours, and F. oxysporum samples were plated on Komada’s medium for 6 days. Emerged weeds in a 1-square-meter area were identified and counted twice in the winter following the fall fumigation and several times during the subsequent summer growing season. Nursery stock establishment, vigor and growth were monitored during the season. Visual evaluations of crop vigor were made on a scale of 1 to 7, where 7 was the most vigorous and 1 was dead or dying plants. Near the end of the growing season, trunk diameter of 10 plants in each subplot was measured 3 inches above the soil surface using a dial caliper. As previously described, rose nursery stock was harvested and graded to commercial standards ratings, but tree nursery stock was not harvested as a part of the experiment. Data were subjected to analysis of variance, and initial analyses indicated that the shank types did not differ in their effect on any of the pest control or crop growth parameters measured. Thus, data from the two shank type treatments were grouped together within surface treatments and reanalyzed with seven treatments and six treatments . The nematode, pathogen and weed density data were transformed [ln ] to stabilize the variance prior to analysis; however, means of untransformed data are presented for clarity. Treatment means were separated using Fisher’s protected least significant difference procedure with α = 0.05.Emission flux. Within a surface treatment, there were no statistical differences in emission flux between the two application shank types, thus data were combined over application rig. However, significant differences in 1,3-D emission flux were observed among surface treatments . Fumigant emission flux from bare plots was two times higher than from water seals and HDPE and nearly 15 times higher than from VIF within 48 hours after treatment. Emission from water-sealed plots was reduced during the sequential water applications, but flux was similar to bare soil plots after 48 hours. HDPE film continued to give lower emission rates than the bare soil and water seals but was significantly higher than VIF. Throughout the monitoring period, VIF-covered plots had the lowest 1,3-D emissions; maximum flux was 11 micrograms per square meter per second , which was at least 90% lower than that from the bare soil plots. Relative to the bare soil treatment, estimated cumulative 1,3-D emission losses for water seals, HDPE and VIF were 73%, 45% and 6%, respectively, which were similar to reports from a previous field study . Headspace 1,3-D concentration. Concentration of 1,3-D immediately below the plastic film indicated that 1,3-D retention is much greater under VIF film than under HDPE . Several other studies have shown that VIF can retain substantially higher fumigant concentrations without negatively affecting nematode, pathogen and weed control efficacy or crop yield . Fumigant distribution in soil. Initial analysis of fumigant distribution in the surface 90 centimeters indicated that there were no differences between the application shanks within a surface treatment in this zone; thus data were combined over application shank types . The 1,3-D concentration was highest near the injection depth, at 45 centimeters and lowest near the soil surface, at 5 centimeters , and at 90 centimeters , but this difference diminished over time.

Plots that had been planted with field peas and vetches in mid-summer were associated with lower flag leaf nitrogen than plots that had received spring plantings of field peas and vetches. Grass cover crops were associated with the lowest leaf nitrogen, suggesting that the ability of grass decomposition to tie up nitrogen can be persistent. Visual growth differences were apparent throughout the winter wheat growing season; wheat in spring field peas and vetch cover crop treatments were taller and much greener than other treatments. This suggests that nitrogen release from legume cover crops can continue for more than 1 year and can potentially have cumulative effects in crop rotations.Chicken manure amendments were the most effective fall-applied amendments for increasing soil nitrate levels at potato planting . Soil nitrate at potato planting in soil amended with chicken manure was greater than 75 pounds of nitrogen per acre , similar to levels in plots treated with field peas and vetches. Potato petiole nitrate levels for plots amended with chicken manure were over 19,000 ppm at early tuber bulking, similar to levels produced by many field peas and vetches. Potato petiole nitrate at early bulking for blood meal and soy meal amendments was similar to levels associated with both chicken manure and 150 pounds per acre of urea fertilizer . Green waste compost applied at all rates, as well as composted steer manure, led to lower soil nitrate at potato planting than did chicken manure, and these amendments did not increase soil nitrate at potato planting compared to the fallow treatment . Green waste compost and steer manure did not increase potato petiole nitrate at early bulking and vine maturity compared to the fallow treatment, flood drain table suggesting that nitrogen in these amendments mineralized too slowly for a single application to benefit a potato crop .

Potato establishment and early season vigor did not differ significantly among treatments, but differences in potato vigor were significant at row closure and tuber initiation . Treatments producing high potato petiole nitrate produced taller, greener potato plants than did treatments producing low potato petiole nitrate. Russet Norkotah total potato yield, average tuber size and cull yield were influenced by cover crops and amendments while Yukon Gold potato yield was similar for most treatments . This trend was not surprising given that Russet Norkotah is more responsive to nitrogen fertilizer than Yukon Gold. For Russet Norkotah, vetch species , chicken manures, steer manure, blood meal and soil protein fertilizer produced higher total potato yields than did the untreated fallow. These treatments, along with five field pea varieties, resulted in a larger average tuber size than did the untreated fallow . Total yield for the treatment with 150 pounds per acre of urea fertilizer was similar to that produced with vetches, chicken manures and blood meal, suggesting that soil nitrogen availability was a primary factor in increasing potato yield . Nitrogen’s important role is also supported by a strong positive correlation between total Russet Norkotah potato yield and potato petiole nitrate at early bulking. The r value for this correlation equaled 0.656 when Russet Norkotah and Yukon Gold data were combined. The only treatment-related effect on total Yukon Gold potato yield was that cover-cropping with spring wheat and fall triticale produced lower total yield than did cover-cropping with legumes . Grass cover crop treatments led to numerically lower soil nitrogen at planting and lower potato petiole nitrate at early bulking, compared to the untreated fallow .

This suggests that the low potato yield following grass cover crops could be due to nitrogen immobilization during potato growth and development. Cover crop and amendment treatments did not cause a substantial increase in tubers with knobs or growth cracks in either Russet Norkotah or Yukon Gold , but the percentage of cull potatoes based on total yield for Russet Norkotah differed among treatments . Both chicken manure treatments, as well as blood meal and soy protein, resulted in higher percentages of culls than did the untreated fallow. An increase in cull percentage often occurs as total yield increases, but Perfect Organic Blend chicken manure also produced a higher percentage of culls than did the treatment with 150 pounds per acre of urea fertilizer. All cover crop treatments led to a percentage of culls similar to or lower than was associated with the treatment with 150 pounds per acre of urea fertilizer . Yukon Gold was chosen for the 2017 trials because Rhizoctonia black scurf and black dot tuber blemish, common problems for organic potato growers, are easy to see on yellow varieties. The severity of black scurf and black dot did not differ according to cover crop species, but in potatoes grown after spring-planted cover crops , 27% exhibited black scurf — compared to 13% in potatoes grown after mid-summer and fall plantings of cover crops. On the other hand, spring plantings of cover crops led to lower black dot severity on tubers than did mid-summer plantings .Economic issues play a major role in the feasibility of using legume cover crops to boost soil nitrogen in a crop rotation. Organic growers must consider the opportunity cost involved in growing cover crops instead of a cash crop as well as the cost of applying an amendment such as chicken manure. The economic analysis required to weigh all benefits and lost opportunity costs is complex, and beyond the scope of this study, but a comparison of monetary costs shows that cover crop production is more expensive than synthetic fertilizer, similar to applying chicken manure and less expensive than applying blood meal and soy meal.

The average cost of bulk urea fertilizer from local suppliers in Northern California in 2018 was $365 per ton, or $60 to supply one acre with 150 pounds of nitrogen . The average cost of bulk dried poultry manure from local suppliers in Northern California was $145 per ton, or $272 dollars to supply one acre with 150 pounds of nitrogen . The cost of bulk blood meal and soy meal represented a nitrogen cost of greater than $3.40 per pound, or over $500 to supply one acre with 150 pounds of nitrogen. The cost of certified organic blood meal, packaged in 50-pound bags, was greater than $7 per pound of nitrogen, or more than $1,000 to supply one acre with 150 pounds of nitrogen. The total cost of field pea and vetch production is estimated at $175 dollars per acre, including the cost of seed, planting, irrigation, management and incorporation .Vetch, field peas, blood meal, soy meal and chicken manure, because they produced potato yields and potato petiole nitrate similar to those produced in plots treated with 150 pounds per acre of urea fertilizer , were feasible alternatives to synthetic fertilizer. Whether organic producers favor cover crops or chicken manure as a nitrogen source depends on several factors, including land availability and the opportunity to grow cash crops. Producers who grow high-value cash crops requiring a full growing season may favor amendments because they can be quickly applied after harvest or before planting. Producers with idle land or with time between cash crops during the growing season may prefer cover crops, as many legumes in this study added over 150 pounds of nitrogen per acre and provided multi-season carry-over of soil nitrogen, and also offer protection from soil erosion. For hay producers, it’s extremely important to leave above ground biomass from legume cover crops in place, instead of haying the residue, because most added nitrogen is contained in legumes’ leaves and shoots rather than their roots. Regardless, both options offer benefits in soil health, and in our study the added nitrogen in both cases broke down into mineralized form in adequate amounts to meet early-season and late-season potato nitrogen needs. The economic benefit of using cover crops and chicken manure is more difficult to justify in conventional potatoes because, in our research, both practices entail higher costs and greater difficulty of application than synthetic fertilizer, which produced similar yields. For organic potato production, using either grass cover crops or a one-time application of compost to increase soil nitrogen is difficult to justify economically. In our research, grow tables 4×8 these treatments had a neutral or negative effect on soil nitrogen compared to fallow treatments. Organic nitrogen in these treatments failed to convert into mineralized form in adequate amounts to increase either potato yield or yield of wheat planted the year after potatoes. Mustard, arugula and radish had a neutral-to-positive effect on potato yield and nitrogen.

Several Brassica species have also been shown to have biofumigation properties, although a reduction in soilborne potato diseases Rhizoctonia solani, Colletotrichum coccodes and Verticillium wilt was not evident in this study. Fallowing for an entire year, starting in spring the year before growing potatoes, is another option that growers with idle land or limited water can consider. In this research, the spring fallow treatment resulted in mineralized nitrogen at potato planting similar to or higher than levels that resulted from the summer fallow and fall fallow treatments . In potatoes, the spring fallow treatment produced petiole nitrate at early bulking similar to that produced by a treatment with 150 pounds per acre of urea fertilizer following barley . The additional nitrogen in the spring fallow treatment was likely related to natural mineralization of soil organic matter, as organic matter in Tulelake soils is naturally high .Rice is one of the most important sources of human energy worldwide and is grown in a wide range of agroecosystems, though paddy systems are the most prevalent . In California, more than 200,000 ha of flooded rice are grown in a waterseeded, continuously flooded system that has successfully suppressed certain nonaquatic weed species such as barn yard grass [Echinochloa crus-galli Beauv.] and bearded sprangletop . Currently, rice growers in California flood fields at the beginning of the growing season and then direct seed pregerminated rice seed into the flooded fields from airplanes. A flood depth of 10 to 15 cm is maintained until approximately 1 mo before harvest, when the field is drained to allow rice harvesting. Repeated use of flooded irrigation in the California rice agroecosystem has since selected for weed species such as late watergrass [Echinochloa oryzicola Vasinger] that are well adapted to the system. In recent years, California has experienced unprecedented drought, with the 2012 to 2014 period being the driest on record . Accordingly, concerns about water usage have increased, particularly for crops like rice that have high water use. Due to the flood irrigation, rice is a visible water user, receiving attention from both the general public and policy makers, and there is increased pressure on rice growers to reduce water use. In California, the only alternative to water seeding currently in use is dry seeding followed byflooding after early postemergent herbicide applications. Recent research, however, indicates that drill seeding into dry soil as practiced in California rice systems does not necessarily reduce crop evapotranspiration, crop coefficient, or irrigation delivery in comparison with the continuously flooded system . A number of alternatives to flood irrigation exist in other rice-growing regions of the world, including an alternate wet and dry system , which reduces water use over the crop growth period through alternating periods of flooding with periods of drying, and saturated soil culture , which reduces water use over the crop growth period by maintaining the soil at the saturation point . Yields in aerobic systems are often lower due to the reduced ability of rice to compete with weeds , and this may be an obstacle to adoption of alternative irrigation systems by growers. In addition to changing the competitive ability of rice with respect to weeds, alternative irrigation systems can shift weed species composition, selecting for some species over others. In California, differences in irrigation during the seedling recruitment period have been shown to shift the emergence of certain weed species when comparing wet- versus dry-seeded systems . In these systems, water seeding favored sedges and broad leaves, whereas dry seeding favored grasses, particularly watergrass and sprangletop species . Later in the season, sedges and grasses dominate over aquatic weeds in saturated, non-flooded soils . For continuously flooded systems, water depth also affects the presence of certain species. Grasses are suppressed by continuous flooding to a depth of at least 5 cm, whereas a deeper flood of about 15 cm suppresses most sedges .

The recommendations comprehensively address bio-fuel production and use, as well as the necessity of agency and private sector stakeholder cooperation for effective implementation of the recommendations . Initially, all federal agencies with authority relevant to bio-fuel production should be identified, their likely responsibilities on the invasiveness issue determined, and their ability to minimize the risk of bio-fuel escape and invasion strengthened as necessary. To reduce the risk of escape, the bio-fuel crops that are promoted should not be currently invasive or should pose a low risk of becoming invasive in the target region. In addition, bio-fuel crops should be propagated in production sites that are least likely to impact sensitive habitat or create disturbances that would facilitate invasion. Most importantly, effective mitigation protocols need to be developed to prevent dispersal of plant propagules from sites of production, transportation corridors, storage areas and processing facilities. Minimizing harvest disturbance can also reduce the potential for dispersal and off-site movement of propagules. Prior to wide scale planting, multi-year eradication protocols should be developed that are based on integrated pest management strategies. Such practices should be readily available, and appropriate information should be distributed with the purchase of bio-fuel crop seeds. These control methods are not only critical for preventing the dispersal of bio-fuel crops from abandoned production sites, they are a necessary component of an effective early detection and rapid response system for bio-fuel crop populations that do escape active management. Throughout this entire process , flood and drain table all stakeholder groups should be engaged, from bio-fuel development to conversion.The question of how species successfully invade new areas has fascinated scientists for over a century .

By studying ruderal and agricultural weeds invading empty niches, Herbert Baker began to identify characteristics associated with invasiveness, which resulted in a list of traits describing the ‘ideal weed’ . Work in subsequent decades examined a wide range of traits using comparative approaches of taxonomically-related species and regional floras . With these studies came an increasing realization that factors contributing to invasiveness are strongly influenced by the stage of invasion, characteristics of the introduced range, and which species groups are being compared. These realizations, combined with discrepancies across studies, resulted in some skepticism that traits associated with invasiveness could be generalized . However, there is support for the idea that invasive species differ from non-invasive native and non-native species in key attributes depending on the environmental context . Here, we explore how ecological and evolutionary theory has refined our understanding of the ‘ideal weed’. We do not provide an exhaustive review of all traits but rather an overview of key functional and evolutionary frameworks in which progress has been made.Baker’s ‘ideal weed’ possessed a general-purpose phenotype , life history traits that permit reproduction from a single individual , rapid growth, and high, continuous seed output . Several of these characteristics are well studied and appear to be common when evaluated across different invasive taxa such as high germination success across environments , selfing , and rapid growth rate , while others are less studied . In recent decades, researchers have broadened the search for ‘weedy’ characteristics to include traits related to resource acquisition and use that underlie rapid growth, competitive ability, and even stress tolerance. Syntheses of regional and global for as have demonstrated that, relative to non-invasive species, invasive species are generally larger, have higher specific leaf area , allocate relatively more biomass to leaves and stems at the expense of roots, and use resources more efficiently .

However, there are exceptions to every rule. Identifying traits associated with invasive species is hindered by differences in how invasiveness is defined, bias in species selection for experiments, and challenges comparing species at different stages of invasion . However, several useful frameworks have been developed to evaluate traits within relevant contexts. First, many researchers recommend controlling for a species’ commonness when selecting species for experiments as comparisons among common invasives and rare non-invasive species may lead to spurious conclusions . For example, invasive species appear to be more competitive than co-occurring natives ; however, many of these studies focus on particularly aggressive and common invaders. In a comparison of annual plants in Germany, Zhang and van Kluenen found that invasive species were stronger competitors only when comparing common invaders with rare natives. In essence, comparing species that are similarly successful should allow researchers to identify traits that promote invasion in particular, rather than commonness more generally. In another effort to standardize how invasiveness is defined, Catford et al. proposed comparing traits of invasive species within invasiveness categories based on four demographic dimensions: local abundance, geographic range, environmental range, and spread rate. One trait may promote invasiveness along one dimension but limit invasion along another . Time since introduction and propagule pressure would ideally be incorporated into invasiveness categories , but these data are not available for many species. Perhaps the most comprehensive effort to link traits to different stages of invasion is that of van Kluenen et al. who proposed a nested, multi-scale approach . Identifying a universal set of traits that explains invasiveness is challenging because traits are dependent on environmental context, including specific abiotic and biotic factors arising from, for example, climate and community composition .

By accounting for spatial scale, the framework proposed by van Kluenen et al. avoids inappropriate comparisons of traits across different stages of invasion and resolves inconsistencies associated with context dependency. For example, studies have found that invasive species can have smaller, similar, or larger seeds compared to native or non-invasive species . However, this inconsistency likely reflects different ecological filters or processes across stages: smaller seeds are likely to be dispersed to new sites, but larger seeds have more resources for establishment and growth . Conversely, some traits may enhance invasiveness at multiple stages of invasion. For example, fast growth rates can assist with colonization of new or disturbed habitats , lead to priority effects , and ultimately affect competition outcomes in established communities . Finally, a trait-based community assembly framework may also elucidate mechanisms of invasion . Community assembly theory allows for both stochastic and niche-based processes at various scales. Species composition within a community is determined by a series of ecological filters that sort species based on their traits . As an example, seed predation is a strong biotic filter on recruitment in some systems and this may favor species with smaller seeds that are more likely to evade predation from rodents . Investigating how trait-performance relationships change when a filter is manipulated can indicate if non-native invaders are succeeding by acting like the natives or by doing something different . Trait analyses can also determine if invasive species occupy empty niches. Work in desert annual communities in the southwest U.S. show that invasive annuals have unique trait combinations that allow them to grow fast and use water efficiently . Below, we expand on how traits and trait plasticity interact with abiotic and biotic filters to regulate invasion.Many invasive species thrive in resource-rich environments . Environments with ample light, water, or nutrient availability could favor fast-growing species that quickly take up available resources. Species associated with a resource acquisitive strategy have trait values aligned with the ‘fast-return’ end of leaf, plant, and root economic spectra . This includes cheaply constructed, short-lived tissues designed for high rates of carbon and nutrient assimilation and biomass allocation patterns that favor light interception and growth . These species may alter the system in a way that prevents slower-growing species from establishing and dominating. For example, hydroponic flood table the proliferation of invasive grasses in many systems suppresses woody seedling establishment via competition for limiting resources or increased fire firequency leading to a type conversion or invasion by other species . Many species can also invade low resource environments and they succeed by employing a wide range of strategies . Community assembly theory predicts that strong abiotic filters in stressful environments will result in co-occurring species with similar traits and there is some evidence for this in invaded systems. For example, species invading low resource systems are similarly or more efficient at using limiting resources relative to native species adapted to those systems .

There is also evidence that invasive species can succeed in low resource environments by possessing resource acquisitive traits. While native and invasive non-native annuals in semi-arid Mediterranean-climate ecosystems are similar with respect to most traits, invasive annuals were taller and had larger seeds and thinner roots—which likely enhances establishment and resource acquisition . Phenological differences, such as early germination, may allow invasive species to avoid competition from co-occurring species in low resource environments . Early phenology coupled with high resource-use efficiency or rapid growth may be particularly effective in low resource environments, such as deserts and coastal sage scrub in the southwestern U.S. . In sum, the fast growth rates and competitive strategies hypothesized by Baker appear to promote invasion in a range of habitats, but the specific physiological traits underlying these strategies differ across environments. Resource acquisition traits may be particularly useful in high resource environments, while efficient resource use or competitive strategies like early phenology may lead to invasion success in low resource environments. Finally, a central tenet of Baker’s ideology is that some invaders display broad environmental tolerance and are able to move past environmental filters by possessing traits that promote high fitness under low and high resource conditions. Some invasive species exhibit broad environmental tolerance by not conforming to growth-stress tolerance tradeoffs. For example, Norway maple is a common invader in North American forests and has high survival under low light conditions and high growth rates in full sun . Tree of heaven is one of the most invasive woody species in Europe and North America and its broad geographic distribution is driven by a combination of traits aligned with high resource acquisition as well as the ability to alter morphological traits and biomass allocation patterns across environments . During the invasion process plants may escape specialist enemies that limit their population growth in the native range . Such escape is typically transient, however, as invaders accumulate new enemies over time . The initial escape from enemies could allow for rapid establishment but, over longer timescales, three traits of invaders may make them particularly adept at overcoming the biotic filter created by enemies and promoting invasion. First, ruderal invaders can escape their enemies by virtue of their high dispersal, short lifespan, and low allocation to defense, fireeing up resources for rapid growth or competitive ability . Second and relatedly, many invaders appear to have high growth rates, which tend to reduce the cost of damage . This high growth rate means that invaders can withstand high amounts of enemy damage with limited effects on fitness . Consistent with this idea, in a multi-species study, invasive vines received just as much herbivory as natives or naturalized species, but were also more tolerant of damage , although other multispecies studies and metaanalyses find that invasives are similarly or even less tolerant to herbivory than natives . Third, native generalist enemies may have reduced preferences for non-native species with which they have no evolutionary history , although this appears not to be a general phenomenon across invasive species . Thus, both innate traits of the invader that Baker hypothesized would facilitate invasion and the match between invader traits and the invaded community may reduce the capacity for enemies to limit invader population growth. Like enemies, mutualists may also be left behind during the invasion process. As a result, successful invaders might be less dependent on mutualists , more generalist and able to interact with a wide variety of partners as predicted by Baker , or rely on co-invasion of mutualist partners . For example, selfng was one of Baker’s ‘ideal weed’ characteristics because it would allow reproduction in the absence of suitable pollinators and at low population densities. Selfers do appear to be over represented in invasive taxa although it is not clear whether this is because of the advantages of selfng when suitable pollinators aren’t available or because of Allee effects. For other species that fail to meet Baker’s criteria of generalized dispersal or pollination mechanisms , like that of highly specialized fgs which require a specific species of wasp pollinator or pines limited by appropriate mycorrhizae, invasion can still occur but only once the mutualist also invades.