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 .