The use of cover crops in rotation with a cash crop is one way to maintain and build SOM (Magdoff and Van Es, 2009). Cover crops are plants grown for the specific purpose of maintaining or improving soil characteristics (Magdoff and Van Es, 2009). Any groundcover planted before, after, or concurrently with the cash crop and then ended before the next crop is seeded may be considered a cover crop (Hartwig and Ammon, 2002). Each cover crop affects the soil differently, and one must be careful to select the appropriate species for each location and farming scenario. Cover crop choice depends largely on the circumstances and objectives of the individual farmer and soil type. In addition to adding SOM, cover crops may also be grown to add nitrogen, reduce soil erosion, increase water infiltration, decrease nutrient loss by leaching, attract beneficial insects, suppress weeds, and/or suppress soilborne pathogens (Magdoff and Van Es, 2009).
In New Mexico, where chile pepper (Capsicum annuum L.) production is the second highest in the country (58,967 t in 2013) (U.S. Department of Agriculture National Agricultural Statistics Service, 2014), there are many obstacles to overcome such as low SOM (0.5% to 1.0%) and high pH (7.5 to 8.0). Many growers also face issues with weed competition and soilborne pathogens (Bosland and Votava, 2000). Chile wilt diseases such as those caused by Phytophthora capsici and Verticillium dahliae can result in devastating yield losses (Walker et al., 2012). Once established, soilborne pathogens can be especially difficult to manage. For this reason, many growers nationwide have adopted the use of various chemical fumigants such as methyl bromide, metam sodium, metam potassium, chloropicrin, or 1,3 dichloropropene (Telone™; Dow AgroSciences, Indianapolis, IN) to suppress yield-detracting weeds and pathogens such as fungi and nematodes (Collins et al., 2006; Martin, 2003).
Synthetic chemical fumigation is a conventional approach to soilborne pathogen management. Many New Mexico chile pepper farmers also rely heavily on the fumigant chloropicrin to reduce soilborne pathogens (Walker et al., 2012). Managing soilborne pests presents many challenges, and chemical fumigation is one of the few choices available to growers. However, the use of chemical fumigants is costly for farmers and results are often inconsistent. Use of chemical fumigants in California can cost growers from $6600 to $7400 per hectare (University of California Agriculture and Natural Resources, 2013). A study conducted in Arizona in 2009 found no significant difference in chile pepper stand establishment or seedling vigor between plots treated with chemical fumigants and untreated plots (Walker et al., 2012). Many chemical fumigants can effectively suppress pathogens, but several of the active compounds are harmful to the environment as well as to humans. For example, methyl bromide is a significant ozone-depleting substance and can also cause various health problems ranging from skin and eye irritation to death (U.S. Environmental Protection Agency, 2000; US-EPA, 2014). For these reasons, methyl bromide was phased out of use in the United States as of January 2005 by the Environmental Protection Agency (EPA), except in cases of quarantine and preshipment and critical use exemptions (US-EPA, 2014). Because of their potential dangers, the EPA is also regulating or restricting other commonly used soil fumigants (US-EPA, 2012). As a result, alternatives to chemical fumigants are needed.
Biofumigants are a type of cover crop that, in addition to other soil benefits, have the ability to suppress soilborne pathogens including fungi or nematodes (Kirkegaard et al., 1999). Biofumigation is a sustainable method of soil management in cash crop rotations that may present additional options for conventional and organic growers. Biofumigants are biologically active (bioactive) cover crops and are frequently referred to as green manures because they are incorporated into the soil as living plant material (Kirkegaard et al., 1999). The term biofumigation refers to the suppression of soilborne pests and pathogens using naturally occurring biocidal compounds, particularly isothiocyanates (ITCs), which are released from bioactive Brassicaceous cover crops on hydrolysis of GSLs (Kirkegaard and Sarwar, 1998). ITCs are chemically similar to methyl isothiocyanate, the active agent from the chemical fumigant metam sodium (Matthiessen and Kirkegaard, 2006).
GSLs are secondary, defensive, antiherbivory compounds that occur in members of the Brassicaceae family (Kirkegaard and Sarwar, 1998). Concentrations of GSLs and the various hydrolysis products vary within each species and within cultivars; therefore, not all Brassica species are well suited as a biofumigant cover crop (Kushad et al., 1999). There are ≈20 different types of GSLs found in Brassica species, and their structures vary based on the organic side chain (aliphatic, aromatic, or indolyl) (Kirkegaard and Sarwar, 1998). When GSLs are hydrolyzed by the endogenous enzyme myrosinase, there are several possible products depending on the soil pH: oxazolidinethiones, nitriles, thiocyanates, and many forms of ITCs (Kirkegaard and Sarwar, 1998). ITCs are typically the most active and the most toxic of the hydrolysis products (Brown and Morra, 1997). However, the toxicity level of different ITCs to different organisms varies (Kirkegaard and Sarwar, 1998).
Unlike chemical fumigants, biofumigants, taken as a whole, may also have the added benefit of increasing SOM by 8% to 23%, suppressing weeds by up to 40%, and increasing soil water infiltration 2- to 10-fold (Collins et al., 2006; Mattner et al., 2008; McGuire, 2003). In addition to these agronomic benefits, biofumigant crops may also be a more economically viable choice for farmers than chemical fumigants. For example, farmers could save $163/ha using green manures, including Brassicas, instead of chemical fumigants (McGuire, 2003). There is the potential for rapid adoption of cover crops that are effective at reducing a major input cost like fumigation and also improving the cash crop health and productivity (Snapp et al., 2004).
The ideal biofumigant crop would therefore have high GSL content in the tissues and also high biomass production to maximize the amount incorporated into the soil (Kirkegaard et al., 1999). Another factor that may be considered when selecting for biofumigant potential is the ability to harvest a primary product such as broccoli or Brussels sprouts, whose residues can then be incorporated into the soil as a biofumigant. This strategy has the benefit of additional income but the drawback of reduced biomass for biofumigation purposes. Biofumigant crops should also be a non-host for nematodes or able to suppress nematode populations in the field.
Southern RKN (Meloidogyne incognita), a plant–parasitic nematode, is of great concern to chile pepper growers in southern New Mexico (Walker et al., 2012). This species is commonly found in the southwestern United States where chile peppers are cultivated and is a serious pathogen that can reduce yields by more than 40% (Thomas, 1995; Thomas et al., 1995). Chile pepper growers may experience reduced plant vigor, yield loss, or complete loss of production by seedling death, depending on the crop’s stage of development (Goldberg, 2001). The use of mustards, both Brassica and Sinapsis species, as bioactive cover crops is becoming common practice with potato growers in Washington to suppress plant–parasitic nematodes (Ramirez et al., 2009). An 8-week field trial in northern Australia found that ‘Weedcheck’ fodder radish (Raphanus sativus) was resistant to Meloidogyne sp. and caused a reduction in the nematode population (Pattinson et al., 2007). A field study in the Czech Republic observed that after incorporating brown mustard and covering it with polyethylene sheets, the number of root galls in carrots (Daucus carota) caused by northern RKN (Meloidogyne hapla) decreased (Douda et al., 2012).
A particular biofumigant crop may work well in a certain geographical area with a specific set of issues (pathogens, nematodes, etc.) and a location-specific primary crop but may not perform as expected in another region with a different crop–pest interaction. The soil environment is complex and varies from state to state, field to field, and even within a field. Individualized evaluation is necessary to determine the impacts biofumigants have on different vegetable cropping systems. In southern New Mexico, little is known about the interaction between biofumigants and the soil, pests, and region-specific crops such as chile pepper.
The primary objective of this research was to evaluate the biofumigation performance of four Brassicas including three mustard cultivars (Brassica juncea ‘Caliente 61’, ‘Caliente 199’, ‘Pacific Gold’) and one broccoli cultivar (Brassica oleracea var. botrytis ‘Arcadia’) in the semiarid climate of southern New Mexico. Biofumigant performance was evaluated based on biomass production and GSL concentrations in Brassica crop tissues; soil changes, including SOM and pH; and chile pepper crop performance, including stand establishment, yield, and vegetative biomass. A second objective of this research was to conduct a greenhouse biofumigant seedling establishment study and a host assay to evaluate the growth and host suitability of the biofumigant species to RKN.
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