- Vegetables: tomatoes
- Crop Production: organic fertilizers, tissue analysis
- Farm Business Management: budgets/cost and returns
- Production Systems: general crop production
- Soil Management: composting, earthworms, nutrient mineralization
Soil amended with green cover crops of Brassica napus or B. juncea generally had higher populations of Fusarium oxysporum and Pythium spp. than the methyl bromide treatment and the fallow control. Laying black polyethylene mulch at incorporation or one month after incorporation did not consistently influence the amount of isothiocynates detected in amended soils. Damping-off and Fusarium wilt on seedless watermelon were not consistently lower in brassica-amended soils compared to the nontreated control or methyl bromide.
Biofumigation refers to the suppression of soilborne pests and pathogens by biocidal compounds released in soil when glucosinolates (GSL), thioglucoside compounds in Brassica green manure or rotation crops, are hydrolyzed (Kirkegaard and Sarwar, 1998). Glucosinolates are sulfur compounds composed of a thioglucose group, a variable carbon side chain (R-group), and a sulphonated oxime (Mayton et al., 1996). There are about 20 different types of GSLs commonly found in brassicas that vary in their structure, depending on the type of organic side chain (aliphatic, aromatic, indolyl) (Kirkegaard and Sarwar, 1998).
Glucosinolates are hydrolyzed by myrosinase, an enzyme present endogenously in brassica tissues, to release a range of hydrolysis products including oxazolidinethiones, nitriles, thiocyanates, and various forms of volatile isothiocyanates (Gardiner et al., 1999; Kirkegaard and Sarwar, 1998; Mayton et al., 1996; VanEtten et al., 1969). The specific hydrolysis products formed depend on the R group of the parent glucosinolate and pH (Gardiner et al., 1999). Myrosinase is located in special cells from which it is released when the leaf tissue is damaged (e.g. when it is rubbed). Reports in the literature document the types of glucosinolates and their quantitative and qualitative differences in plant parts, ontogeny, and season of growth for many Brassica spp (Gardiner et al., 1999; Kirkegaard and Sarwar, 1998; Mayton et al., 1996; Sarwar and Kirkegaard, 1998; Sarwar et al., 1998; Smolinska and Horbowicz, 1999; Smolinska et al., 2003).
Mayton et al. (1996) found that volatile compounds from B. juncea cv. Cutlass were fungicidal in vitro to plant pathogenic fungi, including Fusarium sambucinum. Smolinska et al. (2003) evaluated the sensitivity of four F. oxysporum isolates to different isothiocyanates (ITCs) in vitro. They found that conidial and chlamydospore germination were highly susceptible to inactivation by isothiocyanates leading them to conclude that these two stages in the life cycle of Fusarium are most susceptible. A similar conclusion was reached by Smolinska and Horbowicz (1999), in an experiment in which Fusarium oxysporum chlamydospores exposed to the volatiles from B. juncea lost their viability completely. Soil amended with B. juncea residues had significantly fewer chlamydospores of F. oxysporum (Smolinska, 2000).
Gardiner et al. (1999) tested two cultivars of B. napus, Dwarf Essex and Humus, which are winter hardy. They report that the abundance of most GLS compounds peaked 30 h after incorporation or a little later, and trailed erratically to 20 days after incorporation by which time they were generally below the limit of detection. They also found that the dominant glucosinolate in the roots of both cultivars was 2-phenylethyl (a minor constituent in the shoots) and when they sampled the soil after incorporation, again 2-phenlyethly ITC was the most abundant, followed by benzenepropanitrile. Kirkegaard and Sarwar (1998) also report finding significant amounts of 2-phenylethyl in the roots of brassicas. Therefore, roots might play a more prominent role than shoots in contributing allelochemicals in soil. Because 2-phenylethyl is aromatic–and thus less volatile–it may persist for longer periods in the soil, and be released prior to incorporation (Kirkegaard and Sarwar, 1998). Soil residence times vary among compounds, because the ITC functional groups are reactive and sorption occurs to soil constituents (Gardiner at al., 1999). In addition to the 2-phenylethyl and benzenepropanitrile, Gardiner et al. (1999) also found 3-butenyl, 4-pentenyl, 4-methylthiobutyl, and 5-methylthiopentyl ITCs in small quantities. Detection of 5-methylthiopentanenitrile and 6-methylthiohexanenitrile in soil indicates that glucosinolate hydrolysis in green manures may produce nitriles at the expense of the respective ITCs. Relatively little is known about the activity of 2-phenylethyl ITC in the soil since many previous studies have concentrated on aliphatic types such as methyl ITC (a commercial soil fumigant) or 2-propenyl ITC (allyl) due to its early recognition as the active constituent of mustard oils (Kirkegaard and Sarwar, 1998). Therefore, all isothiocyanates released in the soil following biofumigation will be monitored.
Fusarium wilt of watermelon is a problem where watermelon is grown on short rotation sequences. The pathogen accumulates in soils and remains there indefinitely. Many diploid cultivars of watermelon have resistance to race 1 of F. oxysporum f. sp. niveum. However, only a few cultivars of triploid watermelon have resistance, and these cultivars produce elongated fruit, which is unacceptable to produce buyers. In addition, race 2 of F. oxysporum f. sp. niveum is present in several southern states, and there is no resistance to this race in cultivated varieties (Martyn and Bruton, 1989; Zhou and Everts, 2001). Therefore, biofumigation may be useful to control Fusarium wilt in susceptible cultivars of triploid watermelon and in diploid cultivars in areas where race 2 is present (Martyn, 1987).
Control of other soilborne pathogens also is important. Damping-off caused by Rhizoctonia solani and Pythium spp can be a problem on transplanted watermelon. Watermelon fruits are subject to fruit rot caused by R. solani, Pythium spp, and Sclerotium rolfsii. Therefore, the study will also look at the effect of biofumigation on these fungi with the goal of getting more information out of the study.
- Evaluate the effectiveness of biofumigation in the control of Fusarium wilt of watermelon, compared to control with methyl bromide.
Determine the best time to incorporate green manure prior to laying plastic (i.e., let the biofumigant decompose before tarping, or tarping immediately after plowing under).
Quantify inoculum density of Fusarium oxysporum, Rhizoctonia solani, Pythium spp, Sclerotium rolfsii, and fluorescent Pseudomonas in the soil before and after biofumigation.
Determine glucosinolate concentration in roots and shoots of the brassicas at the time of incorporation.
Quantify glucosinolate breakdown products in the soil after brassica incorporation