However, concentration of GSLs and the hydrolysis products vary within species and cultivars. Therefore, not all brassicaceous crops are well-suited as biofumigants (Kushad et al., 1999). Growers can attain maximum ITC release under field conditions by allowing the proper biofumigant crop to grow until flowering. Plants should be mowed and finely chopped in order to disrupt the plant cells as much as possible. Plant biomass must then be thoroughly incorporated into the soil followed by heavy irrigation and tarping, if possible (McGuire, 2003; Rudolph et al., 2015). Growers interested in using brassicaceous crops for biofumigation should be aware of black leg disease, caused by the fungus, Phoma lingam (sexual stage: Leptosphaeria maculans) which affects a wide range of crops in this family in Washington and Oregon (Ocamb and du Toit, 2014). Growers should also be attentive to isolation distances required for Brassica seed production.
In some cases brassicaceous seed meal (BSM; Fig. 1) may be more advantageous than a biofumigant cover crop. Brassicaceous seed meal is the material remaining after extracting the oil from mustard, canola, or rapeseed seeds. Application of BSM by incorporating it into the soil is quicker than growing a cover crop and the timing of application is flexible. Although BSM does require irrigation upon incorporation, much less water than a cover crop and no fertilizers are needed. A grower can also be certain that frost will not be a limiting factor as with a cover crop, nor will BSM serve as a host to a plant-parasitic nematode (Zasada et al., 2009). It also serves as a source of nitrogen. Additionally, BSM has been shown to alter the soil biology which then aids in the suppression of plant diseases (Cohen and Mazzola, 2006; Mazzola et al., 2015). However, BSM can be costly (~$1,600/ton) and its availability is still currently limited. Recommended application rates of BSM vary between 1 and 7.4 ton/ac (Jonathan Winslow, manager of Farm Fuel Inc., personal communication; Mazzola et al., 2015). Brassicaceous seed meal may be a viable pre-plant biofumigant or an alternative to cover crops.
A study is currently being conducted in Whatcom County in a commercial field of replanted ‘Chemainus’ red raspberry. The primary objectives of the study are to compare BSM to conventional chemical fumigation, and chemical fumigation at half the recommended rate, after raspberry roots have been removed in a continuous red raspberry production system. The study was initiated in Fall 2014 with the one-time application of treatments and data collection will continue through Summer 2017. The experiment is arranged in a completely randomized design with four replications of four treatments assigned to a single row. Each treatment plot is 30 ft long x 6 ft wide with a buffer of 10 ft between plots. Data which will be collected over the life of the study include changes in the soil microbial community structure, root lesion nematode (RLN) populations, and raspberry growth and productivity.
- Root removal followed by BSM application (proprietary mix of Brassica juncea and Sinapis alba)—Fall applied at 1.5 ton/ac at 6 in soil depth with a walk-behind tiller (Fig. 2)
- Root removal followed by full rate metam sodium (Vapam®; Spring applied at 74 gal/ac at 14-16 in depth)
- Root removal followed by ½ rate metam sodium (Spring applied at 37 gal/ac at 14-16 in depth)
- Full rate metam sodium (Spring applied at 74 gal/ac at 14-16 in depth) with no root removal
Figure 2. Incorporating the brassicaceous seed meal.
Raspberry plants have large root systems where both RLN and Phytophthora rubi (the organism that causes Phytophthora root rot) reside. Because these organisms can survive in field soil for years (Duncan, 1980), the few months elapsing between terminating old plants and replanting is likely inadequate to eliminate these organisms from the incorporated root material and prevent future infection. An additional step of removing as much old plant material, which possibly contains these organisms, from the field may improve future disease and nematode management. Raspberry root removal prior to either fumigation or BSM applications may increase treatment efficacy and perhaps also extend the life of new plantings, but there have been no reported results about this potential strategy to date as it is a relatively new strategy. Root removal was performed in the fall of 2014, with a Lundeby plant lifter (Fig. 3). Roots were removed in all plots except for the fumigated control. On the same day, Farm Fuel Inc. ground BSM was applied, incorporated, and followed by a rain event. Fumigation occurred in Spring 2015, prior to planting bare-root ‘Chemainus’ raspberry plants.
Figure 3. Raspberry root removal using a plant lifter.
The following results are from the data collected during Fall 2014 to Fall 2015. Because raspberry plants were newly planted and not fully mature, raspberry root data was not collected in Spring 2015. However, soil surrounding the raspberry roots was collected to estimate RLN populations with no RLN found in any of the soil samples. By Fall 2015, the RLN populations had rebounded. Plots where the soil had been treated with BSM and root removal had significantly higher RLN counts in the raspberry roots compared to either of the treatments where full rate fumigation occurred (Fig. 4). There were no significant differences in the RLN populations in the soil across the different treatments. During the first summer of this study, vegetative growth data was collected instead of raspberry yield because the plants were non-bearing. The number of canes were counted on five different plants in each treatment. There were no significant differences among treatments. Cane height on those same five plants in each treatment plot was also measured and there were also no significant differences among the treatments.
Data collection will continue through the summer of 2017 in order to determine whether differences in RLN populations will persist beyond the first year after treatment application or if further differences among treatments will develop. The bacterial and fungal communities of the different treatments will also be analyzed using molecular techniques.
Figure 4. Fall 2015 RLN populations in raspberry roots. Means with different letters are significantly different at P < 0.05.
Thank you to the grower cooperator for donating land, time, and resources for this project.
Thank you, also, to my advisor, Dr. Lisa DeVetter, and my committee members, Dr. Inga Zasada, Dr. Mark Mazzola, and Dr. Preston Andrews, for their assistance and support with this project.
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