Mustard cover crops as biofumigants for organic strawberry production
Organic strawberry production requires reliable control for soil-borne pests and diseases. Varieties of two species of mustard (Sinapis alba and Brassica juncea) have been identified as producing chemical compounds known as glucosinolates that have been shown to reduce fungus and nematode populations when mowed and incorporated into the soil. This process is known as biofumigation.
Six varieties of mustard have been trialed to test glucosinolate production for disease and nematode control, and biomass production. Test results showed differences in level of glucosinolate production and type of glucosinolate produced. ‘Caliente 199’ (S.alba and B. juncea blend) produced the highest level of the glucosinolate ‘sinigrin’, which has anti-fungal and anti-nematode properties. ‘Idagold’ (Sinapis alba) produced the highest level of the glucosinolate ‘sinalbin’, which has weed suppressing properties. Biomass measurements showed ‘Caliente 199’ to be the highest yielding of the six varieties tested.
There was no significant difference in yield of the strawberries. Eight varieties of strawberries were then planted perpendicularly across the plots. The eight varieties planted were: ‘Wendy’, ‘Galletta’, ‘Brunswick’, ‘Jewel’, ‘Darselect’, ‘Cabot’, ‘Valley Sunset’, and ‘Record’. ‘Cabot’ and ‘Jewel’ were the highest yielding across all mustard treatments. Also, there was significant deer damage in the trial block in the fall of 2015 that reduced yields across all treatments.
Six varieties of mustard were planted with a grain drill in the spring of 2015 in 40’x 80’ plots to test glucosinolate production for disease and nematode control, and biomass production for organic matter. The plots were duplicated for a total of 2 plots for each of the treatments. The varieties included: Kodiak (Brassica juncea), Pacific Gold (Brassica juncea), Ida Gold (Sinapis alba), Caliente 119 (S.alba and B. juncea blend), Caliente 199 (S.alba and B. juncea blend), and Nemat (Eruca sativa– also a Brassica, bred as a nematode trap crop). Eight varieties of strawberries were then planted perpendicularly across the plots. The eight varieties planted were: ‘Wendy’, ‘Galletta’, ‘Brunswick’, ‘Jewel’, ‘Darselect’, ‘Cabot’, ‘Valley Sunset’, and ‘Record’.
The strawberry planting was established on July 30, 2015. Seven soil samples were taken: one on each mustard plot and one control. The planting was successful, although suffered significant deer damage in the fall of 2015. In 2016, strawberry harvest lasted for three weeks, from June 6 to June 26. Marker flags were placed in the field to recreate the mustard plots. Harvested strawberries were brought to the edge of the field within the mustard plot and totaled.
Soil samples from 2015, taken immediately prior to establishing the strawberries, showed organic matter slightly down from before the mustard cover crop in all plots except the control. Research conducted at UNH and published in the journal Nature Communications showed that soil organic matter accumulated from dead microbial cells and microbial byproducts rather than from plants themselves, as previously thought. This suggests that the best way to increase soil organic matter was to promote microbial communities. This suggests that biofumigant mustards may hinder the accumulation of soil organic matter as they are designed to suppress certain microbial communities.
In 2016, strawberry yields were measured and no difference was seen across all mustard plots. ‘Jewel’ and ‘Cabot’ were the high yielding varieties, although no difference was seen in any mustard plots or the control plots. This was expected since both varieties are known as top yielders. Further analysis of the glucosinolate yields showed that ‘Ida Gold’ (Sinapis alba) contains chemically different compounds than the other varieties which are predominantly Brassica juncea. ‘Idagold’ predominantly contained the glucosinolate ‘sinalbin’, while the other varieties predominantly contained the glucosinolate ‘sinigrin’.
Impacts and Contributions/Outcomes
The strawberry yields did not show a difference across mustard plots (see chart: Strawberry Yields after Mustard Cover Crops). Significant deer damage to the plots in the fall of 2015 had an effect across all mustard plots and strawberry varieties. Although we did not see a result in the strawberry yields, we did find interesting results in the mustard trial. It was found that ‘Caliente 199’ (S.alba and B. juncea blend) had the highest biomass yield and highest levels of the glucosinolate ‘sinigrin’, a volatile compound that has been shown to have anti-fungal and anti-nematode properties. Interestingly, ‘Ida Gold’ (Sinapis alba) contained another gluscosinolate, ‘sinalbin’. This non-volatile compound has shown the ability to inhibit weed seed germination. Although measurements were not taken, it was observed there was less overall weed pressure in the ‘Ida Gold’ plots. This is similar to observations in trials of ‘tillage radish’, another Brassica species. It was not determined whether weed suppression was a result of biofumigation or a dense cover crop outcompeting weeds. Planting rate (density) in other cover crops such as winter rye and oats has been shown to effectively suppress weeds.
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