Optimizing Fumigation Efficiency By Doubling Drip Line Number and Using Low Permeability Film in Raised-Bed Production Systems.

Southern California strawberries are planted in raised-beds covered by polyethylene (PE) film and typically are irrigated with two drip lines placed near the bed surface. To control soil borne-pests, fumigants are commonly applied through the drip lines prior to transplanting strawberries, but efficacy depends on adequate lateral and vertical fumigant distribution and the rate of fumigant emission through or around the plastic mulch. To improve fumigation efficacy, in this study we evaluated fumigant distribution and retention after doubling the number of drip lines and using low permeability film (LPF). The trial was conducted in autumn of 2014, in a grower’s field near Oxnard, CA.  The fumigant tested was Tri-Clor EC (a mixture of 94% chloropicrin and 6% other ingredients) and treatments included a full rate (251 kg/ha) or a half-rate (125 kg/ha) applied through either two or four drip lines under LPF, a full rate applied through two drip lines under PE, and a non-fumigated control covered with LPF. The bed width was 114 cm and the spacing between two drip lines was 55 cm. In plots treated with four drip lines, the outer two lines were placed at the standard location and inner two lines were placed near bed center with a spacing of 15 cm to facilitate more effective fumigant distribution in the bed. Different type of drip tapes were used in the trial and the flow rate in the two line treatments was twice to that in the four line treatments. Fumigants applied under LPF had significantly lower emission rates than plots sealed with PE and the peak emission flux under LPF was only 10% of that under PE. No emission was found from uncovered furrows regardless of treatments. LPF clearly improved fumigant retention with the 50% application rate maintaining higher fumigant concentration in the soil compared to the full rate applied under PE. Four drip lines further improved vertical fumigant distribution compared to the two drip lines under LPF. Better weed control and higher early season berry yield were observed in the LPF plots, particularly plots treated with four drip lines, compared to the PE plots and the non-fumigated control. The overall results indicate that increasing drip line number and covering beds with LPF will improve fumigation efficiency and reduce emissions in raised-bed production systems.