There is market demand for organic produced strawberry nursery plants as well as fruit. However, the risks to plant and fruit producers to grow on non-fumigated land is significant as diseases and weed infestations can be costly or devastating to the crop. Soil disinfestation in field with steam is possible on limited areas, but there are engineering issues to overcome. The objective of this work was to evaluate the performance of a prototype field-scale steam applicator. In August and September 2018, we tested a prototype field steamer for soil disinfestation in high-elevation strawberry nursery fields in Northern California, and in fruit fields near Salinas, and Watsonville CA. The steam applicator consisted of a 300 hp Cleaver Brooks steam generator mounted on a trailer towed by a tractor. Steam was injected into a 3 m wide rototiller set to till 40 cm deep. Target temperatures and dwell times were 65°C for 20 minutes. Real-time soil temperatures were measured with Hanna K-type thermocouple thermometers set at 20 cm deep. Hobo Soil temperature recorders were placed immediately behind the steam applicator and left for 24 hours to measure soil temperatures at 10, 20, 25 and 40 cm depths. Soil samples were gathered before and after steam application. Fumigants were used to compare against steam in four out of five test sites. At fruiting sites, steam was applied alone and co-applied with 3,368 kg ha
-1 mustard seed meal (MSM). Weed seed samples in sachets were buried at Salinas only at depths of 15 cm.
Following steam application at Salinas and Watsonville, maximum soil temperature reached 73 and 67°C and time of above 65°C was 108 and 52 minutes, respectively. The steam applicator increased soil temperature of 65°C at a depth of 25 cm for 63 minutes at Salinas and 12 minutes at Watsonville. At Salinas, viability of purslane seeds and microsclerotia population of Verticillium dahliae in the no steam control, MSM alone, steam alone, and steam + MSM were 78, 86, 3 and 1% and 688, 553, 1, and 0.2 microsclerotia g-1, respectively. At Watsonville the percentage reduction in Pythium ultimum by steam alone and steam + MSM was100 and 96%, respectively.
After steam was applied at Sierra Cascade, Plant Science, and Lassen canyon nurseries, maximum 25 cm soil temperature was 71, 66, and 80°C and time of above 65°C was 106.5, 32.5, and 180 minutes.
