Solarization and Steam Heat Combined to Control Soil Pests in Flower and Strawberry

Non-Technical Summary: California flower and strawberry producers are large consumers of methyl bromide (MB). Many acres of flowers and strawberries have already transitioned from MB to alternative fumigants. However, the easiest conversions from MB have already been made. Further MB reduction is limited by caps on alternative fumigant use, and large buffer zones near sensitive sites such as houses. This project aims to overcome limits to conversion away from MB by development of disinfestations systems that use solar (solarization) plus steam heat to kill soil pests. Soil solarization in California coastal areas, where most flowers and strawberries are produced, often does not work because summers there are cool and soil pest control is not reliable. Steam heat has long been used to control weeds and pathogens in greenhouse soils. However, steam disinfestation in open fields has been limited due to difficulty in treating large areas with existing steam applicators and high energy costs for steam generation. Steam injection from a portable steam generator into planting beds covered with clear polyethylene mulch would allow growers to supplement solarization with steam heat. Solarization plus steam would likely improve pest control, and solar heat would reduce the energy needed for soil steaming. The objective is to develop an economically feasible solarization and steam, soil disinfestation system for field-grown cut flowers and strawberries. We propose to inject steam into distribution lines installed in flower or strawberry beds and heat the soil to 70DGC for 30 minutes <P> Approach: Field studies will be conducted to find the optimal combination of solarization and steam to control soil pests in flower and strawberry. Raised beds will be established and prepared as a finished seed beds with starter fertilizer and drip irrigation tape installed. Steam distribution lines with emitters at 8-inch spacing will be installed in the bed at depths of 4 inches with the objective of heating the top 6 inches of soil. Basically we are proposing to adapt the drain steaming methods to our local production systems in flowers and strawberry. The steam lines will be spaced 12-in apart with 8-inch spacing between steam emitters. Pathogen and weed seed samples will be installed in the planting beds prior to tarp installation. Briefly, standard bioassay techniques will be used with weed seed enclosed in gas permeable nylon mesh bags. Weed species to be included in the bioassay are: common chickweed, common knotweed, common purslane little mallow, and yellow nutsedge. Pest samples will be included in both flower and strawberry studies. Flower bulbs enclosed in mesh bags, such as callas, ranunculus, iris, and gladiolus, will be included in the flower studies to determine if the experimental treatments can control volunteer bulbs. After removal from the field plots, bulb viability will be assessed by planting in pots and counting the number of bulbs that sprout. Control of resident weeds will be determined by periodic weed harvest and fresh weight measurement. Data will be subjected to analysis of variance. Bioassay samples of Verticillium dahliae will be prepared in the laboratory, incorporated into autoclaved field soil and placed individually in gas permeable nylon mesh bags. Bags will be buried at 3, 6, and 12 inches below the soil surface prior to treatments and then retrieved afterwards. Standard baiting techniques and/or dilution plating to appropriate selective media will be used to assess viability of each pathogen before and after treatments are applied. Similar techniques have been employed successfully in previous studies to assess the effect of solarization on soil pathogens. The percent reduction in propagules of the various pathogens and the severity of naturally occurring disease outbreaks during the growing season will be subjected to analysis of variance. Temperature recording probes will be installed in the beds at depths of 1, 3, 6 and 12 inches and temperatures will be monitored continuously during the solarization and steam disinfestation process. After the bed has been prepared, clear plastic mulch will be installed and then the beds will be irrigated to bring the soil moisture to sufficient levels for proper heat conduction and solarization.