Drip Fumigation

The most effective registered chemical alternative to methyl bromide plus chloropicrin fumigation for the control of pathogens, nematodes, and weed seeds is either

• chloropicrin mixed with 1,3-dichloropropene (InLine) followed by metam sodium or
• chloropicrin alone followed by metam sodium

Because chloropicrin and 1,3-dichloropropene are less volatile than methyl bromide, they can be applied to raised beds through drip irrigation systems and have been shown to be effective in controlling soilborne pathogens and most weed seeds, resulting in comparable strawberry yields. However, applying these materials through drip systems does not control the pathogens in the edges of raised beds.

Currently, over 55% of the California strawberry acreage is drip fumigated. Drip fumigation is convenient because workers are not required to be in the field during application. However, successful drip fumigation requires adequate soil preparation, a well-designed drip irrigation system, dependable chemigation equipment, well-laid plastic tarp, and timeliness of the process to accommodate longer plantback time.

Soil Preparation

As with all soil fumigation, the first step is to properly prepare and till the soil. Current soil preparation and bed listing practices used after methyl bromide fumigation are generally adequate. Following this, firmly pack the beds and eliminate any dirt clods. If the soil is dry, it may be necessary to preirrigate with enough water to initiate weed seed germination (1–1.5 acre-inch).

Uniform water distribution is necessary in a drip irrigation system and is easiest to obtain on fairly level terrain. On steep or hilly fields, create beds that follow soil contour lines at grades that do not exceed 4 feet uphill or 8 feet downhill from the beginning of the drip line.

When laying the plastic tarp, remove any shanks or chisels to avoid creating channels in the soil, which can result in poor water and fumigant distribution in the soil bed. Repair any holes or tears in the plastic tarp. Avoid embossed tarps to reduce loss of fumigants through volatilization. The use of totally impermeable film (TIF) will enhance weed control in the bed. However, TIF holds fumigants in the soil for longer periods than the standard tarp, so a longer plantback time or bed ventilation for 2 weeks before planting may be required (refer to the pesticide label).

Amount of Water

If chloropicrin or 1,3-dichloropropene is applied at the same time as metam sodium, they react and rapidly degrade in the irrigation water. Instead, they should be applied sequentially with the first application consisting of 1,3-dichloropropene plus chloropicrin or chloropicrin alone followed 5 to 7 days later with an application of metam sodium. Applying the pesticides in this order helps to maximize their effectiveness because 1,3-dichloropropene and chloropicrin are most effective in drier soils whereas metam works best in moist soils.

1, 3-Dichloropropene and Chloropicrin

It is important to use the appropriate amount of water so that the fumigant is evenly distributed throughout the target soil treatment zone. Drip fumigation with recommended amounts of irrigation water will provide good fumigant distribution in soil and reduce fumigant volatilization losses by increasing the amount of fumigant in the water phase and decreasing the total air space available for fumigant diffusion in soil.

• If too little water is used (less than 1.5 inches), the fumigant will be poorly distributed and more likely to volatilize, resulting in less effective control and lower strawberry yields. In addition, with insufficient water and without an emulsifier, fumigants such as 1,3-D or chloropicrin may precipitate in the irrigation pipelines if the concentrations exceed their solubility limits of 2,000 parts per million (ppm).
• However, using too much water may lower the fumigant concentration in the main line below 500 ppm, which can reduce fumigant effectiveness. Also, beds can lose their integrity and become unstable and collapse with excessive water application. Thus, bed stability may limit the volume or application rate of water.

Table 1 (below) lists the recommended amount of water needed to fumigate various soil types to a 24-inch soil depth. For example

• In a sandy loam soil, 2 inches of water in the bed is recommended to fumigate 24 inches deep. With two drip tapes, this provides a 40-inch lateral spread (10 inches on each side of a drip tape).
• If irrigation duration is limited, cutting back to 1.75 inches on sandy loam soils is often acceptable because fumigants move 3 to 5 inches beyond the wetting front and control should extend 2 feet deep. Although the fumigant will volatilize and move beyond the wetted zone, the best treatment appears to occur within the wetted area.
• If the water volume is cut back to the minimum recommendation of 1.5 inches for sandy loam soils, the soil profile will be wetted 18 inches deep with 6 to 8 inches of horizontal spread on both sides of each drip tape.

Metam Sodium or Metam Potassium

Metam sodium and metam potassium are water soluble and generate the active ingredient methyl isothiocyanate (MITC) after being applied to the soil. A minimum of one inch of water is recommended for the sequential application of metam to most soil types.

Drip Tape Flow Rate and Spacing

In drip fumigation, the rate of water flow and the spacing of drip tapes are critical to the even distribution of the fumigant throughout the field as well as in the soil treatment zone. While a water distribution uniformity of 90% is possible in a well designed and operated drip system, at least 80% is necessary for acceptable fumigation.

Drip tapes with a flow rate between 0.3 to 0.7 gallons per minute (gpm) per 100 feet are appropriate for most strawberry soils in California (see Table 1 below). Low-flow drip tape requires longer application time that may become inconvenient. Avoid high-flow drip tape (greater than 0.7 gpm/100 ft) if it causes any wetting of the furrows or runoff, or if it causes the beds to collapse. High-flow tape is not commonly used in California and is not recommended except for soils with high water permeability.

To achieve adequate water distribution uniformity, the pressure in the drip tape throughout the field should not vary more than 3 psi (e.g., from 6 to 9 psi). In addition, the system must be free of leaks and clogged emitters and be flushed and pressure tested before fumigation. It is imperative to use good quality irrigation components and drip tape. Leaks cause fumigant loss and possibly odor and emissions problems.

It may be necessary to reconfigure drip tape in order to obtain good water coverage across the soil bed. For most strawberry beds (sandy loam soils), one drip tape can cover up to 10 inches on each side. Therefore, two drip tapes are recommended for drip fumigation of most strawberry beds. In the two-row strawberry beds (narrow beds with two tapes near the center), spread the tapes as far apart as possible so that the edge of the bed is covered. In the four-row strawberry beds (wide beds with two tapes close to the shoulder), move the tapes a few inches towards the center to treat the middle of the bed. A third drip tape in the center may be needed if the bed top is wider than 40 inches. A third drip tape is also recommended in wide beds on sandy and loamy sand soils where limited lateral water movement may limit fumigant distribution.

Table 1. Estimated water volume needed to treat two feet of soil depth using two drip tapes when applying 1, 3-dichloropropene or chloropicrin or both.¹

Soil type		Volume of application water inches per acre (gallons)2	Application time using 2 tapes (hours)				Comments
			Drip tape flow rate (gpm/100ft)
			0.20	0.34	0.50	0.67
Fine sand and loamy fine sand		1.6 (27,000)	13.9	8.2	5.5	4.1	Pre-irrigation with one inch of water is needed
Sandy loam and fine sandy loam		2.0 (34,000)	17.3	10.2	6.9	5.2	Minimum of 1.5 inches is recommended
Sandy clay loam and loam		2.6 (44,000)	22.5	13.4	9.0	NR	Split application may be required
Clay, clay loam, and silty clay loam		3.2 (54,000)	27.7	16.3	11.1	NR	Soils not common in strawberry production

1	Application time and water volume based on 40-inch average bed width (64 inches center-to-center).
2	One broadcast acre-inch of water is about 27,000 gallons. One acre-inch of water for a 40-inch wide bed is about 17,000 gallons.
NR	not recommended

Determining the Fumigant Concentration in Irrigation Water

Fumigant concentration in the main line may vary from 500 to 1600 ppm, depending on on soil type, fumigant, and water application rate. Below 500 ppm, the efficacy of chloropicrin plus 1,3-dichloropropene (InLine) to control soilborne pathogens may be insufficient. Also, because the solubility of chloropicrin and 1,3-D in water is less than 2000 ppm at 68°F (20°C), exceeding 1500 ppm may result in precipitation of these fumigants in the irrigation pipelines and drip tape.

Fumigant concentration in water can be calculated as follows:

• Chloropicrin: ppm chloropicrin = 119,826 x (# pounds chloropicrin/# gallons water)
• InLine: ppm (chloropicrin / 1,3-D) = 87,872 x (# pounds InLine/# gallons water)

Table 2 shows chloropicrin concentrations as a function of application rate and water volume. A similar table can be prepared for 1,3-dichloropropene plus chloropicrin (InLine) (one gallon weighs 11.2 lb or 6.81 lb 1,3-D and 3.73 lb chloropicrin) but is not provided here because 1,3-dichloropropene requires a special permit to apply.

Other Important Calculations Needed for Drip Fumigation

• Determine the actual treated (bed) area, the total volume of water, and the weight of fumigants to be applied. Strawberry beds usually occupy 50 to 70% of the total area in the acre. Because only the beds are treated, this calculation can be important in determining how much fumigant is necessary for the treatment.
• Calculate the time required for application based on the flow rate of the drip tape.

Table 1 provides the application time for one particular type of bed configuration (40-inch wide with two drip tapes).

Chemigation Equipment for Drip Fumigation

The fumigant cylinders are pressurized with nitrogen gas and metered directly into the irrigation pipeline or manifold. The meter can be a precision needle valve and flow meter, a needle valve and a scale, or a computer-controlled positive displacement meter. Fumigants are injected at low-flow rates and accurate calibration of injection equipment is essential for proper application. Fumigant concentration in the main line may vary from 500 to 1600 ppm, depending on the soil, fumigant type, and water application rate. Refer to the pesticide label for appropriate concentration rates.

Good fumigant mixing with water in the irrigation pipelines is essential. Install a static mixing device after the point of injection to thoroughly mix fumigants with water before being distributed into the irrigation system laterals and drip tape.

The irrigation system must have a standard, single-check valve, a low-pressure drain, and a vacuum-relief valve (a "chemigation" valve) upstream from the injection point to prevent possible contamination of the water source by fumigants. The fumigant injector must be equipped with a check valve to prevent water from flowing back into the fumigant tank and an automatic quick-closing valve to stop fumigant injection when water flow is interrupted or loses pressure. The fumigant automatic shut-off valve can be electrically or hydraulically activated and should be normally closed at the injector. For more information on chemigation equipment, consult the agricultural commissioner office in your county.

Flushing the Pipeline Following Treatment

Many of the fumigants, including chloropicrin and 1,3-dichloropropene (InLine), can damage PVC if left in the pipelines. This does not occur during application of the diluted fumigants but can occur if the lines are not well flushed at the end of the application and the fumigant settles out and accumulates in low points of the distribution system. For this reason, it is critical to flush lines at the end of each application. The required amount of water needed to flush the system can be estimated as three times the volume of the mainline and laterals. Avoid excessive flushing because it will dilute the fumigants around the drip tape.

Safety Rule

Read and understand the fumigant label and follow county permit conditions before starting the fumigation. Know the symptoms and emergency treatments for exposure to the fumigants. Monitor the application system and the field during application.

Table 2. Chloropicrin concentration (ppm)¹ during drip application.

Gallons water/acre	Pounds of chloropicrin (without emulsifier)
	150	160	170	180	190	200	210	220	230	240	250	260	270	280	290	300
19,000	946	1,009	1,072	1,135	1,198	1,261	1,324	1,387	1,451	1,514	1,577
20,000	899	959	1,019	1,078	1,138	1,198	1,258	1,318	1,378	1,438	1,498	1,558
21,000	856	913	970	1,027	1,084	1,141	1,198	1,255	1,312	1,369	1,427	1,484	1,541
22,000	817	871	926	980	1,035	1,089	1,144	1,198	1,253	1,307	1,362	1,416	1,471	1,525
23,000	781	834	886	938	990	1,042	1,094	1,146	1,198	1,250	1,302	1,355	1,407	1,459	1,511
24,000	749	799	849	899	949	999	1,048	1,098	1,148	1,198	1,248	1,298	1,348	1,398	1,448	1,498
25,000	719	767	815	863	911	959	1,007	1,054	1,102	1,150	1,198	1,246	1,294	1,342	1,390	1,438
26,000	691	737	783	830	876	922	968	1,014	1,060	1,106	1,152	1,198	1,244	1,290	1,337	1,383
27,000	666	710	754	799	843	888	932	976	1,021	1,065	1,110	1,154	1,198	1,243	1,287	1,331
28,000	642	685	728	770	813	856	899	941	984	1,027	1,070	1,113	1,155	1,198	1,241	1,284
29,000	620	661	702	744	785	826	868	909	950	992	1,033	1,074	1,116	1,157	1,198	1,240
30,000	599	639	679	719	759	799	839	879	919	959	999	1,038	1,078	1,118	1,158	1,198
31,000	580	618	657	696	734	773	812	850	889	928	966	1,005	1,044	1,082	1,121	1,160
32,000	562	599	637	674	711	749	786	824	861	899	936	974	1,011	1,048	1,086	1,123
33,000	545	581	617	654	690	726	763	799	835	871	908	944	980	1,017	1,053	1,089
34,000	529	564	599	634	670	705	740	775	811	846	881	916	952	987	1,022	1,057
35,000	514	548	582	616	650	685	719	753	787	822	856	890	924	959	993	1,027
36,000		533	566	599	632	666	699	732	766	799	832	865	899	932	965	999
37,000		518	551	583	615	648	680	712	745	777	810	842	874	907	939	972
38,000			536	568	599	631	662	694	725	757	788	820	851	883	914	946
39,000			522	553	584	614	645	676	707	737	768	799	830	860	891	922
40,000				539	569	599	629	659	689	719	749	779	809	839	869	899
41,000				526	555	585	614	643	672	701	731	760	789	818	848	887
42,000					542	571	599	628	656	685	713	742	770	799	827	856
43,000					529	557	585	613	641	669	697	725	752	780	808	836
44,000						545	572	599	626	654	681	708	735	763	790	817

1	ppm = parts per million