• Topics: Nematicides  | Microirrigation  | Nematicides and Miscellaneous  |  Noling, Joseph W  |  Entomology and Nematology 

Metam-Sodium Application Via Drip Irrigation Systems¹

Changes in Florida crop production systems, particularly with respect to irrigation and multiple cropping are mandating reevaluation of nematode management strategies. In Florida, vegetable crops are often produced on raised beds covered by polyethylene mulch. Broad-spectrum soil fumigants, primarily methyl bromide and chloropicrin (MBC), are then used beneath the polyethylene mulch as a preplant soil borne disease control treatment. Water has been traditionally supplied via seep irrigation, but due to declining water levels and increased urban demand, vegetable growers are being forced by water management districts to adopt more water use efficient irrigation practices such as drip irrigation. Increased production costs associated with drip irrigation systems have motivated growers to consider multiple cropping systems in an attempt to amortize these increased irrigation costs over a number of crops.

Nematode problems often evolve as a result of changing from single to multiple cropping systems. This occurs primarily because soilborne disease control is seldom complete following preplant fumigation with MBC. Nematode populations often increase during late season causing little or no damage to the primary fumigated crop, but are often at significant level to cause extensive damage to subsequent crops within the multiple crop sequence. Delays in crop destruction following harvest also contributes to greater nematode population buildup by allowing additional cycles of pest population growth. The research challenge for this newly changing production system involves protection and enhancement of vegetable crop yields following the initial fall fumigated primary crop. The objective of this report is to summarize recent research findings evaluating the chemigational use of Metam-Sodium for nematode control and crop yield enhancement in Florida drip irrigated, multiple-cropping systems.

Metam-Sodium

Metam-Sodium is an infinitely water soluble preplant soil fumigant. When combined with water Metam-Sodium rapidly decomposes into its bioactive chemical, methylisothiocyanate (MIT). MIT is highly volatile and is found in all three phases of the soil-water-air system. Once in the soil, the liquid phase of MIT volatilizes into the gaseous form and moves through open air spaces to contact and kill soil borne pests. Unlike other fumigant nematicides, vapor diffusion of MIT through soil is relatively slow and retarded due to its high affinity for the water phase. Because of the slow diffusion of MIT as a gas and high affinity for the water phase, continuous delivery in irrigation water following premixing has generally resulted in more uniform soil distribution with enhanced nematode control and crop yield when compared with conventional chisel injection methods.

Figure 1. 

Three phases of the soil-water-air system.

The half-life of MIT in soil is in the range of 8 to 14 days, being highly dependent upon environmental conditions, primarily soil temperature and moisture. If soil conditions are both hot and dry, dissipation of MIT from soil may proceed so rapidly that nematicidal concentration of MIT will not accumulate. Surface water seals have been effective in restricting MS loss from soil when mulch is not used. Conversely, if soils are cool and wet, formation of MIT may proceed too slowly for pesticidal concentrations to be reached. At low temperatures, the MIT remains in the soil for an extended period, thus delaying planting date or possibly causing phytotoxicity to a newly planted crop. Field observations also suggest that rainfall or irrigation events which saturate the soil after treatment tend to retain residues for longer periods, particularly in deeper soil layers. In practical terms, soil temperatures in excess of 60°F but less than 90°F at a 3" soil depth are suitable for fumigation. For chisel injection, soil moisture should be in the range of 50 to 80% of field capacity. Optimal soil moisture conditions prior to chemigation injection have not been determined.

Figure 2. 

Phytotoxicity to a newly planted crop.

Nematode Control

Factors which effect nematode control via chemigational delivery of MS has been reasonably well documented. For chemigation, efficacy is largely, if not entirely, dependent upon environmental conditions, drip irrigation system design, MS concentration, duration of activity and nematode exposure, and final soil distribution of MIT. Studies in Israel suggest that the minimum effective concentration of MIT in soil solution for nematode control to be 50 ppm for at least one week.

When a drip irrigation tube is used for delivery, MS is premixed (diluted) with water and continuously delivered in slow drips at discrete locations, dependent on emitter spacings, along the length of the irrigation tubing. Under favorable conditions, continuous application of MS at a constant concentration in irrigation water should result in a soil solution containing the bioactive agent MIT. MS must reach the necessary depth in the critical concentration quickly enough to be effective before the MIT breaks down. Adding MS in one large pulse or in several smaller pulses will lead to areas of high and low MIT concentrations, with the possibility of areas having too low an MIT concentration, especially in the top few inches of the soil.

If MS is to be added through the irrigation system, it is desirable to achieve the distribution of MIT to the necessary depth with the least amount of water. The length of the injection period will depend on soil type, initial soil moisture conditions, and water holding capacity within the volume of the desired wetted zone. For example, longer irrigation runs will be required for soils with greater water holding capacity to compensate for reduced water penetration. Results from California studies suggest that water applications allowing MS to penetrate 18 to 24" on a given soil type is critical for effective nematode control.

Burying the irrigation tube can also influence treatment efficacy. For example, a single injection of MS into the drip irrigation system buried 4" off bed center and 2" deep under the plastic mulch has increased tomato yields 45% over the untreated control and 33% over the conventional method of chisel injection. In similar field trials, application of MS into irrigation tubes buried 2" gave excellent control of root-knot nematode and of purple nutsedge. In this same experiment tomato yields were increased 23% over the untreated controls when the tube was buried to a depth of 2" at an application rate of 50 gals per acre but only 5% when the tube was positioned on the bed surface under new plastic. In most studies, increasing the rate of application to the broadcast equivalent of 100 gals per acre has generally improved both nematode control and crop production, particularly with respect to an increase in fruit size.

Increasing the number of drip emission points has also improved MS effectiveness. Drip emitter spacings of 8" and two drip lines per bed enhanced nematode control and crop response more than did a single drip line per bed with drip emitter spacings of 24". With the single line and drip emitter spacings of 24" the treated wetted zone were uniform, circular, nonoverlapping areas no greater than 6" in radius for individual emitters. Emitters on a closer spacings will generally provide greater uniformity of moisture and MIT distribution in soil. With wider emitter spacing, MS may not reach the planting site (frequently midway between emitters) or move uniformly enough within the root zone to be of efficient use. Studies California and field observations within Florida show that Metam-Sodium treatments were highly effective against root-knot nematode on tomato only when at least 50% of the bed width was treated.

The importance of lateral soil dispersion has also been observed in recent studies in Florida involving use of Metam-Sodium for crop destruction. These studies have indicated that plant proximity to drip tube emitters is very important in terms of defining plant phytotoxic concentrations in soil. In two separate experiments, it was observed that Metam-Sodium application rates as low as 10 to 15 gals per broadcast acre could be effectively used for both tomato and pepper crop destruction purposes if plants were within 2" individual drip emitters. When identical studies were performed at field sites where plants were placed at distances of 6 to 8" from the drip line, rates of 20 to 30 gals per acre were required to achieve the same, near complete plant mortality. Presumably, a two-fold increase in application rate was needed to compensate for the additional travel distance required to contact the plant root zone.

Another important consideration with regard to MS chemigation involves the integrity of the plastic mulch and drip irrigation tube. In all multiple cropping systems, holes in the plastic mulch are created during the planting process of the primary crop. Mulches which are damaged or torn excessively may retain considerably less MIT in soil for sufficient time to be permit pest control. Similarly, drip tubing which is clogged or damaged and discharging water excessively at various points in the field will also prevent uniform MS application. Field observations have indicated that under constant pressure, small pin size holes in the drip irrigation tube can result in discharge rates 2 to 9 times greater than normal rates of discharge for individual emitters. As the frequency of these points of excessive discharge increase, the overall field application rate decreases, thereby contributing significantly to observed failures or inconsistence in nematode control.

Figure 3. 

Drip tubing which is clogged or damaged.

Maintaining soil moisture between the first and second crop planting is also of utmost importance. In fields where soil moisture conditions cannot be easily and uniformly reestablished, chemical dispersal can be seriously compromised during a preplant MS treatment for a subsequent crop. In fields where water is to be discontinued between crops, chemigational treatments with Metam-Sodium should occur soon after final harvest before bedded soil dries to a point where chemical dispersal in soil will be severely restricted.

Summary

The decision whether to chemigationally use Metam-Sodium should be based on need. Examination of plant root systems following harvest of the primary crop will provide valuable information regarding the distribution and severity of the nematode problem within the field. Once this has been determined, strategies which will maximize the outward radial movement of Metam-Sodium will ultimately translate into higher yields because of the increased rooting volume of nematode free soil. Given the sandy nature of Florida soils, narrower bed widths, drip tubes with closer drip emitter spacing, and planting practices which place plants closer to the drip tube may need to be adopted. Growers must also recognize that acceptable results cannot be obtained with Metam-Sodium if drip tubes are clogged or extensively damaged since uniform field application cannot be achieved.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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