Weed Control Options in California Strawberry Without MeBr

S.A. Fennimore ^*1, M.J. Haar ² and H. Ajwa ^3
1,2University of California-Davis, Salinas, CA 93905
³USDA-ARS, Water Management Research Laboratory, Fresno, CA 93727

Introduction

Strawberry production in California is a two-component system consisting of nursery and fruit production, and methyl bromide has been used as the basis for weed control for both components. The phase-out of methyl bromide requires the development of alternative fumigant systems for both nursery and fruit production systems. Alternative fumigants in evaluation include chloropicrin alone, iodomethane plus chloropicrin mixture 50:50, 1,3-D plus chloropicrin mixture 65:35, and metam sodium. Iodomethane was evaluated in nurseries, and several emulsified fumigants were evaluated in a fruiting field. The objective of the work reported here was to evaluate the weed control efficacy of alternative fumigants in both nursery and fruiting fields.

Methods

Iodomethane evaluation in nursery production. The weed control efficacy of iodomethane was compared to methyl bromide in the spring and summer of 2000 at a low elevation nursery at Ballico, CA and a high elevation nursery at Susanville, CA. The effects of iodomethane on weed seed viability, weed densities and time required to hand weed were measured. Iodomethane was applied as a 50:50 mixture with chloropicrin (IM/Pic) at the rate of 350 lbs. per acre (lb./A). Methyl bromide and chloropicrin (MeBr/Pic) was applied for comparison as a 57:43 mixture at the rate of 400 lb./A at Ballico and 355 lb./A at Susanville. Each treatment was replicated four times and the plot size was 2860 ft².

The effect of fumigants on weed seed was tested by burying nylon mesh bags containing seed of common purslane (Portulaca oleracea), prostrate knotweed (Polygonum aviculare) and little mallow (Malva parviflora) prior to fumigation. Following fumigation, bags were retrieved and the percentage viable seed was determined using tetrazolium salts.

Emulsified fumigant evaluation in fruiting fields. Evaluation of emulsified fumigants was initiated in November 1999 in a fruit production field near Salinas, CA. Four weeks after fumigation, the mulch was removed and cultivar 'Selva' was planted in two rows at 12-inch spacing between plants. Emulsified fumigants were applied through two drip lines placed 8 inches apart in the middle of the bed. Drip-applied treatments were: 1,3-D plus chloropicrin mixture in an emulsified formulation (1,3-D + Pic EC) at 253 and 422 lb./A, metam sodium at 213 and 320 lb./A, chloropicrin alone (Pic EC) at 328 lb./A, 1,3-D + Pic EC followed by (fb) metam sodium at 253 lb./A fb 213 lb./A, and 422 lb./A fb 320 lb./A, and Pic EC fb metam sodium at 328 lb./A fb 213 lb./A. Bed shank-applied materials were methyl bromide plus chloropicrin 67:33 (MeBr/Pic) at 400 lb./A, 1,3-D plus chloropicrin mixture 65:35 (1,3-D + Pic) at 410 lb./A and chloropicrin alone (Pic) at 300 lb./A. Each treatment was replicated three times and the plot size was three 52-inch beds wide by 100 ft. long.

The effect of emulsified fumigants on the native weed biomass was measured on February 24, 2000 by harvesting the weeds present on the center bed of each plot (250 ft²). Weeds were identified by species and then biomass was determined. Weed seed bags containing common purslane, little mallow, and prostrate knotweed were placed in nylon mesh bags and buried in all plots prior to fumigation (data not shown).

Results

Iodomethane evaluation in nursery production. Percentage viable common purslane and prostrate knotweed seed was much lower for seed treated with IM/Pic or MeBr/Pic compared to untreated seeds at both sites (Table 1). Differences in percentage viable seed were not found between IM/Pic and MeBr/Pic treatments for common purslane or prostrate knotweed. At Susanville, percentage viable was lower for little mallow seed exposed to MeBr/Pic compared to untreated seed; however a 77.1 percent viable seed is not considered an acceptable level of control. A difference in little mallow seed viability was not observed at Ballico.

The effect of fumigants on emergence of native weed populations was evaluated by periodically determining the density of each weed species. Common lambsquarters (Chenopodium album) and little mallow were the most abundant weeds at Susanville (Table 2). Problem weeds at Ballico were carpetweed (Mollugo verticillata) prostrate spurge (Euphorbia humistrata) and filaree (Erodium spp.). Both IM/Pic and MeBr/Pic reduced the number of common lambsquarters, carpetweed and prostrate spurge for each date. MeBr/Pic reduced the number of filaree compared to IM/Pic for the first sample date, but differences were not found for the second count. The number of little mallow was lower in the MeBr/Pic treated plots compared to controls for the first weed count at Susanville; for other weed counts, there was not a difference.

Less time was required to hand weed plots treated with MeBr/Pic or IM/Pic at Ballico compared to untreated areas (Table 3). A difference was not found between MeBr/Pic and IM/Pic.

These results suggest that the weed control efficacy of IM/Pic 50:50 at 350 lb./A was approximately equal to MeBr/Pic 57:43 at 355 to 400 lb./A.

Emulsified fumigant evaluation in fruiting fields. All fumigant treatments significantly reduced common chickweed (Stellaria media) and shepherdspurse (Capsella bursa-pastoris) bio-masses compared to the untreated check (Table 4). Bur clover (Medicago polymorpha) and little mallow biomass was greater in the MeBr/Pic treatment than in the untreated check. This was likely due to the germination stimulus on bur clover and little mallow seed provided by MeBr/Pic. Results indicate that drip-applied 1,3-D + Pic EC provided better control of bur clover and little mallow than shank-applied 1,3-D + Pic. Similarly, drip-applied Pic EC provided better control of bur clover and little mallow than shank-applied Pic. The weed control provided by drip-applied 1,3-D + Pic EC at 253 or 422 lb./A was not improved by the addition of a sequential application of metam sodium at 213 or 320 lb./A, respectively. Similarly, the weed control provided by drip-applied Pic EC was not improved by a sequential application of metam sodium at 213 lb./A. Metam sodium alone at 213 or 320 lb./A provided poor to fair control of all species.

Sequential applications of metam sodium did not improve the weed control provided by drip-applied 1,3-D + Pic EC or Pic EC.

Last Updated: September 5, 2001