(Second year of funding to complete the study)
Other Key Personnel: Dr. Dick Funt
Our goal is to investigate environmental, biological and cultural factors that may influence the safety of using Spartan (sulfentrazone) on strawberry plantations.
To achieve our goal it will be necessary to do the following:
1. Determine the crop sensitivity under different soil conditions, specifically pH and texture.
2. Determine the lethal dose required for a series of common weed species present in strawberry
crops.
3. Determine sulfentrazone activity at different stages of weed development (ie PRE, Early
POST, and Late POST)
4. Determine sulfentrazone selectivity on different strawberry cultivars used by farmers in Ohio.
Objectives 1 & 4: Field studies were established at Wooster, Ohio (81o58' W longitude, 40o45' N latitude, elevation 310 m) during 2002. Four rates of sulfentrazone were applied one week after planting to strawberries cultivars 'Jewel' and 'Allstar' grown in soil pH levels of 5.0 and 7.0. A three factorial experimental design (2 pH levels x 2 cultivars x 4 sulfentrazone rates) was used with four replications per treatment. Plots consisted of two rows with 10 plants each. Fertilization was applied after planting (40 lb N/A) and early fall (September) as potassium nitrate. Plant growth evaluations (percent stunting, chlorosis, leaf deformation, number of stolons, plant height and diameter) were carried out 1, 3 and 6 weeks after treatment (WAT). Plots were hand weeded every other week and irrigation was provided through central pivots as needed during the summer. The crop was protected from pests in 2003 following OSUE recommendations and ripe berries were harvested approximately every 3 days over 3 weeks. Data were analyzed using general lineal models procedures. When significant differences occurred orthogonal contrasts were used for mean separation at the 5% significance level.
Strawberry tolerance to sulfentrazone applied immediately after planting was affected by soil pH. Strawberry plants were stunted more at soil pH 7 than at pH 5. 'Jewel' was relatively insensitive to sulfentrazone. Plant stunting at soil pH 5 was only detected at the highest rate (0.4 kg/ha) 6 WAA. Visual estimates of stunting indicated that 'Jewel' tolerance 3 WAA was reduced at soil pH 7; however, the cultivar had recovered 6 WAA except for those plots treated with sulfentrazone at 0.4 kg/ha. 'Allstar' was more sensitive to sulfentrazone than was 'Jewel' and recovered more slowly. At soil pH 5, 8% stunting was observed 3 WAA with 0.4 kg/ha, and by 6 WAA stunting was obvious at each rate of the herbicide. 'Allstar' was severely stunted by sulfentrazone at soil pH 7, 3 WAA. Six WAA some recovery had occurred but stunting was still apparent at 0.2 and 0.4 kg/ha. Measures of strawberry plant diameter were not sensitive to differential stunting caused by sulfentrazone rate and soil pH. Plant diameter did not vary within a cultivar across rates of sulfentrazone at 3 or 6 WAA. However, measurement of this variable at 6 WAA illustrated the greater plant size achieved by 'Jewel' compared to 'Allstar' and the enhanced growth of both cultivars at pH 5 compared to pH 7. These data suggest that visual estimates provide a superior integration of the variables involved in the strawberry plant's phytotoxic response to sulfentrazone than do the quantitative measures of plant growth used in this study. Fruit yield was not affected (p= 0.2194) by sulfentrazone at the rates evaluated (0 to 400 g/ha). These results are consistent with those of Wehtje et al. (1997) who reported improved control of Cyperus spp. with sulfentrazone when soil pH was greater than 6.6. Improvements in weed control and greater crop sensitivity may both be attributed to increased sulfentrazone availability in solution for absorption by plants at soil pH > 6.6 (Grey et al. 1997).
Objectives 2 & 3: A greenhouse experiment was performed to determine the response of common groundsel, common mallow, common chickweed, common purslane, redroot pigweed, and yellow wood sorrel at various growth stages to a range of sulfentrazone rates. On April 17, 2003 individual species were seeded in 110 mm plastic pots containing a Promix BX1 that consisted of: Canadian sphagnum peat moss (75-85% volume), perlite, vermiculite, dolomitic and calcitic limestone and a wetting agent. Seeded pots were placed in a greenhouse mist room (25 1 C) set to mist every 10 min for 10 s intervals. Plants were watered as needed and fertilized once a week with 200 ppm of a 20:20:20 (N:P:K) solution. To produce four seedling stages: PRE (preemergence), COT (cotyledon stage), EPOST (early post seedlings with 4 fully expanded leaves) and LPOST (late post seedlings with 10 fully expanded leaves), seeding was done four times 10 days apart. To promote plant hardening and leaf cuticle development, after one week in the mist room seedlings were moved outdoors to benches under direct sun light. The shortest interval between moving seedlings outside and herbicide application was 1 week for plants at COT. A split plot design was used, with weed seedling stages as the main plot and sulfentrazone rate (0, 0.25, 0.50, 0.1, 0.2 and 0.4 kg/ha) as the subplot. Four replications were used. All weed seedling stages were sprayed at the same time. A compressed-air laboratory sprayer equipped with TJ-8003VS flat fan nozzles calibrated to deliver 187 L/ha at a pressure of 276 kPa was used. Weed response to sulfentrazone was estimated by measuring plant height 3 WAA. The experiment was repeated. ANOVA was conducted using SAS GLM procedure, including analysis of interactions between main factors (weed seedling stages, sulfentrazone rates and experiment replication). Seedling height 3 WAA of all sulfentrazone treated weeds was converted to percent of the untreated control. Each weed seedling stage was regressed on sulfentrazone rate using the SAS NLIN procedure. This procedure produced the parameters needed to fit a log-logistic function (Seedfeldt et al. 1995) and obtain the rate-response curves defined by the equation:
% of control height = C + (D - C) /( 1 + (x/GR50)b) [1]
Where C = the mean response of the highest dose; D = mean response of the untreated; b = slope; x= sulfentrazone rate; and GR50 = 50% growth reduction.
Weed seedling growth stage at the time of sulfentrazone application influenced subsequent plant growth. Common chickweed was the only species inadequately controlled by sulfrentrazone, regardless of application rate or stage of growth. Amongst the other, sensitive species, control with applications at PRE and COT were equivalent with essentially complete control achieved when the herbicide was applied at 50 - 100 g ai/ ha; however, yellow wood sorrel was only controlled when the herbicide was applied PRE. Later applications at EPOST and LPOST were progressively less effective. In the e case of common mallow, redroot pigweed and common purlsane LPOST treatments did not provide adequate control. Common groundsel was slightly more sensitive LPOST than EPOST. Groundsel was completely controlled when sulfentrazone was applied PRE at 0.025 kg/ha, and at 0.05 kg/ha to seedlings at the COT stage. These data indicate that sulfentrazone applications should be timed PRE or to coincide with seedling emergence to achieve optimum control.
Literature Cited
Grey, T. L., R. H. Walker, G. R. Wehtje and H. G. Hancock. 1997. Sulfentrazone adsorption and mobility as affected by soil and pH. Weed Science 45: 733-738.
Seedfeldt, S., J. Jensen and P. Fuerst. 1995. Log-logistic analysis of herbicide dose-response relationships. Weed Technology 9: 218-227.
Wehtje, G. R., R. H. Walker, T. L. Grey and H. G. Hancock. 1997. Response of purple (Cyperus rotundus) and yellow nutsedges (C. esculentus) to selective placement of sulfentrazone. Weed Science 45: 382-387.
1 Premier Horticulture Inc., Red Hill, PA, 18076.
For further information contact Doug Doohan Associate Professor, Horticulture & Crop Science or the Ohio IPM Office.
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