Charles A. Mullins

Crop injury was excessive from pyrithiobac (Staple) and this herbicide has little potential for use on summer squash. S- metolachlor (Dual) was used at a low rate to avoid injury. Weed control was poor at this low rate and Dual probably has little value for usage on summer squash. Halosulfuron (Sempra) was much more effective preemergence than post emergence. Sulfentrazone (Authority) was more effective for weed control at the high rate than at the low rate, but a slight yield reduction was evident at the high rate. All the herbicides except pyrithiobac (Staple) and S-metolachlor (Dual) have promise for control of some weed species in summer squash without major crop injury.

Summer squash is grown in large commercial acreage for fresh market and processing (freezing) in Tennessee. An estimated 900 acres of fresh market and 1000 acres of processing summer squash is produced in Tennessee. Fresh market squash has been one of the most profitable vegetable crops in recent years, especially in the fall season. Weed control with hand labor is expensive, and labeled herbicides are not completely effective for weed control, especially of broadleaf weeds and nutsedge. Several new chemicals have been screened in limited trials, and showed some potential for use on summer squash. An experiment was conducted at the Plateau Experiment Station at Crossville, TN in 2000 to evaluate performance of 14 herbicide treatments in summer squash. This trial was supported by the IR-4 program as a new technology trial.

The site was prepared for planting using conventional tillage on May 1. Fertilizer was broadcast at 300 lb/A of 15-15-15 before final disking on May 11. Plots were direct seeded with ‘Pic-N-Pic’ hybrid yellow summer squash on May 11. The seed source was Seedway, and lot number was 7005. Plot size was a 12 by 20 ft plot. One row, 20 ft long with 10 hills (3 seeds/hill) spaced 2 ft apart in the row was seeded down the middle of each plot. Experimental plot design was a randomized complete block with four replications. Each 12 by 20 ft plot was treated with the selected herbicide treatment. Preemergence treatments were applied on May 12. Post emergence treatments were applied on May 26. Herbicide treatments were applied in 27 gal of solution/A using a home assembled plot sprayer with a 12 ft boom equipped with 8004 flat fan nozzles. Compressed air was the pressure source and application pressure was 40 psi. Herbicide treatments, source of herbicides, and herbicide lot numbers are presented in Table 1.

Carbaryl (Sevin XLR) at 1 lb ai/A was applied for insect control on May 17. Predominate weeds were rated for control on June 7. Plant stand was counted, and crop injury was rated on June 7. Eight harvests were made between June 28 and July 14. All data were analyzed by analysis of variance procedures, and means were separated by Duncan’s multiple range tests at the 0.05 level.

Most treatments caused some crop vigor reduction, partly due to chemical burn, and partly due to lack of effective weed control (Table 2). Pyrithiobac (Staple) used in Tmts. 08 and 09 caused the most true chemical injury to the squash. Not only was visible herbicide burn evident, but crop yields were very low. Halsulfuron (Semphra) post emergence (Tmt. 03) did not cause visible herbicide burn, but yields were low, most likely due to less effective weed control. Flumicolorac (Resource) used in Tmt. 6 and flumioxazin (Valor) used in Tmt. 7 were rated high for reduced vigor, but was more a result of poor grass control than actual injury as squash yields were as good as the ethalfuralin check (Tmt.15). S-metolachlor (Dual) used in Tmt. 12 had a relatively high rating for vigor reduction and low squash yields. However, this herbicide did not control broadleaf weeds well and most of the vigor reduction was probably due to poor broadleaf weed control rather than chemical injury.

The treatments were rather specific for weed control (Table 3). Some herbicide treatments controlled broadleaf weeds well while others controlled grasses. A combination of herbicides seems essential for complete weed control. This has also been evident with herbicides presently labeled for use on summer squash. Cultivation is almost essential to control escaped weeds with the labeled herbicides. Halsulfuron (Sempra) pre (Tmt. 02) was one of the most broad spectrum treatments. The post emergence treatment of halsulfuron (Tmt. 03) was less effective on all weeds except common ragweed. Flufenacet (FOE 5043) at both rates (Tmts. 04 and 05) controlled all species well except common ragweed. Flumiclorac (Resource) in Tmt. 06 was not highly effective for barnyard grass control. Flumioxazin (Valor) in Tmt. 07 controlled all prevalent species well. S- dimethenamid (Frontier X 2) used in Tmts. 10 and 11 controlled redroot pigweed and barnyard grass well, but was not highly effective for control of common lambsquarters and common ragweed. S- metolachlor (Dual) in Tmt. 12 did not control any species very well. Rates were very low to avoid crop injury. Sulfentrazone (Authority) was more effective at the high rate (Tmt. 14) than at the low rate (Tmt 13). Sulfentrazone at the high rate controlled all species well except common ragweed.

Based on results of this trial, pesticide label clearance request forms were submitted to the IR-4 for Frontier X 2 and Authority.

Table 1. Weed control treatments, source of herbicides, and herbicide lot number for summer squash trials at The University of Tennessee Plateau Experiment Station at Crossville, 2000.