Pest Management Guidelines
Special Weed Problems
(Reviewed 5/13, updated 5/13)
Annual weed species that may be especially troublesome include hairy nightshade, black nightshade, and annual morningglory. Perennials include yellow and purple nutsedge, field bindweed, bermudagrass, and johnsongrass. Control of these hard-to-control weeds can be enhanced by growing a Roundup Ready cotton variety.
Many weeds, such as hairy, black, and silverleaf nightshade, germinate in the top 1 to 1.5 inches of soil. In fields heavily infested with nightshade or other difficult-to-control weeds, deep plowing with Kverneland-type plows before planting can be used to bury most seeds to a depth of 1 to 1.5 feet to prevent their germination. For weeds that do germinate, hand-rogueing and removing them from the field will reduce seed production and reduce future infestations.
Tank mix applications of trifluralin (Treflan) with prometryn (Caparol) or pendimethalin (Prowl) with prometryn as a preplant incorporated treatment, provides successful control when adequate soil moisture is present. A preplant treatment with the soil fumigant metam sodium can also be effective. Pyrithiobac sodium (Staple) applied at the 4-leaf cotton stage provides excellent control, especially when combined with a cultivation.
Cotton often turns slightly yellow for 1 to 2 weeks if cold weather follows an application. Pima is more sensitive to these symptoms and there is no guarantee that plants will recover sufficiently. Well-timed postdirected (after crop emergence, directed at weeds) treatments of carfentrazone (Shark), oxyfluorfen (Goal), and prometryn (Caparol) when nightshade is in the cotyledon to two-leaf stage also can be successful. Be sure to follow product labels for minimum cotton height to avoid crop injury.
In transgenic cotton varieties, glyphosate (Roundup) in Roundup Ready cotton and glufosinate (Rely) in Liberty Link cotton provide effective nightshade control. (Control is reduced when nightshade is past the fourth-leaf stage.) Cultivation also effectively controls nightshade in the early stages.
Annual morningglory normally does not cause a problem at cotton emergence because it normally germinates later in the season. A postdirected treatment of carfentrazone (Shark), pyraflufen (ET), flumioxazin (Chateau), trifloxysulfuron sodium (Envoke), oxyfluorfen (Goal), diuron (Karmex), or prometryn (Caparol) to morningglory seedlings before they twine onto the cotton plant has provided control.
Tank mix applications of pyrithiobac sodium (Staple) and MSMA, when applied up to the four-to six-leaf stage, have provided excellent control. Be sure to follow product labels for minimum cotton height to avoid crop injury.
For transgenic cotton varieties, two to three applications of glyphosate (Roundup) to Roundup Ready cotton or glufosinate (Rely) in Liberty Link cotton applied before morningglory has more than two leaves provide effective control. In non-Liberty-Link cotton, a late application of glufosinate, before the weed has two leaves, is effective.
To give cotton a head start on nutsedge, sweeps or other shallow cultivating tools can be used to dislodge emerging nutsedge growth before planting. After cotton emergence, the use of precision equipment to cultivate as closely as possible and hand or mechanical thinning can also help reduce nutsedge competition.
Directed sprays after the cotton plants have two or more leaves reduce crop injury. MSMA can be applied broadcast or as a directed spray. Metolachlor (Dual Magnum) can be applied either over the top of cotton plants (from the time the cotyledons appear to the emergence of the first true leaves) or as a directed spray for preemergence control of yellow nutsedge. If these herbicides accidentally get on the growing point of the cotton plant, they will stunt the plant's growth. Because nutsedge is sensitive to competition by shade, early chemical control will allow later shading from the cotton canopy to provide additional control. Rotation to corn with the use of a thiocarbamate herbicide, Roundup Ready corn, or halosulfuron (Sandea), has also significantly reduced both yellow and purple nutsedge infestations. Roundup applied to Roundup Ready cotton effectively suppresses nutsedge growth when applied early over the top and followed by a second, postdirected treatment.
Dry fallowing in summer on sandy loam soils has reduced purple nutsedge populations. Purple nutsedge tubers can be destroyed with repeated summer tillage of dry soil because tubers are susceptible to drying. Springtooth harrows have been the best tool for this method, but this method has not been effective for control of yellow nutsedge.
During fallow periods, apply glyphosate (Roundup) to field bindweed to achieve control. During the growing season, control can be achieved with glyphosate applied through a hooded sprayer in traditional cotton varieties. Belting on the bottom of the hood will keep the spray off the lower leaves and stems of the cotton plant. Adequate growth of field bindweed that is not moisture stressed is essential for this method to work. Two to three applications of glyphosate (Roundup) in Roundup Ready cotton will provide good control. Layby applications of glufosinate (Rely) suppress this weed.
Prevention is the best method to control perennial grasses such as bermudagrass and johnsongrass. Dry fallow during summer when moisture is depleted has significantly reduced bermudagrass and johnsongrass. Cultivation can control these weeds between rows, but hand weeding is not effective. Sethoxydim (Poast), fluazifop-p-butyl (Fusilade), or clethodim (Select Max) applied two to three times per season can provide season-long control. Glyphosate (Roundup) applied with a hooded sprayer to traditional cotton varieties will provide satisfactory control of bermudagrass. For Roundup Flex cotton, over-the-top applications can be made up to the 14th node in cotton; in Roundup Ready varieties they can only be made up to the fourth-leaf stage. Directed sprays, depending on the cotton size, can also be effective.
UC IPM Pest Management Guidelines: Cotton
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