Should Growers Be Concerned About Diuron Carryover to Wheat (Culpepper)?

Growers are strongly encouraged to apply diuron + MSMA at cotton layby for the control of Palmer amaranth.   With rapid adoption of diuron use at layby in cotton, many growers are now questioning the potential for diuron injury to a subsequent wheat crop.   The following restriction is made clear on the Direx label when it is applied at cotton layby: “Cotton, corn, and grain sorghum can be planted during the next spring.  Do not replant treated areas to any other crop within 1 year after the last application, as injury to subsequent crops may result.”

Therefore, the Direx label prohibits planting wheat following a layby application of Direx.  So is this restriction based on sound science or is this proposed potential for damage simply not fully understood?  Our initial research suggests that the potential for damage is simply not understood.  During last winter, we applied Direx from 8 oz/A up to 32 oz/A preemergence on wheat and followed immediately with heavy irrigation.   Injury was not detected with 8 oz/A and less than 10% injury was noted with 16 oz/A.  Therefore, our INITIAL research suggest diuron applied at layby in cotton should pose little to no risk to the following wheat crop as long as at least 90 days has passed (Table 1).  Significant (23 to 43%) wheat injury was noted with 24 to 32 oz/A Direx applied PRE to wheat.  It is important to note that this study was conducted on a soil with less than 1% organic matter which increases the potential for injury as compared to soils with greater than 1% organic matter.

Table 1.  Wheat tolerance to Direx applied PRE.
Rate of Direx applied PRE to Wheat	Maximum Wheat Injury
Direx  8 oz/A	0% d
Direx 16 oz/A	8% c
Direx 24 oz/A	23% b
Direx 32 oz/A	43% a
Experiment conducted in TyTy, GA during the 2010 and 2011 season.  Values with the same letter are not different at P = 0.05.

It is important to remember to always follow all herbicide label restrictions.  Our goal is to work with MANA to address rotational restrictions for Direx to wheat.

Post-Harvest Control of Benghal Dayflower (Tropical Spiderwort) (Prostko)

Although much of our attention and effort has been focused on Palmer amaranth control, other weeds are still problematic in Georgia.  While driving around recently, I noticed many harvested corn fields that were covered with Benghal dayflower a.k.a. tropical spiderwort (Figure 1).  Like Palmer amaranth, it is very important to control these fall populations of Benghal dayflower since plants can go from seed to seed in about 40 days.  Here are a few reminders:

1) Tillage is just as effective as herbicides in controlling post-harvest populations of Benghal dayflower.
2) Herbicides that can be used for the control of post-harvest populations include 2,4-D, paraquat, and Aim.  Please refer to page 56 of 2012 UGA Pest Control Handbook for more information.
3) Plants larger than 6" tall will need split applications of herbicides (Figure 2).