Shawn Askew, Extension Turfgrass Weed Specialist; Jeff Derr, Ornamental Weed Specialist; Mike Goatley, Extension Turfgrass Specialist

 

A fairly recent herbicide of great interest in weed control in Virginia’s warmer climates due to its activity on some of the most difficult to control broadleaf weeds AND a label that includes almost all of the commonly grown turfgrass in Virginia is PBI/Gordon’s AvenueTM South.  This product contains penoxsulam, sulfentrazone, 2,4-D, and dicamba.   Dr. Shawn Askew and Dr. Jeff Derr are consulted for their thoughts on the use of Avenue South based a request for help from a professional consultant.

Background and questions regarding Avenue South:  We have a challenge in Tidewater with having to treat St Augustine lawns along with Tall Fescue (often in the same lawn). Therefore, specialty products like Celsius just won’t work for us. We have been using Speedzone Southern in the past, but control of tough weeds at the St Augustine rate is poor. Dr. Askew has suggested Avenue South and I have checked into this (and we are testing it on a limited basis). My concern with Avenue South is with the potential for phytotoxicity to Tall Fescue. I have heard conflicting views on the potential for Avenue South to injure Tall Fescue.  Also, I am concerned that the rate allowable for use on Tall Fescue will result in less than acceptable control of tough to control weeds (e.g. Virginia Buttonweed and Oxalis). Some specific

  My questions for you today are as follows:

 ·         What are your thoughts on the potential for phytotoxicity to Tall Fescue (at the allowable labelled rate for use on Tall Fescue)?

·         What are your thoughts on being able to control tough weeds with Avenue South when using it at the allowable labelled rate for use on Tall Fescue?

·         Do you have any other thoughts that might be helpful?

Dr. Shawn Askew (@VTTurfWeeds) responds:  We have conducted several trials on Avenue South at up to two times the full rate and have not seen any injury to tall fescue.  We currently have new trials this year and we are looking at an experimental product that contains penoxsulam, the ingredient in Avenue South most likely to cause tall fescue injury, at rates that are equivalent to 1X to 4X of the amount of penoxsulam in a label rate of Avenue South and we are seeing severe injury to tall fescue at all rates.  Here is what I think is happening but I may be wrong on this.  I think the early warming spell this year caused increased turfgrass growth in winter and the combination of Avenue South and subsequent cold weather caused injury responses on tall fescue that we had not seen in the past.  I've sprayed Avenue South on tall fescue in fall, winter, spring, and summer and have never seen a problem on tall fescue until this spring where several reports of injury have come my way.  I'm confident that the problem is the penoxsulam ingredient in Avenue South but we normally don't see tall fescue injury from penoxsulam at appropriate rates.  Even this year, we did not see consistent injury at penoxsulam concentrates equivalent to the 1X rate of Avenue South.  Some plots would have unacceptable injury and others would have essentially no injury.  At higher rates, 2X to 4X, we see consistent and severe injury to tall fescue across the board.  Until this year, 2X was as high as I had evaluated Avenue South and I never saw injury.  So I think the injury some folks have experienced this spring is an interaction between the odd weather that we have had and the herbicide, especially if there were fluctuations in herbicide rate due to application technique but this is not just a rate issue, the product is harming fescue this year at many rates, including the normal labeled rate.  Regardless, it means we must be more careful when using Avenue South on tall fescue.  The folks at PBI tell me that St Augustine is a slam dunk.  Excellent safety and works well on troublesome weeds.  So you are correct that the problem is with tall fescue.  Here are some options:

1) Use Avenue South on fescue only when fescue is still dormant to semi dormant or after active growth but when the threat of cold stress is not expected.  (Another way of saying this is "have faith that the product will not cause problems in a "normal" year as that has been the case in numerous research trials at VA Tech and other universities.  In light of injury reports this spring, I would test the product thoroughly before fully converting.

2) Use Avenue South at a lower rate on tall fescue.  For example, one concentration in the tank, spray St. Aug yards two passes and fescue yards one pass or lower output volume per unit area on tall fescue.  You will probably suffer on weed control in the fescue yards.  One option would be to double spray or increase the volume back to the full rate on areas of fescue lawns that have hard-to-kill broadleaf weeds.  In this way, most of your fescue lawn area gets a lower rate at lower cost and lower risk of injury but you take on a slight risk of injury in areas that have difficult weeds.  I understand, though, that this is complicated and complicated is not always functional.

3) Spike a lower rate of Avenue South in tall fescue with an affordable rate of another herbicide (Escalade II, Turflon, etc).

4) Use Avenue South on St. Aug lawns and a different product on tall fescue lawns but you don't need to clean the tank between lawns.  For example, 1/4 tank left over after spraying St. Aug, just add other product on top and spray fescue lawns.  I understand that this is probably not economical as it requires two trips to many of your neighborhoods but I thought I would throw it out there nonetheless.

I don't have enough experience using Avenue South on hard-to-kill broadleaf weeds.  I've used penoxsulam as an experimental active ingredient to kill wild violet and some other difficult weeds.  I've used Avenue South to kill ground ivy, clover, chickweed, henbit, and some other weeds but there are many broadleaf weeds that I've not evaluated.  I can't say for sure how the fescue rate will perform over all the weeds that may be listed on the label.  I did contact an R&D specialist at PBI and he says that it works great on woodsorrel in fall but it will not outperform other products on the market in spring when plants are more mature and blooming.  In other words, it will not work that well this spring on woodsorrel.  He says that VA buttonweed control at the tall fescue rate will be "so so" and it will take multiple treatments and maybe multiple years with Avenue South alone.  For better VA buttonweed control in St. Aug, spike the product with 0.5 oz/A metsulfuron.  For better VA buttonweed control in tall fescue, spike the product with triclopyr.

We are initiating a new trial to evaluate tall fescue tolerance at 4, 8, and 12 oz Avenue South per acre right now to better explore this tall fescue injury situation.  I'm not really surprised because I've seen things like this in the past but it still amazes me that we can evaluate a product for 5 years and not see injury and then we find substantial injury after it is released due to environment, variety, or other factors.  I'm only just now finding out about these injury concerns.  In fact, the first report was the day of the webinar.  Prior to the webinar, I would have told anyone that Avenue South will not injure tall fescue.  I can't say that now but I'm still pretty confident in the products safety to tall fescue.

I hope this helps.  If you have any observations good or bad about Avenue South, please let me know.  Also, if your colleagues have shared stories or pictures of problems, encourage them to contact me or send me the information.  I will continue to conduct research on this as Avenue South could fit a unique market in the VA Beach area due to its ability to broadly cover cool- and warm-season turf species.

Dr. Jeff Derr responds: We have been getting reports of injury to tall fescue from Avenue South by several of our lawn care companies in Tidewater.  We also were planning to do some work on tall fescue this year with this herbicide.  Shawn could be on target with the weather effect.  We went from hitting a high of 82 back in February followed by below freezing temperatures in March.