Posts Tagged: horseweed

And Now Paraquat

A recent call about the poor control of marestail (horseweed, Conyza canadensis) to glyphosate (Roundup®) wasn't surprising, but that paraquat didnt do the trick was.  It turns out that there is multiple resistance to the materials.  If horseweed is resistant to glyphosate it is possibly going to be resistant to paraguat  which also means that hairy fleabane which has glyphosate resistance could also show resistance to paraquat.  A recent study reports on the increased Conyza resistance to paraquat (Distribution of Conyza sp. in Orchards of California and Response to Glyphosate and Paraquat, Moretti et al, https://doi.org/10.1614/WS-D-15-00174.1):

Resistance to glyphosate in hairy fleabane and horseweed is a problem in orchards and vineyards in California. Population genetic analyses suggest that glyphosate resistance evolved multiple times in both species, but it is unknown if resistance to other herbicides is also present. Two approaches of research were undertaken to further evaluate herbicide resistance in Conyza sp. in the perennial crop systems of California. In the initial study, the distribution of Conyza sp. in the Central Valley, using a semistructured field survey, was coupled with evaluation of the presence and level of glyphosate resistance in plants grown from field-collected seed. In a subsequent study, single-seed descendants representing distinct genetic groups were self-pollinated in the greenhouse and these accessions were evaluated for response to glyphosate and paraquat. Conyza sp. were commonly found throughout the Central Valley and glyphosate-resistant individuals were confirmed in all field collections of both species. The level of glyphosate resistance among field collections varied from 5- to 21-fold compared with 50% glyphosate resistance (GR50) of the susceptible, with exception of one region with a GR50 similar to the susceptible. When self-pollinated accessions from different genetic groups were screened, the level of glyphosate resistance, on the basis of GR50 values, ranged from 1.7- to 42.5-fold in hairy fleabane, and 5.9- to 40.3-fold in horseweed. Three accessions of hairy fleabane from different genetic groups were also resistant to paraquat (40.1- to 352.5-fold). One glyphosate-resistant horseweed accession was resistant to paraquat (322.8-fold), which is the first confirmed case in California. All paraquat-resistant accessions of Conyza sp. identified so far have also been resistant to glyphosate, probably because glyphosate resistance is already widespread in the state. Because glyphosate and paraquat resistances are found across a wide geographical range and in accessions from distinct genetic groups, multiple resistant Conyza sp. likely developed independently several times in California.

conyza avocado

Citrus/Avocado Herbicide Update

With the rains, and in those area where fire took out the competition, weeds are coming back in their glory.  Mustard has painted the hills yellow.  The question comes up, what to do about all that wild growth.  Mechanical control, such as discing or whipping can work great.  Sometimes chemical control is the only answer.  A recent request for an alternative to glyphosate (Round-up) control of marestail (horseweed, Conyza canadensis) which is similar to hairy fleabane (Conyza bonariensis), came to the office from a lemon grower. Glyphosate just wasn't controlling it.  And it's been a problem for a while, even in tank mixes with paraguat and old-line weed killer.  The alternative might be a newer material, such as saflufenacil (Treevix) which has been recently added to the herbicides that can be used on citrus. 

As always before doing "vegetation management" it's best to identify the plant that is the problem

Identify the problem plant (weed)

http://wric.ucdavis.edu/information/weedid.htm

 

or if you know what the plant is, go directly to a listing of the weeds

Listing of weeds, their biology and control

http://wric.ucdavis.edu/information/info_spec_weed.htm

• Susceptibility of Winter Weeds to Herbicide Control (9/08)
• Susceptibility of Spring/Summer Weeds to Herbicide Control (9/08)

 

Or you can go in reverse order and look at your tree crop and see what herbicides are listed

Listing of herbicides by tree crops, including avocado and citrus

http://wric.ucdavis.edu/PDFs/T&V_herbicide_registration_chart.pdf

 

 

 

conyza canadensis

Horseweed Resistance

Researchers have now confirmed that six glyphosate-resistant weed species have been identified in California. Four have been known to exist for some time; they are horseweed (marestail, Conyza sp.), hairy fleabane, rigid ryegrass and annual ryegrass. To that list, junglerice and Palmer amaranth in the Central Valley have been recently added to the list. Additional weeds that have become more of a challenge to control and are on the suspect list are goosegrass and, in the central San Joaquin Valley, the summer grasses sprangletop and witchgrass.

There have never been a lot of herbicides registered for avocado, largely because once they are mature, they self-mulch with their leaves. It's really young orchards that have a weed problem. Growers for the most part have not used pre-emergent herbicides and relied on the many glyphosate products that originally started with Roundup. Sethoxydim (Poast) is registered for non-bearing orchards, but then what can be used as post-emergence herbicides when there is still large open spaces between trees when they are young and bearing? There are still some formulations of paraguat and diquat available for use on avocado, but horseweed is also showing resistance to these materials. Paraquat is also a restricted use material so limits who can apply it. That leaves some of the pre-emergents, such as norfluazon (Solicam), oxyfluorfen (Goal), oryzalen (Surgflan) and simazine (Pricep). These all require water to activate them, and in years with low rainfall efficacy can be erratic. 

From a postemergence standpoint for glyphosate-resistant horseweed the options are pretty slim in avocado. 

• Paraquat is registered and likely to be pretty effective but has regulatory and safety baggage.

• Diquat is registered in nonbearing orchards and likely to be effective.

• Carfentrazone is registered but not very effective on Conyza.

Oxyfluorfen will help glyphosate performance to some extent on Conzya, but not likely to be fully satisfactory. 

From a preemergence standpoint:

• Simazine should be really effective on Conzya.  It would have groundwater and runoff concerns in some areas.

• A combination of oxyfluorfen plus pendimethalin or oryzalin would probably be the best available bet for broad spectrum PRE control with decent crop safety.

• Isoxaben is registered on non-bearing avocado with reasonable Conzya control and good crop safety

Flumioxazin has a non-bearing label and should be pretty good on Conzya.

There has always been concern about the effect on shallow rooted avocado using pre-emergents. This has not been documented, but based on grower experience.

There have never been a lot of herbicides registered for avocado, largely because once they are mature, they self-mulch with their leaves.  It's really young orchards that have a weed problem.  Growers for the most part have not used pre-emergent herbicides and relied on the many glyphosate products that originally started with Roundup.  Sethoxydim (Poast) is registered for non-bearing orchards, but then what can be used as post-emergence herbicides when there is still large open spaces between trees when they are young and bearing?  There are still some formulations of paraguat and diquat available for use on avocado, but horseweed is also showing resistance to these materials.  Paraquat is also a restricted use material so limits who can apply it.  That leaves some of the pre-emergents, such as norfluazon (Solicam), oxyfluorfen (Goal), oryzalen (Surgflan) and simazine (Pricep).  These all require water to activate them, and in years with low rainfall efficacy can be erratic.

We are currently working on getting registration the post-emergence herbicide glufosinate (Rely) registered for avocado.  It's used extensively in other tree crops with success to control many weeds including the Conyza weeds. The registration is being sought through the IR-4 Project (Interregional Research Project Number 4) since 1963 has been the major resource for supplying pest management tools for specialty crop growers by developing research data to support new EPA tolerances and labeled product uses.

 

Photos of young and maturing Conyza

Horseweed

horseweed mature

Herbicide resistant weeds in your neighborhood

It is not always easy to kill weeds with herbicides for several reasons, but if you apply the right material at the right time to susceptible weeds you expect control. But you should never assume it, because resistant weeds rely on this assumption.

Repeated use of herbicides with the same mode of action (usually the same target site within plant) selects for naturally occurring resistance traits in weed population.  The few resistant weeds proliferate since there is no longer competition from susceptible types and if other control measures are not used.

Hairy fleabane (Conyza bonariensis) resistance to the two most commonly used herbicides – glyphosate (Roundup) and paraquat  -  is widely reported in California, including Ventura County. A close relative – horseweed or mare’s tail (Conyza canadensis) has wide-spread glyphosate resistance, as well.

These weeds are most frequently found at road sides, ditches and just about every site with infrequent disturbance. Of course, that is where glyphosate and paraquat are routinely applied.  What’s worse, they produce thousands of wind-dispersed seeds that travel up to 3 miles and carry the herbicide resistance in them to new places. In fact these and other wind-dispersed weed species are increasingly common in our agricultural and urban areas because we fail to control them before seed production. The seed germinate on moist soil surfaces without incorporation and rapidly grow and reproduce.

There are several strategies to manage these herbicide resistant weeds that can be combined:

• Identify what species of weeds you have and select management options specific to your weeds and crop. You can look up susceptibility of weeds to herbicides for most crops at http://www.ipm.ucdavis.edu/PMG/crops-agriculture.html. These susceptibility tables are based on results of University of California research trials in those crops and are updated periodically.
• Always evaluate the efficacy of your herbicide application and look for weeds that escaped treatment. If one or two species survived treatment – they are likely resistant. If most weeds did - perhaps there was a general problem with herbicide application.
• Control weeds when they are small. Large mature weeds are more difficult to kill with herbicides but even resistant weeds can be controlled with herbicides when small. These weeds only reproduce by seed, if you see them flowering it may be already too late.
• To control escaped weeds use herbicide with a different mode of action appropriate to your crop/non-crop site.
• Use mechanical methods of weed removal – there has not been reported resistance to a cultivator or hoe
• Be a good neighbor – communicate with land-owners near you about the wind-dispersed weeds traveling between nearby properties and control them. Even if you manage weeds very well, wind dispersed species can travel to your site from surrounding areas, establish and compete at the time when neither herbicides nor labor for hand-weeding are available.

Herbicide resistance has nowadays been reported for most herbicidal modes of action. Because of its intensive, high-input cropping systems, the United States has more resistant bio-types (131) than any other country. Hopefully we’ll have fewer in Ventura County if we pay attention. California Weed Science Society has a priority focus on the issue and has more information at:

http://www.cwss.org/CWSSJournal/2013_01_CWSSResearch.pdf

 

Young plants of hairy fleabane (top) and Horseweed/mare’s tail (below) should be controlled before flowering.

hairy fleabane

Horseweed

Getting Horseweed and Hairy Fleabane Back Under Control in Central California

Below:  Horseweed on top and Hairy Fleabane on bottom.

 

 

horseweed

hairy fleabane

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