Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: herbicides

Herbicide Injury in Avocado

Although the main objective of herbicide use in avocado orchards (and all crops) is to manage weed populations, sometimes unintentional injury of the crop itself can occur when herbicides are incorrectly applied. Herbicide injury in avocado can reduce yield, decrease fruit, reduce plant vigor, increase susceptibility to diseases and pests, and sometimes result in plant death. Common situations resulting in injury include spray drift, tank contamination, application of the wrong herbicide or rates, and herbicide carryover from a previous crop. The extent of herbicide damage on avocado can vary widely according to factors such as herbicide mechanism of action (MOA) and application rate, route of exposure, plant size and growth stage, soil properties, and weather.

Herbicide injury can be difficult to diagnose properly and is often confused with disease, insect damage, nutrient deficiencies, and other environmental stresses. It is recommended that trained researchers or Pest Control Advisers, who may utilize plant tissue, make diagnoses or soil samples along with plant symptoms, injury progression, and other plant species affected, orchard herbicide use history, weather conditions, and other factors to confirm or rule out injury from herbicides or other causes.

Where the injury occurs can also be an indication of herbicide injury. For example, if injury is on just one side of a tree or trees near another field, it may be an indication of spray drift. If it occurs only along the edge of the skirts, it may be a hint that an uneven ground spray was applied.

The majority of herbicides for use in avocado orchards in California fall into eight MOAs as defined by the Weed Science Society of America. MOAs describe the specific biological processes that are disrupted by a group of herbicides. These processes control the growth and development of plants and when interfered with, can result in plant injury or death.

Table 1: Common herbicides used in avocado, their mechanism of action, and possible injury symptoms

WSSA Group

Mechanism of Action

MOA description1

Example herbicides

Possible injury symptoms1


Acetyl CoA Carboxylase (ACCase) Inhibitors

Inhibits lipid creation in grasses, preventing production of plant cell membranes

Fluazifop-P-Butyl (Fusilade DX), Sethoxydim (Poast)

Chlorosis, necrotic spots, leaf crinkling, leaf distortion


Mitosis Inhibitors

Inhibits cell division in germinating seedlings and lateral roots

Oryzalin (Surflan)

Thickened, shortened lower stems and small, crinkled leaves


Photosystem II Inhibitors

Prevents the transfer of energy generated during photosynthesis, causing a buildup of reactive molecules that damage chlorophyll and cell membranes

Simazine (Princep 4L)

Chlorosis, necrosis progressing from leaf margins toward the center of the leaves, foliar applications will appear as leaf burn


Enolpyruvyl Shikimate-3-Phosphate (EPSP) Synthase Inhibitors

Inhibits the production of three aromatic amino acids and the enzymes and proteins built from them

Glyphosate (Roundup)

Leaves of trees and vines become chlorotic 3 to 7 days after exposure, and margins of new leaves become necrotic


Carotenoid Biosynthesis Inhibitors

Inhibits production of carotenoid pigments, which harvest light and protect chlorophyll from reactive molecules

Norflurazon (Solicam DF)

Plant foliage turns white and appears bleached



Protoporphyrinogen Oxidase (PPO) Inhibitors

Blocks the production of chlorophyll and causes a buildup of reactive molecules that damage existing chlorophyll, carotenoids, and cell membranes

Oxyfluorfen (Goal 2XL), Carfentrazone (Shark EW), Flumioxazin (Chateau)

Drift injury will appear as speckling on leaf tissue. The necrotic spots are sometimes surrounded by a reddish colored ring. Injury from soil applications or residues appears as a mottled chlorosis and necrosis.


Cellulose Inhibitors

Inhibit cell wall synthesis and plant growth

Isoxaben (Gallery 75 DF)

Chlorosis, necrosis, leaf crinkling, leaf distortion, purpling of the leaf, and stunting


Photosystem I Inhibitors

Disrupts photosynthesis, forming reactive molecules that destroy cell membranes

Paraquat (Gramoxone SL)

Drift injury will appear as speckling or necrotic spots on leaf tissue

1Not a complete list. Symptoms listed are likely for established orchards. For detailed descriptions of MOAs and injury symptoms, as well as a searchable database of specific injury images (e.g., “chlorosis, necrosis, stem swelling, etc.” visit


Al-Khatib, K. 2015. University of California Integrated Pest Management Herbicide Symptoms. (accessed 09/05/18)

Faber, B.A., C.A. Wilen, B.D. Hanson. 2016. Weeds. Pages 107-124 in University of California Integrated Pest Management Guidelines for Avocado. (accessed 09/05/2018)

Sosnoskie, L.M., B.D. Hanson. 2013. Understanding herbicide mechanisms (modes) of action and how they apply to resistance management in orchards and vineyards. UC Weed Science Blog Post. // (accessed 09/05/2018)

Weed Science Society of America. Summary of Herbicide Mechanism of Action

According to the Weed Science Society of America. (accessed 09/05/2018)

Photo: Sometimes weeds are tasty, like amaranth and purslane

organic herbicide live
organic herbicide live

Posted on Monday, October 15, 2018 at 6:51 AM
  • Author: Travis Bean
Tags: avocado (289), damage (24), herbicides (18), injury (1), weeds (32)

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)


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

Listing of weeds, their biology and control


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




conyza canadensis
conyza canadensis

Posted on Monday, May 21, 2018 at 5:18 AM
Tags: avocado (289), citrus (338), fleabane (2), herbicides (18), horseweed (6), marestail (4)

What's that Weed? And What Does Herbicide Damage Look Like?

Plant-out-of-place photo galleries:

Horseweed - Conyza canadensis


Herbicide treatment table for citrus:


And if you are wondering what herbicide damage might look like on various plant species (this is heavily weighted to annuals and landscape plants):

Blueberry herbicide damage

Posted on Monday, March 19, 2018 at 6:42 AM
Tags: avocado (289), citrus (338), herbicides (18), herbologist (1), lemon (100), pesticide (9), weeds (32)

Glyphosate formulations - what's the diff (and what's a salt)?

Glyphosate is one of the most widely used herbicides in the world and is extremely important in many of our orchard, vineyard, and annual crops as well as in non-crop and home situations. However, it can be confusing to understand some of the differences among various formulations of glyphosate herbicides.

I'll paraphrase a recurring extension question as “I'm trying to compare the rates and cost-effectiveness of two glyphosate herbicides. One lists the active ingredient as ‘41% glyphosate as the isopropylamine salt' and the other as 48.7% glyphosate as the potassium salt'.  How do I compare these two herbicides?”

First important point, glyphosate is a weak acid herbicide.  The various salt formulations have major impacts on how the herbicide behaves in the jug, and to some degree on how it gets into the plant.  But once in the plant, it is the glyphosate acid that binds to the target enzyme in susceptible plants and causes the herbicidal effect.

What is a salt? From a chemistry perspective, a salt is simply a compound formed by ionic bonding of two ions of opposite charge.  Glyphosate acid has a weak negative charge and the salt is formed when the glyphosate acid is bound to a base that has a positive charge.  In the cartoon below, this is illustrated a little incorrectly - you can see the negatively charged glyphosate acid on the left (C3H8NO5P); however, the "salt+" tagged on the right should really be labeled  "base+" (the combined molecule is actually the "salt").

glyphosate salt cartoon (from ((the "salt" on the right should more accurately be labeled "base"))
What are some common glyphosate salts? There are several glyphosate salts currently available in the market and others have been in the past but are less common now.  Four examples (below) include: isopropylamine, dimethalamine, trimesium, and potassium salts.  (images mostly lifted and modified from

In case it's been a while since your last organic chemistry class, here's a quick refresher on nomenclature. In the isopropylamine salt, that simply means it as a 3-carbon chain (that's “propyl”) and amine group (NH), and the chain is connected in the middle (the “iso” position) rather than on one end.  Dimethylamine indicates two (di) methyl groups and an amine.  Trimesium is a shortened name for trimethylsulfonium which means three methyl groups and a sulfur.  Potassium means, well that one's pretty straightforward and means potassium (K).

How does the salt formulation affect the herbicide? The salt formulations are those of us managing weeds for a couple of important reasons. 

  • First, the different glyphosate salts have different solubility in water (or other solutions). This doesn't have much, if any, effect when we have the herbicide diluted in water to make a spray application (eg. quarts of product in 10 or more gallons of water).  However, it has a big impact on how concentrated the herbicide can be in the formulated product.  (It's useful here to note that pure glyphosate is actually a solid crystal that is dissolved in liquid to make the products we use in the field.) In the example I started with, that is the big driver behind why one product has 41% active ingredient and the other has 48.7%.  From a packaging, shipping, storing, and handling perspective, it's far more efficient to have more concentrated materials (eg less water).  So, we can assume the potassium salt is quite a bit more soluble than the isopropylamine salt form of this herbicide.

  • Second, the different glyphosate salts have different weights. Remember the periodic table of the elements?  The atoms that make up the salts (and any other chemical structure for that matter) have markedly different weights on an atomic scale.  For our purposes, we can think of this as a carbon (C, atomic number 6) weighing six times a hydrogen atom (H, atomic number 1) and a potassium (K, atomic number 19) weighs about three times what carbon does.  Of course, the glyphosate acid weight is the same C3H8NO5P in each formulation – only the salt is different.  So, it is important to remember that in these herbicides the “active ingredient” (AI) is the salt formulation (eg glyphosate isopropylamine) and that each of the different active ingredients has a slightly different weight.

How can I use the percent active ingredient to compare products? I'll be honest here, I find it very confusing to think about the percent AI list on the label of a glyphosate herbicide.  As I indicated above, this does not allow a straight across comparison for two different salts (eg a potassium salt glyphosate is not the same weight as a dimethylamine salt).  More importantly, the percentage listed on the label is actually on a weight basis (weight of glyphosate salt per weight of formulated herbicide).  I don't know about any of you, but I typically measure glyphosate herbicides by volume (fluid ounces, quarts, milliliters) rather than by weight (ounces, pounds, kilos).   So knowing the amount of glyphosate per pound of Roundup Powermax isn't very helpful to me when I don't weigh the liquid.

Instead of focusing on the percent active ingredient, look on the label just below where the percentages are listed for the active ingredient and acid equivalent information. (I'm looking at a Roundup PowerMax label and a Credit41 label, but there are dozens of examples).  In these two herbicides, you can see:

  • Credit41 has 4 lbs per gallon of glyphosate as the isopropylamine salt, which is equivalent to 3 lbs per gallon of glyphosate acid.
  • Roundup PowerMax has 5.5 lbs per gallon of the glyphosate potassium salt, which equates to 4.5 lb/gal of the acid.

So, once we're comparing on an acid equivalent (AE) basis, we can see that that one product is 50% more concentrated than the other (3 lbs vs 4.5 lb ae). If you wanted to make an equivalent rate (AE per acre) you'd have to apply 50% more of the 3 lb product to have the same amount of glyphosate acid.  Similarly, in making cost comparisons, you should be thinking about it in terms of cost per unit of glyphosate acid.  In the above example, if making an equivalent AE rate per acres, the breakeven point would be if the 3 lb material costs 2/3 of what the 4.5 lb material costs.  Don't make the mistake of assuming a quart of one glyphosate is the same as another (or that a 2% solution of two different products is equivalent!  

Check out this publication we did a couple years ago  On the second page, there's a chart that illustrates this concept – if you wanted to apply a rate of ¾ lb ae per acre, you'd need to apply 32 fl oz of a 3.0 lb ae/gal material (eg Credit41, Roundup Original) but only 22 fl oz of a 4.5 lb ae/gal material (eg Roundup PowerMax).

What else is different among the formulations?  This is a really good question without a really clear answer. 

Besides the glyphosate-salt that makes up the active ingredient in each formulation, there are other components of the herbicide product.  Some of these influence physical properties that affect handling, storage stability, etc.  Others are surfactants and other adjuvants that impact how the herbicide penetrates the leaf surface.  The surfactants (types and loading) are proprietary information and are not reported in the same way as the active components are so this can be difficult to ascertain.  However, one way that price points can be reduced in this competitive market (lots of generic glyphosate herbicide due to loss of patent protection) is to reduce or change the surfactant packages.  Thus, even if the active ingredient is the same between two products, they could have substantially different surfactant loading which can impact weed control efficacy (amount of herbicide getting to the target enzyme in the plants).  If you use a low-surfactant-load glyphosate product, you should consider adding appropriate surfactants to make up for it.

I'll wrap this up by saying that I think a weed manager can get similar levels of weed control performance with the wide range of glyphosate herbicides currently available on the market. However, it is really important to make sure you're comparing apples-to-apples in determining rates needed from the various salt formulations and concentrated products.  In my experience, at equivalent AE rates, a 3 lb glyphosate plus a good surfactant can perform similarly to a higher AE and higher surfactant load formulations; however, be sure to sharpen your pencils to make sure the savings from the cheaper AI isn't offset by higher required rates and surfactant additions.



horseweed mature
horseweed mature

Posted on Wednesday, December 27, 2017 at 9:25 AM
  • Author: Brad Hanson
Tags: chemistry (1), formulations (1), herbicides (18), weeds (32)

Italian Ryegrass Resistant to Roundup. Rats.

Article written by UC Davis PhD student Caio Brunharo from his dissertation research.  It was originally posted in the September 2017 "Weed Management Notes" newsletter from the UC Cooperative Extension office in Glenn County by new weed science and agronomy Farm Advisor Mariano Galla (also a UCD PhD student in weed science!).

Take care, Brad 


Italian ryegrass management in perennial crops in California

Caio Brunharo1 and Brad Hanson2

1PhD Candidate, UC Davis; 2UCCE Weed Science Specialist, UC Davis.

Italian ryegrass (Lolium perenne L. spp. multiflorum (Lam.)Husnot) causes yield losses in a variety of cropping systems around the world (Figure 1). This species is highly competitive with annual crops but may also compete with perennial crops particularly during the establishment years when they are most vulnerable to direct competition. In orchards and vineyards, ryegrass infestation can also interfere with cultural practices during the bearing years.


Repeated herbicide use has selected Italian ryegrass populations resistant to a variety of herbicide mode of actions across the world. Glyphosate-resistant Italian ryegrass populations were first reported in California in 2008, and the evolution and spread of these populations in the state made alternative postemergence herbicides an important management strategy against this troublesome species.

Recently, poor control of Italian ryegrass with Gramoxone 2.0 SL was reported in a prune orchard near Hamilton City, California. Greenhouse dose-response experiments and field trials were carried out to evaluate Italian ryegrass response to several postemergence and preemergence herbicides.

Our greenhouse studies confirm that the Italian ryegrass population from Hamilton City is resistant to Gramoxone 2.0 SL, Envoy Plus, Roundup PowerMAX and Osprey, whereas Fusilade DX, Rely 280, Simplicity CA, Matrix and Poast controlled both a known-susceptible and resistant Italian ryegrass population (Table 1). (note: Osprey and Simplicity CA, which are not registered in perennial crops, were included in the study for comparison purposes). Our criteria were that whenever the resistance index (RI) was larger than two and the comparison between biotypes was statistically different (P <0.05), the population was considered as resistant to that particular herbicide. Matrix is an exception, however, because this herbicide controlled both biotypes at well below its recommended field rate.


The field experiment with postemergence herbicides corroborates with data from the greenhouse studies, since glyphosate and paraquat did not adequately control the herbicide-resistant population from Hamilton City. On the other hand, most of the treatments containing Rely 280 were effective for control of the resistant population (Figure 2).


From the preemergence herbicide trial, all treatments containing Alion controlled the resistant population up to 150 days after herbicide application. Chateau, Surflan AS, GoalTender, Prowl H2O, and the tankmixes of Chateau + Prowl H2O and Chateau + Surflan AS exhibited control percentages above 90% with long lasting residual activity (up to 150 days after treatment; Table 2).


Even though several postemergence herbicides controlled Italian ryegrass in our research, it should be noted that ryegrass populations resistant to Fusilade DX, Rely 280 and Poast have been reported elsewhere in the state (data not shown), and overreliance on these herbicides will increase the chances of selection of further cases of resistance. A chemical weed management program in areas infested with Italian ryegrass should include a preemergence herbicide with long residual sprayed in the winter (Alion, Chateau, Surflan, GoalTender or Prowl H2O are possible options) tankmixed with an effective postemergence herbicide. In areas where herbicide-resistant weeds are known to be present, alternative herbicide chemistries should be adopted (rather than increasing the herbicide rate sprayed) in both the winter and spring application. In some cases, a short residual grass herbicide included with the post-harvest burndown application may help reduce recruitment of early-germinating Italian ryegrass plants which will reduce weed pressure and densities to be managed later in the season.



Posted on Friday, September 15, 2017 at 6:48 AM
  • Author: Caio Brunharo and Brad Hanson
Tags: avocado (289), citrus (338), grass (1), herbicides (18), lemon (100), mandarin (68), monocots (1), orange (69), resistance (13), ryegrass (1), weeds (32)

First storyPrevious 5 stories  |  Next 5 stories | Last story

Webmaster Email: