Subtropical Fruit Crops Research & Education
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Subtropical Fruit Crops Research & Education

Posts Tagged: resistance

Glyphosate Resistance in Orchards

There has been a good overall discussion of herbicide resistance found in plants and how they can affect orchard management.  Check out this presentation by UC Cooperative Extension Weedologist, Brad Hanson, in the "past Webinars" section:

And read more about glyphosate resistance in orchards:

Preventing and Managing Glyphosate-Resistant Weeds in Orchards and Vineyards
Rely too much on any one herbicide and you end up with weeds that will resist its effects—and that's just what is happening now with glyphosate (Roundup®). See how you can increase effectiveness by diversifying your weed-management strategies.
Publication Number:: 8501
Managing Glyphosate-Resistant Weeds in Glyphosate-Resistant Crops
Glyphosate-resistant crops made farming a lot easier when they first came out, but many weeds have developed resistance. Herbicide-tolerant crops will only work as part of a more comprehensive, Integrated Pest Management plan.
Publication Number:: 8494

hairy fleabane 1
hairy fleabane 1

Posted on Monday, May 6, 2019 at 6:22 AM
Tags: glyphosate (6), herbicides (18), resistance (13), roundup (2), weeds (32)

The Round UP on Glyphosate Use in Tree Crops

UC Ag Experts Talk:

Managing Glyphosate-Resistant Weeds in Orchard Crops

Description: One hour webinar about glyphosate-resistant weed management in orchards, delivered by Dr. Brad Hanson. One CEU (other) from the DPR is approved.

Time: Apr 24, 2019 3:00 PM in Pacific Time (US and Canada)

Recorded version will be published on UC IPM YouTube channel about a week after the webinar.

The link to register is

More information about the webinar series UC Ag Experts talk:


Dr. Brad Hanson
Cooperative Extension Weed Specialist @UC ANR / UC Davis
Dr. Hanson, an associate Cooperative Extension specialist in the Department of Plant Sciences, specializes in weed management in tree and vine cropping systems. Hanson completed his Ph.D. in plant sciences (with an emphasis in weed science) at the University of Idaho and worked as a research agronomist with the USDA-ARS before joining the UC Davis faculty in 2009. His research interests include weeds, weed control, herbicide resistance, weed biology, invasive plants, pest control in fruit and nut crops and other agricultural production systems.

Webinar logo

horseweed bolting
horseweed bolting

Posted on Tuesday, April 16, 2019 at 8:18 AM
Tags: glyphosate (6), herbicides (18), resistance (13), round up (2), weeds (32)

Herbicide Weed Resistance in California?

UCCE Agronomy and Weed Science Advisor, Merced and Madera Counties

Weeds compete with crops for light, water, and nutrients, which can result in yield reductions. Weeds can also interfere with crop production by serving as alternate hosts for pests and pathogens, providing habitat for rodents, and impeding harvest operations, among other impacts. Natural areas can also be impacted by weed species when they reduce aesthetics and disrupt ecosystem services. As a consequence, growers and land managers employ a variety of control strategies, including the application of herbicides, to manage unwanted vegetation.

Although herbicides can be effective tools for controlling undesirable plants, failures can and do occur. Weeds may escape chemical treatments for several reasons including: the selection of an ineffective herbicide or herbicide rate, improperly calibrated or malfunctioning equipment, applications made at a time when the target species is not susceptible to control, the use of herbicides under adverse environmental conditions, and the evolution of herbicide resistance.

As of 3 January 2019, there are 496 confirmed cases (species x site of action) of herbicide resistance, worldwide. Current reports provided by the International Survey of Herbicide Resistant Weeds ( indicate that 255 different species (148 dicots and 107 monocots) have evolved resistance to 163 different herbicides across 23 of 26 known sites of action. With respect to the United States, 161 unique instances of resistance have been documented. Most resistances (52 cases) are to the acetolactate synthase (ALS) inhibitors followed by the photosystem II (PS II) inhibitors (26 cases), 5-enol-pyruvyl-shikimate-3-phosphate synthase (EPSPS) inhibitors (17 cases), and the acetyl-CoA carboxylase (ACCase) inhibitors (15 cases).

Currently, in California, there are 30 confirmed occurrences of herbicide resistance. Twenty-four of those cases are to a single site of action (Table 1). The most frequently encountered resistances have been to the ALS and EPSPS inhibitors (7 each). Five weed species (late watergrass (Echinochloa oryzicola), barnyardgrass (Echinochloa crus-galli ssp. crus-galli), hairy fleabane (Conyza bonariensis), horseweed (Conyza canadensis), and Italian ryegrass (Lolium perenne ssp. multiflorum)) have populations with documented resistance to up to four herbicide sites of action (Table 2).

Growers and land managers can take several actions to thwart the evolution and spread of herbicide resistant weeds. First and foremost is scouting fields following herbicide applications and keeping careful records of herbicide performance to quickly identify repeated instances of weed control failure. Pesticide applicators should ensure that their equipment is properly calibrated and that they are applying effective herbicides at appropriate rates to manage the target species. Whenever possible, diversify herbicides to reduce chemical selection pressure. If appropriate, incorporate physical and cultural weed control practices into a vegetation management plan. Be sure to control unwanted plants when they are small and never allow escapes to set seed. Clean equipment to prevent seeds of herbicide-resistant weed species from moving between infested and non-infested sites and don't forget that unmanaged roadsides, canal banks, fence lines, and field margins, etc., can serve as a source of propagules.



Table 1. A summary of herbicide resistance in California to single sites of action.




Table 2. Weed species in California with confirmed resistance to multiple herbicide sites of action



horseweed bolting
horseweed bolting

Posted on Tuesday, January 29, 2019 at 7:42 PM
  • Author: Lynn M. Sosnoskie, PhD
Tags: resistance (13), weeds (32)

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

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
conyza avocado

Posted on Friday, June 1, 2018 at 9:32 AM
Tags: avocado (287), citrus (335), fleabane (2), hairy (1), herbicide (11), horseweed (6), marestail (4), pesticide (9), resistance (13)

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 (287), citrus (335), grass (1), herbicides (18), lemon (100), mandarin (68), monocots (1), orange (69), resistance (13), ryegrass (1), weeds (32)

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