- Author: Giuliano Carneiro Galdi
- Posted by: Gale Perez
Roundup Ready technology incorporates genetic resistance to glyphosate into crops and it's an excellent tool for weed control. Initial screening in the early 2000s found good alfalfa crop safety, and many growers currently rely on glyphosate as the only herbicide. Although using the same herbicide over and over is not a good idea because it may accelerate herbicide resistance in weeds, Roundup Ready alfalfa has been successfully used with few to no concerns regarding crop safety. However, the combination of glyphosate and cold weather may cause crop injury, especially in certain regions.
The problem was first observed in 2014 by Steve Orloff, former UCCE farm advisor in Siskiyou County. A Roundup Ready alfalfa field showed injury after glyphosate application followed by a frost event. The main symptoms were plant stunting, chlorosis, and “shepherd's crook”, in which individual alfalfa stems curl over and die (Figures 1 and 2). Yield reductions were also observed for the first cutting. It was clear to Steve the injury was somehow related to the glyphosate application because a section of the field where the wheel line was sitting was not sprayed and it looked perfectly normal.
Interestingly, the injury seen was very similar to symptoms caused by frost and/or bacterial blight caused by Pseudomona syringae – a bacteria that is present everywhere. The bacteria can exacerbate frost damage due to its protein that mimics a crystalline structure and provides a starting point for ice formation, damaging the plant tissue and serving as an entrance port into the leaves and stems. Once into the plant tissue, colonization leads to infection and symptoms about 7-10 days after the frost event. Symptoms on stems start as water-soaked lesions that extend down one side. Leaves become water-soaked and often are twisted and deformed.
Steve was able to replicate the symptoms in field trials conducted in 2015, 2016, and 2017. The field trials showed yield reductions of up to 0.7 ton/acre in the first cutting in Scott Valley. Crop injury was not observed in a similar field trial conducted in 2014, probably because there was no frost event after glyphosate application. Similar impacts were observed in a trial in the intermountain region near Tulelake, CA; additional yield reductions were observed with higher glyphosate rates (Table 1).
Based on this work, a multi-year project started to investigate the effects of glyphosate rate and application timing during various seasons at 24 sites over five years, measuring the impact on alfalfa crop height and biomass yield. Results were published in the Agronomy Journal in 2023 (Loveland et al, 2023). All sites in this study were in the Intermountain West (California and Utah). Results showed that while summer glyphosate application did not injure alfalfa, spring applications reduced crop height at 76% of the sites and biomass at 62% of the sites. In sites where glyphosate application resulted in crop injury, low (22 oz/A) and high (44 oz/A) rates of glyphosate reduced yields by 0.24 ton/A and 0.47 ton/A, respectively. Data also show that the crop height at glyphosate application influenced the degree of injury (Figure 3), with greater yield reductions at 30 – 40 cm than at 5 to 10 cm height.
As alarming as the possibility of injury might sound, its occurrence and degree are widely variable, and most fields will resume normal growth and yields after first cutting. Figure 4 illustrates this complexity and variability throughout experimental sites where harvest yield and crop height were assessed. Note that the locations represented in the following graph are colder than the San Joaquin Valley and the injury happened after glyphosate applications in the spring.
As of 2024, this type of injury has only been reported in the intermountain west due to its high altitude and cooler weather. However, one field I visited in early February in Firebaugh brought my attention back to the issue. The field was planted in the fall of 2023 and had many of the symptoms described above: plant stunting, typical shepherd's crook, chlorosis, and dead stems. While all these symptoms could be exclusively due to bacterial blight infections or frost, parts of the field where glyphosate application was accidentally skipped looked better.
Current UC IPM weed management guidelines for RR alfalfa recommend rotating herbicides with different modes of action to reduce the development of herbicide resistant weeds and avoid glyphosate overuse during the colder winter months. Second, spray glyphosate when the alfalfa is short (< 2”) when using the higher rate (44oz/A) or 4” when spraying at the lower rate (22oz/A). Third, use the lowest glyphosate rate possible according to the weeds present and their stage of development. Finally, pay attention to the weather forecast. Applying glyphosate before frost events increases the likelihood of crop injury, especially in old stands.
Reference
Loveland, L.C., Orloff, S.B., Yost, M.A., Bohle, M., Galdi, G.C., Getts, T., Putnam, D.H., Ransom, C.V., Samac, D.A., Wilson, R., and Creech, J.E. (2023). Glyphosate-resistant alfalfa can exhibit injury after glyphosate application in the Intermountain West. Agronomy Journal, 115, 1827–1841. https://doi.org/10.1002/agj2.21352.
- Author: Travis Bean
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 |
1 |
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 |
3 |
Mitosis Inhibitors |
Inhibits cell division in germinating seedlings and lateral roots |
Oryzalin (Surflan) |
Thickened, shortened lower stems and small, crinkled leaves |
5 |
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 |
9 |
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 |
12 |
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
|
14 |
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. |
21 |
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 |
22 |
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 http://herbicidesymptoms.ipm.ucanr.edu.
References:
Al-Khatib, K. 2015. University of California Integrated Pest Management Herbicide Symptoms. http://herbicidesymptoms.ipm.ucanr.edu (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. http://ipm.ucanr.edu/PMG/selectnewpest.avocado.html (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. http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=9383 (accessed 09/05/2018)
Weed Science Society of America. Summary of Herbicide Mechanism of Action
According to the Weed Science Society of America. https://wssa.net/wp-content/uploads/WSSA-Mechanism-of-Action.pdf (accessed 09/05/2018)
Photo: Sometimes weeds are tasty, like amaranth and purslane