- Author: Tunyalee Martin and Chris Laning
UC Statewide IPM Program
Identifying nontarget crop and ornamental plant damage from herbicides has become much easier with the launch of a new online photo repository by the Statewide IPM Program, University of California Division of Agriculture and Natural Resources.
Herbicides applied to manage weeds may move from the site where it was applied in the air or by attaching to soil particles and traveling as herbicide-contaminated soil. When an herbicide contacts a nontarget plant, a plant it was not intended to contact, it can cause slight to serious injury. Herbicide injury also occurs when the sprayer is not properly cleaned after a previous herbicide application. Herbicide residue can be found in the spray tank, spray lines, pumps, filters and nozzles so a sprayer must be thoroughly cleaned after an application. Dry herbicide particles can be redissolved months later and cause herbicide damage to plants. Economic damage includes reduced yield, poor fruit quality, distorted ornamental or nursery plants, and occasionally plant death.
Accurately diagnosing plants that may have herbicide injuries is difficult. In many cases, herbicide symptoms look very similar to symptoms caused by diseases, nutrient deficiencies, environmental stress and soil compaction. Plant disease symptoms such as mottled foliage, brown spots or stem death and plant pests such as insects or nematodes cause foliage to yellow and reduce plant growth similar to herbicide injury.
Dr. Kassim Al-Khatib, weed science professor at UC Davis and director of the UC Statewide Integrated Pest Management Program (UC IPM), has gathered nearly a thousand photos of herbicide-damaged plants, drawn from his own and others' research. The images are cataloged to show damage that can occur from 81 herbicides in more than 14 specific herbicide modes of action, applied in the field to demonstrate the symptoms or when known herbicide spray has drifted onto the plant.
Each image is characterized with the name of the plant, mode of action of the herbicide, and notes the specific symptoms of damage. Together these photos provide a comprehensive archive of damage to over 120 different crops and ornamental plants by known herbicides, which users can easily compare with what they see in the field.
Also included in the repository is information about the modes of action of various herbicides and an index of example herbicide trade names and active ingredients. Users can learn how unintended injury from herbicide occurs from misapplication and carryover from previous crops in addition to drift and herbicide-contaminated tanks.
The repository can be found at http://herbicidesymptoms.ipm.ucanr.edu. Increased knowledge about what causes herbicide damage and how it occurs can lead to fewer cases of herbicide injury occurring through drift or herbicide-contaminated tanks. Using the repository can increase the skill to correctly identify plant damage. Correctly identifying damage as herbicide injury and not from a plant pest or nutrient deficiency can prevent unnecessary applications of pesticides or fertilizers. Fewer applications can lessen the risk of harm of pesticides and fertilizers to people and the environment.
- Author: Ben Faber
The UC IPM Green Bulletin is a very useful guide to many things pest, weed and disease management. The latest edition is now out.
Check it out. www.ipm.ucdavis.edu/greenbulletin
WHAT'S INSIDE…
Pest Note Updates | Page 2
Understanding Neem- | Page 3
based Pesticides
The Good Side of | Page 4
Pruning
Ask the Expert! | Page 6
SIGN UP…for a free subscription to the Green Bulletin at http://ucanr.edu/subscribegreenbulletin
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/span>- Author: Ben Faber
There is a running debate about whether avocado canopies should be skirted up, raised up so that you can see under the canopy. In doing so, the tree’s tendency is to maintain its bearing volume by increasing a similar amount in height that is lost by removing the bottom layer of canopy. A tree with a full canopy is more cold resistant because it traps heat inside the canopy and is not so prone to cold winds. In an inversion freeze, though, warming air from irrigation, wind machines and orchard heaters is less likely to circulate when the skirts block air movement. A low skirt also impedes a uniform application of water from microsprinklers, and hence fertilizer distribution. A low skirt also has more fruit lying on the ground which is more uneven in coloration and more prone to disease and possibly food safety issues. A raised skirt also promotes more air circulation within the canopy which can reduce the incidence of some other diseases of both fruit, stems and branches.
A raised skirt, though exposes the base to light, and if there is no leaf mulch, there are more weeds to control. In the case of hillsides, because of gravity and wind exposure, leaves tend to blow away. The roots are now more exposed to drying because of increased evaporative loss. Loss of leaves is also a major disease problem, since leaves and organic matter are the first lines of defense (after proper irrigation management) against Phytophthora root rot. It is the microorganisms breaking down the leaves that create a hostile environment for the Phytophthora pathogen. In fact, in releasing enzymes to break down organic matter, the microorganisms also break down the cell walls of Phytophthora which are made of the same material as leaves. An orchard with no leaves is wide open to root rot infection.
So I propose something modest. On flat ground where trees are more prone to frost damage, and less subject to winds blowing away leaves that the trees are skirted. On slopes, though where winds blow away leaves and the trees are less subject to low lying cold, that the skirts are left. To maintain a more even water distribution, though, windows are cut into the canopy on the side facing the microsprinkler so that the canopy does not interfere with water spray.
- Author: W Thomas Lanini
In recent years, several organic herbicide products have appeared on the market. These include Weed Pharm (20% acetic acid), C-Cide (5% citric acid), GreenMatch (55% d-limonene), Matratec (50% clove oil), WeedZap (45% clove oil + 45% cinnamon oil), and GreenMatch EX (50% lemongrass oil), among others. These products are all contact-type herbicides and will damage any green vegetation they contact, though they are safe as directed sprays against woody stems and trunks. These herbicides kill weeds that have emerged, but have no residual activity on those emerging subsequently. Additionally, these herbicides can burn back the tops of perennial weeds, but perennial weeds recover quickly.
These products are effective in controlling weeds when the weeds are small and the environmental conditions are optimum. In a recent study, we found that weeds in the cotyledon or first true leaf stage were much easier to control than older weeds. Broadleaf weeds were also found to be easier to control than grasses, possibly due to the location of the growing point (at or below the soil surface for grasses), or the orientation of the leaves (horizontal for most broadleaf weeds).
Organic herbicides only kill contacted tissue; thus, good coverage is essential. In test comparing various spray volumes and product concentrations, we found that high concentrations at low spray volumes (20% concentration in 35 gallons per acre) were less effective than lower concentrations at high spray volumes (10% concentration in 70 gallons per acre). Applying these materials through a green sprayer (only living plants are treated), can reduce the amount of material and the overall cost
http://www.ntechindustries.com/weedseeker-home.html
Adding an organically acceptable adjuvant has resulted in improved control. Among the organic adjuvants tested thus far, Natural wet, Nu Film P, Nu Film 17, and Silwet ECO spreader have performed the best. The Silwet ECO spreader is an organic silicone adjuvant which works very well on most broadleaf weeds, but tends to roll off of grass weeds. The Natural wet, Nu Film 17 and Nu Film P work well for both broadleaf and grass weeds. Although the recommended rates of these adjuvants is 0.25 % v/v, we have found that increasing the adjuvant concentration up to 1% v/v often leads to improved weed control, possibly due to better coverage. Work continues in this area, as manufacturers continue to develop more organic adjuvants. Because organic herbicides lack residual activity, repeat applications will be needed to control new flushes of weeds.
Temperature and sunlight have both been suggested as factors affecting organic herbicide efficacy. In several field studies, we have observed that organic herbicides work better when temperatures are above 75F. Weed Pharm (acetic acid) is the exception, working well at temperatures as low as 55F. Sunlight has also been suggested as an important factor for effective weed control. Anecdotal reports indicate that control is better in full sunlight. However, in a greenhouse test using shade cloth to block 70% of the light, it was found that weed control with WeedZap improved in shaded conditions. The greenhouse temperature was around 80F. It may be that under warm temperatures, sunlight is less of a factor.
Organic herbicides are expensive at this time and may not be affordable for commercial crop production. Because these materials lack residual activity, repeat applications will be needed to control perennial weeds or new flushes of weed seedlings. Finally, approval by one's organic certifier should also be checked in advance as use of such alternative herbicides is not cleared by all agencies.
- Author: Oleg Daugovish
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.