- 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
X
/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.
- Author: Kurt Hembree
While both horseweed and hairy fleabane have been here since farming began in the region, it’s only since about 2003 that they have become such an obvious problem, particularly in tree and vine systems and non-crop areas.
In the past, the traditional use of combinations of pre- and postemergence herbicides and/or cultivation was adequate to manage them. However, recent changes in environmental regulations, economics, herbicide use patterns (toward more postemergence-only programs), treatment timing, and glyphosate-resistant biotypes have all contributed to the problem. Other factors contributing to their spread include, high seed production, wind dissemination, lack of seed dormancy requirement, preference for undisturbed areas (i.e. tree and vine rows), and adaptability to both moist and dry soils.
To get these weeds back under control, it is important that growers, managers of non-crop areas, and other land owners all do their part to help resolve the issue. Regardless of control tactics used, preventing new seed production is a must to be successful. It is also critical to understand what we’re dealing with when it comes to timing management efforts.
Although considered summer annuals, they have also been seen emerging in early October in the southern San Joaquin Valley (see Figure). During this time, they appear to go through an over-wintering or “survival” stage, where root growth seems more important than leaf production. So, by the time spring emergence occurs in mid-February, plants that actually emerged several months earlier may only appear to look the same as those that just emerged. This may help explain why some late-winter or early-spring applications of postemergence herbicides are not as effective.
Relying only on postemergence products can make horseweed and hairy fleabane very problematic. Consider including effective soil-residual herbicides (Table 1) where possible. Once under control, apply treatments every 2nd or 3rd year to maintain their control. Also consider making split-applications in Oct/Nov and again in Jan/Feb if you have seen them emerge during these periods in your specific area. If you farm in a groundwater protection area (GWPA), you will have to get a permit to use some of these products (refer to your county agricultural commissioner for local GWPA regulations). It is important to know, that while most of the effective materials on these weeds fall under GWPA regulations, they can still be used in many cases and should be considered.
Table 1. Preemergence herbicides for horseweed and hairy fleabane control in tree and vine crops in California |
|
Herbicide |
Notes |
Bromacil (Hyvar X) |
Citrus >4 years, GWPA permit needed, 3-4 lb/A in fall and winter, HW=C, HF=C |
Bromacil + Diuron (Krovar) |
Citrus >3 years, GWPA permit needed, 3 lb/A in fall and winter, HW=C, HF=C |
Diuron (Karmex, Direx, etc.) |
Established fields, GWPA permit needed, 2 lb in fall and winter, HW=P, HF=P |
Isoxaben (Gallery T&V) |
NB fields only, 10.6 oz/A, HW=C, HF=C does not control grasses |
Flumioxazin (Chateau) |
Bearing almond/pistachio/grape, NB others, 6 oz/A fall and winter, HW=C, HF=P |
Norflurazon (Solicam) |
Established fields, GWPA permit needed, 2.5-5 lb, adjust to soil type, HW=P, HF=P |
Oxyfluorfen (Goal, etc.) |
NB citrus, bearing/NB others, 6-8pt/A, HW=P, HF=P |
Simazine (Princep, etc.) |
Established fields, GWPA permit, 2 qt or 2 lb fall + winter, mix w/diuron, HW=C, HF=P |
Thiazopyr (Visor) |
Bearing/NB citrus, NB others, 4 pt/A winter or 2 pt in fall and winter, HW=P, HF=P |
NB = non-bearing only HW = horseweed HF = hairy fleabane C = effective control P = partial control This is not a complete list of registered products available. Check with your pesticide dealer for other products available. It is not a written recommendation for herbicide use. Always read and follow all label recommendations. |
Sensitivity to postemergence herbicides decreases the older horseweed and hairy fleabane get. Use higher label rates of effective materials (Table 2), proper coverage, and treat when they have
Table 2. Effect of glyphosate rate and timing on control |
|
Hairy fleabane growth stage and lb ai/A for good control |
Horseweed growth stage and lb ai/A for good control |
3-6 leaf = 0.5 |
5-8 leaf = 1.0 |
7-12 leaf = 1.0 |
11 leaf to 4” bolted = 2.0 |
13-19 leaf = 1.5 |
4” to 12” bolted = 4.0 |
20-21 leaf = 2.0 |
|
>25 leaf = erratic |
|
Prather, UC KAC 1999 and Shrestha et. al., UC KAC 2005 |
Table 3. Herbicides and rates for control at
Herbicide |
Rate/A |
Rely + AMS |
1 to 1.5 gal |
Gramoxone Inteon, etc. + NIS or COC |
2.0 |
Roundup Weathermax, etc. |
47 fl oz |
Shark EW + COC |
2 fl oz |
2,4-D (Dri Clean, etc.) |
1.55 lb |
Glyphomax, etc. + Chateau |
64 fl oz + 4 oz |
Read and follow label for rates and recommendations. |
Also consider using spray additives (citric acid, ammonium sulfate, spreaders, etc.), if allowed on the label, to improve activity. Tank-mixing various postemergence products can also work well (i.e. glyphosate at 2 lb ai/A plus 2,4-D at 1.5 lb ai/A or Chateau at 2-4 oz/A) are effective treatments. There are numerous products sold in California that contain glyphosate, but they do not all contain the same amount of active ingredient. Read the label carefully to make sure you are using the correct amount of product that will give you a rate of at least 2.0 lb ai/acre (Table 4). If you are using recommended label doses and herbicide timing and are using properly calibrated and operating spray equipment and you still have some of these weeds escaping control, contact your local farm advisor and chemical representative to make sure you do not have an herbicide-resistant biotype. If it is determined that you do, you will need to make changes to your weed management program as soon as possible to eradicate the problem.
In addition to appropriate herbicide selection and use, cultivation can also play an important role in horseweed and hairy fleabane management. Use shallow cultivation to dislodge small plants (
Table 4. Comparative rates of some glyphosate products |
|
Herbicide |
Fl oz for 2.0 lb ai/A |
Touchdown Hitech |
42 |
Roundup Weathermax |
47 |
Touchdown Total |
50 |
Roundup Original |
64 |
Glyphomax |
64 |
Touchdown |
70 |
Managing horseweed and hairy fleabane can seem like a daunting task. However, with the proper selection and use of chemical and mechanical tools, management can be possible. One thing to keep in mind when attacking these two weeds, other weed species may also be waiting for their opportunity once you have got these out of the way. So, it’s a good idea to routinely monitor your fields following each herbicide application and check for any kind of weed escape or shift in the types of weeds present.
Below: Horseweed on top and Hairy Fleabane on bottom.