May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
This is the time of year when people start to regret not controlling weeds earlier in wheat. Uncontrolled weeds begin to become more visible and both the crops and weeds are too big for most herbicides. The weeds not only produce millions of little time bombs (seeds) but can contaminate the crop, increase moisture and make harvest difficult. There are some herbicide options at this point, but none are great and all of them can only be applied when the wheat is at least in the hard dough stage and the crop is essentially done.
If the wheat is still developing both Simplicity and Osprey will control some big broadleaf and grass weeds. They may not be killed but they likely will stop growing. They can only be applied up to the jointing stage and most vegetables cannot be planted for 5 to 9 months following simplicity or 10 months following Osprey. The growth regulators (2,4-D, dicamba, MCPA and others) can only be applied up to tillering or the heads could be distorted and yield reduced.
All of the preharvest herbicide options can only be applied when the wheat is in the hard dough stage and the crop is essentially finished. So, if the wheat is too far along now, you likely will have to wait. A few of the options include:
Do not use on wheat grown for seed. There is a zero tolerance for this herbicide in wheat for export to some countries and it also has the potential to reduce seed germ. This may be an option, however, for grain intended for other uses that is seriously contaminated with weeds. Applying glyphosate too early will damage the crop. There is a preharvest interval of 14 days and this treatment is likely to be slow. Application will have to be by air and drift should be avoided as glyphosate is none selective.
This is a contact herbicide and will work as a desiccant without systemic activity. There is a preharvest interval of 3 days and results can be slow. Systemic activity is not necessary since this will be used as a burndown harvest aid treatment. Paraquat is not registered for wheat.
Growth Regulators (2,4-D, Clarity, Dicamba)
Drift Hazard These are highly systemic and volatile. They will need to be applied by air and will be difficult to use after March. May be an option in isolated areas. There is a 7 to 14 day preharvest interval and the plant back interval can be from 1 to 6 months to many vegetable crops. It will not kill grasses such as volunteer sudan or Johnsongrass.
This is a contact, fast acting herbicide similar to Aim or Shark. It will not control grasses and has a preharvest interval of 3 days. Some markets will not accept grain treated with Sharpen so check first.
This is a systemic that will not work well if the weeds are stressed. It only controls broadleaf weeds.
Preharvest control of grasses such as sudan, summer and winter annuals. Most of the grass herbicides used in wheat (Discover, Tacoma, Simplicity and Osprey) are restricted to application before the boot stage of the wheat and have a plant back restriction of from 1 to 10 months. The use of the postemergence grass herbicides such as Clethodim and sethoxydim, will control most grasses but could reduce the viability of the wheat seed.
In the past couple of weeks, the reports of INSV in fields has increased dramatically. INSV has been found in fields in Yuma/Gila Valley, Wellton, Tacna, Roll, and Imperial Valley.
PCAs have reported thrips pressure as low this year and most fields have infection less than 1% but some fields have been reported to have higher incidence. The virus has been detected in direct seeded field as well as transplants imported from Salinas, CA.
Impatiens necrotic spot virus, also known as INSV is a tospovirus closely related to Tomato spotted wilt virus. Infected plants usually have leaves with brown to dark brown necrotic areas. Sometimes the symptoms may be confused with “chemical burn”. As necrosis progresses the leaf browns or die out. Plants infected in early stage may become stunted and die, or become unmarketable.
What makes this virus of high economic importance?
The plants become unmarketable which is the ultimate economic loss. But there are factors that facilitate the virus outbreak.
The first one is efficient transmission by its vector (s). The virus is transmitted by western flower thrips, Frankliniella occidentalis.
If you remember our virus transmission series in past newsletters, thrips transmit viruses in persistent propagative manner. Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus throughout its lifespan. The virus can multiply in the vector system and often times the virus particles are also passed on to the insect offspring. Adult thrips can transmit these viruses only if acquired in the larval stage of development. Larval thrips will feed on a virus-infected plant, pupate, and emerge as a winged adult capable of transmitting the virus. The thrips then will carry the virus for life.
The next contributing factor is host range. INSV infects large number of ornamental and vegetable plants. We are talking 600 species of plants that are susceptible to INSV and thrips love flowers.
While it may not be practical to remove all your ornamentals in fear of INSV, it is definitely practical to monitor thrips population in your field. As the legend says “When in doubt, scout”.
And if you need diagnosis, drop the samples in the clinic! But then if you have immunostrips, you don’t have to make the drive to the Ag Center!
DIY testing: Impatiens necrotic spot virus (INSV)
As visual diagnosis of the virus is confusing and could even be misleading at times, it is very important to confirm a symptomatology via clinical diagnosis.
The good news is there are tools available for quick and easy diagnosis of INSV. You can order the immunostrips from Agdia (https://orders.agdia.com/agdia-immunostrip-for-insv-isk-20501)
The immunostrips cost anywhere from $5-20 depending on how much you buy. They perform better when they stay refrigerated until just before use.
Immunostrips are quick and easy tool to use. The kit comes with a buffer bag and immunostrip.
One band means that the positive control worked which means the system worked. Sometimes you see no bands at all. This means the system did not work and you have to repeat the test.
If you are seeing symptoms in your field please let Bindu Poudel-Ward know via email (email@example.com or text (928-920-1110). Please keep a note of weed species you are consistently seeing in your fields and keep the thrips population under check.
Mark C. Siemens
Vol. 12, Issue 9, Published 5/5/2021
Automated thinning machines have been commercially available since 2012. These machines identify crop plants and intermittently deliver an herbicidal spray or dose of liquid fertilizer to thin the stand to the desired plant spacing. Some growers have converted older machines to spot apply pesticides to crop plants rather than thin lettuce. Spot spraying just the crop plant makes sense – it reduces applied chemical amount by about 1/3rd as compared to band spraying and by roughly 90% as compared to broadcast. I have heard reports of improved efficacy with this technique, perhaps due to better coverage, however this potential benefit has not been validated in formal trials.
A drawback with automated thinning machines is their high cost. Retail prices for machines are approximately $25,000 per seed line, or about $200,000 for a 4-row, 2-line machine. Another option might be to use automated systems designed for spot spraying weeds. These devices have been commercially available since the mid 90’s and function similarly to automated thinning machines in that they use optical sensors to detect plants and solenoid activated spray assemblies to intermittingly spray unwanted plants (Fig. 1). The cost of these devices is quite reasonable – about $3,000 per unit, or about $24,000 for a 4-row, 2-line machine.
Automated spot sprayers are typically used in agriculture to control weeds in fallow fields (Fig. 2), but could easily be adapted to apply pesticides or even fertilizer to vegetable crops. Spot applying foliar fertilizers to vegetable crops is an interesting concept and is being investigated in California with lettuce.
Another potential use of spot sprayers is to control herbicide resistant weeds. The device can be positioned between crop rows to spot spray a non-selective herbicide to target weeds. Placing the sprayer in a hooded enclosure prevents unwanted drift onto crop plants. We are conducting trials using this technique in cotton this season (Fig. 3). We are also looking for collaborators interested in trying the device as a pesticide and/or fertilizer spot applicator in vegetable crops for this upcoming season. If you are interested collaborating or would like to see a demo of the device, please feel free to reach out to me.
Corn earworm: First significant CEW moth activity since mid-November; particularly active in Dome/Wellton/Tacna areas.
Beet armyworm: Moth counts remain very low consistent with seasonal temperatures, but below average for this point in the season.
Cabbage looper: Slight increase in activity, but moth counts remain unusually low for this time of season.
Whitefly: Adult movement is at seasonal low consistent with temperatures and lack of melons or cotton.
Thrips: Thrips activity beginning to pick up, particularly in Tacna and Yuma Valley. Movement is still below average for February.
Aphids: Seasonal aphid counts peaked in early February and tending down last week. Counts remain high in Gila Valley and Wellton. Above average for this time of year.
Leafminers: Adult activity remains light in most trap locations. Trap counts increasing slightly in the South Gila Valley.