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.
Last year we had a lot of watermelon fields infected with Fusarium from Winterhaven to Yuma, Wellton, and Mohawk Valley. Rain, and overwatering of fields when plants set fruits might have contributed to the disease development.
Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is one of the oldest described Fusarium wilt diseases and the most economically important disease of watermelon worldwide. It occurs on every continent except Antarctica and new races of the pathogen continue to impact production in many areas around the world. Long-term survival of the pathogen in the soil and the evolution of new races make management of Fusarium wilt difficult.
Symptoms of Fusarium can sometimes be confused with water deficiency, even though there is plenty of water in the field. In Yuma valley we have seen fusarium problem in some overwatered fields.
Initial symptoms often include a dull, gray green appearance of leaves that precedes a loss of turgor pressure and wilting. Wilting is followed by a yellowing of the leaves and finally necrosis. The wilting generally starts with the older leaves and progresses to the younger foliage. Under conditions of high inoculum density or a very susceptible host, the entire plant may wilt and die within a short time. Affected plants that do not die are often stunted and have considerably reduced yields. Under high inoculum pressure, seedlings may damp off as they emerge from the soil.
Initial infection of seedlings usually occurs from chlamydospores (resting structure) that have overwintered in the soil. Chlamydospores germinate and produce infection hyphae that penetrate the root cortex, often where the lateral roots emerge. Infection may be enhanced by wounds or damage to the roots. The fungus colonizes the root cortex and soon invades the xylem tissue, where it produces more mycelia and microconidia. Consequently, the fungus becomes systemic and often can be isolated from tissue well away from the roots. The vascular damage we see in the roots is the defense mechanism of the plant to impede the movement of pathogen.
Disease management include planting clean seeds/transplants, use of resistant cultivars, crop rotation, soil fumigation, soil solarization, grafting, biological control. An integrated approach utilizing two or more methods is required for successful disease management.
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.
The Yuma County Leaf Wetness Network remains in place for the 2018/19 vegetable season. Growers and PCAs may access information generated by the network by entering the following internet address: http://188.8.131.52:460
Upon entering the address above, you will be transferred to internet page that provides a series of tabs at the top of the page. Simply click on the tabs to access the information of interest.