May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Herbicide resistant weeds have received a lot of attention in recent years. It is often misunderstood. Three of the most misunderstood concepts regarding herbicide resistance are: 1- Weed tolerance and weed selection are not resistance,2- Weed resistance is not universal and does not affect every weed of a certain species from field to field or within a field and weed resistance often takes much longer than insect resistance that is more common and occurs faster.
No Herbicide controls all weeds. Those weeds that are not controlled are tolerant. They never were controlled by that particular herbicide and they are often selected for and become more prevalent over time if the same herbicide is used. Resistant weeds, on the other hand, were controlled at one time by a particular herbicide and have naturally developed a trait that stops the herbicide from working. These resistant weeds survive from generation to generation and become more prevalent over time.
Weed resistance does not occur in all weeds in a field at the same time. It can be just one plant of trillions in a field. As this plant survives the herbicide and goes to seed it becomes more widespread in the field and in other fields. We conducted a trial in Parker last year where sprangletop survived Glyphosate in one field and was killed by the same treatment down the road. If your neighbor has resistant weeds it doesn’t mean that you do too.
Lastly, insect resistance to insecticides has occurred in this region for many years and was the first exposure that many pest control advisers and growers had to pesticide resistance. The principals are the same although insects generally produce multiple generations per season and mutations that facilitate resistance occur faster than for weeds. Annual weeds often produce only one or two generations per season and resistance takes much longer.
With harvesting time getting closer to many field crops now, we are seeing higher incidence of bacterial diseases. We had few reports of bacterial spot on lettuce, cilantro, arugula, and parsley. Most times bacterial symptoms are not expected or simply ignored because we think the desert is too dry and bacterial diseases require high humidity. But as the plants grow bigger the space and aeration in between plants decreases, thus creating a humid microclimate. It is even more common on produces/herbs like cilantro, arugula, parsley etc. where the crops are grown densely, and sprinkle irrigation is used.
Initial symptoms of bacterial leaf spot are water-soaked lesions on leaves. The lesions develop into spots that are varying shades of tan or brown (see picture ‘B’ on parsley whereas advanced spots on cilantro can be black (see picture ‘A’ on cilantro). The lesions are usually limited by leaf veins and thus have an angular, square, or rectangular appearance, a typical feature of bacterial infection. Lesions tend to be relatively small about 1/8 to 1/4 inch (3–6 mm) in diameter and are visible from both the top and bottom of leaves. Under favorable conditions, free moisture from rain or sprinkler irrigation, leaf spots may coalesce and cause considerable blighting of the entire foliage.
Pseudomonas syringae pv. apii (Psa) and P. syringae pv. coriandricola (Psc). cause bacterial leaf spot on most vegetable. Pseudomonas syringae pv. apii (Psa) can cause leaf blight in celery and fennel as well. Though the problem is documented as more of a problem in cilantro and less in celery, in severe condition the disease can result in unmarketable produce in any host. The bacteria can be seedborne. However, water from rain, sprinkler irrigation, and heavy dews and fogs will splash bacteria from infected plants onto adjacent healthy foliage resulting in heavy infestation.
To manage the disease, always use tested/treated seeds, rotate crop with non-host to reduce inoculum level, switch from sprinkler to furrow irrigation to limit secondary spread, avoid excessive use of nitrogen fertilizer. If sprinkle irrigation has to be used, use light and more frequent irrigation, or irrigate in the morning or early afternoon so the plants dry off during the day. Copper spray/copper based fungicide provide limited control against the pathogens.
In next few weeks in the Clinic:
Because of the recent increase of Covid-19 related cases in Yuma, the Yuma Ag center is open only in limited capacity. Samples have to be dropped in the bench outside the main building. Please fill out the form provided when you drop samples. Our last day to take samples in the clinic for 2020 will be 18th of December. We will be using the remaining few days of the year to update the lab records, data, and prepare for 2021. Thank you for your love, support, and patience this year.
The Yuma Plant Health Clinic and Plant Pathology program wishes you safe and happy holidays!
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://220.127.116.11:460
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.
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 late January.
Whitefly: Dult movement is at seasonal low consistent with temperatures and lack of melons or cotton.
Thrips: Activity remains lower than normal for this point late January. Increased movement noted in Roll/Tacna.
Aphids: Seasonal aphid counts peaked during the past 2 weeks, suggesting movement with recent winter storms and lack of desert vegetation. Counts were particularly high in North Yuma and Gila Valleys, and Bard. 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.