May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Puncturevine is a weed that even the most avid environmentalist has a difficult time liking. Fortunately, it is not a native plant and there are products available to control it. It was brought to California from Asia, Africa and the Mediterranean region and has spread across the county. It can be a serious problem in orchards, turf and on ditch banks. It is a summer annual and will die from frost, but the seed heads are most troublesome when they have matured. Puncturevine (Tribulus terrestris) is in the caltrop family and has several common names including goatheads, Mexican sandbur, caltrop and many other names that cannot be printed here. It is appropriately named because of the hard spike like seed pods that can puncture tires injure animal feet and mouths. It commonly has four spikes that are arranged so that when three of the spikes are on the ground, the fourth will point upward. It can grow in dry areas but thrives in wet summers. The seed heads that you see now will germinate in the pod next spring. One plant can form a dense mat running 20 feet or more and produce up to 5000 seeds. Seeds can survive around 5 years. The most common way that it is controlled in residential areas is by pulling and hoeing. In large landscaped parks, schoolyards etc. herbicides can be useful. Triluralin and Balan applied in the spring prior to emergence will provide partial but not complete control. After it is established the systemic growth regulators like 2,4-D, dicamba and others will work but they will injure other broadleaf plants if they come in contact. Biological control of puncturevine is possible but it might take up to two years to work. There are two weevils: A seed weevil (Miclarinus lareynil) and a stem weevil (Micarlinus lypriformis) that are specific to puncturevine. The seed weevil deposits its eggs in the immature seed head and feed on and destroy the seed before they pupate. The stem weevil lays its eggs in the stems, branches and root crown. They were imported in the mid 1970’s and released in both Arizona and California. Surveys conducted in California indicated that puncturevine decreased by as much as 80% in the years after release. It appeared to be effective in Arizona as well but puncturevine started to build up about 10 years ago and has continued. They both over winter as adults in plant material. Field borders and ditch banks are kept more weed free now than they were in the 60’s and 70’s and overwintering spots have declined. Part of the reason for the increase of puncturevine could be the reduction in overwintering sites. Weevils can be purchased but will be difficult to establish and maintain.
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.
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.