May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Clovers can be very difficult to control weeds here, but it is also a major crop and common ornamental. Clovers can survive under poor growing conditions and are not controlled with glyphosate and seem to get worse every year. There are more than 50 types and 300 species of clover and they can be easily misidentified. They are all in the legume (Fabracea) family and can use a bacterium (rhizobium) in the soil to convert nitrogen in the atmosphere to a form that they and other plants can use for fertilizer. There are only 4 or 5 clover species that are agricultural pests here. The ones we get the most questions on are white and yellow sweet clover. These are in the Melilotus family. White sweet clover (Melilotus albus) is tall for a clover and can get 3 to 5 foot in height. The leaves are thinner than most clovers and this difficult to control weed lives at least 2 years and sometimes longer. Glyphosate and most of the contact herbicides do not control it. The plant growth regulator herbicides work best. Yellow sweet clover (Melilotus officinalis) is less common here. The flowers are yellow, and it is not as tall and vegetative as white sweet clover. Yellow is more common at higher elevations. California burclover (Medicago polymorpha) and Black medic (Medicago lupina) are in the same genus as alfalfa and are more of a problem in landscapes, parks and golf courses than in agricultural fields here. They do not grow upright and spread below the crop or turf. The true clovers are in the Trifolium genus and include white and strawberry clover. These creep along the ground and root at the nodes of the stem. These are more of a urban landscape weed and not considered an agricultural problem. Creeping woodsorrel or Oxyalis looks like a clover but it is not related. It is a turf weed that spreads rapidly along the ground and can live for several years. Preemergent herbicides are effective against all these clovers before they become established. The postemergence herbicides that are most effective in controlling these clovers are the plant growth regulators. Contact herbicides and glyphosate are generally ineffective.
It is about that time of the year/growing season when you start seeing bacterial diseases. With the rain we got last week and as plants get more vegetative growth bacterial issues become more prevalent. Cilantro and parsley are two crops grown in desert southwest that often suffer from bacterial leaf spots. Most times, the disease incidence is also high because of sprinkler irrigation used in these cropS. On both crops, 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 parsley and cilantro. 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 are likely seedborne in both crops. 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. Copper spray/copper based fungicide provide limited control against the pathogens.
Weed escapes are easy to spot in vegetable fields at harvest time. Some growers have these weeds pulled, bagged and removed by hand from the field because they are unsightly and to reduce seedbank loads. This can be a costly operation. An alternative solution might be to use high voltage electricity to kill these weeds. The idea of using electricity to “zap” weeds is not new. Machines for agriculture applications were developed decades ago and commercially available in the late 1970’s. Although the devices worked, they were not widely adopted due in part to the availability of low cost and efficacious herbicides.
Because of environmental concerns, herbicide resistant weed issues and increased organic production, non-chemical, high voltage weed control technology is seeing a resurgence. There are now five companies, three established within the last four years, offering or developing machines for commercial agriculture. Although configurations differ, all machines operate using the same principles. To explain, consider the example of the machine shown in Fig. 1. The unit comprises high voltage electrodes (8-15 kV) positioned above the crop canopy, an electric generator and a soil engaging coulter connected to ground. During operation, when an electrode touches a weed protruding above the canopy, current flows through the plant back to the generator via the ground contacting coulter. Current flow combined with electrical resistance in the plant causes rapid heating and plant fluids to vaporize. This ruptures cell walls and kills the plant. Although there are few recent reports in the literature, prior research on dated machines showed that the technique can provide better than 98% weed control in moderate weed densities (15,000 weeds/acre) at travel speeds of 2 mph (Diprose & Benson, 1984).
Modern approaches that utilize high voltage electricity in combination with smart machines to spot treat weeds are being developed. The idea is to use camera imagery and artificial intelligence to locate weeds and high voltage electricity to kill them. One such machine being developed by the MASCOR Institute1 and the Zasso Group is an autonomous robot equipped with cameras, on-board computers and robotic arms (Fig. 2). As the machine moves through the field, high voltage electrodes mounted on the movable, computer controlled robotic arms zap weeds. Another unit is being developed by Stekettee and RootWave. It is tractor pulled and designed to travel at 3 mph. Stekettee’s machine vision system identifies the weeds and RootWave’s high voltage electric technology shocks the weed with a pulsed 5 kV charge. Power is supplied by a generator connected to the tractor’s PTO. Both systems are in late stages of development with field tests conducted in 2020.
These systems appear promising and if they prove to be effective and economical, may be something to look for in the future.
1Reference to a product or company is for specific information only and does not endorse or recommend that product or company to the exclusion of others that may be suitable.
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.
Beet armyworm: Moth activity has declined in most trap location and remains below average for this point in the season.
Cabbage looper: Cabbage looper activity remains unusually low for early November. Trap catches picked up a bit in Wellton and Gila Valley.
Whitefly: Adult movement has been about average for this time of year. Activity highest in Wellton near fall melons being harvested.
Thrips: Thrips activity picking up significantly in several trap locations; activity increased significantly in Bard and Dome and Gila Valleys.
Aphids: Aphid numbers peaked last week in many locations, particularly in the Gilas and Yuma Valleys.
Leafminers: Adult activity increased significantly last week, particularly in the Wellton area near melons.