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.
Downy mildew has always been one of the major problem for PCAs and growers in the desert southwest. The symptoms observed are green to yellow angular spots on the upper surface of the leaves and fluffy growth on the lower side (See Picture). Symptoms usually start from older leaves. As disease progresses the lesion turn brown and dry up and in some occasions, the disease can become systemic causing dark discoloration of vascular tissue.
Favorable condition for disease development:
The pathogen Bremia lactucae thrives in damp, cool condition, with moisture present on leaves. Spores are short-lived but dispersed efficiently by wind during moist period. Cultivated lettuce is the main host of the pathogen but it has also been reported to infectartichoke, cornflower and strawflower.
Why is downy mildew difficult to manage?
One of the main reason that hinders the disease management is the complexity of the pathogen. Bremia lactucae consists of multiple races (pathotypes), and new races continue to occur as pathogen evolves. The pathogen is one of the fastest evolving plant pathogen. And each pathotypes have developed insensitivity to fungicides to different extent.
One of the best practice is to grow resistant cultivar, but there are limitations. As the pathogen is highly variable and dynamic, resistant cultivars are not a permanent solution as the pathogen overcomes the resistance by evolving into virulent strains and isolates.
Preventative application of fungicides are effective to some extent. Reducing leaf wetness and humidity by using drip or furrow irrigation can be helpful. However, weather condition like rain during cool weather as we had in past couple of weeks is conducive to development of epidemics and we have very little control on that matter.
It Takes a Village:
And better yet the whole nation or world! Downy mildew is a bigger problem than we think. It is just not a problem in Arizona and California, it is a nationwide, worldwide problem. Thousands of plant pathologist/scientists/labs are working hard everyday to combat the disease.
Dr. Michelmore’s lab in University of California-Davis has been working on downy mildew for years and they need our help to build the database for next several years. Please let me (Bindu Poudel) or anyone in University of Arizona know if you see symptoms in the field. We can come collect the samples and send it to UC-Davis. The goal is to racetype as many isolates as possible to understand the genetic variability, the more information we have about the pathogen, the more it helps the scientific community come up with better management practice, better resistant cultivars etc.
Learn more about the Bremia Project:
Bremia Project Database:
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.
Results of pheromone and sticky trap catches can be viewed HERE.
Results of pheromone and sticky trap catches can be viewed HERE.
Corn earworm: CEW moth activity increased a bit in the past 2 weeks but remains well below average for late spring.
Beet armyworm: Moth counts increased slightly, but remain very low consistent with seasonal temperatures, and below average for this point in the season.
Cabbage looper: Significant increase in activity in Dome Valley, Gila Valley and Tacna, but moth counts remain unusually low for this time of year, as they have all season.
Whitefly: No adult movement recorded across all locations and overall low numbers consistent with temperatures.
Thrips: Thrips adult movement beginning to pick up considerably, particularly in Yuma and Dome Valleys. Movement is below average for late March.
Aphids: Seasonal aphid counts down considerably compared with the Feb and Jan. Counts highest in Bard and Gila Valley. Below average movement for this time of year. Majority of species found on traps were green peach aphid.
Leafminers: Adult activity up slightly in some locations, but well below average for late season.