May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Malva (malva parviflora) is one of the oldest and most pervasive weeds that that we deal with here. It is also known as little mallow or cheeseweed and is in the same family as cotton, okra and hibiscus. It is often classified as a winter annual but survives all year in this region. It has a deep taproot and can grow in compacted clay or sand and in freezing conditions and high temperatures It provides a refuge for insects and diseases that can damage several crops.
Malva is easy to identify both as a seedling and mature plant. The seedlings are distinctively heart shaped and the mature plant is broad and palm shaped. It is very vegetative and can grow to 6 ft.
The deep tap root of this weed makes it difficult to cut out after it is established. Its response to herbicides id somewhat unusual. It is very sensitive to contact herbicides that do not move into the plant. These include Goal, Sharpen Gramoxone , Rely,Aim and others. However, it is not sensitive to systemic herbicides like 2,4-D and Glyphosate. It reproduces from seed and can be controlled preemergence with many of the same preemergence herbicides used in cotton like Prowl or Treflan. The seed pods are wheel shaped which is where the name cheeseweed comes from. Each seed pod contains 10 to 12 seeds
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.