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:
Vol. 12, Issue 7, Published 4/7/2021
Over the last several years, there has been a tremendous amount of research activity towards the development of autonomous agriculture vehicles. A quick internet search will reveal over 50 companies or university research groups working in this space. A question I get often from groups developing such platforms is “What is a good agricultural application for our lightweight “robot”?”. It’s a great question, and for Arizona vegetable production, it’s also one that I’m not sure I have a satisfying answer for.
The calls I get regarding autonomous robots are mostly related to automated weeding applications. Automated weeding machines are commercially available, but their adoption has been limited not because of labor costs for tractor operation, rather it is the lack of the development of a functional and cost-effective means for identifying and removing weeds.
For decades, researchers have been attempting to develop sensing systems that are able to reliably detect weeds. Techniques such as 2-D and 3-D color imaging, x-rays, hyperspectral sensing and artificial intelligence have been tried (Slaughter, 2014; Bender et al., 2020). The best performing systems provide about 96% accuracy, meaning that 4% of the crops plants are identified as weeds and would be destroyed by the weeder. For high value vegetable crops like lettuce with gross revenues of roughly $10,000 per acre, killing 4% of the crop equates to $400 per acre of losses. Economically, this does not make sense as hand weeding labor costs are typically $300 per acre or less. The other main issue is that current automated weeding technologies are not highly precise and provide only partial control. Our studies with these types of machines have shown that these systems remove only about 1/3rd of the in-row weeds (Lati, et al., 2016) and a follow up hand weeding operation is often necessary. To be highly cost effective, elimination of the hand weeding step is needed.
In short, my recommendation to research groups asking about applications for autonomous robots is that their time and technical skills would best be served developing reliable crop/weed differentiation systems and a technique to remove a very high percentage of weeds.
Bender, A., Whelan, B. & Sukkarieh, S. 2020. A high‐resolution, multimodal data set for agricultural robotics: A Ladybird's‐eye view of Brassica. J. Field Robotics. 37(1): 73-96.
Lati, R.N, Siemens, M.C., Rachuy, J.S. & Fennimore, S.A. (2016). Intrarow Weed Removal in Broccoli and Transplanted Lettuce with an Intelligent Cultivator. Weed Technology, 30(3), 655-663.
Slaughter, D.C. The biological engineer: Sensing the difference between crops and weeds. Autonomous robotic weed control systems: A review. Computers and Electronics in Agriculture 61(2008): 63-78.
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.