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.
Last year we had a lot of watermelon fields infected with Fusarium from Winterhaven to Yuma, Wellton, and Mohawk Valley. Rain, and overwatering of fields when plants set fruits might have contributed to the disease development.
Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is one of the oldest described Fusarium wilt diseases and the most economically important disease of watermelon worldwide. It occurs on every continent except Antarctica and new races of the pathogen continue to impact production in many areas around the world. Long-term survival of the pathogen in the soil and the evolution of new races make management of Fusarium wilt difficult.
Symptoms of Fusarium can sometimes be confused with water deficiency, even though there is plenty of water in the field. In Yuma valley we have seen fusarium problem in some overwatered fields.
Initial symptoms often include a dull, gray green appearance of leaves that precedes a loss of turgor pressure and wilting. Wilting is followed by a yellowing of the leaves and finally necrosis. The wilting generally starts with the older leaves and progresses to the younger foliage. Under conditions of high inoculum density or a very susceptible host, the entire plant may wilt and die within a short time. Affected plants that do not die are often stunted and have considerably reduced yields. Under high inoculum pressure, seedlings may damp off as they emerge from the soil.
Initial infection of seedlings usually occurs from chlamydospores (resting structure) that have overwintered in the soil. Chlamydospores germinate and produce infection hyphae that penetrate the root cortex, often where the lateral roots emerge. Infection may be enhanced by wounds or damage to the roots. The fungus colonizes the root cortex and soon invades the xylem tissue, where it produces more mycelia and microconidia. Consequently, the fungus becomes systemic and often can be isolated from tissue well away from the roots. The vascular damage we see in the roots is the defense mechanism of the plant to impede the movement of pathogen.
Disease management include planting clean seeds/transplants, use of resistant cultivars, crop rotation, soil fumigation, soil solarization, grafting, biological control. An integrated approach utilizing two or more methods is required for successful disease management.
There are many innovative automated weeding technologies coming out of Europe. One of these is the autonomous weeding robot being developed by Ecoroboti (Yverdon-les-Bains, Switzerland). The device is lightweight and solar powered. Early prototypes used a spider like, three-axis delta robot to precisely deliver herbicides to target weeds. Videos of the device were futuristic and intriguing to watch. The company has since moved on to a simpler weeding robot equipped with a fixed boom for spot spraying weeds (Fig. 1 & 2). The autonomous robot has some specifications that are plausible for use in Arizona vegetable production. Machine travel speed is 2.2 mph and work rate is 15 acres/day (10 hour day). Spot spray resolution is reasonable at 2.5 inch2 (1.5 x 1.5 inch). This is accomplished using a series of 52 nozzles mounted on an 80 inch wide boom (Fig. 2). The machine uses computer imaging and artificial intelligence for crop/weed differentiation to identify and target weeds.
This past summer, the system was tested in sugar beets in Germany. Results showed the system correctly sprayed about 80% of the weeds. For a first time, real-world, field scale test, this outcome is encouraging.
There are some limitations however. According to product literature, the machine’s artificial intelligence system will identify a crop plant as a weed approximately 5% of the time. Given the high value of vegetable crops, killing 5% of the crop as a trade-off for robotically controlling weeds is probably not viable. It should be noted that this level of crop/weed recognition performance is consistent with other artificial intelligence-based systems reported in the literature.
Don’t give up hope though. This type of technology is advancing rapidly, and may become feasible in the future. Computing speed and sensor capabilities are advancing all the time. A review of literature indicates that systems that combine 3-D morphology, optical color and accurate location data with deep learning techniques may be a viable approach to reliably differentiate crops from weeds. It will be interesting and exciting to watch this technology as it develops. That’s for sure.
As I have mentioned before, automated thinning and weeding technologies are advancing at a very rapid pace. If you know of a new technology that would be of interest and appropriate for this newsletter, please feel free to contact 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://22.214.171.124:460
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.