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.
Mark C. Siemens
Vol. 12, Issue 9, Published 5/5/2021
Automated thinning machines have been commercially available since 2012. These machines identify crop plants and intermittently deliver an herbicidal spray or dose of liquid fertilizer to thin the stand to the desired plant spacing. Some growers have converted older machines to spot apply pesticides to crop plants rather than thin lettuce. Spot spraying just the crop plant makes sense – it reduces applied chemical amount by about 1/3rd as compared to band spraying and by roughly 90% as compared to broadcast. I have heard reports of improved efficacy with this technique, perhaps due to better coverage, however this potential benefit has not been validated in formal trials.
A drawback with automated thinning machines is their high cost. Retail prices for machines are approximately $25,000 per seed line, or about $200,000 for a 4-row, 2-line machine. Another option might be to use automated systems designed for spot spraying weeds. These devices have been commercially available since the mid 90’s and function similarly to automated thinning machines in that they use optical sensors to detect plants and solenoid activated spray assemblies to intermittingly spray unwanted plants (Fig. 1). The cost of these devices is quite reasonable – about $3,000 per unit, or about $24,000 for a 4-row, 2-line machine.
Automated spot sprayers are typically used in agriculture to control weeds in fallow fields (Fig. 2), but could easily be adapted to apply pesticides or even fertilizer to vegetable crops. Spot applying foliar fertilizers to vegetable crops is an interesting concept and is being investigated in California with lettuce.
Another potential use of spot sprayers is to control herbicide resistant weeds. The device can be positioned between crop rows to spot spray a non-selective herbicide to target weeds. Placing the sprayer in a hooded enclosure prevents unwanted drift onto crop plants. We are conducting trials using this technique in cotton this season (Fig. 3). We are also looking for collaborators interested in trying the device as a pesticide and/or fertilizer spot applicator in vegetable crops for this upcoming season. If you are interested collaborating or would like to see a demo of the device, please feel free to reach out to me.