May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
The off-target movement of Dicamba and 2,4-D that was applied to resistant cotton and soybeans in the Midwest and south has been in the news for the last few years. It has exploded recently with the cancellation of these uses by a Federal court and the EPA. There have
not been serious problems with these herbicides here in Arizona. The focus of this article is to explain why not.
Much of the cotton, corn and soybeans grown in the U.S. was Glyphosate resistant for several years. The number of Glyphosate resistant weeds have increased every year and new concerns about the effect of Glyphosate on human health and environmental safety have arisen. In response to this problem, new varieties of cotton, corn and soybeans have been developed that are resistant to a couple of old but highly effective herbicides, Dicamba and 2,4-D. These herbicides have high vapor pressures and can volatilize after application. They change from a liquid or solid to a gas after application and can move, sometimes long distances, in the air. Any herbicide can drift onto sensitive crops during application and cause injury. This is different than volatilization. New formulations of 2,4-D and Dicamba have been developed, however, that have significantly lower volatility. Studies have shown that while the potential is lower that it still can occur. Volatilization of both Dicamba and 2,4-D have caused widespread problems to field crops, trees, parks, school yards, landscapes… to cause “Silent Spring “type conditions in the Midwest and south. Volatility problems occasionally occur but widespread and serious problems have not been encountered in Arizona.
The extreme conditions that exist during the summer in the low deserts of Arizona all contribute to herbicide volatility. High temperature, low humidity and the occurrence of temperature inversions have always made it difficult to use volatile herbicides here. Growers and Pest Control Advisers have learned to be cautious when using these products. They are rarely used after daytime temperatures go much above 90 degrees.
Acreage and Crop Diversity
Crops are grown in Arizona on a smaller scale and more intensively than they are in the Midwest and south. This is especially true in the southwestern counties. The acreage of cotton in 2019, for instance, was 173,000 acres in Arizona,4,350,000 in Texas,1,305,000 in Georgia,550,000 in Oklahoma and 497,000 in Alabama. This smaller scale allows Arizona growers to practice more careful management. Sprayers are cleaned more carefully or dedicated to spraying only volatile products. Fields and surrounding area are checked more frequently. In high acreage states, tens of thousands of acres can be treated at the same time with the same products. The amount of herbicide in the environment at those times is very high. In Arizona not only are crops grown on a smaller scale, but they are more diversified. It is not uncommon to see 3 ,4 or more different crops being grown at the same time on a 20 acre block. When fields are sprayed it is done very carefully.
Newer formulations and Varieties
New formulations of both Dicamba and 2,4-D have been developed that are much less volatile than the old formulations. Although some of these are promoted as non-volatile, they can still move. Some studies have shown that they are 30 to 50% more stable. It will be variable and dependent on many factors. It is important to choose those cotton varieties that have been developed to tolerate Dicamba or 2,4-D. According to Randy Norton, U of Az. Cotton Specialist, upwards of 60% of the cotton varieties being grown in Arizona this year are Dicamba resistant. Randy states that these varieties were selected for their yield and lint quality more so than their resistance to Dicamba.
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.
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://188.8.131.52:460
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.