In desert agriculture we have an abundance of motivation to manage water efficiently.  Water is the first limiting factor in crop production systems in the desert Southwest.  Agriculture is responsible for the use of approximately 70% of all the freshwater use in Arizona and some estimates indicate about 80% of the Colorado River water diversions.  
 
Crop production systems in the desert Southwest are experiencing increasing pressure to document and improve irrigation efficiency.  Following more than two decades of drought in the western U.S., many watersheds are experiencing critical water shortages.  A great deal of scrutiny is being applied to crop production systems in this region.
 
There are several ways to define irrigation efficiency.  The primary definitions for irrigation efficiency include agronomic, economic, and environmental orientations.
 
In my view, the agronomic efficiency of irrigation is fundamental and at the core of why we irrigate, which is for the benefit of the crop being grown.  The crop plants are the centerpiece of the production process and the soil-plant system itself represents the primary objective of irrigation management.
 
Agronomic Irrigation Efficiency
 
Agronomic (crop and soil) considerations are centered on our ability to provide irrigation water for the sustainable production of a crop in the field.  The three primary demands for crop water management include: 1) providing water for seed germination and stand establishment, 2) providing irrigation water to match crop-consumptive water use and avoid crop water stress, and 3) provide sufficient irrigation water to leach soluble salts from the root zone so that the soils can support crop production in a sustainable manner (Figure 1).  
 
Agronomic efficiency at the field level focuses on the crop water demand (CWD) through crop consumptive use (crop evapotranspiration, ETc, which is the combination of evaporation and transpiration from the crop) and leaching requirements (LR) which are dependent upon the crop and salinity of the irrigation water.  
 
Agronomic efficiency can be estimated by considering the difference between ETc + LR = CWD and the volume of irrigation water applied (IWA).


The leaching requirement (LR) can be estimated by use of the following calculation:

Where: 
 
EC_w = salinity of the irrigation water, electrical conductivity (dS/m)
EC_e = critical plant salinity tolerance, electrical conductivity (dS/m)
 
This is a good method of LR calculation that has been utilized extensively and successfully in Arizona and the desert Southwest for many years.  This method considers the quality of the irrigation water being used and the salinity tolerance of the crop.  
 
We can easily determine the salinity of our irrigation waters (EC_w) and we can find the critical plant salinity tolerance level from readily available tabulations of salinity tolerance for many crops (Ayers and Westcot, 1989).  Additional direct references are from Dr. E.V. Maas’ lab at the University of California (Maas, 1984: Maas, 1986; Maas and Grattan, 1999; Maas and Grieve, 1994; and Maas and Hoffman, 1997).  
 
To deal with crop water management at the field level agronomically, the crop and soil factors, there are some good tools that we can easily access for assistance. 
 
Dr. Jeremy Weiss, program manager for the University of Arizona AZMET system, developed some valuable tools last year that provide actual crop evapotranspiration (ETc) estimates from several AZMET sites in the lower Colorado River Valley and for several key vegetable crops, including lettuce (iceberg and romaine), broccoli, cauliflower, cabbage, and spinach.  
 
He has made some nice modifications to this tool and I think you will find the enhancements very useful and beneficial.
 
This tool provides accumulations of ETc values for this current season utilizing the crop planting date selected by the user and any range of time up to the present.  The Kc values presented in FAO-56 are used for appropriate stages with each crop.  The reference evapotranspiration (ETo) measurements are taken directly from each AZMET site listed.  Thus, by this method ETc estimates are made by use of equation 3.  These provide good crop-water use estimates for our crops in this region.


To access this crop-water estimate tool please refer to the following link:
 
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates/
 
If we enter information for a lettuce crop planted on 1 September 2023 through 10 November for the Yuma South site, we get the following information from this tool:
 
“Estimated total water use at the AZMet Yuma South station for lettuce since the planting date of September 1, 2023 and through the end date of November 10, 2023 is 11.47 inches. Total precipitation over this period is 0.40 inches.”
 
Similar estimates are provided by this model for several AZMET weather stations in the Yuma production area: Roll, Yuma North Gila, Yuma South, and the Yuma Valley.  The Yuma Valley station is at the University of Arizona Yuma Agricultural Center and the Yuma South site is near Somerton, Arizona.  A map of AZMET sites can be found in the following link:
 
https://ag.arizona.edu/azmet/mapsites.htm
 
When we include the leaching requirement for the crop, an overall estimate of field level irrigation efficiency can be made.  We can estimate the leaching requirement as follows assuming an electrical conductivity for Colorado River water of 1.1 dS/m and the lettuce crop salinity tolerance of 1.3 dS/m.
 
 Leaching Requirement (LR) = ECw/((5 X ECe)-ECw)
 
LR = 1.1 dS/m / (5 X 1.3) – 1.1 = 1.1/5.4 = 0.20 = 20% leaching requirement
 
LR = 11.5 X 0.2 = 2.3 ~ 2.3 inches
 
If 12 inches of irrigation water was required in this field for total crop consumptive use, then the total crop water demand in this case is consumptive use plus the leaching requirement: 
 
Total crop water demand (in this example) = 12.0 + 2.3 = 13.8 or ~ 14 inches total
 
Considering the total crop-water demand from this AZMET crop-water use tool plus the leaching requirement, we can compare that with the amount of irrigation water that was actually applied to the field.  This can provide an estimate of the agronomic efficiency for a given field of lettuce, or any of the for other leafy green vegetable crops that we commonly grow in this area and select from tool’s menu.  
 
It serves us well to measure and understand the relationship between crop demand (consumptive use plus leaching requirements) versus the irrigation water that is applied to a given field.  
 
References
 Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9), D05109.
 
Ayers, R.S. and D.W. Westcot. 1989 (reprinted 1994). Water quality for agriculture.  FAO Irrigation and Drainage Paper 29 Rev. 1. ISBN 92-5-102263-1. Food and Agriculture Organization of the United Nations Rome, 1985 © FAO.
https://www.fao.org/3/t0234e/t0234e00.htm
 
Erie, L.J., O.A French, D.A. Bucks, and K. Harris. 1981. Consumptive Use of Water by Major Crops in the Southwestern United States.  United States Department of Agriculture, Conservation Research Report No. 29.
 
Khokhar, T. 2017.  World Bank. 
https://blogs.worldbank.org/opendata/chart-globally-70-freshwater-used-agriculture
 
Maas, E.V. 1984. Crop tolerance to salinity.  California Agriculture, October 1984. https://calag.ucanr.edu/archive/?type=pdf&article=ca.v038n10p20
 
Maas, E.V. 1986. Salt tolerance of plants. Appl. Agric. Res., 1, 12-36.
 
Maas, E.V. and S.R. Grattan.  1999. Crop yields as affected by salinity, Agricultural Drainage, Agronomy Monograph No. 38.
 
Maas, E. V., and Grattan, S. R. (1999).  Crop yields as affected by salinity, agricultural
drainage, Agronomy Monograph No. 38, R. W. 
 
Maas, E. V., and Grieve, C. M. 1994. “Salt tolerance of plants at different growth stages,” in Proc., Int. Conf. Current Developments in Salinity and Drought Tolerance of Plants. January 7–11, 1990, Tando Jam, Pakistan, 181–197.
 
Maas, E. V., and Hoffman, G. J. (1977).  “Crop salt tolerance: Current assessment.”

Last week, we initiated our first trial of the season examining the use of band steam for controlling Fusarium wilt of lettuce. The premise behind this research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae (FOL) the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing FOL disease incidence by 45-98% (Matheron and Porchas, 2010).

In our trials, we are using steam heat to raise soil temperatures. Steam is delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). As the device travels through the field, steam is injected into the soil in narrow bands centered on the seedline. Soil temperatures are raised to about 190°F. After cooling (< ½ a day), the crop is planted into the disinfested soil.

Trials conducted in prior years show the technique holds promise; however further work is needed. In these studies, a 4” wide by 2” deep band of soil was treated. Although soil assay results showed that steam treatment significantly reduced the number of Fusarium colony forming units by over 95% as compared to the untreated control, this didn’t always translate into reductions in disease incidence. At one field site with a known history of FOL, band-steam provided no benefit with virtually all lettuce plants succumbing to the disease (Fig 2a). However, at another site, disease incidence was reduced by more than 40%, plants appeared to be healthier and more vigorous, and yield was increased by more than 90% (Fig. 2b). One explanation for this difference is that at the first site, Fusarium inoculum loads were very high and the width and/or depth of soil treated by steam wasn’t large enough to protect seedling roots, which are highly susceptible to FOL, from infection. The aim of this year’s trial is to determine the width and depth of soil that needs to be disinfested to prevent disease. In the current study, we are examining bands of soil receiving steam treatment that range in size from 4” wide by 2” deep to 6” wide by 4” deep. 

Stay tuned for final trial results and reports of our other studies examining the efficacy of using steam heat to control sclerotinia lettuce drop, pythium and weeds.
As always, if you are interested in trying band-steam on your farm or would like more information, please feel free to contact me.

References
Matheron, M. E., & Porchas, M. 2010. Evaluation of soil solarization and flooding as management tools for Fusarium wilt of lettuce. Plant Dis. 94:1323-1328.

Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council.  We greatly appreciate their support.
A special thank you is extended to Fatima Corona, Roberto Delgado, Chad VanMatre, Matt McGuire and JV Farms for their generous assistance and allowing us to conduct this research on their farm.

Fig. 1. Band-steam applicator principally comprising a 35 BHP steam generator
mounted on a bed-shaper applicator sled. The device injects steam into the soil
as beds are formed.



Fig. 2. Lettuce seedlings at field sites with (a) very high and (b)
moderate levels of Fusarium wilt of lettuce inoculum. Band-steam
(left) and untreated (right) plots are shown.

Surveying the Yuma area, we have observed a lot of Hairy Fleabane (Conyza bonariensis) and received calls from PCA's regarding this weed and its control. 
We have observed that the application of Glyphosate is not showing good efficacy in controlling this species in parts of Yuma and Phoenix area. 
Resistance to glyphosate has been reported in some grain growing areas of Queensland and northern New South Wales and other cropping regions across Australia ⁽¹⁾ as well as Spain ⁽²⁾.
Other cases of resistance to other herbicides such as paraquat, and 2,4-D have been confirmed ⁽³⁾.
In the International Herbicide Resistance Weed Database it is reported that in Switzerland that both Conyza canadiensis (Horseweed) and Conyza bonariensis (Hairy Fleabane) presented resistance to a HRAC Group 9 herbicide last year. We found resistance reported in California only and not in Arizona ⁽⁴⁾.

Some of our PCA amigos and field technicians have reported having problems finding a good treatment for fleabane due to possible glyphosate resistance. We included Glufosinate and Embed Extra in a trial last year. The images below show good results of an application of Rely at a high rate (82floz) with two applications at a 2-week interval. The second picture shows the efficacy of Embed Extra (2 pt.) following the same application schedule. Weeds were ~2-3” at the time of application. Some growers have reported good results with glufosinate in Waddell AZ. Sharpen has also been used by Yuma citrus growers.  
A study showed that plants grown at 90% relative humidity presented more translocation of glufosinate than those grown at 35%. Relative humidity had greater effect than temperature on glufosinate toxicity to Palmer amaranth, redroot pigweed, and common waterhemp ⁽⁵⁾. In a trial conducted by Barry Tickes from UA nutsedge control was better in an August application with a ~40% RH than a June application with ~20% RH.
As always please check labels and registration before using these treatments.

Figure 1. Hairy fleabane control with Glufosinate


Figure 2. Hairy Fleabane control with Embed Extra

References:

1. https://bnrc.springeropen.com/articles/10.1186/s42269-020-00316-w

2. https://cdnsciencepub.com/doi/10.1139/cjps-2018-0254

3. https://bioone.org/journals/weed-technology/volume-35/issue-4/wet.2021.11/Cross-resistance-to-diquat-in-glyphosate-paraquat-resistant-hairy-fleabane/10.1017/wet.2021.11.short

4. https://www.weedscience.org/Pages/Species.aspx

5. Coetzer, E., Al-Khatib, K., & Loughin, T. (2001). Glufosinate efficacy, absorption, and translocation in amaranth as affected by relative humidity and temperature. Weed Science, 49(1), 8-13. doi:10.1614/0043-1745(2001)049[0008:GEAATI]2.0.CO;2

 Area wide Insect Trapping Network   VegIPM Update, Vol. 14, No. 21, Oct 18, 2023           
 
Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth increased in Tacna/Roll/Dome last week, and areawide about average for mid-October.
 
Beet armyworm:          
Trap counts highest in Tacna, Wellton and Dome, but below average for October.
 
Cabbage looper:         
Cabbage looper numbers increased in Tacna/Roll/Wellton but still below average.
 
Diamondback moth:    
DBM moths are beginning to appear in traps in Yuma Valley, trending below average for early October.
 
Whitefly:                      
Adult movement increased in the past 2 weeks and above average for mid-October.
 
Thrips:                         
Thrips adult activity down over the past week, and trending below average in October.
 
Aphids:                        
Winged adults continue to be captured for the season, consistent with heavy winds from W-NW. Can expect aphid flights to increase in the coming weeks.
 
Leafminers:                  
Adult activity increased significantly in Tacna associated with maturing melons. Trending above average for mid-October.