Following more than two decades of drought in the western U.S., many watersheds are experiencing critical water shortages.  In the Colorado River basin, the two largest reservoirs, Lakes Powell and Mead, are now at less than 20% capacity.  In contrast, these reservoirs were nearly full in 2000.  With agriculture responsible for 70-80% of the Colorado River water diversions, a great deal of scrutiny is being applied to crop production systems in this region.
 
Crop production systems in the desert Southwest are experiencing increasing pressure to document and improve irrigation efficiency.  There are several ways to define irrigation efficiency.  The primary definitions for irrigation efficiency include orientations with water conveyance and delivery systems (engineering efficiency), and also agronomic, economic, and environmental efficiencies.
 
It comes as no surprise that as an agronomist and soil scientist I tend to focus on the agronomic efficiency of irrigation 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.  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 fundamentals that we can refer to for assistance.  
 
Dr. Jeremy Weiss, program manager for the University of Arizona AZMET system, has recently developed two valuable tools 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.  
 
The first tool provides accumulations of ETc values over the previous week and a range of dates and the planting date selected by the user.  In both models, 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.
 
 

 
 
To access this first crop-water estimate tool please refer to the following link:
 
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates/
 
The second tool provides accumulations of ETc values over the crop production cycle from planting (or the wet date) and the date of harvest.  
 
This second tool gives us the opportunity to estimate agronomic efficiency of irrigation management.  With this tool we can review total crop water use estimates (ETc) and include the leaching requirements (LR) to compare with the irrigation water applied (IWA) as described in Equation 1.
 
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates-post-harvest/
 
For example, using lettuce (either iceberg or romaine) with a 10 October 2022 planting date through 9 January 2023, we can see from this model the reference evaporation, listed as “water use” (ETo) and the cumulative crop evapotranspiration (ETc, or total crop-water use) of lettuce since the selected planting date. 
 
Since planting on 10 October 2022 through 9 January 2023, a total crop water use (evapotranspiration), or the ETc, has been 11.27 ~ 11.3 inches in the Yuma Valley.  We can also determine crop water use (ETc) last week (3-9 January), has been 0.52 inches in the Yuma Valley.
 
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 = 11.3 X 0.2 = 2.26 ~ 2.3 inches
 
Thus, total crop water demand in this case = 11.3 + 2.3 = 13.6 or ~ 14 inches total
 
Contrasting this estimate of total crop-water use with the amount of irrigation water that was applied to the field can provide an estimate of the agronomic efficiency for a given field of lettuce.  We can do the same thing using this tool for other primary leafy green vegetable crops.
 
We are working in times of decreasing water availability and increasing scrutiny in agriculture.  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.”

It’s unfortunately a very great season to be a plant pathologist…

We have confirmed the first sample of Fusarium wilt on lettuce submitted to the Yuma Plant Health Clinic from Yuma County. The stunted seedlings looked like any other typical case of damping-off at the seedling stage. When plated on culture media, subsequently confirmed Fusarium colonies grew abundantly from the declining plant tissues. If you’re not already on guard and scouting, this is a warning that Fusarium is active in Yuma County.

Adding on to this early alert, we’ve received a surge of submissions of young brassicas to the clinic. Several severely wilted and declining plants from around Yuma County have cultured positive for Pythium, likely as an opportunistic invader coming in on the back of all the early-season rain that brought stress to seeds and young transplants. Growers may want to consider oomycides, but only if the seedling disease is first confirmed to be Pythium. Remember, many seedling diseases caused by true fungi are indistinguishable from those caused by Pythium.

If you have any concerns regarding the health of your plants/crops please consider submitting samples to the Yuma Plant Health Clinic for diagnostic service or booking a field visit with me:

 

Chris Detranaltes
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109

Last fall, we established trials investigating the use of band-steam to control weeds and Fusarium wilt of lettuce in iceberg and romaine lettuce. Band-steam is where, prior to planting, steam is injected in narrow bands, centered on the seedline to raise soil temperatures to levels sufficient to kill weed seed and soilborne pathogens (>140 °F for > 20 minutes). After the soil cools (<1 day), the crop is planted into the strips of disinfested soil.

In the study, we utilized the prototype band-steam applicator (Fig. 1) described in a previous UA Veg IPM articles (Vol. 11 (13) to inject steam into the soil as beds are shaped. The steam applicator was configured to treat a 4” wide by 3” deep band of soil. Three rates of steam (Low, Standard, High) were applied by varying travel speed. The “Standard” rate was where steam was applied at rates needed to reach the target soil temperatures (>140 °F for > 20 minutes). Higher and lower applications rate were examined to ensure target temperatures were met/exceeded to get a better understanding of the efficacy of steam treatment, and to determine if higher travel speeds (less fuel consumption) could be used and still provide effective pest control.

Results showed that application of steam was highly effective at controlling weeds (nettleleaf goosefoot predominant species). At the Standard application rate, over 80% of the weeds were controlled. At High application rates, weed control approached 100%. What was particularly encouraging was that at the Low steam application rate where travel speeds were 60% faster than Standard, and target temperatures were not met, weed control was still very good – about 75%.

Steam treatment was also effective at controlling Fusarium wilt of lettuce. Disease incidence in iceberg and romaine lettuce were reduced by more than 50% as compared to the untreated control (Table 2). Crop plants were noticeably larger and more vigorous throughout the growing season in all steam treated plots (Fig. 2). At the Standard and High application rates of steam, this translated into significant yield increases in iceberg (>300%) and romaine (>90%) lettuce. Significant yield increases were also found at the Low application rate of steam – iceberg (>200%) and romaine (>60%).

The results are very promising, but it is important to note that steam treatment is not an end-all cure for Fusarium wilt disease. At the trial site, disease inoculum levels were considered moderate. However, when inoculum levels are very high, our trials have shown that a 4” wide by 3” band of soil is not effective at controlling the disease. We hypothesize that a wider and/or deeper band of treated soil is needed for effective control. This fall, we will be initiating trials to examine this. We will also be investigating the use of band steam to control pythium and nematodes in carrot. Trial results will be presented in future articles - so Stay Tuned.
As always, if you are interested in seeing the machine operate or would like to test the machine on your farm, please feel free to contact me.

Acknowledgements
This work is supported by the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

Fig. 1. Band-steam applicator principally comprising a 35 BHP steam generator mounted on a bed-shaper applicator sled.


Fig. 2. Iceberg lettuce planted in beds treated with steam (left) prior to planting and untreated (right).

Hairy fleabane is a summer annual weed from the Asteraceae family and occasional biennial in our area found in cultivated areas and citrus orchards, alfalfa fields, roadsides and ditch banks, landscapes and around buildings. The seedlings are oval to egg shaped and very similar in size (~0.4-0.8mm) to horseweed (Conyza canadiensis), which makes it very hard to distinguish one from the other in the field. 
Hairy fleabane grows about 4 feet, branches from the bottom and leaves are pubescent also stems are covered with stiff hairs. The growth habits of horseweed (marestail) are different in that they grow up to 10 feet tall and branches from the upper half of the plant.
You can find Fleabane flowering right now in the Yuma area especially the Yuma Mesa. The IPM Team has received calls from our friends PCAs and growers stating that the Fleabane survives the application of Glyphosate, which has been reported for both conyzas. So, we got some trials in progress looking for options to control Fleabane as well as other weeds that have shown tolerance to Roundup such as White sweet clover or malva in different crops.
The results from the evaluations will be shared in this newsletter.

Reference:
1. UC/IPM Retrieved from http://ipm.ucanr.edu/PMG/WEEDS/hairy_fleabane.html

This time of year, John would often highlight Lepidopteran pests in the field and remind us of the importance of rotating insecticide modes of action. With worm pressure present in local crops, it’s a good time to revisit resistance management practices and ensure we’re protecting the effectiveness of these tools for seasons to come. For detailed guidelines, see Insecticide Resistance Management for Beet Armyworm, Cabbage Looper, and Diamondback Moth in Desert Produce Crops .

VegIPM Update Vol. 16, Num. 20

Oct. 1, 2025

Results of pheromone and sticky trap catches below!!

Corn earworm:  CEW moth counts declined across all traps from last collection; average for this time of year.

Beet armyworm: BAW moth increased over the last two weeks; below average for this early produce season.

Cabbage looper: Cabbage looper counts increased in the last two collections; below average for mid-late September.

Diamondback moth: a few DBM moths were caught in the traps; consistent with previous years.

Whitefly:  Adult movement decreased in most locations over the last two weeks, about average for this time of year.

Thrips: Thrips adult activity increased over the last two collections, typical for late September.

Aphids: Aphid movement absent so far; anticipate activity to pick up when winds begin blowing from N-NW.

Leafminers: Adult activity increased over the last two weeks, about average for this time of year.