After more than 22 years of severe drought, watersheds across western North America are experiencing critical water levels that threaten agricultural and domestic supplies.  The Colorado River shortage is emblematic of these shortages.
 
These external factors are significantly impacting crop production systems at the field level in the desert Southwest.  Irrigation districts in central Arizona, particularly Pinal County, have lost access to Colorado River water allocations via the Central Arizona Project because of Tier 1 and Tier 2a reductions from the Drought Contingency Plan.  Reductions have also been imposed on Nevada and Mexico allocations.  Water levels in Lakes Powell and Mead are at less than 25% capacity and continue to decline due to insufficient action to arrest this worsening situation.
 
Since diversions to agriculture represent approximately 80% of the Colorado River water use, the basic arithmetic on the balancing of the river’s water budget will depend significantly on how agricultural diversions are ultimately managed in the process.  Further reductions of Colorado River water diversions to agricultural districts seems rather inevitable but many uncertainties remain with discussions and negotiations ongoing.
 
Water is the first and most limiting agronomic factor in crop production.  To deal with these challenging conditions from the water shortages 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 a valuable tool that provides 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.  Accumulations of ETc values over the previous week and a range of dates are shown based on a date of planting, which is selected by the user.  In this model, 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.
 
To access this crop-water estimate tool please refer to the following link:
 
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates/
 
For example, using lettuce (either iceberg or romaine) with a 1 November planting date through 11 December, 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 1 November through 11 December, a total crop water use, or the ETc, has been 3.57 inches in the Yuma Valley.  Last week, from 5-11 December, ETc has been 0.56 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 = 9.8 X 0.2 = 1.96 ~ 2 inches
 
Thus, total crop water demand in this case = 10 + 2 = 12 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. 

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

Want to see this Prototype Band-Steam Applicator in action? Click on the link or the image

https://www.youtube.com/watch?v=AEY4wHo7vgo

Last Thursday April 28, 2022 the EPA issued a notice of intent to suspend (NOITS) DCPA, which when effective, will prevent the sale, distribution, and use of the technical-grade product containing the pesticide dimethyl tetrachloroterephthalate (DCPA).
Please see all details of the NOITS by clicking the following link:
https://www.epa.gov/pesticides/epa-issues-notice-intent-suspend-herbicide-dcpa
 
Dacthal was first registered in 1958^[1] and we are constantly adjusting it to our changing cultural practices.  Recently some evaluations were done of Dacthal’s safety to iceberg lettuce applied Pre and Post-Transplanting. The product showed promising results. It is a mitotic inhibitor and kills germinating weeds by stopping cell division. This mode of action is similar to preemergence herbicides such as Prowl, Kerb, Balan, Trifluralin and others but its chemistry and how it moves in the soil differs. Dacthal is absorbed by both shoots and roots of germinating seedlings although most of the activity is from shoot absorption. When absorbed by the shoots, it will move upward into the plant. It is not absorbed by foliage and can be safely applied over the crop. Dacthal adheres strongly to soil particles. The best time to apply Dacthal is when the soil is moist but not saturated^[2]. 
We are also including in this update the statement from registrant AMVAC Corporation for more information regarding notice of intent to suspend DCPA: https://www.amvac.com/news/amvac-regulatory-issues-statement-regarding-dachtal-dcpa.
 
 
 
References:

1. https://en.wikipedia.org/wiki/Dimethyl_tetrachloroterephthalate
2. https://cals.arizona.edu/crop/vegetables/advisories/more/weed193.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.