On 31 July, the Secretary of the Interior, through the Bureau of Reclamation (Bureau), announced the proposed Post-2026 Operational Guidelines for coordinated management of Lake Powell and Lake Mead, accompanied by the
Final Environmental Impact Statement (Final EIS). A formal Record of Decision is expected shortly.
The new guidelines replace the 2007 Interim Guidelines, which expires in 2026. They respond to more than two decades of persistent drought, declining runoff, increasing temperatures, and the recognition that long-term Colorado River flows
are insufficient to meet current demands.
Since 2023, the Basin States have worked to develop a consensus replacement for the expiring guidelines; however, no agreement was reached. Consequently, the Bureau developed the new operational framework pursuant to its authority under the Colorado River Basin Project Act of 1968.
Unlike previous operating rules, the Final EIS does not prescribe fixed operating criteria through 2036. Instead, it establishes an adaptive management framework, with operating guidelines generally updated every two years.
This approach allows reservoir operations to respond to changing hydrologic conditions while accommodating future agreements among the Basin States, Tribes, Mexico, and the federal government. This represents one of the most significant changes in Colorado River management since adoption of the 2007 Interim Guidelines.
Historical Colorado River Allocations
The legal basis for Lower Basin water deliveries remains the U.S. Supreme Court decision in Arizona v. California (1963) and the 1964 Consolidated Decree. As these legal allocations remain unchanged, the EIS proposed Post-2026 Guidelines do not alter the Law of the River; rather, they establish how deliveries may be temporarily reduced during shortages to protect the operational integrity of Lakes Mead and Powell. That will certainly be a target of considerable legal review.
Table 1. Proposed operational deliveries of Colorado River Water to Lower Basin States for 2027-2028 based on the final Environmental Impact Statement from the U.S. Bureau of Reclamation, 31 July 2026.
*MAF, million acre-feet
The analysis also assumes Mexico contributes approximately 250,000 acre-feet (about a 17% reduction), resulting in 1.5 million acre-feet of total annual system conservation during 20272028. In addition, the Lower Basin proposal includes 700,000 acre-feet of voluntary conservation, supported in part through federal incentive payments.
The Final EIS incorporates elements of the conservation framework proposed by the Lower Division States in May 2026. These proposed reductions are based on modeling assumptions, not permanent changes to legal allocations.
Key Points
Fundamental Changes After 2028
A significant aspect of the Final EIS is the transition to adaptive management beginning in 2029.
Rather than relying on predetermined Lake Mead elevation triggers and fixed shortage tiers, future operations will be established at approximately two-year intervals within the framework defined by the Final EIS. Based on the proposal in this final EIS, under severe drought conditions, Lower Basin shortages could approach 3 million acre-feet (up to 3.6 million acre-feet in representative modeling that includes Mexico).
Unless superseded by future negotiated agreements, shortages are expected to be distributed according to the priority system established under the Law of the River. Consequently, no fixed schedule of annual state allocations exists for 2029-2036. Annual deliveries will instead depend on reservoir storage, projected hydrology, negotiated conservation agreements, and future operational decisions.
Implications for Arizona Agriculture
The Final EIS confirms that while Arizona's legal allocation remains unchanged, actual annual deliveries may be substantially reduced during shortages. Because of existing water-right priorities, Arizona, particularly Central Arizona Project agricultural users, is expected to continue bearing a disproportionate share of conservation, a trend already established under the 2019 Drought Contingency Plan.
At the same time, the adaptive framework provides greater flexibility by allowing operations to be updated every two years in response to changing conditions, new conservation agreements, and improved hydrologic information.
Conclusions
The Post-2026 Operational Guidelines do not modify the legal allocations established under Arizona v. California. Instead, they fundamentally change how those allocations are managed during shortages by replacing fixed shortage schedules with an adaptive management framework.
The Final EIS demonstrates that substantial conservation will be required throughout the Lower Basin, while the Upper Basin states are not subject to mandatory reductions under the current plan.
For Arizona, this marks a long-term transition from predictable shortage tiers to a more dynamic management approach based on reservoir conditions, negotiated conservation, and the priority system that continues to govern Colorado River water rights.
Improving irrigation water management has always been extremely important in Arizona crop production systems and that it increasingly critical every day.
References
The Final EIS is available on the project website at: http://www.usbr.gov/ColoradoRiverBasin/post2026/final-eis/
For further information, please visit the project website at: https://www.usbr.gov/ColoradoRiverBasin/post2026/
Few things in Yuma County are more frustrating than watching lettuce wilt in the field. Even the best-managed fields are not immune to the many destructive soilborne pathogens capable of causing significant economic losses in lettuce production fields.
It is an unfortunate reality that Fusarium wilt, corky root, bottom rot, and lettuce drop are encountered and require management nearly every season in Yuma to produce a healthy crop. Although advances in resistant cultivars and integrated disease
management have improved our ability to reduce some of the crop losses to these diseases, management options for soilborne diseases, in general, remain severely limited and often are ineffective once symptoms progress to root or crown rot and irreversible
wilting. Strong, durable genetic resistance remains the strategic priority and most viable option when it comes to the management of these pathogens.
To select the right genetic resistance package, accurate field history and proper diagnosis of the cause of plant health challenges on that ground is critical. One of the greatest obstacles in plant disease diagnosis is that symptoms alone are rarely
what we call pathognomonic, that is, they are seldom unique enough to identify down to a single disease or environmental stressor. Temperature, soil moisture, plant age at time of infection, and varietal susceptibility all influence how diseases and
disorders develop and how symptoms are expressed in the plant as a response. As a result, the same pathogen can produce different symptoms under different conditions, while entirely different pathogens may produce nearly identical symptoms. Add to
this the complexity of the frequent occurrence of co-infections with multiple pathogens colonizing the same host at the same time and the ability to provide field diagnoses or diagnosis from pictures can quickly become confusing, overwhelming, or
downright impossible.
This known variability in symptom expression underscores the importance of confirming a diagnosis rather than relying solely on field observations, especially when the time comes for selecting resistant varieties to be planted in fields with a history
of soilborne diseases. Ideally, lettuce shouldn’t be planted on ground that is contaminated with soilborne pathogens, but this isn’t always feasible in our production systems.
Despite my obligatory warning for the difficulty and unreliability of diagnosing plant diseases and disorders from symptoms alone, I thought it would be a good chance to highlight a few key trends for two infamous diseases that we frequently observed
during last season’s atypically warm and rainy weather. I’m talking of course about Fusarium wilt and corky root. My hope is that these observations may prove helpful when scouting wilting fields, explaining your observations, and, importantly,
making it easier to document on our submission sheet what you’re seeing in the field if you choose to drop off samples to the clinic for culture testing and confirmation diagnoses. Of course, I remain happy to take a look and sample from the
symptomatic field as well. Over the 2025-2026 season the plant clinic received 20 samples culturing positive for Fusarium wilt, 16 for corky root, and 8 for a co-infection with both pathogens simultaneously.
In the field, the two diseases often appear indistinguishable from aboveground symptoms alone. Both commonly produce wilting, which may be more pronounced particularly during the heat of the day then seemingly “recover” in the evenings. Plants often lose their characteristic healthy, green waxy luster and instead develop a dull, matted appearance that is more apparent when compared with neighboring healthy plants or fields and as the wilting intensifies. As the diseases progress, the outermost leaves surrounding the crown may rot and collapse, further contributing to the appearance of a sick and declining plant. At this point, secondary microorganisms of both the bacterial and fungal kind may set in and produce signs of growth as they feed on the dead tissue, potentially appearing to be related to the cause of the disease and complicating diagnosis.
Below ground, corky root primarily affects the outermost tissues of the root system, causing feeder roots and the taproot to develop yellow to brown (sometime with specks of green) lesions that eventually crack and become roughened. To me, severely affected roots appear as if they are covered in mesquite bark. In contrast, Fusarium wilt primarily invades the plant's vascular system. Although root deterioration may occur (especially on small feeder roots), the hallmark of Fusarium wilt is vascular discoloration within the taproot and crown, where the water-conducting xylem tissues become tan to reddish-brown. These belowground differences, together with laboratory culture or other laboratory testing, provide the evidence needed to confidently distinguish between the two diseases.
Figure 1: Symptoms on a sample culturing positive for Fusarium wilt alone. Left.) outside view of the root system mostly absent of any rot symptoms, and right.) dissected root showing interior vascular discoloration. Note the bright white cortical tissue between the vascular tissue (center) and epidermis (outermost layer of cells, the “skin”) and near absence of overall rot of inner taproot tissue that is commonly observed with other soilborne diseases. The vascular system appears reddish due to the colonization with Fusarium oxysporum f. sp. lactucae.
Figure 3: Symptoms on a sample co-infected with Fusarium wilt and corky root. Left.) outside view of the root system showing significant barky lesions, and right.) dissected root shows a barky vascular dry rot towards the tip of the taproot with reddish-brown discoloration moving up towards the crown.
The images above are from some of the more diagnostically clear samples we have received in the clinic. As mentioned earlier, timing of infection, local microclimate, and varietal genetics can play a huge role in defining how a plant expresses disease
symptoms, so these observations of textbook symptomology may not be as clear in every field with wilting that you manage. For these reasons, laboratory testing remains the gold standard for disease diagnosis. Submission of symptomatic plants to the
diagnostic clinic allows for pathogen isolation and culture, providing definitive identification that can distinguish between diseases with similar symptoms. Accurate diagnosis is the foundation of effective disease management, ensuring that control
measures are directed at the true causal agent rather than its look-alikes. The Yuma Plant Health Clinic is here to help to that end.
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:
Christopher Detranaltes, Ph.D.
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364
– Room 109
It is with mixed emotions that I write to inform you that this will be my last University of Arizona Vegetable IPM Update. The reason – I am retiring. Thank you for your support over the years. It’s been an honor and a privilege working with you and serving this ag community.
Figure 1. Automated Thinning & Weeding Technologies Field Day.
(Photo credits: Rosa Bevington)
Chemigation is an effective method for applying pre-emergence herbicides such as Kerb (pronamide) and Prefar (bensulide) in lettuce production when sprinkler irrigation is used for crop establishment. However, successful applications depend on delivering
the proper herbicide rate uniformly across the field.
An efficacy trial was conducted at the Yuma Ag Center, supported in part by the Western IPM Center to evaluate different rates of Kerb and Prefar chemigated through overhead sprinkler irrigation using customized venturi system (Figure 1).
Hydrovant adjuvant was used in some of Kerb treatments to evaluate herbicide movement in the soil and crop injury.
Figure 1. Customized venturi chemigation unit
Figure 2. Venturi injector components and operating principle
Choosing the Correct Venturi Injector
One of the most common questions from growers is, "What size venturi should I use?" The most important factors in venturi selection are irrigation flow rate and operating pressure. The website below give some information about venturi injectors:
https://help.dripdepot.com/support/solutions/articles/11000130649-selecting-a-mazzeiventuri-injector
A properly sized venturi should:
General Venturi Sizing Guidelines
A venturi injector should be selected based on irrigation flow rate and operating pressure, not simply on pipe diameter. As a general guide:
Table 1. *Actual selection should always be verified using manufacturer performance charts.
System flow rate can be determined from an inline flow meter or estimated using sprinkler discharge rates and the number of sprinkler nozzle heads.
Calibrating the Venturi
Before every chemigation application:
Timing Kerb Applications
According to Dr. Jesse Richardson (Corteva Agriscience), Kerb applications should be timed according to crop development rather than number of days after irrigation. The preferred timing is when lettuce cotyledons are just about to emerge.
Recommended application guidelines include:
Proper timing and irrigation management can significantly improve Kerb performance.
Venturi Chemigation Units Available for Testing
To support further evaluation of chemigation technology in vegetable production systems, venturi chemigation units are available for use by interested growers, PCAs, Extension personnel, and researchers.
These units can be used to:
Growers interested in exploring chemigation as a herbicide application method are encouraged to contact us to discuss potential demonstration or research opportunities.
Looking Ahead: What Happens if Sprinkler Irrigation Shifts to Drip Irrigation?
Many lettuce production systems are gradually increasing reliance on drip irrigation for improved water-use efficiency. While this transition offers water advantage, it may create new challenges for soil-applied herbicides.
Kerb and Prefar perform well under sprinkler irrigation because water distribute the herbicide uniformly across the bed surface and moves it into the weed germination zone.
Under potential drip irrigation:
Research Questions
Several important questions remain unanswered:
Plan for next trial: I am planning to run a trial to find a way to mechanically incorporate Prefar and find an appropriate method to apply Kerb.
Potential approaches for Prefar applications include:
Potential approaches for Kerb applications include:
Take-Home Message
Venturi chemigation remains an efficient and effective tool for applying Kerb and Prefar in sprinkler-irrigated lettuce. Proper venturi sizing, calibration, and application timing are essential for maximizing herbicide performance.
As lettuce production systems continue to modernize, the future IPM program may combine mechanically incorporated residual herbicides, drip irrigation, precision cultivation, and robotic weed management tools. The challenge will be maintaining the excellent weed control growers currently achieve with sprinkler-activated Kerb and Prefar while improving irrigation efficiency and sustainability.
In Arizona's desert vegetable production system, beet armyworm, diamondback moth, and cabbage looper frequently occur together in brassica crops. While late-instar larvae of these lepidopteran species are generally easy to distinguish, identifying eggs and newly hatched larvae can be difficult, making early identification challenging. Recognizing these pests throughout their life cycle—from egg to adult—allows growers and Pest Control Advisors (PCAs) to detect infestations sooner, select appropriate management tactics, and optimize insecticide application timing before significant crop injury occurs.
Identification of Eggs
Diamondback moth eggs are small, oval, and flattened. They are pale yellow to light green and are usually laid singly or in small clusters of two to eight eggs, most often on the upper surface of brassica leaves. Eggs are commonly deposited
in small depressions on the leaf surface.
Cabbage looper eggs are hemispherical with a flattened base attached to the leaf. They are laid singly on either the upper or lower leaf surface and are yellowish white to greenish in color.
Beet armyworm eggs are the easiest to recognize because females deposit them in masses of approximately 50-150 eggs, usually on the underside of leaves. The egg masses are covered with whitish scales, giving them a distinctive cottony appearance.
Figure 1. Diamondback moth eggs (A), cabbage looper eggs (B), and beet armyworm eggs (C) on foliage.
Characteristics of Larvae
Newly hatched diamondback moth larvae are colorless to pale white with a dark head capsule. As they develop, they become green and remain slender with tapered ends and feature a pair of posterior prolegs, forming a “V” shape. Small white patches and a few short hairs are often visible along the body.
Figure 2. Newly hatched (A) and late instar (B)diamondback moth larvae.
Newly hatched cabbage looper larvae are dusky white and quickly become pale green after feeding. They have only three pairs of prolegs, two pairs on the abdomen and one pair at the anal end, resulting in a "loop shape" when they move. Mature cabbage looper larvae have two stripes on top and two others on each side. They are hairy initially, but the number of hairs decreases rapidly as larvae mature.
Figure 3. Newly hatched (A) and late-instar (B)cabbage looper larvae.
Young beet armyworm larvae are pale green to yellow and have a smooth, relatively hairless body that is slightly thicker than cabbage loopers. Pale longitudinal stripes become visible by the third instar, while older larvae develop darker coloration and a distinct lateral stripe.
Figure 4. Newly hatched (A), second instar (B), and 5th instar (C) beet armyworm larvae.
Behavioral Clues
Diamondback moth larvae wriggle vigorously when disturbed and frequently suspend themselves from leaves on a silken thread.
Cabbage looper larvae often rear into a "cobra-like" posture or drop from the plant when disturbed.
Beet armyworm larvae commonly curl into a "C" shape and may also drop on a silk thread before remaining motionless until the disturbance has passed.
Feeding Habits
Diamondback moth larvae initially mine inside leaves, creating the characteristic "windowpane" injury before feeding externally on foliage, terminals, flower buds, and seed stalks. This species feeds exclusively on brassicas.
Young cabbage looper larvae feed primarily on the underside of leaves, leaving the upper epidermis intact. Larger larvae chew large irregular holes through leaves but generally avoid feeding along the leaf margins.
Early-instar beet armyworm larvae feed together beneath leaves and often produce light webbing over the feeding site. Older larvae disperse and feed individually.
Identification of Pupae
Although pupae are encountered less frequently during routine scouting, they provide another useful diagnostic feature.
Diamondback moth pupae are small, slender, and light green when first formed, gradually turning brown before adult emergence. They develop inside a loose, open-mesh silk cocoon attached to leaves, stems, or other plant surfaces. The delicate silk cocoon is one of the easiest ways to recognize this species.
Figure 5. Newly pupated diamondback moth (A) and older diamondback moth pupa (B).
Cabbage looper pupae are green initially and later become dark brown. They are enclosed within a thin, transparent silk cocoon attached to leaves, crop residue, or nearby plant material. The cocoon is denser than that of the diamondback moth but still relatively delicate.
Figure 6. Newly pupated cabbage looper (A) and older cabbage looper pupa (B).
Beet armyworm pupae differ from the other two species because they usually develop below the soil surface inside an earthen cell. The pupa is smooth, shiny reddish-brown, and lacks a silk cocoon. Finding pupae in the soil rather than on foliage strongly suggests it is beet armyworm.
Figure 7. Beet armyworm pupa.
Identification of Adults
Diamondback moth adults are the smallest of the three species, measuring about 6–8 mm long. Their narrow grayish-brown wings fold tightly over the body at rest, producing a series of pale cream-colored diamond-shaped markings along the back, which give the insect its common name.
Cabbage looper adults are considerably larger than diamondback moths and have mottled grayish-brown forewings. A distinctive silver-white figure resembling the number "8" or the letter "Y" occurs near the center of each forewing and is the easiest feature for identification.
Beet armyworm aults are medium-sized moths with grayish-brown forewings that contain irregular pale markings and a small light-colored kidney-shaped spot near the center. Their hindwings are pale white with darker margins.
Figure 8. Diamondback moth (A), cabbage looper (B), and beet armyworm (C) moths.
Host Range
Diamondback moth feeds exclusively on brassica crops and brassica weeds.
Cabbage looper feeds on numerous crops, including brassicas, lettuce, spinach, celery, parsley, peas, potatoes, tomatoes, cotton, and beets. They can also be found on some weed species.
Beet armyworm has a broad host range that includes brassicas, lettuce, cotton, beets, peppers, tomatoes, and many weed species.
Take-Home Message
Successful IPM begins with accurate pest identification. Observing egg location, larval appearance, feeding injury, behavior, pupation site, and adult wing patterns provides reliable clues for distinguishing beet armyworm, diamondback moth, and cabbage looper in the field. Early and accurate identification enables growers and PCAs to improve scouting, optimize treatment timing, and achieve more effective pest management.For iceberg lettuce, saving water is not as simple as cutting irrigation. Lettuce is a fresh-market crop, and even small water-stress effects can reduce head size, uniformity, firmness, and marketable yield.
The first question is: deficit compared to what? Deficit irrigation should always be clearly defined. Is the treatment applying 90%, 75%, or 50% of the estimated crop water requirement? Is the baseline based on AZMET reference ET, local crop coefficients, soil moisture sensors, or measured crop water use?
Yuma Already Leads on Water Efficiency
Before getting into the trial, it is worth remembering what Yuma iceberg lettuce growers and irrigation districts have already built. Over generations, they have developed one of the most productive and water-efficient agricultural systems in the Colorado
River Basin through precise field leveling, well-maintained delivery infrastructure, careful irrigation management, and continued conservation investments. On a “crop-per-drop” basis, or production per acre-foot of water, regional assessments
suggest Yuma agriculture runs significantly more efficiently than other agricultural areas in the Colorado River Basin.
“That leadership matters. Yuma does not just grow crops; it helps feed the nation. Protecting Yuma’s water resources supports not only the local community, but also the families, businesses, and consumers across the country who depend on a steady supply of winter vegetables” (Yuma Water, 2026). The Bureau of Reclamation’s July 2026 Final Environmental Impact Statement emphasizes adaptive management, cooperation, innovation, and long-term water-supply reliability- the same values Yuma growers and irrigation districts have been putting into practice on the ground for decades.
An Added Pressure: The Weather Has Been Warming
On top of that, the growing environment itself is shifting. A recent University of Arizona Cooperative Extension analysis by Mohammed (2025), based on nearly four decades of AZMet data from 1987 through 2025, showed that both daytime and nighttime temperatures in Yuma Valley have been climbing gradually. Warmer days can increase crop water demand and soil evaporation. Warmer nights can raise the plant’s maintenance respiration, which is energy the crop spends just to keep functioning. That extra pressure is one more reason to look carefully at any strategy that promises to save water in lettuce.
The Trial: Three Levels of Irrigation, Side by Side
Iceberg lettuce was planted on October 28, 2025, at the Yuma Agricultural Center on a clayloam field. Germination was established by sprinkler irrigation, and the crop was then managed under subsurface drip irrigation. Three irrigation levels were compared
side by side:
Full irrigation — meeting 100% of the estimated crop water requirement.
Moderate deficit — applying 75% of the estimated full requirement.
Severe deficit — applying 50% of the estimated full requirement.
Seasonal water use, or crop evapotranspiration, was monitored with soil moisture sensors. At harvest, marketable yield was recorded, and crop water productivity was calculated as the kilograms of lettuce produced per cubic meter of water the crop used.
What We Saw
The story from this season was fairly clear. The full-irrigation crop produced the highest marketable yield, the highest water use, and, importantly, the highest crop water productivity of the three treatments (Figure 1). In other words, reducing irrigation did not come out ahead on a per-drop basis in this trial. The deficit treatments used less water, but they lost proportionally more yield, so crop water productivity declined rather than improved.
Figure 1. Crop water productivity (CWP, kg m⁻³) of iceberg lettuce under three irrigation levels at the
Yuma Agricultural Center, 2025–2026 season. In this trial, full irrigation produced the highest CWP;
reducing water did not improve efficiency.
This means that applying 75% or 50% of the estimated crop water requirement carried a real production cost in this trial. Iceberg lettuce needs enough water to support stand establishment, root activity, head development, cooling, nutrient uptake, and salinity management. When irrigation is reduced too far, the crop may save water, but it can lose marketable yield and quality. Iceberg lettuce is bought on appearance and quality; water stress that hurts head quality can cost more than the water it saves.
References
Mohammed, A. T. (2025). Long-term trends in annual maximum temperature, annual minimum temperature, and annual mean temperature in Yuma Valley, Arizona (1987–2025). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/long-term-trends-annual-maximum-temperature-annual-minimum-temperature-and-annual
Mohammed, A. T. (2026, July 15). Effects of full and deficit irrigation strategies on lettuce yield, seasonal crop evapotranspiration, and crop water productivity in Yuma, AZ [Conference presentation]. ASABE Annual International Meeting, Indianapolis, IN, United States.
U.S. Bureau of Reclamation. (2026). Final Environmental Impact Statement: Colorado River Interim Operating Guidelines. U.S. Department of the Interior.
