There have been ongoing negotiations for several years directed at developing the new Colorado River management guidelines that will go into place in 2026 when the 2007 interim guidelines expire.
The goal of negotiations among the seven U.S. Colorado River basin states has been to submit a single document proposing guidelines to the Bureau of Reclamation (BoR) within the U.S. Department of the Interior. However, the upper and lower basin state groups have been dealing with significant differences and were not able to come to a consensus agreement. As a result, the upper and lower basin delegations each submitted separate proposals to the BoR in early March.
Both proposals are under review by the BoR officials who have indicated that they will be working with the basin states in the next six months to develop a consensus draft proposal for post-2026 river management by the end of 2024.
To help review the differences between the two proposals, I have summarized the basic parameters in Tables 1 through 4 (Davis, 2024a and Hager, 2024a).
To help review the differences between the two proposals, I have summarized the basic parameters in Tables 1 through 4 (Davis, 2024a and Hager, 2024a).
Lower Basin Proposal Summary
Table 1. Outline of lower basin proposal for Colorado River water management based on reservoir levels of Lakes Powell and Mead.
|
Reservoir Levels (%) |
Lower Basin Reductions (MAF/Year)* |
Upper Basin Reductions (MAF/Year) |
|
70 |
0 |
0 |
|
59-69 |
0-1.5** |
0 |
|
39-58 |
1.5 |
0 |
|
≤ 38 |
X |
X |
Note:
*MAF = million acre-feet
**Up to 1.5 MAF/year reductions for the entire lower basin depending on the reservoir levels between 59-69%. Divisions of water reductions among the lower basin states is not specified.
X = 1.5-3.9 MAF/year total reductions for the total Colorado River system. The specific amounts of water reductions in each of the upper and lower basins with reservoir levels at 38% or less are not specified in the current lower basin proposal.
Table 2. Annual reductions in Colorado River water allocations for the lower basin states when the reservoir water levels are between 39-58% in both reservoirs, based on the lower basin proposal.
|
Lower Basin State |
Reduction (acre-feet) |
|
Arizona |
760,000 |
|
California |
440,000 |
|
Nevada |
50,000 |
|
Mexico* |
250,000 |
*Contingent upon Mexico’s agreement with the reductions.
Based on the lower basin proposal, reservoir levels at 38% of capacity and lower would result in reductions from 1.5 to 3.9 MAF/year, depending on the levels of water depletion in Lakes Powell and Mead. The exact splits in water reductions between the upper and lower basins in this range of reservoir depletion is not specified in the lower basin plan, at least based on the materials used for this review. Accordingly, the share of water reductions that Arizona and California would take are not specified either.
For reservoir levels of less than or equal to 38%, the cuts would gradually tighten until the reservoirs fall below 23% of capacity. At that point, all Colorado River basin states would have to take maximum collective reductions of 3.9 million acre-feet a year.
Upper Basin Proposal Summary
Table 3. Outline of reductions in water releases from Lake Mead based on the upper basin proposal.
|
% of Capacity |
Release Reductions (MAF/year)* |
|
90 |
0 |
|
70-90 |
0-1.5 |
|
20-70 |
1.5 |
|
≤ 20 |
1.5-3.9 |
*MAF = million acre-feet
Specifically for Lake Mead, the Upper basin states proposal includes points:
— If Mead is at least 90% full, the Lower Basin states would take no cuts in their supplies.
— If Mead is between 70% and 90% full, the Lower Basin states would lose up to 1.5 million acre-feet per year.
— If Mead is 20% to 70% full, Lower Basin states would lose 1.5 million acre feet a year.
— If Mead is 20% full or lower, Lower Basin states would have to take cuts of up to 2.4 million acre-feet on top of the 1.5 million they’re already taking.
For Lake Powell, the Upper Basin states proposed a series of shrinking water releases depending on the lake’s elevation:
— If Powell is 81% to 100% full, the lake will release anywhere from 8.1 to 9 million acre-feet of water annually to send to Lake Mead for use by Lower Basin states.
— If Powell stands anywhere from 20% of 81% full, the lake will release somewhere between 6 million and 8.1 million acre-feet a year to Mead.
— If Powell is less than 20% full, it will send to Mead 6 million acre-feet a year.
Table 4. Outline of water releases from Lake Powell based on the upper basin proposal.
|
% of Capacity |
Lake Powell Water Release (MAF/year)* |
|
81-100 |
8.1-9 |
|
20-80 |
6.0-8.1 |
|
< 20 |
6.0 |
*MAF = million acre-feet
It is important to note that the upper basin proposal puts the entire reduction of Colorado River water use on the lower basin states. Essentially, the upper basin states see the need for Colorado River water reductions as lower basin responsibility entirely.
To help keep the conversation interesting, the Gila River Indian Community (GRIC) Governor Stephen Roe Lewis announced on 13 March 2024 at the University of Arizona Water Resource Research Center Annual Conference in Tucson, Arizona (WRRC, 2024) that their community is opposed to the lower basin proposal and with a letter to the BoR, GRIC is seeking to reinforce their existing rights to water and gain a stronger place and voice in the negotiations regarding future access to Colorado River water via the Central Arizona Project (CAP). Sixteen other tribes in the Colorado River basin signed the 11 March letter from GRIC to the BoR. Eight of those tribes are from Arizona (Davis, 2024b and Hager, 2024b).
The GRIC and some other tribes are pushing for the BoR to find other supplies of water to replace any water that is cut from CAP deliveries or provide financial compensation, particularly to cover existing and/or potential tribal water rights for Colorado River water.
There are many interesting ramifications that are possible from these recent developments. The BoR has two proposals to review from the upper and lower basins and now the added dimension of the GRIC opposition. These recent requests from GRIC and some other tribes will need to be taken into consideration along with the proposal elements from both the upper and lower basins.
References
Davis, Tony. 2024a. States are far apart on new Colorado River water-saving plans sent to U.S. agency. Arizona Daily Star, 13 March 2024.
First, I want to thank everyone who participated in last week's Vegetable Pest Losses Survey.
This year's survey included the return of the lettuce disease losses section. While several diseases were present and managed last season, downy mildew accounted for the majority of disease management costs by a wide margin. This really underscores the
impact that last spring's unusually rainy weather had on disease development across the Yuma lettuce production region.
No one can predict exactly what this upcoming spring will bring, but there has been discussion about the possibility of a strong El Niño leading to an extended monsoon season. If that proves true, the conditions would once again support spring
downy mildew development. The pathogen only needs about 3 to 4 hours of continuous leaf wetness to infect lettuce, so periods of overnight moisture, prolonged morning dew, or frequent rainfall when inoculum (spores) are present increase disease risk.
With that in mind, this seems like a good opportunity to review what is known about downy mildew and discuss strategies for its management.
Resistance in lettuce to Bremia lactucae, the causal oomycete pathogen behind downy mildew, is inherited in a gene-for-gene fashion, meaning one major gene product in the plant host interacts with one major gene product in the pathogen. When
resistance is present, this leads to an incompatible interaction between plant and pathogen and results in complete immunity to infection. Resistance genes in these types of interactions most often encode a protein molecule that acts like a burglar
alarm. These molecular sensors in the host bind to proteins secreted specifically by the pathogen, and when that happens a storm of defense responses is activated in the plant that excludes further infection. This is not the only mode of genetic resistance
found in plants, but it is often the most drastic and effective against obligate parasites like downy mildew.
But this simple gene-for-gene interaction often puts incredible selection pressure on the pathogen populations to change such that they can get around the resistance. By losing the molecule that the plant detects in order to initiate a defense response,
the pathogen becomes unrecognizable to the resistance genes a plant variety may have. Just like spraying the same mode of action over and over again leads to a pest population developing resistance to a pesticide, the same selection applies to genetic
resistance. The longer a resistance gene is deployed in a region, the more likely the pathogen population is to change in response until that resistance gene is no longer effective at managing the disease.
One of the biggest challenges with lettuce downy mildew is that B. lactucae is constantly changing over time. It exists as many different races, where each race has a different reaction to the resistance genes bred into lettuce varieties. You
can think of these races as different versions of the same pathogen. A lettuce variety that resists one race may still be susceptible to another.
These races are identified by testing them against a panel of lettuce varieties with known resistance genes. In the western United States, races are named by the International Bremia Evaluation Board-U.S. (IBEB-US) and are given names with a number followed by the country’s abbreviation, such as 8US, 9US, or 10US. The populations found in the western U.S. are different from those found in Europe, so each region uses its own independent naming system.
The downy mildew population has changed considerably over time. Earlier races (1US through 4US) are now rarely found in commercial lettuce production. During the 2000s and 2010s, races 5US through 8US became the most common. Race 9US was recognized after being detected repeatedly between 2015 and 2017, and the newest officially recognized race, 10US, was designated in 2025. Below is a pie chart showing the relative frequency of the races identified from 114 Yuma County downy mildew samples between 2023-2024:
Figure 1: Pathotyping results of 114 lettuce samples from Yuma County collected between 2023 and 2024. Data source: https://bremia.ucdavis.edu/bremia_database_main.php
The results show that much of the downy mildew population found in Yuma County is made up of novel strains of Bremia lactucae that have not yet been officially classified as a race. An official race is only recognized after it has been shown to be stable and widespread over multiple locations and growing seasons. These newer strains may disappear over time, or they may eventually become established and earn an official race designation. In the meantime, this means growers and lettuce breeders in Yuma County are often dealing with strains that can dodge the resistance in some lettuce varieties before those strains are common enough to be officially recognized. It also highlights why relying on resistance alone is not enough to manage the disease.
Table 1: Pathotyping and fungicide sensitivity results of samples from Yuma County collected in 2025.
This trend appears to be continuing. All of the downy mildew samples sent for race testing last season were identified as novel strains rather than known, officially designated races.
It's impossible to predict exactly how these new strains will respond to the resistance genes found in today's commercial lettuce varieties. However, because they have not been previously characterized, they are more likely to overcome existing genetic
resistance than the races we already know about.
New strains develop naturally over time. They can arise when different strains exchange genetics (i.e. intermate) or through random mutations. When growers plant varieties with similar resistance packages over large areas, the pathogen population
is placed under strong selection pressure. Any strain that happens to acquire the ability to infect those resistant varieties gains a major advantage and gets to reproduce without competition where other strains cannot. Over just a few disease
cycles, those successful strains can become much more common in the population until they are the predominant strain overall.
An important point to remember is that the resistance bred into commercial lettuce varieties is not wearing out or becoming weaker over time. The genetics in the lettuce remain just as effective as when the variety was released. What changes is the
pathogen. As the downy mildew population evolves new strains emerge that can bypass resistance genes that previously worked very well.
That means that varieties carrying resistance to races 5US through 10US are still doing exactly what they were designed to do. They continue to suppress those known races and help prevent them from becoming widespread in commercial fields. So, if
you experience significant downy mildew in a field planted with a variety that has a strong resistance package, the culprit is most likely one of these newer, uncharacterized strains rather than a failure of the variety itself.
Unfortunately, Bremia lactucae can evolve much faster than scientists can identify new races and breeders can develop and release resistant varieties. That's why no resistance package should be viewed as a stand-alone solution.
This is also why extension, researchers, and the seed and crop protection industries place so much emphasis on the integrated pest management (IPM) concept. Genetic resistance is an essential tool, but it works best and remains the most sustainable when combined with other management practices. For novel strains that can slip past host resistance, timely fungicide applications and other disease management strategies become especially important for maintaining control.
Figure 2: Mean disease severity by treatment. Disease severity was determined by rating 10 plants within each of the five replicate plots per treatment using the following rating system: 0 = no downy mildew present; 1 = downy mildew present on bottom leaves of plant; 2 = downy mildew present on bottom leaves and lower wrapper leaves; 3 = downy mildew present on bottom leaves and all wrapper leaves; 4 = downy mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = downy mildew present on entire plant. Disease severity is displayed as the mean of five replicates across all three lettuce varieties and bars show a 95% confidence interval around the mean calculated from individual treatment data. Compact letter display (CLD) above boxes show significantly different treatments (Kruskal-Wallis ANOVA and Dunn’s test). Boxes sharing the same letter(s) are not significantly different from one another. Fb = “followed by” in the rotation. Not all products are registered yet for use in lettuce. The inclusion of specific fungicide products or formulations in these trials does not constitute an endorsement or recommendation over other labeled products.
The most effective way to manage lettuce downy mildew is to use an integrated approach. Plant varieties with a strong resistance package against races 5US through 10US, and pair that resistance with timely, full-label-rate fungicide applications when environmental conditions favor disease. This combination provides the broadest and most reliable protection against both known races and the novel strains that continue to emerge in Yuma County.
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 109VegIPM Update Vol. 17, Num. 15
July 22, 2026
Results of trap catches below!!
Whitefly: Adult activity remains steady across locations; above average for this time of the year, especially high numbers seen in North Gila Valley. Historically, whitefly numbers peak in July.
Thrips: Adult thrips activity remained low over the last two weeks. About average for this time of the year. Historically, thrips numbers remain low until Sept-Oct.
