In a recent article in this space, I provided a summary of some recent work from the Dr. J S. Famiglietti program at Arizona State University (ASU). Dr. Famiglietti is a Global Futures Professor and hydrologist with the School of Sustainability, and he serves as the director for ASU’s Arizona Water Innovation Initiative.
The work I recently summarized was published in the Geophysical Research Letters journal (Abdelmohsen et al., 2025) with the American Geophysical Union (AGU). That article describes the declining groundwater supplies in the lower Colorado River basin (LCRB). The methods for conducting this analysis are described in other publications (Rodell, Famiglietti et al., 2004; Rodell, Houser et al., 2004).
Additional work from the Famiglietti group has recently been published with their findings regarding a global review of terrestrial water storage (TWS) as a critical indicator of freshwater availability. In their methodology, they again used NASA GRACE/GRACE-FO data to show that the continents have undergone unprecedented TWS loss since 2002 (Chandanpukar, et al. 2025).
Some results of this study are summarized in Figures 1 and 2 revealing areas of intense TWS loss in this century. Mexico and Central America, the desert Southwest and other areas in the United States, i.e., California and the Ogallala Aquifer on the high plains, are clearly identified as having severe depletions of TWS, both surface and groundwater. These three regions are collectively identified as one large southwestern North American-Central American mega-drying region.
This work represents one piece of an ensemble of studies conducted by Famiglietti and his colleagues in recent years (Castle, 2014; Chandanpurkar, 2021; Famiglietti, 2014; Famiglietti, 2019; Famiglietti and Ferguson, 2021; Khorrami, 2023; Mohan, 2023; Rodell, 2018; Rodell, Famiglietti, 2004; Rodell, Houser, 2004; Scanlon, 2023;Voss, 2013; and Xu, 2023). Their methods are well-established, and they have been thoroughly scrutinized and reviewed.
Important conclusions from this study show that because of the increasing aridification along with increasing demand among many areas across the entire planet between 2002 to 2024, the availability of surface water and the levels of groundwater storage have substantially decreased.
The depletion of TWS is impacting many regions of the world. These depletions are particularly severe in many arid and semi-arid regions. As surface water sources have declined with increasing aridity in many areas, that has contributed to increasing demand for groundwater resources.
The current water crisis in Iran demonstrates the degree of severity that many parts of the world are experiencing, Kowser and Nader, 2025. The basic needs for Iran in dealing with their water crisis is like many other countries and regions. The key points that need immediate attention include improved and well-integrated agronomic programs (i.e., crop-soil-water management), incentive-based polices, strict legal enforcement, and ongoing evaluation and oversight. Like many regions, including Arizona and the desert SW of the U.S., the Iranians need immediate action, both short and long-term.
Arizona agriculture will be continually forced to deal with the increasing urban population and the diminishing water supplies. The recent results coming from Abdelmohsen et al., (2025) provide a valuable assessment of groundwater supplies in the LCRB and Arizona and the importance of conservation measures. As groundwater supplies are further diminished, that will likely direct more attention surface water sources and intensify the competition for all TWS sources. The recent work from Chandanpukar, et al. (2025) further demonstrates the importance of our water conservation needs.
This also reinforces the critical need for Arizona to develop functional groundwater legislation for areas beyond the established AMAs. It is imperative that we manage the water resources we have with great care and discipline. No one is going to come save us and there are no other water resources we can draw from. We need valid and realistic information to work with (i.e. Chandanpukar et al., 2025) and we need practical and effective conservation guidelines employed.
Fig. 1. Global map of long-term TWS trends from GRACE/FO.
(A) Trends in TWS (cm year−1) from February 2003 to April 2024 (see Materials and Methods). Mega-regions (regions exceeding −0.2 cm year−1 and connecting previously reported TWS hot spots) are outlined in black and labeled 1 to 4 corresponding to the main text. (B) Zonal sum of TWS trends for all (black) and non-glaciated regions (red). Source: Chandanpukar, et al. 2025.
Fig. 2. Mapping robustness of TWS trends.
(A) Drying and wetting land regions from where the TWS trend sign has been persistent and less sensitive to the increasing GRACE/FO record length. (B) Ratio of local interannual variability of detrended TWS anomalies to their long-term local trends. The red and blue color bars indicate regions with decreasing TWS trend and increasing TWS trend from Figure 1. Source: Chandanpukar, et al. 2025.
References
Abdelmohsen, K., Famiglietti, J. S., Ao, Y.Z., Mohajer, B., & Chandanpurkar, H. A. 2025. Declining freshwater availability in the Colorado River basin threatens sustainability of its critical groundwater supplies. Geophysical Research Letters, 52,e2025GL115593. https://doi.org/10. 1029/2025GL115593
Castle, S.L., B.F. Thomas, J. T. Reager, S. C. Swenson, M. Rodell, J. S. Famiglietti. 2014. Groundwater depletion during drought threatens future water security of the Colorado River Basin. Geophys. Res. Lett. 41, 5904–5911.
Chandanpurkar, H.A., J. T. Reager, J. S. Famiglietti, R. S. Nerem, D. P. Chambers, M.-H. Lo,B. D. Hamlington, T. H. Syed. 2021. The seasonality of global land and oceanmass and the changing water cycle. Geophys. Res. Lett.48,e2020GL091248.
Chandanpukar, H.A.,Famiglietti, J.S., Gopalan, K, Wiese, D.N., Wada, Y., Kakinuma, K., Reager,J.T., and Zhang, F. 2025. Science Advances, Vol. 11, Issue 30, 25 July 2025. DOI:10.1126/sciadv.adx0298
Famiglietti, J.S. 2014. The global groundwater crisis. Nat. Clim. Chang. 4,945–948.
Famiglietti, J.S. 2019. A map of the future of water. Trend Magazine, 3 March2019.
Famiglietti, J.S. and G. Ferguson. 2021. The hidden crisis beneath our feet. Science 372,344–345.
Khorrami, M.,M. Sherzaei, K. Ghobadi-Far, S. Werth, G. Carlson, and G. Zhai. 2023. Groundwater volume loss in Mexico City constrained by In SAR and GRACE observations and mechanical models. Geophys. Res. Lett.50,e2022GL101962.
Kowsar, N. and A. Nader. 2025. Iran’s TapsAre Nearly Empty: After five straight years of drought, the country is running dry. Foreign Policy, 7 August 2025.
Mohan, C., T. Gleeson, T. Forstner, J. S. Famiglietti, I. de Graaf. 2023. Quantifying groundwater’s contribution to regional environmental-flows in diverse hydrologic landscapes. Water Resour. Res. 59,e2022WR033153 (2023).
Rodell, M., J. Famiglietti, D. N. Wiese, J.T. Reager, H. K. Beaudoing, F. W. Landerer, M.-H. Lo. 2018. Emerging trends inglobal freshwater availability. Nature 557, 651–659(2018).
Rodell, M., Famiglietti, J.S., Chen, J., Seneviratne, S. I., Viterbo, P., Holl, S., &Wilson, C. R.2004. Basin scale estimates of evapotranspiration using GRACE and otherobservations. Geophysical Research Letters, 31(20), L20504. https://doi.org/10.1029/2004GL020873
Rodell, M., Houser, P. R.,Jambor, U., Gottschalck, J., Mitchell, K., Meng, C.‐J., et al. 2004. The global land data assimilation system. Bulletin America Meteorology Social, 85(March),381–394. https://doi.org/10.1175/BAMS853381
Scanlon, B.R., S. Fakhreddine, A. Rateb, I. deGraaf, J. S. Famiglietti, T. Gleeson, R. Q.Grafton, E. Jobbagy, S. Kebede, S. R. Kolusu, L. F. Konikow, D. Long, M.Mekonnen, H. M. Schmied, A. Mukherjee, A. MacDonald, R. C. Reedy, M.Shamsudduha, C. T. Simmons, A. Sun, R. G. Taylor, K. G. Villhoth, C. J.Vorosmarty, and C. Zheng. 2023. Global water resources and the role of groundwater in a resilient water future. Nat. Rev. Earth Environ. 4,87–101.
Silvertooth,J.C. 2025. Groundwater assessment in the lower Colorado River basin. Universityof Arizona, Vegetable IPM Newsletter, Vol. 16, No. 14.
Voss, K.A., J. S. Famiglietti, M. Lo, C. R. de Linage, M. Rodell, and S.C. Swenson. 2013. Groundwater depletion in the Middle East from GRACE withimplications for transboundary water management in the Tigris-Euphrates-WesternIran region. Water Resour.Res. 49, 904–914.
Xu, L., J. S.Famiglietti. 2023. Global patterns of water-driven human migration. WIREsWater 10, e1647
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 109Vegetable season is well underway and early planted vegetable crops are already about one month old. Many fields have or soon will need to be cultivated. As such, I thought it would be timely to “repost” this video on “new” technologies for cultivating weeds - 1) a camera-guided side-shift hitch and 2) finger weeders, an in-row weeding tool (Fig. 1). The video shows the device operating in seedling corn, but it will work similarly in other crops such as broccoli or cauliflower.
We have found the devices work well. Trials conducted in cotton over 3 years showed that use of camera-guidance improved weed control by more than 30% and finger weeders removed about 45% of the in-row weeds. Weed control using the two technologies together simultaneously was roughly 90% for broadleaf weeds and about 85% for all weed species including grasses.
If you are interested in trying these technologies, please contact me. We still have the equipment and I’d be happy to work with you.
Fig. 1. Technologies for precision cultivation and in-row weeding include a) a
camera-guided side-shift hitch attached to a cultivator and b) in-row weeding
tools (finger weeders).
Watch the Video Below
Dr. John Palumbo never sought the spotlight, but everything he did—every trial, field visit, and conversation—pointed us toward something better: better science, better decisions, better farming.
I’ve looked up to John since my undergraduate days. He was the model of what I hoped to become: a scientist grounded in integrity, driven by purpose, and deeply connected to the people and land he served. A career-long dream came true when I asked him to join my Ph.D. committee—and he said yes. A defining moment in my journey that I’ll always carry with me.
The work we hoped to do with him is now a reminder of how much he still had to offer. But more than anything, I’m grateful. Grateful that I got to learn from him, talk with him, and be challenged by him. I especially loved when I’d share an idea and he would provide feedback like, “Now I don’t like the sound of that, and I’ll tell you why...” To me that meant he was listening closely. He cared. He was genuine.
John didn’t just give us answers—he gave us perspective and the confidence to ask better questions. His perspective came from a place of hard-earned wisdom which he shared humbly and freely. We’ve lost a giant, but his impact remains in our work, our conversations, and the values he quietly instilled. His presence is deeply missed—and deeply lasting.As the desert vegetable season ramps up, organic growers face tough pest management challenges with limited insecticide options. To help address this, University of Arizona researchers at the Yuma Agricultural Center conducted field trials during the 2024–2025 season, testing OMRI-listed products on six key pests in leafy greens and Brassicas.
This article highlights results for products like Entrust, Pyganic, XenTari, and M-Pede, evaluated for their performance against diamondback moth, beet armyworm, whiteflies, green peach aphid, pale striped flea beetle, and western flower thrips. The results can help growers fine-tune their spray programs and make more informed decisions this season.
Click here to read the full report: Organic-Allowed Insecticide Options for the Management of Six Major Insect Pests in Arizona’s Vegetable CropsWeed management is one of the most significant challenges in organic production systems, especially in high-value leafy greens like lettuce. With limited tools available, growers should combine strategies to suppress weeds effectively. One approach is precision irrigation management. By delivering water directly to the crop root zone through systems like subsurface drip irrigation, growers can reduce soil surface wetting, making conditions less favorable for weed germination. Yuma’s organic growers are doing a pioneering job in adopting these strategies, and when paired with sensor-based irrigation scheduling, the practice not only conserves water but also helps disrupt the weed life cycle.
Read my recently published Extension article for the full details: Can Precision Irrigation Be an Effective Approach to Suppress Weed Pressure in Organic Lettuce?VegIPM 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.
