Water and nutrient demands coincide with the fruiting cycle of most crop plants. The efficient management of irrigation water and plant nutrients is enhanced by tracking crop development in the field and making decisions on plant demand and condtion. The use of heat units (HUs) with 86/55 oF upper and lower thresholds can be applied to warm season crops in the desert Southwest in relation to the thermal environmental impacts on the development of all crop systems (Brown, 1989), including chiles, (Figures 1 and 2).
Crop Phenology Relationship to Water and Nitrogen Demand
Phenological guidelines have been developed for many crops, including New Mexico (NM) type chiles (Soto-Ortiz and Silvertooth, 2007 and Silvertooth, et al, 2010; Figure 1). This phenological guideline can be used to identify or predict important stages of crop development that impact physiological requirements. For example, a phenological guideline can help identify stages of growth in relation to crop water use (consumptive use) and nutrient uptake patterns (Figure 3).
This information allows growers to improve the timing of water and N inputs to improve production efficiency. For most crops HU based phenological guidelines can be used to project critical dates such as harvest or crop termination. Many other inputs and applications related to crop management (e.g., pest management) can be derived from a better understanding of crop growth and development patterns.
Arizona chile production, consisting of NM type chile varieties, is mostly located in Cochise County in southeastern Arizona. The 2025 Arizona chile crop has been progressing steadily in line with the basic phenological model shown in Figure 2. For a set of fields near Pearce, Arizona with several different NM type varieties and planting dates ranging from 2 – 22 April 2025, current heat unit accumulations after planting are 1,785 – 1,957 (Figure 4a-d). The crop is moving into peak bloom and the period of peak N and water demand.
This is very good time in the season to evaluate these fields in relation to current irrigation and N fertilization needs.
Figure 1. Typical relationship between the rate of plant growth and development and
temperature. Growth and development ceases when temperatures decline below the
lower temperature threshold (A) or increase above the upper temperature threshold (C).
Growth and development increases rapidly when temperatures fall between the lower
and upper temperature thresholds (B).
Figure 2. Basic phenological guideline for irrigated New Mexico-type chiles.
Figure 3. Basic phenological guideline for irrigated New Mexico-type chiles with periods
of peak water and nutrient demand, including optimum N application window.
Figure 4a. New Mexico type chile, variety Carne Duro, 31 July 2025.
Figure 4b. Crown set chiles, Carne Duro variety, 31 July 2025.
Figure 4c. Early fruit set, Carne Duro variety, 31 July 2025.
Figure 4d. Mid-canopy fruit set, Carne Duro variety, 31 July 2025.
References
Brown, P. W. 1989. Heat units. Bull. 8915, Univ. of Arizona Cooperative Extension,
College of Ag., Tucson, AZ.
Silvertooth, J.C., P.W. Brown, and S. Walker. 2010.Crop Growth and Development for
Irrigated Chile (Capsicum annuum). University of Arizona Cooperative Extension
Bulletin No. AZ 1529
Soto-Ortiz, R. and J.C. Silvertooth. 2007. A Crop Phenology Model for Irrigated New
Mexico Chile (Capsicum annuum L.) The 2007 Vegetable Report. Jan 08:104-122.
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 109Last week, we initiated our first on-farm demonstration of soil steaming of the season with our self-propelled steam applicator. The machine is designed to inject steam into the soil and raise soil temperatures to levels sufficient to kill weed seed and soilborne pathogens (140°F for > 20 minutes). After the soil cools (< ½ day), the crop is planted into the disinfested soil.
In this trial, we are examining the viability of soil steaming for controlling weeds in organic carrot at the field scale level (plot size > ½ ac). The machine performed well in that it was able to reach target soil temperatures at reasonable travel speeds (> 0.4 mph), provide uniform temperature distribution across the bed and form nicely shaped beds suitable for subsequent planting. Stay tuned for reports of weed control efficacy, crop yield and overall profitability as compared to the grower standard.
We are seeking collaborators to conduct similar field-scale trials/demos in Yuma, AZ. The primary objectives are to assess the viability of soil steaming at the field-scale level and obtain grower feedback on the device’s commercial potential. The machine can be adjusted to work with most bed configurations including narrow (40”, 42”) and wide (80”, 84”) beds, and is suitable for use in conventional or organic crops (soil steaming is organically compliant). To date, the device has been successfully trialed in iceberg lettuce, romaine lettuce, baby leaf spinach and carrot crops.
If you are interested in an on-farm demo of soil steaming, please let me know. I’d be happy to work with you.
Fig. 1. On-farm demonstration of a self-propelled steam applicator for weed and
disease control
Acknowledgements
This project is sponsored and funded in part by the Arizona Specialty Crop Block Grant Program and the Propane Education and Research Council (PERC). We greatly appreciate their support.
In 2010 we started publishing the Arizona Vegetable IPM Updates to a small number of friends. It was embraced by the community because of contributions from Mike Matheron, John Palumbo and Barry Tickes in the Plant Pathology, Entomology and Weed Science areas. Mike, John and Barry are the original IPM dudes.
I was honored to be part of the team editing and sending the Newsletter for almost
16 years.
This is my last update, and I would like to thank the UA team and the agricultural community for the opportunity to work with you.
To better support our stakeholders with pest management, we are launching an Insecticide Resistance Monitoring Program in Calvin’s Lab. This initiative will include seasonal insecticide efficacy bioassays using the most commonly used insecticides. We will target key pests of major crops grown in Arizona, with a particular focus on those affecting vegetables and citrus crops.
A major goal of this program is to establish baseline susceptibility data for Plinazolin’s target pests in vegetables and citrus crops. This will enable us to monitor potential changes in susceptibility over time and make informed, timely adjustments to pest control strategies.
Insecticide resistance monitoring is a critical component of Integrated Pest Management (IPM). Understanding the baseline susceptibility of insect pests is essential for early detection of resistance and the long-term success of new insecticide modes of action. It is especially important to gather this data for Plinazolin before its widespread adoption in our region.
To support this effort, we will collect insects from vegetable- and citrus-growing regions throughout Arizona. Your collaboration is essential. We will reach out to request access to field sites where we can collect target insect populations. Additionally, we encourage you to contact us directly if you experience insecticide control failures or suspect resistance in your fields.
Our current target pests include:
• Diamondback moth
• Beet armyworm
• Cabbage looper
• Western flower thrips
• Citrus thrips
• Aphids
• Whiteflies
Please let us know if there are additional pests you would like us to consider.
Thank you for your continued support. We look forward to working with you to address Arizona’s most pressing pest management challenges.As we approach the start of the upcoming season, it's a perfect time to revisit irrigation efficiency strategies, a critical component for both conventional and organic cropping systems across Arizona.
Extension professionals in Central Arizona requested a resource to help growers better understand how to manage irrigation more effectively. In response, we developed a short video explaining key strategies for efficient water use, including how to integrate weather station tools into your decision-making process.
This video provides simple, practical guidance on:
Arizona is fortunate to have a robust network of weather stations called Arizona Meteorological Network (https://azmet.arizona.edu/). I highly encourage you to identify the one closest to your location and start leveraging it as part of your irrigation planning and scheduling. These data-driven strategies can help you make more informed, sustainable, and profitable decisions in the field. If you have any questions or need help getting started, please feel free to reach out.
Wishing you a productive and water-wise season for your organic/conventional cropping systems ahead!
Watch the Video: Irrigation Efficiency for Farms (YouTube)
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.
