Identifying important stages of crop growth and development (crop phenology) for in-season management and harvest dates is important with melon (Cucumis melo ‘reticulatus’ L.) crop production. To monitor and predict plant development it is best to measure actual thermal conditions in the plant’s environment. Plant growth is a direct response to temperature and environmental conditions. Measuring “thermal time” is a more reliable measure and predictive tool for plant development as opposed to a calendar.

Thus far, 2026 has been an abnormally warm year for Arizona and the desert Southwest. Accordingly, crop development has been accelerated by conventional calendars but closely following the thermal calendars that plants respond to.

Various forms of temperature measurements and units commonly referred to as heat units (HU), growing degree units (GDU), or growing degree days (GDD) have been utilized in numerous studies to predict phenological events for many crop plants (Baskerville and Emin, 1969; Brown, 1989; Baker and Reddy, 2001; and Soto, 2012). A graphical depiction of HU computation using the single sine curve procedure is presented in Figure 1 (Brown, 1989).

A little over twenty-five years ago we began working on the development and testing of a phenology model for desert cantaloupe production for Arizona conditions. The basic cantaloupe phenology model is shown in Figure 2 (Silvertooth, 2003; Soto et al., 2006; and Soto, 2012). Since cantaloupes are a warm season crop, we use the 86/55 ºF thresholds for phenological tracking.

This melon crop phenology model was developed under fully irrigated and well-managed conditions, primarily in the lower Colorado River Valley. That is important since non-irrigated fields are more likely to experience water stress, which significantly disrupts crop development patterns.

Key stages of growth or “guideposts” indicated in Figure 2 represent general average or “target” values that are subject to a slight degree of natural variation, which is normal.

Referring to the data from the Arizona Meteorological Network (AZMET) and several locations in the Yuma area, the HU accumulations (86/55 ºF thresholds) from 1 January 2026 to a set of four possible 2026 planting dates are listed in Table 1. The HU accumulations from 1 January to 1 April 2026 are listed in Table 2.

The HU accumulations after planting (HUAP) for these four possible planting dates for three Yuma area locations to 1 April 2026 are shown in Table 3. The HUAP values in Table 3 are simply the difference between the values in Tables 1 and 2. An example for the Yuma Valley (UA Yuma Ag Center), 15 January 2026 planting date is: HU 1318- 95 HU = 1223 HUAP.

It is rather easy to test and evaluate this crop phenology model in the field under various planting dates, varieties, and conditions. Tracking HUs and reference to this phenology model (Figure 2) can serve as a check for melon crop development in the field. Based on the estimates in Table 3, the earliest planting dates are ready or close to crown fruit harvest and later planting dates are for small golf-ball and slightly large sized melons. Calendar wise, this is rather early but right on track based on seasonal HU accumulations.

Note: Yuma AZMET site is at the University of Arizona Yuma Agricultural Center. The North Gila AZMET site is located west of the Laguna Dam Road near County 3E. The Roll site is located near E. County 8th Street and S. Avenue 35E.

References:
Baker, J.T., and V.R. Reddy. 2001. Temperature effects on phenological development and yield of muskmelon. Annals of Botany. 87:605-613.

Baskerville, G.L., and P. Emin. 1969. Rapid estimation of heat accumulation from maximum and minimum temperatures. Ecology 50:514-517.

Brown, P. W. 1989. Heat units. Ariz. Coop. Ext. Bull. 8915. Univ. of Arizona, Tucson, AZ.

Silvertooth, J.C. 2003. Nutrient uptake in irrigated cantaloupes. Annual meeting, ASA-CSSA-SSSA, Denver, CO.

Simonne, A., E. Simonne, R. Boozer, and J. Pitts. 1998. A matter of taste: Consumer preferences studies identify favorite small melon varieties. Highlights of Agricultural Research. 45(2):7-9.

Soto, R. O. 2012. Crop phenology and dry matter accumulation and portioning for irrigated spring cantaloupes in the desert Southwest.  Ph.D. Dissertation, Department of Soil, Water and Environmental Science, University of Arizona.

Soto-Ortiz, R., J.C. Silvertooth, and A. Galadima. 2006. Nutrient uptake patterns in irrigated melons (Cucumis melo L.). Annual Meetings, ASA-CSSA-SSSA, Indianapolis, IN. 

Table 1. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2026 on four
possible 2026 planting dates utilizing Arizona Meteorological Network (AZMET) data for
each representative site.


Table 2. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2025 on 12 April
2026 utilizing Arizona Meteorological Network (AZMET) data for each representative site
in the Yuma Valley, North Gila Valley, and Roll.

Table 3. Heat unit accumulations (86/55 ºF thresholds) after planting (HUAP) as of 10
April 2026 from four possible 2026 planting dates and three sites in the Yuma area
utilizing Arizona Meteorological Network (AZMET) data for each representative site.
Note: the values in Table 3 are determined by taking the difference between the HUs for each representative site and four planting dates in Tables 1 and 2.

Yuma Valley: https://azmet.arizona.edu/application-areas/heat-units/station-level-summaries/az02
Yuma North Gila: https://azmet.arizona.edu/application-areas/heat-units/station-level-summaries/az14
Roll: https://azmet.arizona.edu/application-areas/heat-units/station-level-summaries/az24

Figure 1. Graphical depiction of heat unit computation using the single sine curve procedure. A sine curve is fit through the daily maximum and minimum temperatures to recreate the daily temperature cycle. The upper and lower temperature thresholds for growth and development are then superimposed on the figure. Mathematical integration is then used to measure the area bounded by the sine cure and the two temperature thresholds (grey area). (Brown, 1989)


Figure 2. Heat Units Accumulated After Planting (HUAP, 86/55 oF)

 

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.

Below are the results from last season's downy mildew fungicide efficacy trial, where we compared 12 fungicide programs under Yuma growing conditions.

 

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 109

At the 2025 Southwest Ag Summit Field Demo a couple of weeks ago, many of the latest technologies were demonstrated in the field. Most were related to pest control. Several of the technologies demonstrated are new to the Yuma, AZ area. The recent technologies presented included two types of high precision spot sprayers for weeding and applying beneficial pesticides (Fig. 1a, Fig. 1b) and an AI based lettuce thinner. Manufacturers stated the technologies will be getting even better/more versatile soon. Both precision spot sprayer companies are developing models for small lettuce so that the machines can also be used for lettuce thinning. Also new was that the AI lettuce thinner technology is being further developed so that the device can be used also be used as a weeder. Several “older” pest control technologies also seemed to garner a lot of interest. These included an implement designed for cultivating high density crops (Fig. 1d), a camera guided cultivator equipped with in-row weeding tools (Fig. 1e) and a field scale drone sprayer (Fig. 1f).

Fig. 1. Newer pest control technologies demonstrated at the 2025 Southwest Ag Summit
Field Demo included a) Ecorobotix1 Ara high precision spot sprayer, b) Verdant Robotics
Sharp Shooter, c) Niqo Robotics AI based lettuce thinner, d) Oliver high density
cultivator, e) Steketee IC Light camera guided cultivator and f) DJI Agras T50 drone
sprayer.

This winter at the Yuma Agricultural Center, we evaluated whether Kerb (pronamide) can be effectively applied through sprinkler irrigation using a custom venturi chemigation unit, rather than a traditional ground sprayer. We also tested whether adding Hydrovant fA improves performance.

This trial was conducted under very heavy Sudan grass pressure and in cloddy seedbed conditions, making it a strong stress test of these programs.

Read the full report HERE

Key Takeaways (14 DAT – Preliminary)

1. Chemigated Kerb appears feasible and effective.
   Early results support venturi chemigation as a practical option in desert lettuce.
2. Hydrovant at 1% v/v reduced Kerb performance on Sudan grass.
   No phytotoxicity observed, but higher adjuvant rate did not improve control.
3. Seedbed quality matters.
   Cloddy conditions reduced Prefar efficacy. A fine, firm seedbed remains critical for pre-emergent success.
4. Under severe Sudan grass pressure,
   Prefar remains a suppression tool, but performance depends heavily on soil conditions.

This is a single, unreplicated screening trial. Always follow label directions and consult your PCA before adjusting programs.

Visitors are welcome to walk the plots at YAC.

Figure 1. Early results (14 DAT) show strong goosefoot control with chemigated Kerb
programs. 

This trial was made possible through partial funding from the Western IPM Center, with additional support from the University of Arizona Cooperative Extension and industry collaborators. Co-PIs on the project: Macey Keith and Wilfrid Calvin.

Insect pests are constantly adapting to their environment. Over time, they have evolved ways to overcome control tactics. Repeated use of the same insecticides or tactics places strong selection pressure on pest populations. This allows the few individuals that can survive treatments to reproduce, leading to populations that are increasingly difficult to control, a process known as resistance.

Insects can develop resistance in several ways. Some break down insecticides more efficiently or develop changes at the target site that reduce product effectiveness. Others avoid exposure altogether by changing their feeding or movement behavior. In some cases, insects even develop thicker cuticles that slow the absorption of insecticides.  

Production practices can influence the rate of resistance development. Large, uniform cropping systems and repeated use of the same control tools reduce opportunities for susceptible insects to persist, accelerating resistance.  

Figure 1: Chart illustrating how insecticide  resistance develops when
insecticides with the same mode of action (MoA) are  used repeatedly

What Should You Do?
An integrated pest management (IPM) approach remains the best defense against resistance.  

Key practices include:

• Rotating insecticides with different modes of action  
• Avoiding unnecessary applications  
• Preserving beneficial insects  
• Incorporating non-chemical tools when possible  
• Maintaining refuges to sustain susceptible pest populations  

Reducing selection pressure is critical. The more intensively a single tactic is used, the faster resistance will develop. A diversified management approach will help extend the life of available tools and improve long-term pest control. 

Daytime and nighttime temperatures have risen well above the recent norm, with important implications for crop development, irrigation demand, and pest management.

Yuma Valley has experienced an unusually warm spring pattern since late February 2026. Based on AZMET Yuma Valley station data, daily maximum and minimum air temperatures from January through March 29, 2026, diverged markedly from the 2020–2025 pattern, with the greatest separation occurring during March. In the most recent period, daytime temperatures repeatedly rose into the mid-90s and above 100°F (Figure 1), while nighttime temperatures frequently remained near or above 60°F (Figure 2). Relative to the recent historical pattern, both daytime and nighttime conditions have remained consistently warmer than expected for this point in the production season.

This warming trend is agronomically important because crops respond not only to extreme temperatures but to cumulative thermal exposure over time. When both maximum and minimum temperatures remain elevated for prolonged periods, crop development accelerates, the growing season becomes compressed, and management schedules can shift quickly. In several vegetable production systems, this season’s heat pattern appears to be advancing crop maturity and harvest readiness by as much as three weeks. While earlier harvests may appear favorable at first, such rapid progression often reduces the amount of time the crop has to accumulate biomass and achieve full yield potential.

In desert vegetable systems, crop growth and maturity are strongly linked to heat-unit accumulation. Under warmer-than-normal conditions, crops move more quickly through vegetative development, canopy expansion, and market maturity. However, earlier maturity is not necessarily equivalent to better productivity. Yield depends not only on developmental timing, but also on the duration of active growth. Crops need adequate time to intercept sunlight, expand leaf area, produce carbohydrates, and partition assimilates into the harvestable portion of the plant. When the season is shortened by sustained heat, plants may reach harvest stage before fully developing the biomass needed for optimum size, weight, or quality. The result can be smaller plants, lighter fresh weight, reduced head development, and lower total marketable yield.

Warm nighttime temperatures may be especially important in explaining this response. Plants photosynthesize during the day, but they respire continuously, including throughout the night. When nights remain warm, respiration rates increase, and a larger proportion of the carbohydrates produced during the day is consumed for maintenance rather than retained for growth and yield formation. This reduces carbon-use efficiency and limits dry matter accumulation. In practical terms, the crop may mature earlier while simultaneously losing some of its ability to build harvestable biomass. Warm nights also reduce the recovery period plants normally experience after hot daytime conditions, creating sustained thermal stress across the full 24-hour cycle.

The recent heat pattern is also likely increasing crop water demand. High daytime temperatures elevate evapotranspiration, and warm nights can prolong plant metabolic activity and maintain greater atmospheric demand. As a result, irrigation requirements may now be higher than expected for late March. Fields managed under fixed irrigation intervals may begin to experience short-term moisture deficits if irrigation is not adjusted to current weather conditions and crop demand. Even brief periods of water stress during rapid growth can reduce cell expansion, canopy development, and final yield. At the same time, excessive irrigation in response to heat can create additional problems, including nutrient leaching, fluctuating root-zone aeration, and reduced fertilizer-use efficiency. Under these conditions, irrigation decisions should be based on crop stage, soil moisture status, and weather-based demand rather than on calendar assumptions alone.

From an IPM perspective, abnormal warming may also alter pest pressure and the timing of management decisions. Higher temperatures can accelerate insect development, shorten generation times, and increase the rate of population growth. At the same time, crop phenology is also advancing more quickly, which narrows the window for scouting, threshold-based treatment decisions, and timely intervention. In effect, both the crop and the pest may be moving ahead of schedule. Heat-stressed plants may also be less tolerant of pest injury because they are already operating under shortened growth duration and higher respiratory demand. Under these conditions, pest damage that might otherwise have been manageable under normal spring temperatures could become more economically significant.

Other agronomic practices may also need adjustment. Nutrient demand can shift when crop development accelerates, and fertilizer programs designed around the normal seasonal calendar may no longer align well with actual crop uptake. Harvest logistics may also be affected if multiple planting blocks mature earlier than expected. Labor scheduling, harvest sequencing, cooling, and market timing can all become more difficult when crop development is compressed. Thus, the current warming pattern should be viewed not only as a temperature anomaly but as a whole-system production issue affecting physiology, water management, pest pressure, and field operations.

Bottom line
The abnormal warming pattern that developed in Yuma Valley from late February through March 2026 may be substantially shortening the vegetable production window. Although earlier maturity may move harvest ahead of schedule, the physiological and agronomic costs can be considerable. A compressed season reduces the time available for biomass accumulation, while elevated nighttime temperatures increase respiration and reduce carbon retention. Together, these conditions may lower yield potential, increase irrigation demand, intensify pest management pressure, and complicate agronomic decision-making across the production system.

Figure 1. Daily maximum air temperature recorded at the AZMET Yuma Valley
station from January 1 through March 29, 2026, compared with the 2020–2025
temperature pattern. 



Figure 2. Daily minimum air temperature recorded at the AZMET Yuma Valley
station from January 1 through March 29, 2026, compared with the 2020–2025
temperature pattern. 

VegIPM Update Vol. 17, Num. 2

Jan. 21, 2026

Results of pheromone and sticky trap catches below!!

 

Corn earworm:  Few CEW moths were found in traps over the last two weeks; about average for mid-January.

Beet armyworm: BAW were collected across all locations over the last two weeks; and activity slightly increased in most areas. Above average for mid-January.

Cabbage looper:  Cabbage looper moth activity remains at all locations and above average for mid-January.

Diamondback moth: Adult activity slightly decreased across most locations. About average for mid-January.  

Whitefly: Adult activity remains low across all locations, about average for this time of year.

Thrips: Thrips adult activity remains low across locations, about average for this time of year.

Aphids: Winged aphid numbers increased over the last two weeks; below average for mid-January.

Leafminers: Adult leafminer activity remained low over the last two weeks, about average for this time of season.