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)
On March 30th, I shared a newsletter highlighting an increase in celery samples exhibiting symptoms consistent with black heart and black streak abiotic disorders, associated in timing with the mid-March heat wave. At that time, affected plants showed elongated necrotic pitting accompanied by pronounced horizontal cracking along the inner petioles. In more severe cases, there was complete breakdown of young tissues within the heart, including petioles and emerging leaves. Importantly, diagnostic testing on those early samples returned negative results for the common viral pathogens typically considered in the differential diagnoses fitting these symptoms.
Early black heart symptoms
Late black heart symptoms + secondary colonization
Black streak of celery
The YPHC has since received a new sample that changes part of that picture. This most recent submission tested positive for celery mosaic virus (CeMV). In addition to sunken discolored stalk lesions that could potentially be mistaken for early black heart or black streak, the plants in this sample exhibited severe foliar symptoms. Leaf symptoms included deformation including curling and marginal distortion. The leaves also showed distinct mosaic patterning, yellowing, and streaking, all of which are typical characteristics for viral infections.
Sunken, discolored lesions on celery petioles
Broad yellowing and mosaic pattern (left) and severe leaf distortion
and mild mosaic (right) on the foliage
Mosaic, yellowing, and leaf distortion combined on the same petiole
Additional symptoms associated with CeMV can include overall plant stunting and abnormal growth habit (Greer et al. 2024). In some cases, outer petioles may distort severely and begin growing horizontally, giving the plant a flattened appearance. Symptom severity can vary depending on the strain of the virus and environmental conditions during host infection. Be on the lookout for these additional foliar symptoms if you’re seeing sunken petiole lesions as a possible indicator of viral infection. Early infections, symptoms on different varieties of celery, or symptoms caused by different viral strains of CeMV may not look identical in severity or expression as the sample photographed here. CeMV is transmitted by aphids in a non-persistent manner and may also be spread mechanically through wounds introduced during field activities (Karanfil & Randa-Zelyüt 2025). It has not been reported to be seedborne. Fungicides are not effective against plant viruses, and while insecticides may reduce aphid populations, they are generally not effective at preventing virus transmission due to the rapid transmission following the initiation of aphid feeding that occurs before the insecticide has a chance to act on the vector (Greer et al. 2024). We will continue monitoring all incoming celery samples to determine whether this represents an isolated case or a broader trend developing alongside the ongoing celery abiotic disorders.
Sources and further reading:
Greer, C., Koike, S., & Turini, T. “Celery Mosaic Virus.” Agriculture: Celery Pest Management Guidelines. University of California Agriculture and Natural resources, 2024. URL: https://ipm.ucanr.edu/agriculture/celery/celery-mosaic-virus/#gsc.tab=0 (Accessible 4/28/2026)
Karanfil, A., & Randa-Zelyüt, F. “Celery Mosaic Virus.” Compendium of Phytopathogenic Microbes in Agro-Ecology. Springer Nature, 2025. 153–160.
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)
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:
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. 11
May 27, 2026
Results of sticky trap catches below!!
Whitefly: Adult activity continues to increase across locations, yet these numbers do not reflect what we are seeing in the field on melon insecticide efficacy trials at YAC, numbers for untreated plots and melon virus incidence are well above what we have seen in other years. Historically whitefly numbers peak in July.
Thrips: Adult thrips activity remained steady across locations over the last two weeks. Historically, western flower thrips numbers peak in May.
