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)
Fall melon season is approaching, and one recurring question I've been hearing is: Will viruses be as bad this fall as they were in the spring?
The incidence and severity of melon viruses this past spring were unprecedented across Yuma County, Imperial County, and northern Mexico. This is supported by the volume of feedback we received from growers, PCAs, and industry representatives who attended the June 2nd melon virus incident response meeting. As a result, predicting what we can expect is going to happen this fall is difficult. We have no recent, if any, experience with virus pressure at this scale in spring melon to guide our expectations for the upcoming fall season. At this point, predictions are more of an educated guess without the guidance of past observations.
To quickly recap, the three main viruses that affected cucurbits this spring were cucurbit yellow stunting disorder virus (CYSDV), cucurbit chlorotic yellows virus (CCYV), and watermelon chlorotic stunt virus (WmCSV). All three are transmitted by the Biotype B whitefly (Bemisia tabaci), whose populations overwintered at unusually high levels between 2025 and 2026. Between melon seasons, these viruses persist in a wide range of crop and weed hosts, many of which show few or no visible symptoms of infection. Unfortunately, these asymptomatic plants can still serve as reservoirs, allowing both the viruses and their whitefly vectors to bridge the melon-free gap between cropping seasons and provide a source of inoculum for newly planted fields. It is an unfortunate reality that neither the whitefly vectors nor many of the alternate host plants (weeds) show symptoms or suffer ill effects while carrying these viruses. As a result, they can stealthily maintain virus populations between melon seasons and serve as a source of infection for newly planted fields.
Below is a compilation of reported host plants for CYSDV, CCYV, and WmCSV. This list reflects the viruses’ confirmed hosts identified to date but is unlikely to be exhaustive. Additional weed and crop species may also be capable of serving as reservoirs for these viruses but have yet to be discovered or reported. Note that many of these plants may grow throughout the region as weeds, native vegetation, commercial crops, or in backyard gardens:
Table 1: Primary and alternate hosts of CYSDV, CCYV, and WmCSV reported to date.
I can see this upcoming melon season unfolding in one of two ways. On one hand, the most intuitive prediction is that the high virus inoculum and abundant whitefly populations present during the spring melon season will carry over into the fall, resulting in early and significant virus pressure. On the other hand, the intensive whitefly management programs implemented throughout the spring may have suppressed vector populations to provide knockdown to pre-winter 2025 levels, resulting in lower virus incidence early in the season than at the start of spring.
Regardless of which scenario plays out, proactive and preventative management of both whiteflies and weed reservoirs remains the most effective strategy for minimizing virus pressure in fall melons. This approach targets two critical stages of the disease cycle by reducing the initial sources of virus inoculum and limiting the whitefly vectors responsible for further virus spread.
Dr. Palumbo developed a management guide for whiteflies and CYSDV in fall melons in 2024. The recommendations are based on research findings from two key publications and provide practical guidance for reducing virus risk throughout the season, from planting through netted fruit (Castle 2017a and 2017b).
Table 2: Insecticide Use Guidelines for Whitefly /CYSDV Management in Fall Melons
The earlier melons become infected with one or more of these viruses, the greater the impact on plant growth, fruit development, and ultimately yield. Even when infection cannot be completely prevented, delaying virus establishment can substantially reduce losses in both yield and fruit quality. Protecting young plants from early whitefly feeding, and virus infection, is therefore one of the most important management objectives to reducing losses.
In field trials comparing at-plant soil applications of flupyradifurone (trade name Sivanto), dinotefuran (Venom), imidacloprid (Admire Pro), and cyantraniliprole (Verimark), Dr. Palumbo and colleagues found that flupyradifurone and dinotefuran provided the greatest protection against both whiteflies and lowest final incidence of CYSDV (Castle et al. 2017b). All products were applied as a single soil shank injection at planting, allowing systemic uptake and protection during crop establishment.
Across both spring and fall trials, flupyradifurone consistently produced the lowest whitefly densities and the lowest incidence of CYSDV. Dinotefuran was the second most effective treatment, significantly reducing both whitefly populations and CYSDV incidence, although its performance was somewhat less consistent than flupyradifurone. In contrast, at-planting treatment with imidacloprid and cyantraniliprole did not consistently reduce CYSDV incidence.
Further reading:
Castle, S., Palumbo, J., Merten, P., Cowden, C. and Prabhaker, N. (2017a), Effects of foliar and systemic insecticides on whitefly transmission and incidence of cucurbit yellow stunting disorder virus. Pest. Manag. Sci., 73: 1462-1472. https://doi.org/10.1002/ps.4478
Castle, S.J., Palumbo, J.P., Merten, P. (2017b), Field evaluation of cucurbit yellow stunting disorder virus transmission by Bemisia tabaci. Virus Res., 241:220-227. doi: https://doi.org/10.1016/j.virusres.2017.03.017
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
Finger weeders are an in-row weeding tool made from flexible rubber. Pairs are centered on the seed row and overlapped slightly to remove in-row weeds. Our experience has been that finger weeders are effective at removing small weeds (< 3-4 leaf stage), but not large, well anchored weeds. A Texas A&M University colleague shared that they were able to regularly remove 3-4inch tall Palmer Amaranth with finger weeders using a cultivator configuration developed by organic cotton grower Carl Pepper. I was pretty impressed by their video. I think you will be too. Check it out by here or by clicking on the image below.
Acknowledgements
Credit and thanks are extended to Kyle R. Russell, Texas A&M University, for capturing and sharing the video.
Fig. 1. Large Palmer Amaranth being removed from the plant row by finger weeders –
slow motion video. (Video credit: Kyle R. Russel, Texas A&M University. Cultivator
configuration credit: Carl Pepper, Lubbock, TX.)
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.
Two weeks ago, we discussed how unusually high maximum and minimum air temperatures in Yuma Valley were likely accelerating crop development and shortening the vegetable production window. Mean relative humidity (RH) provides an additional perspective on this spring’s weather pattern because it affects crops through a different pathway than temperature. While temperature strongly influences crop development rate, respiration, and time to maturity, RH is more closely related to atmospheric drying, crop water relations, canopy microclimate, and some aspects of pest management and spray application conditions.
Mean RH at the AZMET Yuma Valley station from January 1 through March 29, 2026, was more variable than the temperature pattern discussed in the earlier blog (Figure 1). Unlike maximum and minimum air temperature, which showed a clearer and more sustained departure from the 2020–2025 pattern, RH fluctuated more over time. Even so, several periods during March 2026 appear to have had relatively lower RH than the recent historical pattern, including part of the late-March period highlighted in the figure. This distinction is important because lower RH under warm conditions can increase the drying power of the air and contribute to greater atmospheric demand for water.
Figure 1. Mean relative humidity at the AZMET Yuma Valley station from January 1
through March 29, 2026, compared with the 2020–2025 pattern.
This RH pattern should be interpreted differently from the maximum and minimum temperature trends. Higher maximum temperatures can accelerate heat-unit accumulation and move crops more quickly toward maturity. Higher minimum temperatures can increase nighttime respiration and reduce carbon-use efficiency by increasing the fraction of assimilated carbon used for maintenance. Lower RH, in contrast, does not directly speed crop development or increase respiration in the same way. Instead, it can increase evaporative demand and strengthen the gradient that drives water loss from the crop and soil surface. When relatively lower RH occurs during an already warm period, crop water demand may increase further, especially in actively growing fields.
From an agronomic standpoint, this means that RH may add to the stress associated with the temperature pattern rather than duplicate it. In fields where irrigation timing, soil moisture, or root-zone conditions are already marginal, periods of lower RH may increase the likelihood of transient midday stress, reduced leaf turgor, and lower growth efficiency. This does not mean that lower RH alone caused crop stress or yield loss, but it may have increased atmospheric pressure on crops that were already developing under unusually warm conditions. For this reason, RH can help explain why water management may have become more challenging during March.
The RH pattern may also have relevance for IPM. Hot and relatively dry conditions can favor some stress-associated arthropod pests in certain crop systems, particularly when the crop is already under heat or water stress. At the same time, relatively drier air may make conditions less favorable for some moisture-dependent foliar diseases. However, these responses are highly dependent on the crop, pest, pathogen, irrigation method, canopy structure, and leaf wetness duration. Therefore, this figure should not be interpreted as showing a universal increase or decrease in pest or disease pressure. Rather, it suggests that the field environment may have shifted in ways that could influence pest dynamics and disease favorability.
Relative humidity may also matter operationally for crop protection. Under hot and relatively dry conditions, spray droplets can evaporate more rapidly, especially during the warmest part of the day. This can make application conditions less forgiving and may increase the importance of timing and coverage. Again, this does not necessarily mean reduced efficacy, but it does mean that lower RH can add practical challenges to foliar application under already warm spring conditions.
Overall, the mean RH does not replace the temperature story described in the blog from two weeks ago. Instead, it adds another layer to it. The earlier temperature data suggested that crops in Yuma Valley may have been developing more rapidly than normal because of unusually warm days and nights. The RH pattern presented here suggests that, during some periods in March, the atmosphere may also have been relatively drier than normal, potentially increasing evaporative demand and adding to crop water-management and IPM complexity. In that sense, this figure helps explain how the spring environment may have become more demanding even beyond temperature alone.
What this may mean going forward
Going forward, this pattern suggests that unusually warm periods combined with lower RH could increase the risk of faster crop development, higher water demand, and more challenging IPM timing in Yuma Valley. If these conditions become more frequent, growers may need to place even greater emphasis on weather-based irrigation scheduling, close field scouting, and timely management decisions.
VegIPM Update Vol. 17, Num. 14
July 8, 2026
Results of trap catches below!!
Whitefly: Adult activity remains steady across locations; above average for this time of the year. Historically, whitefly numbers peak in July.
Thrips: Adult thrips activity decreased over the last two weeks. About average for this time of the year. Historically, thrips numbers remain low until Sept-Oct.
Diamondback Moth: No diamondback moths have been collected in the traps since May 19th. Based on the past six years summer collection data, no DBM is collected in the traps in the summer months (Jun-Aug) until September.
