In the last two editions of this UA Vegetable IPM Newsletter, I have presented a melon (Cucumis melo ‘reticulatus’ L.) crop phenology model (Figure 1; Silvertooth, 2026) based on heat unit accumulations (86/55 ºF thresholds). This model can be used for predicting and tracking crop development and identifying important stages of crop growth and development (crop phenology).
I have utilized data from AZMET for three locations in the Yuma area as examples for this season. The HU accumulations from 1 January 2026 for a set of four possible 2026 planting dates are listed in Table 1. The HU accumulations from 1 January 2026 to 10 May 2026 for these sites are listed in Table 2.
The HU accumulations after planting (HUAP) for these four possible planting dates for three Yuma area locations to 10 May 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 15 January 2026 planting date is: 1793 - 95 HU = 1698 HUAP for this case.
The information in Table 3 can help serve as a reference to check for melon crop development in the field against this phenological model in Figure 1. With this information we check actual field condition, evaluate crop status, and make some projections on crop development and management.
An important point of reference with the use of this or any similar model is the degree of variability in the normal plant growth and development patterns. Based on a 95% confidence interval, the standard deviations for each benchmark stage of growth associated with the model in Figure 1 are listed in Table 4. Commonly, we use a general rule of thumb in using this model in the field considering natural variation of approximately 100-150 HU, broadening from early in the season to later stages of development. That can commonly equate to 7-10 days later in the season.
Crop development can be delayed or accelerated based on several factors, some of which are listed in Table 5. A constructive aspect of using a model like this is considering the driving forces in crop development and how they interact.
Reference:
Silvertooth, J.C. 2026. 2026 Melon (Cantaloupe) Crop Growth and Development. University of Arizona Vegetable IPM Newsletter, Volume 17, No.9, 24 April 2026.
Table 2. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2026 to 26 April
2026 utilizing Arizona Meteorological Network (AZMET) data for each representative
site.
HT = Heat Unit Total Accumulation.
Table 2. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2026 to 26 April
2026 utilizing Arizona Meteorological Network (AZMET) data for each representative
site.
HT = Heat Unit Total Accumulation.
Table 3. Heat unit accumulations (86/55 ºF thresholds) after planting (HUAP) from four
possible 2025 planting dates and three sites in the Yuma area on 26 April 2026 utilizing
Arizona Meteorological Network (AZMET) data for each representative site. Each value
is rounded to the next whole number. 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.
Table 4. Standard deviations for the phenological model in Figure 1 based on heat unit
accumulations (86/55 ºF thresholds) after planting (HUAP) based on 95% confidence
intervals.
Table 5. Factors that can delay or stimulate crop phenology.
Figure 1. Melon (cantaloupe) phenological development model expressed in Heat Units
Accumulated After Planting (HUAP, 86/55 oF).
Few things in Yuma County are more frustrating than watching lettuce wilt in the field. Even the best-managed fields are not immune to the many destructive soilborne pathogens capable of causing significant economic losses in lettuce production fields.
It is an unfortunate reality that Fusarium wilt, corky root, bottom rot, and lettuce drop are encountered and require management nearly every season in Yuma to produce a healthy crop. Although advances in resistant cultivars and integrated disease
management have improved our ability to reduce some of the crop losses to these diseases, management options for soilborne diseases, in general, remain severely limited and often are ineffective once symptoms progress to root or crown rot and irreversible
wilting. Strong, durable genetic resistance remains the strategic priority and most viable option when it comes to the management of these pathogens.
To select the right genetic resistance package, accurate field history and proper diagnosis of the cause of plant health challenges on that ground is critical. One of the greatest obstacles in plant disease diagnosis is that symptoms alone are rarely
what we call pathognomonic, that is, they are seldom unique enough to identify down to a single disease or environmental stressor. Temperature, soil moisture, plant age at time of infection, and varietal susceptibility all influence how diseases and
disorders develop and how symptoms are expressed in the plant as a response. As a result, the same pathogen can produce different symptoms under different conditions, while entirely different pathogens may produce nearly identical symptoms. Add to
this the complexity of the frequent occurrence of co-infections with multiple pathogens colonizing the same host at the same time and the ability to provide field diagnoses or diagnosis from pictures can quickly become confusing, overwhelming, or
downright impossible.
This known variability in symptom expression underscores the importance of confirming a diagnosis rather than relying solely on field observations, especially when the time comes for selecting resistant varieties to be planted in fields with a history
of soilborne diseases. Ideally, lettuce shouldn’t be planted on ground that is contaminated with soilborne pathogens, but this isn’t always feasible in our production systems.
Despite my obligatory warning for the difficulty and unreliability of diagnosing plant diseases and disorders from symptoms alone, I thought it would be a good chance to highlight a few key trends for two infamous diseases that we frequently observed
during last season’s atypically warm and rainy weather. I’m talking of course about Fusarium wilt and corky root. My hope is that these observations may prove helpful when scouting wilting fields, explaining your observations, and, importantly,
making it easier to document on our submission sheet what you’re seeing in the field if you choose to drop off samples to the clinic for culture testing and confirmation diagnoses. Of course, I remain happy to take a look and sample from the
symptomatic field as well. Over the 2025-2026 season the plant clinic received 20 samples culturing positive for Fusarium wilt, 16 for corky root, and 8 for a co-infection with both pathogens simultaneously.
In the field, the two diseases often appear indistinguishable from aboveground symptoms alone. Both commonly produce wilting, which may be more pronounced particularly during the heat of the day then seemingly “recover” in the evenings. Plants often lose their characteristic healthy, green waxy luster and instead develop a dull, matted appearance that is more apparent when compared with neighboring healthy plants or fields and as the wilting intensifies. As the diseases progress, the outermost leaves surrounding the crown may rot and collapse, further contributing to the appearance of a sick and declining plant. At this point, secondary microorganisms of both the bacterial and fungal kind may set in and produce signs of growth as they feed on the dead tissue, potentially appearing to be related to the cause of the disease and complicating diagnosis.
Below ground, corky root primarily affects the outermost tissues of the root system, causing feeder roots and the taproot to develop yellow to brown (sometime with specks of green) lesions that eventually crack and become roughened. To me, severely affected roots appear as if they are covered in mesquite bark. In contrast, Fusarium wilt primarily invades the plant's vascular system. Although root deterioration may occur (especially on small feeder roots), the hallmark of Fusarium wilt is vascular discoloration within the taproot and crown, where the water-conducting xylem tissues become tan to reddish-brown. These belowground differences, together with laboratory culture or other laboratory testing, provide the evidence needed to confidently distinguish between the two diseases.
Figure 1: Symptoms on a sample culturing positive for Fusarium wilt alone. Left.) outside view of the root system mostly absent of any rot symptoms, and right.) dissected root showing interior vascular discoloration. Note the bright white cortical tissue between the vascular tissue (center) and epidermis (outermost layer of cells, the “skin”) and near absence of overall rot of inner taproot tissue that is commonly observed with other soilborne diseases. The vascular system appears reddish due to the colonization with Fusarium oxysporum f. sp. lactucae.
Figure 3: Symptoms on a sample co-infected with Fusarium wilt and corky root. Left.) outside view of the root system showing significant barky lesions, and right.) dissected root shows a barky vascular dry rot towards the tip of the taproot with reddish-brown discoloration moving up towards the crown.
The images above are from some of the more diagnostically clear samples we have received in the clinic. As mentioned earlier, timing of infection, local microclimate, and varietal genetics can play a huge role in defining how a plant expresses disease
symptoms, so these observations of textbook symptomology may not be as clear in every field with wilting that you manage. For these reasons, laboratory testing remains the gold standard for disease diagnosis. Submission of symptomatic plants to the
diagnostic clinic allows for pathogen isolation and culture, providing definitive identification that can distinguish between diseases with similar symptoms. Accurate diagnosis is the foundation of effective disease management, ensuring that control
measures are directed at the true causal agent rather than its look-alikes. The Yuma Plant Health Clinic is here to help to that end.
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
For those interested in in-row weed control, checkout the quality video below (Fig. 1) on how finger weeders (Fig. 2) are being used on a 3,600 acre organic cotton farm in Texas. The grower states that finger weeders help him achieve 95-97% weed control and are a highly cost-effective tool for lowering hand weeding costs.
Fig. 1. Carl Pepper discusses how finger weeders are used to control in-row weeds on
his 3,600 acre organic cotton farm. Click here or on image to view. (Photo credit: Tilmor
LLC, Dalton, OH)
Fig. 2. Finger weeders, an in-row weeding tool, operating in seedling cotton. Finger
weeder pairs are centered on the seed row and overlapped slightly to loosen soil in the
row and uproot small weeds.
As we wrapped up our chemigation trial, I would like to extend our sincere thanks to all the participating partners and to every grower, PCA, and representative who took time from a busy schedule to visit and observe the trial.
With spring well underway across the Yuma valley, weed proliferation is accelerating rapidly. Both in winter vegetable crops approaching harvest and in newly planted, emerging warm-season crops. Proactive and effective weed management remains essential for crop protection, harvest efficiency, and long-term field productivity within this desert production system.
The Yuma Integrated Weed Management Program supports growers and pest control advisors by providing locally relevant, research-based guidance to address current and emerging weed challenges.
Crops We Typically Support
Our program works closely with the Yuma production system to address weed management challenges in:
Common Weed Challenges This Time of Year
During April in Yuma Valley, we typically observe:
Timely management decisions made now can significantly reduce weed pressure throughout the season and help prevent seed accumulation for future crops.
How the Yuma Weed Management Program Help
We provide direct, practical support to growers and pest control advisors, including:
Our focus is on solving the real-world, field-level weed problems that are unique to the Yuma region.
Get in Touch — We Are Here to Help
If you are experiencing weed problems in your fields, have concerns regarding herbicide efficacy or crop response, or need assistance planning current or future weed management programs, we encourage you to reach out.
Growers — Contact us for weed identification, control options, and management strategies tailored to the current season and your specific crops.
PCAs — We welcome collaboration and field observations. Working together allows us to address weed challenges more effectively across the entire Yuma production system.
Early communication leads to better outcomes. The Yuma Weed Management Program, operating within the broader Integrated Pest Management framework, is available throughout the season to help you manage weeds effectively and sustainably.Background
The University of Arizona Specialty Crops IPM program supports the agricultural community by delivering research-based solutions for insect pest management, with a strong emphasis on desert production systems in Yuma and surrounding regions. The program addresses key challenges, including insecticide efficacy, resistance management, biological control, and the development of practical, economically viable IPM strategies through applied research and targeted Extension and outreach efforts. The program integrates research and Extension to improve insect pest management by developing and facilitating the adoption of farm-ready tools and promoting sustainable production systems that reduce reliance on chemical inputs.
I need your help!
I am conducting this survey to gather honest, anonymous feedback on my program focus, activities, and accomplishments so far. Your input will help me to improve the program and serve you more effectively. All responses are anonymous and will remain confidential. I greatly appreciate your honest responses to help me improve my work. Please click on the link below to take the survey.
Survey link: https://uarizona.co1.qualtrics.com/jfe/form/SWCV_5nz4KTx9reNsavY
Thank you for your time and support!Reference evapotranspiration, or ETo, is a daily measure of atmospheric water Reference evapotranspiration, or ETo, is a daily measure of atmospheric water demand from a well-watered reference surface. For growers in Yuma Valley, where almost every drop of water a crop receives comes from irrigation, ETo is one of the most useful numbers available for deciding when and how much to irrigate.
How ETo changes over the year
The graph shows daily ETo at the AZMET Yuma Valley station from May 2023 through early 2026 (Figure 1). The pattern repeats each year: ETo is low in winter, climbs through spring, peaks during the hot summer period, and declines again in fall. From June through August, daily ETo often rises into the 0.30 to 0.40 inch per day range, with some peak days approaching 0.44 inch per day. During December through February, daily ETo often falls below 0.10 inch per day. This seasonal swing is much more important for irrigation management than the weak downward trendline.
That is a four- to five-fold difference in daily crop water demand between the peak of summer and the depth of winter. A grower irrigating a full-canopy crop in July is managing a very different situation than the same grower irrigating in January, even if the field looks similar.
Figure 1. Daily reference evapotranspiration (ETo) at the AZMET Yuma Valley station
from May 2023 through early 2026.
What high and low ETo numbers actually mean
Four weather factors drive ETo up or down: solar radiation, air temperature, relative humidity, and wind speed. When all four push in the same direction, bright sun, high heat, dry air, and strong wind- ETo climbs quickly and crop water demand rises with it. When conditions are mild and humid, ETo stays low, and irrigation needs ease off. The table below gives a practical guide to what different ETo ranges mean for a Yuma Valley grower and how to respond:
These ranges are general guides. The actual water use of any specific crop also depends on its growth stage and canopy cover, which is captured by the crop coefficient (Kc). A young transplant with little canopy uses far less water than a mature plant at full cover, even when ETo is the same.
Turning ETo into an irrigation target
Multiply daily ETo by the crop coefficient (Kc) for your crop and growth stage to get estimated crop water use:
Crop ET = ETo × Kc
For example, if ETo is 0.30 inches per day and your crop is at a growth stage with a Kc of 0.80, the crop is using about 0.24 inches of water that day. At a peak summer ETo of 0.40 inches per day with the same Kc, water use climbs to 0.32 inches per day. That difference adds up quickly across a field over a week. Staying current with daily ETo during high-demand periods matters more than it does in the cooler months, when ETo is low and there is more time to respond.
What this means for irrigation management
During warm, sunny, dry, and windy periods, particularly from April through September, check ETo daily and adjust irrigation accordingly. During cooler months, ETo is low enough that there is more flexibility, but it still varies with weather and should not be ignored entirely.
Current daily ETo for Yuma Valley is available from the AZMET network at ag.arizona.edu/azmet. Use the daily ETo value along with your crop’s Kc to estimate how much water to apply. Combining that number with soil moisture information and knowledge of your irrigation system’s output will give you the most reliable basis for scheduling. During peak summer demand, staying close to that daily target is one of the most effective things a grower can do to protect yield and avoid water stress.
