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).
Galls observed on the stems of begonias submitted to the clinic
Recently, we received a submission of begonias to the Yuma Plant Health Clinic that immediately presented a diagnostic challenge. At first glance, the symptoms seemed to point toward a familiar set of suspects. The plants had developed severe swelling of the stems, symptoms characteristic of physiological oedema or bacterial Agrobacterium-associated crown gall. But, as is often the case in plant diagnostics, appearances can be deceiving.
The begonias had been grown indoors in high-humidity terrariums and planter boxes. The symptoms were not associated with a particular begonia variety, nor were they consistently expressed across all plantings or cuttings. When the plants were removed from their planting substrate, however, another clue emerged: additional small galls were scattered throughout the root systems of those plants also showing the stem swellings. That observation changed the diagnostic picture considerably.
Galls observed on the roots of begonias submitted to the clinic
Dissection of both the swollen stem tissue and the roots revealed an abundance of eggs, juveniles, mature female nematodes, and giant cells, a classic signature of root-knot nematode (RKN) infection. Root-knot nematodes belong to the genus Meloidogyne, a group of microscopic roundworms that are among the most important plant-parasitic nematodes worldwide. Their typical symptom is familiar to many resulting in the formation of characteristic galls on roots.
These galls are not simply a physical wound created by the nematodes. Rather, RKNs actively manipulate their host. During infection, the nematodes secrete salivary compounds that alter plant growth and development, effectively hijacking normal cellular processes and redirecting them toward the formation of specialized feeding structures known as giant cells.
These giant cells become a nutrient-rich feeding site for the developing and mature nematodes. In doing so, the nematode establishes a sustained drain on the plant's resources. As infection progresses, the resulting disruption to the root system can interfere with nutrient acquisition and water uptake. Aboveground, this typically translates into stunted growth, declining foliage, wilting, and, in severe infestations, plant death. With more than 3,000 plant species reported as hosts to different root-knot nematode species worldwide, including many field and vegetable crops, the annual impact of these pathogens is enormous. Much research has been done on RKN in various crops, and reports of stem swellings occurring naturally is almost non-existent.
Yet the begonia case raises an especially interesting question: Why did a pathogen best known for producing root galls appear to be responsible for such dramatic swelling in the stems? This is where the environment becomes an essential part of the diagnosis.
The symptom is not the pathogen. It is a reaction of the plant to the presence of a pathogen. One of the most important principles in plant pathological diagnostics is that the same pathogen does not necessarily produce the same symptom under every set of environmental conditions. Likewise, similar symptoms can arise from entirely different causes.
Temperature, humidity, water availability, substrate conditions, light, nutrition, plant age, and even the way a plant is cultivated can all influence how a disease develops and how its symptoms are expressed. In other words, the symptom we see is the product of an interaction between the host, the pathogen, and the environment. Change any one of those components, and the resulting disease can change dramatically. This recent begonia case is a particularly useful reminder of this principle.
Root-knot nematodes are well known for their belowground symptoms, but reports of rare and isolated cases of infections on aerial plant tissues, including stems, leaves, and flowers, are reported in only one publicly available extension bulletin from 1985. These symptoms remain unusual and have rarely been observed unless aerial tissues are artificially inoculated, and they have only been documented in a limited number of hosts.
So, when confronted with the unusual stem symptoms in a begonia, it would be easy to focus exclusively on the appearance of the tissue and pursue diagnoses such as oedema or crown gall. These plants were being cultivated in enclosed, high-humidity conditions, where moisture levels around the plant were substantially different from those experienced by a plant growing in a more open environment. Such conditions can alter transpiration rates, water movement, tissue hydration, pathogen development, and the plant's own physiological responses. They also give rise to the wet, humid conditions necessary for oedema to develop or crown gall bacteria to thrive.
The small root galls were an important clue. Microscopic examination then provided the confirmation: eggs, juveniles, mature females, and the characteristic giant cells associated with root-knot nematode feeding. These signs of RKN were found in both the root and stem swellings. Understanding the environment in which the hosts were being grown, with its high moisture conditions, then provided supporting evidence that the nematodes had favorable environmental conditions to migrate into much higher foliar tissues than is typically seen in RKN infestations and produce an unusual symptom. Swollen galls on the stem told us that something was wrong, and if we relied on the most common culprits associated with this symptom alone, namely oedema or crown gall, we would have gotten the diagnosis wrong.
The begonia case is a fascinating example of how flexible disease expression can be. A pathogen with a reputation for being almost exclusively soilborne and producing galling symptoms on root tissues, under particular host and environmental circumstances, can be associated with symptoms in tissues where we do not ordinarily expect to find them.
Ultimately, environment is not simply a backdrop against which plant disease occurs. It is one of the determinants of how disease develops, how severe it becomes, and critically for anyone trying to ID the pathogenic cause, what it looks like.
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.
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.
