Being able to accurately track crop development and then to describe and predict important stages of crop growth and development (crop phenology) and harvest dates is important for improving melon (Cucumis melo ‘reticulatus’ L.) crop management (e.g. fertilization, irrigation, harvest scheduling, pest management activities, labor, and machinery management, etc.). It is best to monitor and predict plant development based on the actual thermal conditions in the plant’s environment. Thermal conditions are a more reliable measure and predictive tool for plant development as opposed to a calendar, simply because plant growth is a direct response to temperature and environmental conditions.
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).
Twenty-five years ago we began working on the development and annual 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. 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.
Referencing 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 2025 to a set off our possible 2025 planting dates are listed in Table 1. The HU accumulations from 1 January 2025 to 30 March 2025 are listed in Table 2.
The HU accumulations after planting (HUAP) for these four possible planting dates for three Yuma area locations to 30 March 2025 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 2025 planting date is: 718.7 HU - 73.1 HU = 645.6 ~ 646 HUAP.
It is rather easy to test and evaluate this crop phenology model in the field under various planting dates, varieties, and conditions. 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 2.
For melon crops in the lower Colorado River Valley, we would currently expect to find fields planted and watered up in mid-January to have small melons approaching golf-ball size and fields planted in early March should be starting to show fresh blooms soon.
Table 1. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2025 on four possible 2025 planting dates utilizing Arizona Meteorological Network (AZMET) data for each representative site.
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
Table 2. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2025 to 30 March 2025 utilizing Arizona Meteorological Network (AZMET) data for each representative site.
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 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.
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 super imposed 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 °F)
I sent this out early this year, but I am resending the information as we have had few calls about the disease in desert southwest. Looks like we traded downy mildew for botrytis this year..
Botrytis rot is not considered a major problem in lettuce, but it can cause significant damage/loss when the field conditions are favorable for the pathogen. Cool wet conditions are favorable for the pathogen. Symptoms include water-soaked, brownish-gray to brownish-orange, soft wet rot that occurs on the oldest leaves in contact with the soil. Old leaves are more susceptible than young leaves and the fungus
can move into the healthy parts. Fuzzy gray growth can be observed in the disease area which is characteristic of the pathogen. In worse cases, the entire plant can collapse. Romaine cultivars, transplanted lettuce that are big and have leaves touching the soil are more susceptible.
The pathogen: Botrytis cinerea
Botrytis cinerea affects most vegetable and fruit crops, as well as a large number of shrubs, trees, flowers, and weeds. Outdoors Botrytis overwinters in the soil as mycelium on plant debris, and as black, hard, flat or irregular sclerotia in the soil and plant debris, or mixed with seed. The fungus is spread by anything that moves soil or plant debris, or transports sclerotia. The fungus requires free moisture (wet surfaces) for germination, and cool 60 to 77 F, damp weather with little wind for optimal infection, growth, sporulation, and spore release. Botrytis is also active at low temperatures, and can cause problems on vegetables stored for weeks or months at temperatures ranging from 32 to 50. Infection rarely occurs at temperatures above 77 F. Once infection occurs, the fungus grows over a range of 32 to 96 F.
Masses of microscopic conidia (asexual spores) are produced on the surface of colonized tissues in tiny grape-like clusters (see picture). They are carried by humid air currents, splashing water, tools, and clothing, to healthy plants where they initiate new infections. Conidia usually do not penetrate living tissue directly, but rather infect through wounds, or by first colonizing dead tissues (old flower petals, dying foliage, etc.) then growing into the living parts of the plant.
Disease management:
The Desert Difference: A Showcase of Ag Tech Opportunities for Growing in the Desert begins TODAY Wednesday, November 13th with a Field Demo Day at the Yuma Agricultural Center. The educational workshop will feature 13 of the latest automated and robotic technologies for pest control and improved vegetable production being demonstrated in the field. Registration begins at 7:00 am and the program starts at 7:30 am (agenda below).
The Field Demo Day is part of at wo-day event. The second day will be a standard conference with keynote speakers, breakout sessions and trade booths. The event will be held Thursday, November 14th at the Yuma Civic Center. Details of the event and Conference Day (Day 2) activities can be found here.
Looking forward to seeing everyone at both events!
Fig. 1. Field Demo Day agenda (Day 1) for The Desert Difference: A Showcase of Ag
Tech Opportunities for Growing in the Desert event. More information about the event
and Conference Day activities (Day 2) can be found here.
Today, the EPA posted in the Federal Register an Emergency Order suspending the Registrations of All Pesticide Products Containing Dimethyl Tetrachloroterephthalate (DCPA). We will include the link to the official document at the end of the article.
The notice says in the II. Emergency Order paragraph the following:
“Effective immediately, no person in any state may distribute, sell, offer for sale, hold for sale, ship, deliver for shipment, or receive and (having so received) deliver or offer to deliver to any person any pesticide product containing DCPA. Additionally, in accordance with FIFRA section 6(a)(1), EPA has elected not to permit the continued use of existing stocks, consistent with its policies applicable to cancellations where the Agency has identified significant risk concerns. See 56 FR 29362, 29367, June 26, 1991 (FRL-3845-4)”.
Also, the same paragraph in the document states clearly: “Accordingly, this Emergency Order expressly prohibits any person from using any pesticide product containing DCPA for any purpose. However, EPA will allow continued distribution of existing stocks of DCPA for the express purpose of returning any DCPA product to the registrant of such products”.
You can find and download the document posted in the journal today following this link:
https://live-azs-vegetableipmupdates.pantheonsite.io/sites/default/files/2024-08/240807_EPA_DCPA_ORDER_2024-17431.pdf
References:
We have conducted a field efficacy trial evaluating the efficacy of 14 biological insecticides alone or as a tank mix against lepidopteran pests, including diamondback moth (DBM), beet armyworm(BAW), and cabbage looper (CL). The insect pressures were relatively low when we initiated the insecticide applications; the CL number was never high enough to be considered for statistical analysis and treatment comparisons.
We applied all insecticides at the highest label rate when sprayed alone or at mid-rate when sprayed as a mixture of two insecticides using an application volume of 40 gal/ac. The adjuvant, Oroboost, was added to each of the insecticide treatments at a rate of 0.4% v/v. Most of the insecticides evaluated in our trial are registered for lepidopteran control except for M-Pede, BotaniGard, and PFR-97.
The results of our trial showed that Xentari, Xentari + Pyganic, and Entrust provided the highest level of BAW suppression. We also found that other insecticides/mixes, including Aza-Direct, Dipel, Dipel +Pyganic, Gargoil, Grandevo, Venerate, M-Pede, and PFR-97, may also cause some levels of BAW suppression (Figure 1A). Xentari and Dipel + Pyganic provided the best DBM suppression, followed by Xentari + Pyganic, Dipel, and Entrust, which provided 50-60% of DBM suppression (Figure 1B). Pyganic alone did not control either BAW or DBM (Figure 1A&B).
Table 1. List of bioinsecticides evaluated
Figure 1. Means Beet armyworm larvae (A) and Diamondback moth (B) per cabbage plant as affected by bioinsecticide sprays.
Estimation of evapotranspiration for specific crops (ETc) is important for irrigation scheduling and agricultural water management. ETc for crops such can be estimated using the following equation:
ETc = ETo x Kc
Where ETo is the evapotranspiration (ET) of a reference crop (usually grass or alfalfa), which is commonly called reference ET Reference ET (ETo) is defined as the ET from a 3-6" tall cool season grass that completely covers the ground and is supplied with adequate water. ETo is commonly used mostly in the eastern, southern, southeastern, and western U.S.
Reference ET (ETr) assumes a reference surface of tall grass (or alfalfa-20" tall) that completely covers the ground and is supplied with adequate water. ETr is more commonly used in the Midwest.
Both ETc and ETo can be expressed in units of water depth per unit of time, such as inches per day, inches per week, or inches per month. ETo is usually estimated using equations that use weather variables as inputs. These variables include solar radiation, air temperature, wind speed, and relative humidity. Reference ET or ETo can be obtained from AZMET Weather Data (https://cales.arizona.edu/AZMET/az-data.htm)
Figure 1: The Arizona Meteorological Network
The Kc is an adjustment factor called the “crop coefficient,” which mainly depends on the type of crop and its growth stage. Usually determined experimentally. Each agronomic crop has specific crop coefficients to predict water use rates at different growth stages and could be obtained from university extension or agricultural research center.
Example 1:
A lettuce crop is at the KcD growth stage, with a crop coefficient (Kc) of 0.80 (as indicated in the Kc table). The reference evapotranspiration (ETo) from October 20 to 24 is 1.20 inches over a 7-day period since the last irrigation, based on AZMET data. Determine the actual crop evapotranspiration (ETc), which also represents the total irrigation requirement, assuming the irrigation system operates at 100% efficiency.
Solution:
ETc=ETo *Kc
Kc @ KcD is 0.80
ETo = 1.20 inches
ETc: 1.20 inches * 0.80
= 0.96 inches is actual crop water use which is total irrigation requirement that needs to apply.
This time of year, John would often highlight Lepidopteran pests in the field and remind us of the importance of rotating insecticide modes of action. With worm pressure present in local crops, it’s a good time to revisit resistance management practices and ensure we’re protecting the effectiveness of these tools for seasons to come. For detailed guidelines, see Insecticide Resistance Management for Beet Armyworm, Cabbage Looper, and Diamondback Moth in Desert Produce Crops .
VegIPM Update Vol. 16, Num. 20
Oct. 1, 2025
Results of pheromone and sticky trap catches below!!
Corn earworm: CEW moth counts declined across all traps from last collection; average for this time of year.
Beet armyworm: BAW moth increased over the last two weeks; below average for this early produce season.
Cabbage looper: Cabbage looper counts increased in the last two collections; below average for mid-late September.
Diamondback moth: a few DBM moths were caught in the traps; consistent with previous years.
Whitefly: Adult movement decreased in most locations over the last two weeks, about average for this time of year.
Thrips: Thrips adult activity increased over the last two collections, typical for late September.
Aphids: Aphid movement absent so far; anticipate activity to pick up when winds begin blowing from N-NW.
Leafminers: Adult activity increased over the last two weeks, about average for this time of year.
