Two weeks ago at the Cotton Beltwide Conferences in San Antonio, I shared results from our Arizona organic cotton IPM work—and the response was immediate. Questions, follow-ups, and hallway conversations kept circling back to the same point: Arizona is different.

Arizona produces the highest-yielding cotton in the U.S. while using the lowest amount of insecticides in the Cotton Belt, the result of more than 35 years of coordinated, area-wide IPM. That same system could keep Lygus pressure low enough for organic cotton to be successful—but only if management decisions reflect organic realities.

In vegetable-focused regions like Yuma, cotton is treated as a rotational crop rather than a primary cash crop. Winter vegetables drive regional profitability and account for a significant share of Arizona’s certified organic acreage. Even so, Yuma still represented roughly 10% of the state’s cotton acreage last year, producing about 10,000 acres out of approximately 100,000 acres statewide. Cotton fits strategically as a summer rotation that benefits from the landscape-level pest suppression already in place.

Over the last 35 years, Arizona cotton IPM has evolved from an average of 11 insecticide sprays per season to fewer than two statewide, driven by decades of disciplined decision-making by growers and PCAs using selective insecticides, transgenic technologies, and conservation biological control.

Since 1996, those IPM-first choices have saved more than $700 million and prevented over 40 million pounds of insecticide active ingredient from being applied, creating the area-wide pest suppression that defines Arizona cotton today.

That foundation is what makes organic cotton feasible in this region: our field data showed limited benefit from certain OMRI-listed insecticides for Lygus, reinforcing a message shared at Beltwide that, in Arizona organic cotton, the smartest spray may be no spray at all.

Not all sunshine and cotton bolls. Important challenges remain in Arizona organic cotton IPM, and we’ll be sharing more details soon.

This work was funded in part by Western IPM Center, which also featured the project—read the story HERE.

Figure 1. The Arizona cotton IPM continuum illustrating how shifts in IPM tools and practices reduced statewide insecticide use over the past 35 years.

Click HERE to read more about our Arizona Cotton IPM Story.

Justus von Liebig was a 19th century German scientist (1803-1873) who is regarded as one of founders of organic chemistry.  He is also commonly considered as the “father of agricultural chemistry and the fertilizer industry.”

Liebig developed several important analytical methods used in a broad range of applications as well as many other important contributions and developments.  Liebig was one of the great chemistry teachers of the 19th century that served as a foundation for the robust German chemical industry of the 20thand 21st centuries.

In 1840 Justus von Liebig presented a concept that is known as the “Law of the Minimum”, which was built upon a theory that had first been developed by Carl Sprengel, a German botanist (1787-1859).  The Law of the Minimum basically states that a plant’s rate and extent of growth and overall health is dependent on the amount of the scarcest of the essential nutrients that are available to the plant (Figure 1; Liebig, 1840 and van der Ploeg et al., 1999).

The Law of the Minimum has been further applied into a general model of all organisms and biological functions, including the limiting effects of other environmental factors i.e., sunlight and water in terrestrial ecosystems, as well as excesses of nutrients and other environmental factors (Bruuselma and Nigon, 2023; Davidson, 2016; and Mosaic, 2023).

An important way to consider the Law of the Minimum is that the growth of plants, or crops, is not dependent on the total amount of nutrients available, but rather by the scarcest nutrient or resource (i.e., water).  This is particularly important in relation to nutrients such as nitrogen, which is the nutrient required in largest amounts by plants, and it is the most common limiting plant nutrient.  In desert agriculture, water is commonly the first most common limiting factor in plant growth and development, closely followed by bio-available nitrogen.

The Law of the Minimum is important to understand in managing fertilizer and other agronomic inputs to a crop.  This has been demonstrated when fertilizer prices are high, particularly for nitrogen and phosphate fertilizers, and growers may be inclined to reduce or eliminate fertilizer applications.

Liebig’s Law of the Minimum is most applicable for nutrients and plant growth factors that are mobile in the soil.  This is particularly relevant for nitrogen, which is available to plants in the nitrate form (NO3--N), which is mobile in the soil.  Thus, (NO3--N) moves with soil-water. 

Accordingly, the Liebig Law of the Minimum also pertains to water in a soil-plant system.  Other growth factors or nutrients will not compensate for a deficiency in a given nutrient or plant growth factor.  Plant-available nitrate-nitrogen (NO3--N) or water are good examples, there are no substitutes.

The Liebig Law of the Minimum is an important concept in soil fertility and plant nutrition and overall agronomic crop management in an irrigated production system.

Figure 1.  Graphic illustration of the Law of the Minimum with shortest stave in
the barrel representing the most limiting nutrient in the soil-plant system.

References:
Bruulsema, T, and Nigon, L.L. Crops & Soils Magazine, November–December 2023 American Society of Agronomy.  pp.  54-59.

Davidson, D. 2016.  Nutrient Management Magazine.https://www.no-tillfarmer.com/articles/5648-no-till-notes-how-to-plan-your-summer-fertility-program?v=preview

Liebig, J. 1840.  Die organische Chemie in ihrer Anwendung auf Agri- Sprengel, C. 1831. Chemie fu¨ r Landwirthe, Forstma¨nner und Cameralisten (Chemistry for agronomists, foresters, and agricultural econo-cultur und Physiologie (Organic chemistry in its applications to agri-culture and physiology). Friedrich Vieweg und Sohn Publ. Co., mists). Volume 1. Vandenhoeck und Ruprecht Publ. Co., Go¨ ttingen, Germany. Braunschweig, Germany.

Mosaic.  2023.  How Law of the Minimum Impacts Crops' Nutrient Use. In: No-Till Farmer. https://www.notillfarmer.com/articles/12637-how-law-of-the-minimum-impacts-crops-nutrient-use

Vander Ploeg, A.R;   Böhm, W.; and M. B. Kirkham.  1999.  On the Origin of the Theory of Mineral Nutrition of Plants and the Law of the Minimum. Soil Sci. Soc. Am. J. 63:1055–1062.

Cladosporium leaf spot on spinach has been coming into the Yuma Plant Health Clinic in increasing numbers as of late.

Most samples have been brought in displaying early lesions which start on the upper leaf surface (adaxial side). The lesions form a sunken cavity visible under a hand lens. As the spot progresses, chlorosis can be observed on the lower leaf surface (abaxial side) corresponding to the lesions above. With disease progression, lesions on both leaf surfaces become necrotic, papery, and tan, with little to no surrounding yellowing or haloing. Early lesions are usually tan to white or yellowish and measure less than 3 mm in diameter. Under favorable environmental conditions and high leaf moisture, individual spots may coalesce into larger, irregular blotches that form dark green fuzzy growth as the fungus begins to sporulate.

Cladosporium leaf spot is caused by the fungal pathogen Cladosporium variabile and possibly other closely related Cladosporium species which have shown pathogenicity in lab assays. Disease development is favored by the high humidity and extended periods of leaf wetness Yuma has been experiencing. The pathogen can be seedborne, making the use of clean or disinfested seed especially important. However, spread between fields via wind and water is also likely contributing to the current outbreak of this disease.

Research has shown that QoI fungicides (FRAC Group 11), also referred to as the strobilurins, can be effective in managing this disease when applied preventively (Higgins 2020). However, in Arizona, these products are not labeled for control of Cladosporium leaf spot on spinach. While this year may represent an outlier associated with our atypical winter weather, repeated outbreaks in future seasons would suggest the need to re-evaluate long-term management strategies should this disease become a recurring issue. Please continue to evaluate the economic impact this disease is having on your spinach production, as documentation of yield loss, quality reductions, and increased management costs will be critical in determining the need for research support and potential registration of additional management tools.

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:

Chris Detranaltes, Ph.D.
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109

As part of their efforts to promote agtech, Western Growers is developing a freely available image library of key specialty crops. The idea is to create a database of labeled/annotated images that startups and researchers can use as training sets to develop AI software for automated/robotic machines. Sets of images of mature iceberg, romaine, broccoli, cauliflower and strawberry crops have been completed and are available at https://github.com/AxisAg/GHAIDatasets/tree/main/datasets. These images are useful for creating AI models for automated harvesting machines.

Last season, in collaboration with Axis Ag, Inc., we worked to expand the database to include images of crops at all growth stages. This will allow users to develop AI tools for crop thinning, weeding and crop health monitoring. Francisco Calixtro, a UofA Yuma student majoring in Ag Systems Management, spent the winter collecting images of various vegetable crops throughout their growth cycle using an Amiga¹ (farm-ng, Watsonville, CA) robot equipped with a camera. These images will be sorted, labeled and uploaded to the image library. We’ll announce when these data sets become available. 

Special thank you to Jason Mellow, Axis Ag, Inc. and to the many growers who allowed us to capture images of their fields.

Fig. 1. Francisco Calixtro, UofA Yuma student, operates an Amiga¹ (farm-ng,
Watsonville, CA) robot equipped with a camera to capture images of various
vegetable crops at different growth stages. Images will be labeled and
uploaded to a freely available image library to facilitate development of AI
software for automated/robotic machines. (Photo credits: Jason Mellow and
Francisco Calixtro)

[1] Reference to a product or company is for specific information only and does not endorse or recommend that product or company to the exclusion of others that may be suitable.

The Southwest Agricultural Summit that recently took place in Yuma included a breakout session on Thursday, 21 February 2024 titled “New Developments in Weed Control”.
In this session Jose Antonio Cabrera representing BASF technical services for Coastal CA and Arizona provided a review of the new technologies being developed by his company in the Weed Control area. This includes new active ingredients as well as novel encapsulation technologies. The breakout session also included the lecture “Registration Support for Pest Management Tools in Specialty Crops, The IR-4 Project: Purpose and Process” by Roger B. Batts. “The IR-4 Project was established in 1963 by the U.S. Department of Agriculture to ensure that specialty crop farmers have legal access to safe and effective crop protection products. Helps growers address pest management concerns, develops data necessary for the registration of safe and effective pest management solutions with the U.S. Environmental Protection Agency¹.
Roger is the Weed Science biologist from the NC State University IR-4 headquarters. If you have questions such as: What is the IR-4 Mission? Why is IR-4 Needed? What are the programs within the IR-4 project? you will find some answers in the IR-4 Presentation Slides here.
We are currently conducting trials in Yuma in coordination with this project that could result in the addition of new labels and weed control tools for our Arizona vegetable growers.
Thank you for attending the SW Ag Summit Weed Science breakout session.
 
 
Reference:

1. https://www.ir4project.org/

Get your free copy of the Weed Seedling Identification Pocket Guide at the Yuma
Agricultural Center.

Biological control is one of the key tools for pest management in organic crop production. By maintaining permanent habitats and food sources for the pests’ natural enemies (good bugs) in the vicinity of your farms, you can ensure the continuous availability of the natural enemies. When the growing season starts, the good bugs will be readily available to attack the pests before they become established in the crops. 

Researchers have found that planting a diversity of flowering plants (e.g., sweet alyssum, nasturtium, milkweeds, common cryptantha, hillside vervain, wild petunia, etc.) on a small portion of your farms or the farms’ border can provide adequate food and shelter allowing to maintain abundant and diverse natural enemy species, including syrphid flies, tachinid flies, lacewings, parasitic wasp, etc. that will attack aphids, thrips, lepidopterans, and more. 

As you plan for the next season, please consider planting flowering plants on your farms’ borders or on dedicated patches to conserve natural enemies and enhance your biological control.

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.