Chile (Capsicum annuum) is an important crop in the desert Southwest. The Capsicum annuum species is the most common group of chiles that we encounter and these chiles are common and an important part of our southwestern cuisine. The New Mexico type chile (aka, Anaheim) is the most common type of chile crop in the portion of the desert Southwest often referred to as the “southwestern chile belt” and it includes production areas in southeastern Arizona, southern New Mexico, west Texas, and northern Chihuahua and Sonora, Mexico.
There are five domesticated species of chile peppers. 1) Capsicum annuum is probably the most common to most of us and it includes many varieties such as bell peppers, wax, cayenne, jalapeños, Thai peppers, chiltepin, and all forms of New Mexico chile. 2) Capsicum frutescens includes malagueta, tabasco, piri piri, and Malawian Kambuzi. 3) Capsicum chinense includes what many consider the hottest peppers such as the naga, habanero, Datil, and Scotch bonnet. 4) Capsicum pubescens includes the South American rocoto peppers. 5) Capsicum baccatum includes the South American aji peppers (Guzman and Bosland, 2017). Some examples are shown in Figure 1 with some other chile pepper species as well.
Figure 1. Capsicum annuum pod types include A) Asian, B) Cayenne, C) Chile de Arbol, D)
Chiltepin, E) Hungarian Paprika, F) Jalapeño, G) New Mexican, and H) Poblano. A
Capsicum baccatum pod type is I) Aji. Capsicum chinense pod types are J) Bhut Jolokia, K)
Habanero, and L) Scorpion. A Capsicum pubescens pod type is M) Rocoto and a
Capsicum frutescens pod type is N) Tabasco. Source: Guzman and Bosland, 2017.
Crop Phenology
Understanding of the basic stages of plant growth in relation to time or age (crop phenology) and basic crop needs, i.e., water and nutrients, is an important part of crop management. There is a strong relationship among the phenological, morphological and physiological changes that occur with crop development. Understanding physiological function within a plant at various stages of growth and development is critical in efficiently managing crop inputs such as water and plant nutrients. Plants respond to environmental conditions, and temperature is the primary developmental force in plant development when water is not limited. Heat units (HUs) can be used as a good measure of “thermal time” that drives plant development. Thus, HUs can be used as a management tool for predicting and identifying stages of growth and physiological needs. We can then use that information for more efficient timing of irrigation and nutrient inputs to crop and pest management strategies. Figures 2 and 3 describe the basic HU concept with upper and lower thresholds.
Crop phenology models describe how crop growth and development are impacted by the environment, weather and climate, and provide an effective way to standardize crop growth and development among different years and across many locations (Baskerville and Emin, 1969; Brown, 1989).
People still use days after planting to estimate stages of crop development but HUs are more accurate and reliable under irrigated conditions where water is non-limiting factor. Water stress will alter, retard, phenological plant development, and it is a major source of variation irrespective of temperature conditions. Thus, HU based methods of predicting and tracking crop growth can be more consistently and effectively used in irrigated crop production systems.
Chiles are a warm season, perennial plant with an indeterminant growth habit that we grow and manage as an annual crop. The fruiting cycle begins at the crown stage of growth (Figures 4 and 5) and continues until the plant reaches a point of “cut-out” with a hiatus in blooming as the plant works to mature the chile fruit that the crop has set and established. People familiar with cotton plant growth and development will understand similarities in this pattern.
Figure 6 describes the basic phenological baseline for New Mexico – type chile and was developed from field studies conducted in New Mexico and Arizona (Silvertooth et al., 2010 and 2011; Soto et al., 2006; and Soto and Silvertooth, 2007).
Heat units accumulated after planting (HUAP) for any date of planting to the present date can be easily accessed in the Arizona Meteorological Network (AZMET) website.
In the 2026 season, New Mexico-type chiles in southeastern Arizona and southern New Mexico experienced a good start to the season after planting. In tracking a set of New Mexico-type chile varieties, their development is tracking very closely to the HU model shown in Figure 6. For example, crown formation and first flowers have been consistently observed at approximately 1,300 – 1,450 HUAP among numerous New Mexico type varieties in southeastern Arizona this season. Many fields are progressing into peak bloom (Figure 6), which is also the growth stage of peak water and nutrient demand, particularly nitrogen.
Figure 2. Typical relationship between the rate of plant growth and development and
temperature. Growth and development cease when temperatures decline below the lower
temperature threshold (A) or increase above the upper temperature threshold (C). Growth
and development increases rapidly when temperatures fall between the lower and upper
temperature thresholds (B).
Figure 3. Heat unit calculation with the sine curve method using upper and lower
temperature thresholds (Brown, 1989). The 86/55 ºF thresholds are used, consistent with
most warm season crops.
Figure 4. Photo of a New Mexico chile plant with crown formation and the first fruiting
branch shown withing the red circle.
Figure 5. Photo of a New Mexico chile plant with crown formation and early first fruiting
branch development. Source: Courtesy of Mr. Ed Curry, Pearce, AZ.
Figure 6. Basic phenological guideline for irrigated New Mexico-type chiles.
References:
Baskerville, G.L. and P. Emin. 1969. Rapid estimation of heat accumulation from maximum and minimum temperatures. Ecology 50:514-517.
Bosland, P.W., E.J. Votava, and E.M. Votava. 2012. Peppers: Vegetable and spice capsicums. Wallingford, U.K.: CAB Intl.
Brown, P. W. 1989. Heat units. Bull. 8915, Univ. of Arizona Cooperative Extension, College of Ag., Tucson, AZ.
Guzmán, I. and P.W. Bosland. 2017. Sensory properties of chile pepper heat - and its importance to food quality and cultural preference. Appetite, 2017 Oct 1;117:186-190. doi: 10.1016/j.appet.2017.06.026.
Silvertooth, J.C., P.W. Brown, and S. Walker. 2010. Crop Growth and Development for Irrigated Chile (Capsicum annuum). University of Arizona Cooperative Extension Bulletin No. AZ 1529
Silvertooth, J.C., P.W. Brown and S. Walker. 2011. Crop Growth and Development for Irrigated Chile (Capsicum annuum). New Mexico Chile Association, Report 32. New Mexico State University, College of Agriculture, Consumer and Environmental Science.
Soto-Ortiz, Roberto, J.C. Silvertooth, and A. Galadima. 2006. Crop Phenology for Irrigated Chiles (Capsicum annuum L.) in Arizona and New Mexico. Vegetable Report, College of Agriculture and Life Sciences Report Series P-144, November, University of Arizona.
Soto-Ortiz, R. and J.C. Silvertooth. 2007. A Crop Phenology Model for Irrigated New Mexico Chile (Capsicum annuum L.) The 2007 Vegetable Report. Jan 08:104-122.
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
This week, we initiated a trial investigating the effectiveness of applying fungicides post-emergence with an alternative tool, a point injection applicator, to control the pathogen which causes Fusarium wilt of lettuce. The hypothesis is to inject fungicide where it is needed to protect plants from the soilborne disease – in the root zone, without injuring crop roots. We are doing this with a point injection applicator, a device designed to apply ag chemicals post emergence with minimal root damage and soil disturbance. The applicator utilizes hollow pointed tips attached to a rotatable wheel to inject liquid products into the root zone at precise intervals and depths (Fig. 1). The advantage of point injection compared to conventional shank injection is that root pruning is minimized, reducing the number of wounds that provide a pathway for the pathogen to infect the plant. Stay tuned for trial details and results.
Fig. 1. Point injection applicator operating in iceberg lettuce.
Fig. 2. Click here or the image above to see the point injection system in action.
Last year, I was contacted by one of our PCAs regarding a concerning observation of escaped Palmer amaranth plants in a cotton field that may indicate potential glyphosate resistance.
Palmer amaranth continues to be one of the most problematic weeds in row crops due to its aggressive growth habit, high seed production, and documented ability to evolve herbicide resistance.
Field Observation & Seed Collection
Following the PCA report, I visited the field and began collecting seeds from the suspected escaped Palmer amaranth plants during the 2025 cotton growing season.
Sampling collection of Palmer Amaranth, August 18th, 2025
Research Effort and Funding
In collaboration with other colleagues, we submitted a grant proposal aimed at supporting a structured study of Palmer seed collection and greenhouse bioassays to test herbicide response across Yuma and Pinal County locations to map for resistance. Unfortunately, the proposal was not funded. Despite this setback, I decided to move forward by self-funding a preliminary experiment to begin generating data and insights.
Greenhouse Bioassay Initiation
The collected Palmer amaranth seeds were planted on June 4th, 2026, in a greenhouse setting. The initial germination rate of the collected seeds was 77%
Greenhouse Bioassay Experiment Setting
Experimental Plan
The next phase of this experiment involves:
This dose-response approach will help determine whether the population exhibits traits consistent with glyphosate resistance.
Field & Greenhouse Documentation
This study will be accompanied by images documenting the process, including:
What’s Next
The plants will continue to be carefully monitored as they develop to an appropriate stage for spraying. Once herbicide applications are completed and responses evaluated, results will be compiled and shared.
Stay tuned for updates as we assess whether this Palmer amaranth population represents a new case of glyphosate resistance in our region. If you have encountered similar escapes or suspect resistance in your fields, please feel free to reach out. Collaboration is key to staying ahead of herbicide resistance challenges
Whiteflies are among the most important insect pests of broccoli, cauliflower, cabbage, and other cole crops grown in Arizona. Whitefly feeding reduces plant vigor and contaminates harvestable portions with honeydew and sooty mold. With the unusually high whitefly abundance this spring and summer, it is crucial to be vigilant as you are planning for the fall growing season. As always, IPM approaches are the most effective way to manage the pest, regardless of the crop.
Resistant variety: Although highly resistant brassica varieties are limited, growers should select varieties with available tolerance whenever possible. These varieties can reduce pest damage, lower production costs, and minimize environmental impacts associated with insecticide use.
Sanitation: Crop residues should be destroyed immediately following harvest. Maintaining weedfree field borders, canal banks, roadsides, and fallow areas helps reduce breeding sites for whiteflies.
Figure 1. Immediate disking of crop residues after harvest reduces whitefly carryover
compared with leaving crop residues on the soil surface.
Crop placement: Whenever possible, avoid planting your brassica crops directly adjacent to or downwind of significant whitefly hosts like cotton, alfalfa, and melon. Delaying planting until after adjacent whitefly host crops are harvested and terminated can play a significant role in reducing whitefly pressure in your brassica crops.
Crop health: Maintaining healthy, vigorously growing crops can help reduce the impact of whitefly feeding. Proper irrigation management, balanced fertility programs, and avoidance of excessive nitrogen applications can reduce plant stress and improve crop tolerance. Excessive nitrogen promotes lush vegetative growth that may increase the crop's attractiveness to whiteflies.
Scouting: Regular scouting is essential for timely management decision-making. Fields should be monitored beginning at crop emergence and inspected at least weekly throughout the season. Scouting twice weekly during head formation is encouraged. Placing yellow sticky traps in your fields may help monitor flight activity and population increases. Particular attention should be given to fields located near cotton, alfalfa, melon, or recently harvested crops where whitefly movement is likely. Monitoring should include assessments of adults, eggs, nymphs, and beneficial insect activity.
Figure 2. The whitefly Bemisia tabaci. A) Male (smaller) and female adults feeding on a
leaf; B) circular pattern of eggs laid by B. tabaci; C) exuvia of Bemisia that is left after adult
emergence; D) Bemisia life cycle.
Conservation of natural enemies: Natural enemies can help suppress whitefly populations and should be conserved. Predators such as lacewings, lady beetles, minute pirate bugs, and big-eyed bugs, along with some parasitoid wasps, help suppress whitefly populations. Although natural enemies alone may not prevent economic damage during heavy infestations, preserving beneficial insects can reduce whitefly pressure and improve overall pest management.
Figure 3. A minute pirate bug feeding on whitefly eggs and nymphs.
Management with organic insecticides: Organic insecticides, including Pyganic, neem-based products, insecticidal soaps, microbial products containing Beauveria bassiana (e.g., BotaniGard), and Kaolin clay (Surround), can provide whitefly suppression when applied early and with thorough coverage. Multiple applications at short intervals are often necessary because most organic insecticides have limited residual activity and primarily target exposed life stages. Thorough coverage of the undersides of leaves is essential for effective suppression, as whiteflies are concentrated on the lower leaf surfaces.
Organic insecticides should be viewed as population suppression tools rather than rescue treatments. Once heavy infestations become established, satisfactory control is difficult to achieve with any currently available organic product.
Key Take-Home Messages
Following my recent article on Vapor Pressure Deficit (VPD), I received four thoughtful questions from colleagues in Arizona and California. While each question approached the topic from a different perspective, all pointed to a broader concept: the role of crop microclimates in shaping plant growth, water use, and plant health. This article takes a closer look at crop microclimates and why they matter in agricultural production systems. However, the conditions measured by a weather station (Figure 1) may not be identical to those experienced within a crop canopy. The environment surrounding leaves is often referred to as the crop microclimate.
Figure 1. The Arizona Meteorological Network (AZMET) weather station at the Yuma Valley Agricultural Center. Weather stations provide standardized measurements of ambient environmental conditions; however, environmental conditions within crop canopies may differ from those measured at the station, creating crop-specific microclimates.
As crops grow and develop, they can modify the environment within the canopy by intercepting sunlight, shading the soil, and influencing temperature, humidity, and air movement around the plant. Crop architecture, canopy density, growth stage, irrigation practices, and soil moisture conditions can all contribute to these microclimatic differences. Understanding crop microclimates is important because plants respond to the conditions they experience rather than solely to the conditions measured above the canopy. Environmental factors such as temperature and humidity influence plant growth, transpiration, and physiological processes. Likewise, many plant diseases are affected by conditions within the canopy, including temperature, humidity, and the presence of moisture on plant surfaces. Weather-station measurements remain extremely valuable because they provide a consistent and standardized description of environmental conditions. At the same time, understanding crop microclimates may provide additional insight into crop responses, water use, and plant health.
Many questions remain regarding how environmental conditions measured by weather stations relate to those experienced within different crop canopies and under different management practices. Continued research will help improve our understanding of these relationships and their implications for irrigation management, crop performance, and disease development. The recent discussions generated by the VPD article highlight the importance of considering both the broader environment measured by weather stations and the microenvironment experienced by plants. Together, these perspectives can help us better understand the complex interactions among crops, management practices, and environmental conditions.
