Good plant health is dependent on a strong and healthy root system. Similarly, a healthy plant is dependent on a healthy soil. A soil with good aggregation and physical structure is conducive to better water-holding capacity and root penetration and development. Soil health, strong root system development, and plant health are three inter-related and fundamental components of vigorous and healthy crop plants.
Root systems provide the foundation for plant development. Roots are responsible for all water and nutrient uptake by the plant, and they provide the physical anchoring and support of the plant structure.
Each plant and crop species has its own “personality” and growth habits, that includes root systems. Accordingly, root systems have unique characteristics among plants species (Gardner, Pearce, and Mitchel, 1985; Moore et al., 1998).
Young plant roots, particularly at the time of germination and stand establishment, are generally the most sensitive plant part to soil and water salinity. In fact, seedling plant sensitivity to salinity can often be measured by approximately ½ of the tabulated salinity tolerance guidelines.
In general plant root systems constitute 30-50% of the total plant dry matter. When post-harvest plant residues are incorporated into the soil, the root systems provide a significant contribution to that plant material and final carbon (C) contributions to the soil, which is an important factor contributing to soil health.
The first thing a seed develops in the germination process is a primary root that grows downward into the soil. We often refer to this as the “stinger” root that extends from a germinating seed. New cells are formed at the tip of the primary root as it extends downward into the soil forming a “thimble-shaped” cluster of cells called a root cap (Figure 1).
The root cap serves as a type of shield that helps the root penetrate the soil and it protects the developing root tissue. As the root grows downward into the soil the root cap cells are sloughed off creating a slimy surface that helps lubricate the root as it extends deeper into the soil (Moore et al., 1998).
The growing point (apical meristem) for the developing root is just behind the root cap. This growing point is the zone of new cell formation that facilitates root growth and replaces the cells that are sloughed off as the root grows through the soil. The new cells elongate and serve to extend the roots further into the soil (Figure 1).
The most active parts of the plant root system for mineral nutrient and water uptake are in the tiny root hairs that are formed in zone behind the apical meristem. Root hairs are only formed in the relatively new and freshly developed root tissue (Moore et al., 1998). The root hairs are extremely small, tender, and physiologically active. Healthy fresh young roots and root hairs should be clean and white.
Root hairs are often referred to as “feeder roots” due to their high-level of activity in securing water and nutrients from the soil for the growing plant. In the process of transplanting, it is important to protect the feeder roots as much as possible and promote their health to ensure rapid adaptation to the new soil environment.
Young plants have the capacity to develop basic aboveground tissue to perform sufficient photosynthesis for establishment and growth. Above ground growth is dependent on the plant’s ability to take up mineral nutrients and water from the soil from the root system. Sometimes it can appear that plants are not growing rapidly while the young crop is investing energy and resources into root system development.
Energy for root development is dependent upon the photosynthetic capacity of the plant. This demonstrates the synchrony required between plant shoots and roots and this is the foundation for complete and subsequent plant growth and development.
The depth of the roots will vary according to the soil physical conditions and effective soil depth, soil fertility and salinity management, plant-available water, and of course the natural rooting characteristics of the plant.
In general, there are two basic types of plant root systems. Broadleaf plants (dicotyledonous) and coniferous plants (gymnosperms) commonly have a taproot system the extends downward through the soil developing root branches from the primary root stem (Figure 2).
Grass plants and their relatives (monocotyledonous plants) produce fibrous root systems that branch extensively and radiate out into the soil from the plant base (Figures 2 and 3).
In general, taproots tend to be deeper with extensive branching from the primary root, develop woody tissue on older roots, and are generally long-lived. In contrast, fibrous roots tend to be smaller, short-lived, with less branching (Moore et al., 1998).
As roots age, they become more fully developed in conducting nutrients and water to the growing points of the plant, both above and belowground. In all cases, the young and freshly developed root hairs (feeder roots) are the primary zone of water and mineral nutrient uptake.
As root systems age, the older roots will die, and new root tissue is formed. As dead roots are sloughed off, the discarded tissue is attacked by naturally occurring, beneficial soil organisms (bacteria, fungi, protozoa, and worms) the release of mineral nutrients and produce soil organic matter. Turnover of root tissue is an important aspect of plant contributions to soil carbon (C), organic matter, and general soil health.
We do not directly see the plant root systems, and we cannot watch root hair development. But it is good to be conscious of root system development since all mineral nutrients, water uptake, and structural support are provided through the roots.
In field evaluations it is necessary to sacrifice a few plants occasionally and evaluate root system health and development. Healthy plant roots should have white and clear tissues on their surface. Examples are shown in Figures 3 and 4.
Accordingly, it is good to review and understand normal root structure and function as we work to manage crop plants for optimum growth, development, and yield.
Figure 1. Basic root tip anatomy.
Figure 2. Examples of taproot and fibrous root systems.
Figure 3. Healthy fibrous root systems on a cereal
grain crop. Source: Grain Central, 2021.
Figure 4. Clean and healthy lettuce plant roots.
Source: Fifth Season Gardening.
References:
Gardner, Pearce, and Mitchell. 1985. Physiology of Crop Plants. The Iowa State University Press.
Moore R., W.D. Clark, and D.S. Vodopich. Botany. The McGraw-Hill Companies. 1998. ISBN: 0-697-38363-1
To view this article as a PDF, click here and hit download.
First, I want to thank everyone who participated in last week's Vegetable Pest Losses Survey.
This year's survey included the return of the lettuce disease losses section. While several diseases were present and managed last season, downy mildew accounted for the majority of disease management costs by a wide margin. This really underscores the
impact that last spring's unusually rainy weather had on disease development across the Yuma lettuce production region.
No one can predict exactly what this upcoming spring will bring, but there has been discussion about the possibility of a strong El Niño leading to an extended monsoon season. If that proves true, the conditions would once again support spring
downy mildew development. The pathogen only needs about 3 to 4 hours of continuous leaf wetness to infect lettuce, so periods of overnight moisture, prolonged morning dew, or frequent rainfall when inoculum (spores) are present increase disease risk.
With that in mind, this seems like a good opportunity to review what is known about downy mildew and discuss strategies for its management.
Resistance in lettuce to Bremia lactucae, the causal oomycete pathogen behind downy mildew, is inherited in a gene-for-gene fashion, meaning one major gene product in the plant host interacts with one major gene product in the pathogen. When
resistance is present, this leads to an incompatible interaction between plant and pathogen and results in complete immunity to infection. Resistance genes in these types of interactions most often encode a protein molecule that acts like a burglar
alarm. These molecular sensors in the host bind to proteins secreted specifically by the pathogen, and when that happens a storm of defense responses is activated in the plant that excludes further infection. This is not the only mode of genetic resistance
found in plants, but it is often the most drastic and effective against obligate parasites like downy mildew.
But this simple gene-for-gene interaction often puts incredible selection pressure on the pathogen populations to change such that they can get around the resistance. By losing the molecule that the plant detects in order to initiate a defense response,
the pathogen becomes unrecognizable to the resistance genes a plant variety may have. Just like spraying the same mode of action over and over again leads to a pest population developing resistance to a pesticide, the same selection applies to genetic
resistance. The longer a resistance gene is deployed in a region, the more likely the pathogen population is to change in response until that resistance gene is no longer effective at managing the disease.
One of the biggest challenges with lettuce downy mildew is that B. lactucae is constantly changing over time. It exists as many different races, where each race has a different reaction to the resistance genes bred into lettuce varieties. You
can think of these races as different versions of the same pathogen. A lettuce variety that resists one race may still be susceptible to another.
These races are identified by testing them against a panel of lettuce varieties with known resistance genes. In the western United States, races are named by the International Bremia Evaluation Board-U.S. (IBEB-US) and are given names with a number followed by the country’s abbreviation, such as 8US, 9US, or 10US. The populations found in the western U.S. are different from those found in Europe, so each region uses its own independent naming system.
The downy mildew population has changed considerably over time. Earlier races (1US through 4US) are now rarely found in commercial lettuce production. During the 2000s and 2010s, races 5US through 8US became the most common. Race 9US was recognized after being detected repeatedly between 2015 and 2017, and the newest officially recognized race, 10US, was designated in 2025. Below is a pie chart showing the relative frequency of the races identified from 114 Yuma County downy mildew samples between 2023-2024:
Figure 1: Pathotyping results of 114 lettuce samples from Yuma County collected between 2023 and 2024. Data source: https://bremia.ucdavis.edu/bremia_database_main.php
The results show that much of the downy mildew population found in Yuma County is made up of novel strains of Bremia lactucae that have not yet been officially classified as a race. An official race is only recognized after it has been shown to be stable and widespread over multiple locations and growing seasons. These newer strains may disappear over time, or they may eventually become established and earn an official race designation. In the meantime, this means growers and lettuce breeders in Yuma County are often dealing with strains that can dodge the resistance in some lettuce varieties before those strains are common enough to be officially recognized. It also highlights why relying on resistance alone is not enough to manage the disease.
Table 1: Pathotyping and fungicide sensitivity results of samples from Yuma County collected in 2025.
This trend appears to be continuing. All of the downy mildew samples sent for race testing last season were identified as novel strains rather than known, officially designated races.
It's impossible to predict exactly how these new strains will respond to the resistance genes found in today's commercial lettuce varieties. However, because they have not been previously characterized, they are more likely to overcome existing genetic
resistance than the races we already know about.
New strains develop naturally over time. They can arise when different strains exchange genetics (i.e. intermate) or through random mutations. When growers plant varieties with similar resistance packages over large areas, the pathogen population
is placed under strong selection pressure. Any strain that happens to acquire the ability to infect those resistant varieties gains a major advantage and gets to reproduce without competition where other strains cannot. Over just a few disease
cycles, those successful strains can become much more common in the population until they are the predominant strain overall.
An important point to remember is that the resistance bred into commercial lettuce varieties is not wearing out or becoming weaker over time. The genetics in the lettuce remain just as effective as when the variety was released. What changes is the
pathogen. As the downy mildew population evolves new strains emerge that can bypass resistance genes that previously worked very well.
That means that varieties carrying resistance to races 5US through 10US are still doing exactly what they were designed to do. They continue to suppress those known races and help prevent them from becoming widespread in commercial fields. So, if
you experience significant downy mildew in a field planted with a variety that has a strong resistance package, the culprit is most likely one of these newer, uncharacterized strains rather than a failure of the variety itself.
Unfortunately, Bremia lactucae can evolve much faster than scientists can identify new races and breeders can develop and release resistant varieties. That's why no resistance package should be viewed as a stand-alone solution.
This is also why extension, researchers, and the seed and crop protection industries place so much emphasis on the integrated pest management (IPM) concept. Genetic resistance is an essential tool, but it works best and remains the most sustainable when combined with other management practices. For novel strains that can slip past host resistance, timely fungicide applications and other disease management strategies become especially important for maintaining control.
Figure 2: Mean disease severity by treatment. Disease severity was determined by rating 10 plants within each of the five replicate plots per treatment using the following rating system: 0 = no downy mildew present; 1 = downy mildew present on bottom leaves of plant; 2 = downy mildew present on bottom leaves and lower wrapper leaves; 3 = downy mildew present on bottom leaves and all wrapper leaves; 4 = downy mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = downy mildew present on entire plant. Disease severity is displayed as the mean of five replicates across all three lettuce varieties and bars show a 95% confidence interval around the mean calculated from individual treatment data. Compact letter display (CLD) above boxes show significantly different treatments (Kruskal-Wallis ANOVA and Dunn’s test). Boxes sharing the same letter(s) are not significantly different from one another. Fb = “followed by” in the rotation. Not all products are registered yet for use in lettuce. The inclusion of specific fungicide products or formulations in these trials does not constitute an endorsement or recommendation over other labeled products.
The most effective way to manage lettuce downy mildew is to use an integrated approach. Plant varieties with a strong resistance package against races 5US through 10US, and pair that resistance with timely, full-label-rate fungicide applications when environmental conditions favor disease. This combination provides the broadest and most reliable protection against both known races and the novel strains that continue to emerge in Yuma County.
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 109New automated/robotic ag technologies are coming out all the time. Ever wonder how they function in the “real world” and whether they are cost effective? Western Growers recently released a case study report on the economic impact of Stout Industrial Technology, Inc.’s Smart Cultivator on overall weeding costs. The study tracked expenses, productivity, and labor savings of the machine operating over one year on five types of lettuce crops and 2,700 acres at Triangle Farms in Salinas, CA. It is a well done, detailed study with machine costs and labor savings broken down by crop type and acreage. It’s an easy read and worth the time for those interested in the economic and overall feasibility of automated mechanical weeding. Check it out here or by clicking the image below. I don’t want to be a spoiler, but I was surprised to learn that costs for hand weeding lettuce in Salinas, CA were so high - $525/acre (conventional) and $750/acre (organic) and that in these conditions, the return on the $330K investment for the machine was less than one year when used on 2,700 acres.
Stay tuned. Western Growers plans to release four more automation technology case study reports within the next year. Upcoming reports include grower case studies experiences with the automated weeding machine from Ecorobotix; and with autonomous ag platforms from Burro, GUSS Automation, and Bluewhite. Their first report, which examined the economics of Carbon Robotics’ Laser Weeder at the commercial scale, can be found here.
Fig. 1. Western Growers case study report on the economic impact of Stout Industrial
Technology, Inc.’s weeding machine on weeding costs in lettuce on 2,700 acres at
Triangle Farms, Salinas, CA. Click here or on the figure to view. (Photo credit: The
Western Growers Centerfor Innovation & Technology)
Recently, a sample of different species of sprangletop weed was sent to me by a PCA for identification purposes, highlighting the importance of accurately recognizing this troublesome group of grassy weeds. Sprangletop can appear similar across various species, but correct identification is crucial for effective management and herbicide selection. Sprangletop species belong to the genus Leptochloa and are generally summer annuals or short-lived perennials that thrive in wet or irrigated environments, often impacting specialty crops, orchards, and rangelands. The most common types you may encounter include Mexican sprangletop (Leptochloa fusca ssp. uninervia), green sprangletop (Leptochloa dubia), and bearded sprangletop (Leptochloa fascicularis).
Key Identification Features
Practical Tips
When inspecting sprangletop, focus on panicle shape and density, spikelet arrangement, and the presence or absence of awns. Note leaf blade length and texture as well as sheath color. Ligule characteristics—whether hairy, membranous, or jagged— can also aid identification. Correctly identifying the specific sprangletop type helps tailor weed control strategies, especially herbicide selection, since control efficacy can vary among species. For growers and PCAs encountering sprangletop challenges, collecting samples and seeking expert identification support is highly recommended, as illustrated by the recent field sample I received. Understanding these distinctions enhances integrated weed management efforts and supports cleaner, more productive crops.
| Mexican sprangletop (Leptochloa fusca ssp. uninervia) |
Green sprangletop (Leptochloa dubia) |
Bearded sprangletop (Leptochloa fascicularis) Source: https://weedid.missouri.edu/weedinfo.cfm?weed_id=473
References:
Flea beetles can be serious pests of vegetable crops. Unmanaged populations can lead to substantial crop losses and cosmetic damage, particularly to leafy vegetables and Brassica crops. Although several flea beetle species attack vegetable crops, the most damaging species is the pale striped flea beetle. This beetle has a very broad host range and is an important pest in all leafy vegetables, Brassica crops, carrots, beets, and cucurbits. They can also occur in field crops such as alfalfa, corn, cotton, sugar beets, and Sudan grass. Additionally, the pest can be found on several weed species, including purslane, lambsquarter, and pigweed, thus proper management of weeds in and around your plots can help with the management of the pest. On leafy vegetables and Brassica crops, pale striped flea beetle adults cause most of the damage by attacking the emerging cotyledons of direct-seeded plants and the tender new growth of transplants during stand establishment.
Organic insecticide options for the pale striped flea beetle are limited. We observed some inconsistent results from fall 2024 & 2025 trials. Results from fall 2024 trial demonstrated that some organic insecticides including Biolink (insect & bird repellent), a tank mix of Biolink + Pyganic, and a tank mix of Entrust + M-Pede provided 55, 51, and 46% suppression of pale striped flea beetle, respectively (Fig.1). However, these insecticides only provided minimal suppression of the pest in our fall 2025 trial (Fig. 2). In fall 2024, insecticide treatments were applied using chemigation through sprinklers during the last 40 minutes of germination water while, in 2025, insecticides treatments were applied at cotyledon stage using a backpack sprayer. This indicates that applying these insecticides using chemigation through sprinklers during the last 40 minutes of germination water might be the best timing to enhance the efficacy of these organic insecticides against the pale striped flea beetle. Among the insecticides evaluated in our fall 2025 trial, Captiva Prime resulted in more pale striped flea beetle suppression (Fig. 2).
Figure 1. Insecticide efficacy trial against pale striped flea beetle, fall 2024.
Figure 2. Insecticide efficacy trial against pale striped flea beetle, fall 2025.
Additional Reading Materials
1- Calvin et al. 2025. Organic-Allowed Insecticide Options for the Management of Six Major Insect Pests in Arizona’s Vegetable Crops. https://extension.arizona.edu/publication/organic-allowed-insecticide-options-management-six-major-insect-pests-arizonas
2- Palumbo, JC. 2018. Insect Management on Desert Produce and Melons: Pests at Stand Establishment. https://vegetableipmupdates.arizona.edu/sites/default/files/2021-09/180808_pests_at_stand_establishment_2018.pdf
Managing nitrogen efficiently is one of the most important and complex challenges for lettuce growers in the desert Southwest. In Yuma Valley, where over 90% of the nation’s winter lettuce is produced, irrigation and fertilization must be closely coordinated to ensure nitrogen (N) remains available in the crop root zone. Because nitrate-N is highly mobile in soil, both under- and over-application can impact crop yield, input costs, and environmental quality. Traditionally, growers and crop advisors have relied on periodic soil sampling and laboratory testing to assess nitrate levels. While accurate, these methods are time-consuming and represent only a snapshot of field conditions at the time of sampling. This limitation often makes it difficult to track rapid changes in nitrogen availability following irrigation or fertilizer events.
Introducing Near-Real-Time Nitrate Sensing Technology
Recent technological advances are helping close that information gap. Near-real-time nitrate sensors are designed to monitor soil nitrate-N directly in the field, transmitting readings continuously to an online dashboard that growers can access on their computer or smartphone. These sensors, along with soil moisture probes, offer the potential to better understand nitrogen movement and crop uptake dynamics across the growing season.
During the 2024–2025 lettuce season, the University of Arizona Yuma Agricultural Center conducted an evaluation of this emerging technology using the AquaSpy (Inc.) nitrate-N sensor system under both organic and conventional iceberg lettuce production.
Fertilization and Sensor Setup
The conventional treatment received 200 lbs N acre⁻¹ of synthetic fertilizer applied pre-plant. The organic system received 2,000 lbs acre⁻¹ of chicken manure (4-4-2) before planting and 1,800 lbs acre⁻¹ of organic fertilizer (9-6-1) side-dressed midseason. Nitrate and soil moisture sensors were installed after crop emergence between two healthy lettuce plants, with probes positioned vertically at depths of 3, 6, 9, 12, 15, and 18 inches (Figure 1). The sensors collected hourly data on nitrate-N and soil moisture and transmitted it to a cloud-based platform for remote access.
Figure 1. Nitrate-N sensor-soil moisture sensor from AquaSpy and soil moisture sensor
from Sentek were installed between two healthy plants in the organic lettuce production
field at the Valley Research Center at the University of Arizona, Yuma Agricultural Center,
Yuma, Arizona.
Results and Observations
The sensors performed well under both systems, capturing seasonal changes in nitrate-N levels that corresponded with fertilizer applications and irrigation events (Figures 2 and 3). The data showed a clear rise in soil nitrate-N following the January 8, 2025, side-dress application of organic fertilizer and subsequent decreases following irrigation cycles, reflecting nitrogen redistribution in the soil profile. Comparisons with laboratory-analyzed soil samples confirmed that sensor readings followed the same general trends, with differences typically within 3–4 ppm of lab values. Although no formal statistical analysis was conducted in this initial evaluation, the consistency between methods demonstrated that near-real-time sensors can effectively capture nitrogen dynamics in the active root zone. Soil moisture levels strongly influenced sensor performance. Under persistently dry conditions, nitrate readings tended to stabilize or decline, likely due to reduced nitrate mobility and limited diffusion in the soil solution. These results reaffirm that maintaining uniform and adequate soil moisture is essential for both crop uptake and accurate sensor measurements.
Figure 2. Soil nitrate concentrations in the first and second foot of the soil profile under
the conventional lettuce system during the 2024–2025 growing season.
Figure 3. Soil nitrate concentrations in the first and second foot of the soil profile under
the organic lettuce system during the 2024–2025 growing season.
Ongoing Evaluation
This study represents an initial evaluation of nitrate-N sensor technology under Yuma Valley conditions. Additional research is underway to expand testing across different fields, soil types, and seasonal moisture conditions. Future work will focus on calibration, long-term accuracy, and integration with precision irrigation systems to better support adaptive nitrogen management strategies. The early results are encouraging: near-real-time nitrate-N sensing has the potential to become a valuable component of precision agriculture in the desert Southwest, helping growers maintain productivity while conserving water and nutrients.
To read the full Extension article, visit: https://extension.arizona.edu/publication/performance-evaluation-nitrate-nitrogen-sensingtechnologies-organic-and-conventional
VegIPM Update Vol. 17, Num. 15
July 22, 2026
Results of trap catches below!!
Whitefly: Adult activity remains steady across locations; above average for this time of the year, especially high numbers seen in North Gila Valley. Historically, whitefly numbers peak in July.
Thrips: Adult thrips activity remained low over the last two weeks. About average for this time of the year. Historically, thrips numbers remain low until Sept-Oct.
