The cycling of nitrogen (N) in the soil is one of the most important aspects of soil health. Nitrogen cycling in the soil includes a multitude of interactions with microbes and plants.
The basics of soil N were described in a recent article posted in this UA Vegetable IPM newsletter (Silvertooth, 2024). The overall N reactions in the soil system are described in the N Cycle, Figure 1.
Figure 1. The nitrogen cycle.
Most of the N in soil is present as organic N in the soil organic matter. As a general estimate, there are approximately 1,000 pounds N per acre of this form for every one percent organic matter in the soil. However, soil organic matter (SOM) is relatively stable and resistant to further decay. So, most of the organic N in the soil is not biologically available during any given growing season (Havlin, et al. 2014;Thompson and Troeh, 2005; Warren, et al., 2017; and Weil and Brady, 2017).
Normally, about two percent of the N from SOM will be released each year to mineral forms when soil is cultivated. Arizona soils typically have approximately 1%organic matter. Thus, 10 to 20 pounds of N per acre is a typical amount of N present in unfertilized soils after cultivation and seed bed preparation.
We commonly assume that 1 acre-furrow slice of soil (soil that is 6 inches deep covering an area of 1 acre (43,560 ft.2))contains approximately 2 million pounds of soil. Thus, a soil in Arizona with 1% organic matter will have approximately 20,000 lbs. of SOM per acre and that will contain about1,000 lbs. of total N (5% N in the SOM), most of that being in an organic form that is not directly plant-available.
If 2% of the N in the SOM does mineralize in one year, that would release approximately 20 lbs. of N per acre per year. Accordingly, it is common to find 10 to 20 pounds of mineral plant-available N (NO3--N) per acre. That is a typical amount of mineral plant-available N present in unfertilized soils after cultivation and seed bed preparation. Higher levels of residual NO3--N is commonly a remnant of prior fertilizations.
Nitrogen Mineralization and Immobilization
Because N release from SOM is dependent upon decay by microorganisms, which themselves require N, the amount of N available for a crop is in constant flux. Unlike plants, which get their carbon (C) as carbon dioxide (CO2) from the air, many microorganisms get C from decaying organic matter.
Nitrogen availability depends upon the relative amount of C and N in the SOM, its resistance to decay, and the environmental conditions supporting microbial activity. Figure 2 illustrates how mineral N becomes more concentrated in SOM as soil organic matter decays.
Figure 2. The carbon to nitrogen ratio (C:N) declines and narrows as residues decay
until mineral nitrogen finally becomes available.
(Source: B. Raun In: Warren, et al., 2017)
Note that mineral N is not released during the first stages of decay. This is because N that is released is immediately consumed by active microorganisms. With time, remaining SOM becomes more resistant to decay, microorganisms die off, and there is more mineral N present than can be consumed by the few active microorganisms.
The first step of measurable mineral N release from SOM is usually in the form of ammonia NH3. The ammonia readily reacts with soil moisture to form ammonium NH4+. These two reactions can be stated simply as:
1) Organic nitrogen → NH3 (gas)
2) NH3 + H2O → NH4+ + OH- (ammonia + water → ammonium + hydroxide)
The process of converting or transforming N from organic compounds to inorganic compounds is called mineralization. This results in increasing N available for crops. The N form that is directly available to plants is nitrate-N (NO3--N).
When the reverse happens, plant-available N, primarily nitrate-N (NO3--N) is absorbed by crops or microorganisms. This process is called immobilization and results in a decrease in the amount of N in the soil solution that is immediately available for crops.
These processes of mineralization and immobilization and their interacting nature for a typical field soil situation are illustrated in Figure 3 starting with 2,060 lbs. of total soil N in this case partitioned among the SOM, crop residues, and mineral N pools.
Figure 3. Interacting pools of soil nitrogen.
(Source: B. Raun In: Warren et. al., 2017)
Approximately 98 percent of the soil N is unavailable for plant uptake. This large reservoir of organic N provides an important buffer against rapid changes in available N and plant stress. The small reservoir of mineral N can often be slowly replenished by mineralization (Figure 2.2).
Supplemental N as fertilizer usually must be added to support high, economic crop production levels when native levels of soil N mineralization are not sufficient. This is common in Arizona agricultural soils.
Example Case:
Immediately following fertilization with 120 pounds N, the system may be illustrated by Figure 4a. Note the increase of 120 pounds of N in the mineral N pool in addition to the original 60 lbs. of mineral N in this case (Figure 3), for a total of180 lbs. of mineral N. While the organic N pool is static at 2000 lb./acre in this example. In this case there is a total of 2,180 lbs. Nin the soil.
Addition of fertilizer N will stimulate microorganism activity, resulting in consumption of mineral N by soil microbes as they attack and break down some of the crop residues (immobilization) as illustrated in Figure 4b. Note that the total amount of N among the three pools (mineral, organic, and crop residues) is the same at 2,180lbs. N/acre.
The immobilized N will be present as microbial tissue and other new material in the organic pool. Note that the N present in the crop residue has dropped from 30 to 20 lbs. N/acre in this example due to microbial decomposition.
An important point to follow in this example is the transformations taking place with the consistent base of 2,180 lbs. of N partitioned into various forms. This is a rather simplified example, but it illustrates the natural dynamics taking place in the soil-plant system with the cycling of N and the challenges of managing N fertilizers effectively.
Figure 4. Relative amounts of organic and mineral nitrogen in soil immediately after
fertilizing (a) and several days after active immobilization (b).
As indicated by the two arrows pointing in opposite pathways, mineralization and immobilization are both taking place simultaneously. Immobilized fertilizer N will again become available in a few weeks if conditions favor crop uptake.
This exchange in N forms is often referred to as the mineralization-immobilization transformation (MIT) process and it is a core element of N cycling and management for crop production.
There is a constant flux taking place in soils among these pools of soil N through the MIT process and it is important to be aware of and understand this process when managing fertilizer N inputs for crop production.
References
Havlin, J.L., Beaton, J.D., Tisdale, S.L. and Nelson, W.L. 2014. Soil Fertility and Fertilizers; An Introduction to Nutrient Management. 6thEdition, Prentice Hall, Upper Saddle River, NJ.
Silvertooth, J.C. 2024. Soil Health – Nitrogen. University of Arizona Vegetable IPM Newsletter, Volume 15, No. 22,
Troeh, F.R. and Thompson, L.M. (2005) Soils and Soil Fertility. Sixth Edition, Blackwell, Ames, Iowa, 489.
Warren, J., H. Zhang, B. Arnall, J. Bushong, B. Raun, C. Penn, and J. Abit. 2017. Oklahoma Soil Fertility Handbook. Published Apr. 2017; Id: E-1039
Weil, R.R. and Brady, N.C. (2017) The Nature and Properties of Soils. 15thEdition, Pearson, New York.
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 109Time flies. February is right around the corner and that means it is farm equipment/ag trade show season. Upcoming regional events include the World Ag Expo, Tulare, CA, February 13-15, the Southwest Ag Summit, Yuma, AZ, February 20-22, and AgroBaja, Mexicali, Mexico, March 7-10. If you are interested in the latest ag tech and farm machinery, all are outstanding events and well worth attending. For more information, click on the links below.
Fig. 1. Upcoming farm equipment/ag trade show events: World Ag Expo, Tulare,
CA, February 13-15, 2024 theSouthwest Ag Summit, Yuma, AZ, February 20-
22,2024 and AgroBaja, Mexicali, Mexico, March 7-10, 2024.
We did some trials at the University of Arizona Yuma Agricultural Center in broccoli to evaluate and compare Napropamide (Devrinol) liquid formulation 2XT versus the Dry formulation DF-XT.
This product inhibits the production of fatty acids in plants, which is crucial for plant development. It affects primarily the meristematic cells which are in growing points of the stems and roots. We saw activity especially on seedling development that we show in some pictures at the end of the article.
Some growers expressed their concern on the safety of different levels of incorporation with sprinkler irrigation. Therefore, we established a test in which we applied the product as a broadcast application after planting. Then we used different levels of incorporation in some sections using 12, 24 and 36 hours of sprinkler irrigation. No difference was observed with the incorporation level in our trial. We observed temporary phytotoxicity from 4-10%. The data was obtained from visual evaluations. Also, a 0.5 to 1” height reduction was exhibited when compared to untreated plots.
It is common that growers and PCAs make management decisions on herbicide applications in different crops knowing that some injury is expected. Such is the case for alfalfa, wheat, spinach, lettuce and in this case broccoli. Frequently slight stunting it is not noticed because commercial fields don’t have untreated areas for comparison.
We talked to PCA’s and growers at the SW Ag Summit and asked for their experience with napropamide this past season. Some noticed the broccoli exhibited similar levels of phyto, which they considered economically tolerable.
We noticed that good soil prep was important for before the application of the product and avoiding direct contact of the seed with the product for best results.
On weed control we noticed good activity on nettleleaf goosefoot and lambsquarter.
At the 45day we collected data counting SMALL and LARGE weeds and at the 60 day evaluation we noticed most of the SMALL goosefoot and lambsquarter in the high rate of napropamide plots stayed small (pic. below).
The push-pull strategy, a stimulo-deterrent diversionary strategy, combines behavior-modifying stimuli that manipulate the distribution and abundance of insect pests and/or natural enemies. When your main crop is intercropped with plant species that can mask the host (main crop) appearance or emit undesirable volatiles (smells) that divert the pests away from the main crop (push), on the other hand, other plants in your intercropping system can be extremely attractive using stimuli that are highly apparent and attractive to the pest, hence trapping the pest (pull) (Fig. 1). Insects use visual, chemical, or tactile cues. Thus, by intercropping the main crop with plants that emit more attractive smells, are more visually appealing, or release undesirable smells, one can cause the pest to be trapped and repelled from the main crops, resulting in effective control of the pests.
Figure 1. Pictorial representation of push-pull strategy.
In Brazil, the push-pull strategy has been found effective in managing major kale pests. They found that using mustard as a preferred host pulled the pests away from the kale crops, while marigold plants increased the beneficial arthropod population which provided additional control of the pests (da Silva et al. 2022; https://www.sciencedirect.com/science/article/pii/S1049964421003029). My
lab plans to evaluate the efficacy of similar systems for insect pest management in organic vegetable crops in Arizona.
In Salinas, California, intercropping lettuce with sweet alysum has favored some measurable aphid control. Sweet alyssum attracts and feeds hoverflies, which then lay eggs in lettuce, producing hoverfly larvae that consume aphids. In this video, Dr. Brennan describes in detail how this system works. This research was conducted about a decade ago, but I believe this could be an important tactic to consider for aphid control in lettuce. We also plan to evaluate this system for aphid management in lettuce in Arizona lettuce growing regions.
Figure 2. Graphical representation of Lettuce-Alyssum intercropping system for aphids control. (Image source: Brannan 2013).
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.
