A good stand and uniform plant population is a critical step in the crop production process. Crops that require expensive seeds and have tight production schedules require extra diligence in securing a good stand, which can be challenging. A review of
the fundamental stages of seed germination and seedling development can be helpful in early season field evaluations and management.
Seeds are living organisms, and they need healthy embryonic tissue to germinate. Most plant species have food reserves to carry the seedling through germination and emergence. The embryo and food reserves are covered in a seed coat and the seeds are stimulated,
or they “wake up”, in response to appropriate moisture, temperature, and light conditions that serve to initiate the germination process (Moore et al., 1998).
There are three distinct steps in the process of seed germination.
Several cool season plant species, including lettuce experience thermodormancy (Reynolds and Thompson, 1971), which is a state of dormancy induced by high temperatures that prevent germination. This is an adaptive trait that protects seeds from sprouting
during hot conditions that can create plant stress and poor-quality growth (Gardner et al., 1985 and Maynard and Hochmuth, 2007).
A range of optimum temperatures for a select group of common vegetable crops is shown in Table 1. Lettuce has an optimum temperature range for germination of 40-80°F with an optimum temperature of 75°F and a maximum temperature of 85°F.
The soil temperature conditions commonly experienced in the low deserts in August, September, and October when thousands of acres of lettuce are being planted and established, commonly exceed these lettuce temperature ranges for germination.
Table 1. Crop germination temperature ranges for a select set of vegetable crops. Source: Kemble and Musgrove, 2006.
The temperature limits for lettuce have been modified to some extent by plant breeding and improvement programs that have produced lettuce varieties that can better accommodate hot conditions. However, there are limits to a plant’s genetic elasticity in this regard. Development of some genetic tolerance to heat is being accomplished in crops such as lettuce with modern crop improvement programs. But we must realistically recognize that we are not changing a cool-season plant to a warm-season plant.
Several aspects of crop and soil management can help mitigate the extreme effects of heat on the soil surface. Perhaps the best and most common is the use of sprinkler irrigation systems to both moisten the soil and cool surface temperatures to a range that encourages lettuce seed germination and emergence (Figure 1).
Figure 1. Sprinkler irrigation on a recently planted lettuce field, Yuma Valley, Arizona.
The first step in germination is water entering the plant (imbibing) water from the soil. In dry lettuce seeds for example, this is primarily a physical process driven by the high-water potential gradient between the dry seed and the surrounding moist
soil medium. For many seeds, including lettuce, a key entry point for water is the micropyle, which is a small pore or opening in the seed coat. In many species, water enters through the micropylar end, which is where the radicle (embryonic root)
will later emerge.
The first thing to emerge from the seed is the primary root, called the radicle, which we often refer to as the “stinger” (Figure 2). The primary root serves to both anchor the plant to the ground and it also begins to absorb water.
After enough water is absorbed into the seed, the shoot emerges. In dicot plants (most broadleaf plants), the shoot has three main parts: the cotyledons (seed leaves), the hypocotyl, the section of shoot below the cotyledons, and the epicotyl, the section of shoot above the cotyledons (DuPont, 2025).
Figure 2. Germination stages for lettuce and other dicotyledonous crops. Source: Seeds and Seedling Biology, Penn. State University, 2025.
These early stages of germination and emergence shown in Figure 2 are critical in establishing a healthy plant, a strong stand, and a good plant population. Root development includes an elongation of the radical and the formation of lateral and secondary roots. The epicotyl will be elongating simultaneously, proceeding towards soil emergence (Figure 3).
Figure 3. Epigeal and hypogeal emergence. Source: Michael Knee, The Ohio State University. Further illustrations of emergent lettuce plants are shown in Figures 4 and 5.
During the emergence process when a dicot plant is in the “hook” stage, it is a critical time when the hook is trying to push through the soil surface. Plant species with small seeds are commonly not very strong with this process and that can be exacerbated with any soil crusting. This is another reason sprinkler irrigation is often used to diminish soil crusting and facilitate germination and emergence in the low desert vegetable crop production areas.
Figure 4. Lettuce cotyledons and first true leaves. Source: Harvest to Table.
Figure 5. Stages of emergence. Source: Spider Farmer.
Planting cool season crops, i.e., leafy green vegetables, in the lower Colorado River Valley in August, September, and October is a challenging operation. Seedling germination and early development are critical stages of growth that have lasting impact on the vigor and health of the crop.
References:
DuPont, T. Seed and Seedling Biology. 2025. Pennsylvania State University Cooperative Extension, ART 3208.
Gardner, Pearce, and Mitchell. 1985. Physiology of Crop Plants. The Iowa State University Press.
Kemble, J., and M. Musgrove. Soil Temperature Conditions for Vegetable Seed Germination. Alabama Cooperative Extension, 2006.
Knee, M. Development of Seed to Plant. 2024. The Ohio State University.
Maynard, D., and G. Hochmuth. Knott's Handbook for Vegetable Growers. Vol. 5. Hoboken, N.J.: John Wiley and Sons, 2007.
Moore R., W.D. Clark, and D.S. Vodopich. Botany. The McGraw-Hill Companies. 1998. ISBN: 0-697-38363-1
Reynolds, T. and P.A. Thompson.1971. Characterization of the high temperature inhibition of germination in lettuce (Lactuca sativa). Physiologia P1.24:544-547.
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 109It is with mixed emotions that I write to inform you that this will be my last University of Arizona Vegetable IPM Update. The reason – I am retiring. Thank you for your support over the years. It’s been an honor and a privilege working with you and serving this ag community.
Figure 1. Automated Thinning & Weeding Technologies Field Day.
(Photo credits: Rosa Bevington)
As lettuce operations are transitioning to drip irrigation rather than overhead sprinklers to maximize water efficiency in the lower Colorado River, farm management practices must adapt, especially weed control. Pre-emergence tools like Prefar 4-E (bensulide) and Kerb SC (propyzamide) remain vital for managing purslane, chenopodium species, annual bluegrass, and key broadleaves. However, both active ingredients depend on specific soil placement and water delivery to form an effective barrier against germinating weed seeds.
Historically, standard overhead sprinkler passes provided the uniform surface coverage and water volume needed to activate Prefar and wash Kerb SC into the target seed zone. As fields transition to surface or subsurface drip irrigation, moving away from overhead water creates a distinct field challenge: drip lines push moisture laterally and downward, often leaving the top 1 to 2 inches of soil too dry to activate surface-applied herbicides.
Matching the Tool to the Mechanism: Because these two herbicides interact with soil and water differently, switching to drip requires tailored incorporation strategies:
Insect pests are constantly adapting to their environment. Over time, they have evolved ways to overcome control tactics. Repeated use of the same insecticides or tactics places strong selection pressure on pest populations. This allows the few individuals
that can survive treatments to reproduce, leading to populations that are increasingly difficult to control, a process known as resistance.
Insects can develop resistance in several ways. Some break down insecticides more efficiently or develop changes at the target site that reduce product effectiveness. Others avoid exposure altogether by changing their feeding habits or movement patterns. In some cases, insects even develop thicker cuticles that reduce or slow down the absorption of insecticides.
Production practices can influence the rate of resistance development. Large, uniform cropping systems and repeated use of the same control tools reduce opportunities for susceptible insects to persist, accelerating resistance.
Figure 1: Chart illustrating how insecticide resistance develops when insecticides with the same modeof action (MoA) are used repeatedly.
What Should You Do to Mitigate Resistance in Insect Pests?
An integrated pest management (IPM) approach remains the best defense against resistance.
Key practices include:
- Using crop varieties that are resistant to pest attack
- Rotating insecticides with different modes of action (MoA)
- Avoiding unnecessary insecticide applications
- Preserving beneficial insects by selecting insecticides with no or minimal adverse effects on them
- Incorporating non-chemical management tools whenever possible
- Maintaining refuges to sustain susceptible pest populations
- Eliminating crop residues after harvest to remove food sources and breeding sites for pests.
No insecticide remains fully effective over the long-term if overused. Reducing selection pressure by avoiding consecutive applications of insecticides from the same IRAC group is critical. The more intensively a single tactic is used, the faster resistance will develop. Resistance can increase production costs, reduce marketable yield, and ultimately affect grower competitiveness. A diversified pest management approach helps preserve the effectiveness of available tools and supports long-term, sustainable pest control.
Additional Reading Materials
- Calvin W., M. N. Keith, and B. McGrew. 2025. Guidelines for effective management of diamondback moth in brassica crops. University of Arizona Extension Publication. az2143. https://extension.arizona.edu/publication/guidelines-effective-management-diamondbackmoth-brassica-crops
- Calvin W. 2026. Effects of the Maximum Dose of Common Insecticides and Incipio on Diamondback Moth Populations Collected from Arizona and California Brassica Crops (Fall 2025– Winter/Spring 2026). https://acis.cals.arizona.edu/agriculturalipm/vegetables/vipm-archive/vipm-specialty-crops-view/effects-of-the-maximum-doseof-common-insecticides-and-incipio-on-diamondback-moth-populations-collected-fromarizona-and-california-brassica-crops-(fall-2025-winter-spring-2026)
Yuma is heading into fall produce season after an unusually warm year. By July 31, 2026, the season had accumulated 553 heat units (86/55°F) more than the 2020–2025 average. Through mid-August, nighttime temperatures were the warmest for that period in the AZMet Yuma Valley record, with 15 of 17 nights never dropping below 80°F. So the simple question is: if we go into produce season with above-average days and above-average nights, what should we expect? So, the useful answer starts with a distinction. Warm days and warm nights are not the same problem. They affect different parts of the crop, on different timelines, and they call for different management responses.
Warm Days, Part One: A Water-Demand Problem You Can Calculate Hot afternoons raise vapor pressure deficit and pull more water from the soil and crop. But the size of that effect is smaller than many people assume, and it is predictable. Averaged over 2020–2025, standardized reference evapotranspiration (ETo) during the establishment period was 1.69 inches per week in September, 1.26 inches in October, and 0.93 inches in November, a 45% decline across the window. Meanwhile, the spread between the warmest and coolest September in that record was only about one inch over the whole month. That comparison is the point: planting date moves water demand far more than how hot the year is. A warm September is not a reason to add a blanket percentage to every irrigation set. It is a reason to watch ETo and soil moisture closely during the two or three weeks when the crop has a limited root system and no canopy to protect the bed.
Warm Days, Part Two: A Germination Problem
This is the issue that can cost stands, and it is easy to miss because it looks like a water problem. Lettuce germination can be inhibited above roughly 81°F, and the effect is
stronger in darkness, which is exactly where the seed is. Prolonged exposure can push imbibed seed into secondary dormancy, at which point cooling the bed may no longer rescue it. Mean September soil temperature at 4 inches at the AZMet Yuma Valley
station over the past six years was 85, 85, 86, 87, 89, and 93°F. Every year was above the threshold. In a warm year, the margin gets worse rather than better. More water alone does not fix this. Variety selection, seed priming,
planting during the coolest part of the day, and sprinkler cooling of the seed zone are more targeted tools. Also, station soil temperature is a regional index, not the temperature in your bed. A bare, tilled, sunlit seedbed can run warmer. Put a
thermometer in the bed at seeding depth and decide on that number.
Warm Soils and Surface Drying
Warm conditions also affect the seedbed and root zone. Faster surface drying can concentrate salts near the seed line, especially where moisture is not uniform. This matters during lettuce establishment
because small seedlings have limited roots and are sensitive to both heat and salinity stress. The goal is not simply more water, but more uniform moisture in the right zone. Bed temperature, seed-zone moisture, and salt distribution should be checked
together before and after planting.
Warm Nights: A Marketability and Quality Problem
Nights get less attention than hot afternoons, but this year they are the more distinctive signal. Lettuce spends energy overnight on maintenance respiration. When nights stay warm,
more of the day’s photosynthate is burned rather than stored, and there is less recovery time after daytime stress. Later in the crop cycle, that can affect head development, firmness, uniformity, and tip burn risk. The cost lands on marketable
yield. Lettuce is bought on quality, and stress that reduces uniformity or head structure can cost more than the water it appeared to save. The encouraging part is that nighttime minimum temperatures usually fall from about 74°F in September to
about 62°F in October, so this pressure eases quickly with planting date.
Both Together: Pest Timing Can Move Up
Insect development is influenced by accumulated heat, so a season running 553°F-days ahead can also move pest timing ahead. That does not necessarily mean higher pest pressure; it means
the timing may shift.
What to Expect, in Short
Notice that four of those five pressures ease substantially between September and October. That is the practical thread running through all of this: in a warm year, planting date is one of the most powerful tools available, and the two weeks around establishment are where precision pays. A hot start does not call for guesswork, and it does not automatically call for more water or more inputs. Yuma growers have built their success by responding to heat with precision: better timing, closer measurement, stronger scouting, and careful root-zone management before produce season begins.
