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.

1. Imbibition. Water from the soil moves into the seed, the seed coat softens and swells.  
2. Interim or lag phase. In this part of germination, the internal seed physiology is activated, and cells begin to respire, and the seed begins to build proteins and metabolize its stores of food.
3. Radicle and root emergence. The cells in root tissue start to elongate and divide and this begins pushing the radicle, the embryonic root, out of the seed.

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

Few things in Yuma County are more frustrating than watching lettuce wilt in the field. Even the best-managed fields are not immune to the many destructive soilborne pathogens capable of causing significant economic losses in lettuce production fields. It is an unfortunate reality that Fusarium wilt, corky root, bottom rot, and lettuce drop are encountered and require management nearly every season in Yuma to produce a healthy crop. Although advances in resistant cultivars and integrated disease management have improved our ability to reduce some of the crop losses to these diseases, management options for soilborne diseases, in general, remain severely limited and often are ineffective once symptoms progress to root or crown rot and irreversible wilting. Strong, durable genetic resistance remains the strategic priority and most viable option when it comes to the management of these pathogens.

To select the right genetic resistance package, accurate field history and proper diagnosis of the cause of plant health challenges on that ground is critical. One of the greatest obstacles in plant disease diagnosis is that symptoms alone are rarely what we call pathognomonic, that is, they are seldom unique enough to identify down to a single disease or environmental stressor. Temperature, soil moisture, plant age at time of infection, and varietal susceptibility all influence how diseases and disorders develop and how symptoms are expressed in the plant as a response. As a result, the same pathogen can produce different symptoms under different conditions, while entirely different pathogens may produce nearly identical symptoms. Add to this the complexity of the frequent occurrence of co-infections with multiple pathogens colonizing the same host at the same time and the ability to provide field diagnoses or diagnosis from pictures can quickly become confusing, overwhelming, or downright impossible.

This known variability in symptom expression underscores the importance of confirming a diagnosis rather than relying solely on field observations, especially when the time comes for selecting resistant varieties to be planted in fields with a history of soilborne diseases. Ideally, lettuce shouldn’t be planted on ground that is contaminated with soilborne pathogens, but this isn’t always feasible in our production systems.  

Despite my obligatory warning for the difficulty and unreliability of diagnosing plant diseases and disorders from symptoms alone, I thought it would be a good chance to highlight a few key trends for two infamous diseases that we frequently observed during last season’s atypically warm and rainy weather. I’m talking of course about Fusarium wilt and corky root. My hope is that these observations may prove helpful when scouting wilting fields, explaining your observations, and, importantly, making it easier to document on our submission sheet what you’re seeing in the field if you choose to drop off samples to the clinic for culture testing and confirmation diagnoses. Of course, I remain happy to take a look and sample from the symptomatic field as well. Over the 2025-2026 season the plant clinic received 20 samples culturing positive for Fusarium wilt, 16 for corky root, and 8 for a co-infection with both pathogens simultaneously.  

In the field, the two diseases often appear indistinguishable from aboveground symptoms alone. Both commonly produce wilting, which may be more pronounced particularly during the heat of the day then seemingly “recover” in the evenings. Plants often lose their characteristic healthy, green waxy luster and instead develop a dull, matted appearance that is more apparent when compared with neighboring healthy plants or fields and as the wilting intensifies. As the diseases progress, the outermost leaves surrounding the crown may rot and collapse, further contributing to the appearance of a sick and declining plant. At this point, secondary microorganisms of both the bacterial and fungal kind may set in and produce signs of growth as they feed on the dead tissue, potentially appearing to be related to the cause of the disease and complicating diagnosis.

Below ground, corky root primarily affects the outermost tissues of the root system, causing feeder roots and the taproot to develop yellow to brown (sometime with specks of green) lesions that eventually crack and become roughened. To me, severely affected roots appear as if they are covered in mesquite bark. In contrast, Fusarium wilt primarily invades the plant's vascular system. Although root deterioration may occur (especially on small feeder roots), the hallmark of Fusarium wilt is vascular discoloration within the taproot and crown, where the water-conducting xylem tissues become tan to reddish-brown. These belowground differences, together with laboratory culture or other laboratory testing, provide the evidence needed to confidently distinguish between the two diseases.

Figure 1: Symptoms on a sample culturing positive for Fusarium wilt alone. Left.) outside view of the root system mostly absent of any rot symptoms, and right.) dissected root showing interior vascular discoloration. Note the bright white cortical tissue between the vascular tissue (center) and epidermis (outermost layer of cells, the “skin”) and near absence of overall rot of inner taproot tissue that is commonly observed with other soilborne diseases. The vascular system appears reddish due to the colonization with Fusarium oxysporum f. sp. lactucae.

Figure 3: Symptoms on a sample co-infected with Fusarium wilt and corky root. Left.) outside view of the root system showing significant barky lesions, and right.) dissected root shows a barky vascular dry rot towards the tip of the taproot with reddish-brown discoloration moving up towards the crown. 

The images above are from some of the more diagnostically clear samples we have received in the clinic. As mentioned earlier, timing of infection, local microclimate, and varietal genetics can play a huge role in defining how a plant expresses disease symptoms, so these observations of textbook symptomology may not be as clear in every field with wilting that you manage. For these reasons, laboratory testing remains the gold standard for disease diagnosis. Submission of symptomatic plants to the diagnostic clinic allows for pathogen isolation and culture, providing definitive identification that can distinguish between diseases with similar symptoms. Accurate diagnosis is the foundation of effective disease management, ensuring that control measures are directed at the true causal agent rather than its look-alikes. The Yuma Plant Health Clinic is here to help to that end.

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

It 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:

• Prefar 4-E (Mechanical Placement): Prefar binds tightly to soil particles and does not leach easily. Under drip, it demands precise mechanical incorporation into the top 1 to 2 inches immediately after application. Options include shallow rolling cultivators (Lillistons), rotary hoes, power tines, or integrating the spray pass directly into final bed-shaping operations before drip line installation.
• Kerb SC (Water Mobility & Timing): Kerb SC relies heavily on water movement to carry it into the active root/germination zone. Under drip, growers must balance mechanical soil mixing with strategic irrigation timing, such as running short, high-volume drip cycles to encourage capillary wicking to the bed surface, or using a brief initial sprinkler pass solely to activate the herbicide before switching to drip. 

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 mode

of 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

1. 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 
2. 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

1. Water demand: modestly higher during warm periods, but still driven by ETo, crop stage, wind, solar radiation, humidity, and planting date. Track it; do not guess it.
2. Emergence: the highest-risk item. September seedbed temperatures can already sit near or above the lettuce germination threshold.
3. Salinity: faster surface drying can concentrate salts near the seed line. Aim for uniform seed-zone moisture, not simply more volume.
4. Quality: warm nights can affect recovery, head structure, firmness, uniformity, and tipburn risk, with pressure easing as nights cool in October.
5. Pests: accumulated heat can move insect development ahead of the calendar, so scouting should follow crop stage, degree-day conditions, and field observations.
   

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.

VegIPM Update Vol. 17, Num. 5

March 4, 2026

Results of pheromone and sticky trap catches below!!

Corn earworm:  CEW moth activity seen in some areas, but numbers have been low; About average for this time of season.

Beet armyworm: BAW numbers decreased across most locations since last collection. Above average  for this time of the season. Historically BAW numbers peak in March/April.

Cabbage looper:  Cabbage  looper moth activity decreased across most sites since last collection, but  remains higher than average for late-February. Historically cabbage looper moth  numbers peak in March/April. 

Diamondback moth: Adult activity remains steady across most locations. About average for this time of the year. 

Whitefly: Adult activity remains low across all locations, about average for this time of year.  

Thrips:  Thrips adult activity remains steady across locations, about average for this time of year. Expect numbers to continue increasing as temperatures warm up.

Aphids:  Winged aphid numbers decreased at most sites since last collection; below average for this time of the year.

Leafminers: Adult leafminer activity remains steady, high numbers seen in Yuma Valley, about average for this time of the season.