Norman E. Borlaug completed his Ph.D. in plant pathology at the University of Minnesota in 1942. He worked briefly as a microbiologist for the DuPont de Nemours Foundation. In 1944 he accepted a position as a geneticist and plant pathologist and began work at the International Maize and Wheat Improvement Center (CIMMYT, Centro Internacional de Mejoramiento de Maíz y Trigo) based in Ciudad Obregón, Sonora, Mexico.
Norman Borlaug (left) and his colleague George Harrar in a wheat field in the Yaqui Valley, Sonora, Mexico in 1944.
In joining CIMMYT, Borlaug had the responsibility of organizing and directing the Cooperative Wheat Research and Production Program in Mexico, which was a joint program with the Mexican government & Rockefeller Foundation. This program utilized scientific research in a fully integrated agronomic system utilizing genetics, plant breeding, plant pathology, entomology, soil science, and cereal technology.
The primary objective of Borlaug and his team of colleagues was the development of semi-dwarf varieties of wheat with improved disease resistance, particularly stem rust caused by Puccinia graminis f. sp. tritici (Borlaug, 1968). In the 1940s, Mexico imported a substantial portion of its wheat because of low productivity and severe disease pressure. Within twenty years Borlaug and his team were realizing spectacular success in developing high-yielding short-strawed, diseaseresistant wheat varieties.
One of the key developments by Borlaug and his colleagues was a breeding strategy that combined shuttle breeding between contrasting environments, in this case of the Yaqui Valley of Sonora near Ciudad Obregón, Sonora and Puebla, Mexico with rigorous disease screening. This approach accelerated genetic gains and produced broadly adapted wheat cultivars with durable disease resistance.
Borlaug’s team also developed the incorporation of dwarfing genes into spring wheat varieties. Traditional wheat cultivars commonly developed long stalks and tall plants that were prone to lodging, particularly if provided sufficient water and nitrogen (N) fertilization. This lodging and vegetative growth tendency was limiting yield potential. The new semi-dwarf cultivars allocated more biomass to grain production with a higher harvest index (Pingali, 2012).
The release of two principal varieties, Lerma Rojo 64 and Sonora 64, dramatically increased yields in Mexico and by 1963, Mexico became a net exporter of wheat. Borlaug and the CIMMYT program next directed attention to wheat production in southern Asia and between 1965 to 1970, wheat yields nearly doubled in Pakistan and India, greatly improving food security.
The collective increases in yield from these programs have been labeled the “Green Revolution”, referring to the rapid increase in global agricultural productivity that occurred between the 1940s and the 1970s through the development of improved crop varieties. These developments were especially important in Asia and Latin America, where food shortages and rapid population growth raised concerns regarding widespread famine (Evenson and Gollin, 2003).
The Green Revolution is often credited with saving over a billion people from starvation and in 1970 Borlaug was Awarded the Nobel Peace Prize in recognition of his contributions to world peace through increasing food supply. He is the first and only agronomist to win the Nobel Peace Prize.
The program was a systems approach that consisted of agronomic practices that included improved irrigation, fertilization, and mechanization. Although many scientists, governments, and institutions contributed to the Green Revolution, the work of Norman Borlaug was central to its success.
The Green Revolution developments were also extended beyond wheat. Similar advances occurred in rice breeding through the development of semi-dwarf rice cultivars such as IR8 at the International Rice Research Institute. These plant breeding innovations have contributed to major increases in cereal grains and other crops across many parts of the world in the past 50 years (Khush, 2001).
Borlaug repeatedly and quite emphatically pointed out the fact that their work with the CIMMYT program was only one small step, and it was far from perfect. He consistently encouraged the next generation of agronomists and agricultural scientists to continue working diligently to address food security in the fight against what he often referred to as the “population monster” facing humanity.
Borlaug and the CIMMYT program substantially reduced hunger and demonstrated the importance of science-based agricultural innovation. Due to his contributions to global food security, Borlaug remains widely recognized as one of the most influential agronomists and plant breeders of the twentieth century.
References
Borlaug, N.E. 1968. Wheat breeding and its impact on world food supply. Proc. Third Int. Wheat Genet. Symp. 1:1–36.
Evenson, R.E., and D. Gollin. 2003. Assessing the impact of the Green Revolution, 1960 to 2000. Science 300:758–762. doi:10.1126/science.1078710.
Khush, G.S. 2001. Green revolution: The way forward. Nat. Rev. Genet. 2:815–822. doi:10.1038/35093585.
Pingali, P.L. 2012. Green Revolution: Impacts, limits, and the path ahead. Proc. Natl. Acad. Sci. USA 109:12302–12308. doi:10.1073/pnas.0912953109.
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 109Weed control in wheat and barley in the Southwestern United States faces significant challenges, including high winter temperatures, extensive irrigation, and limited crop rotation options. Although wheat and barley are naturally competitive crops, research indicates that weeds can reduce yields, hinder harvesting, increase grain impurities, cause grain discoloration, and lead to insect or mold problems during storage if not managed effectively.
An Integrated Pest Management (IPM) approach—which combines prevention, cultural practices, and the appropriate use of herbicides—offers the most sustainable and reliable weed control within wheat production systems in low-desert regions.
Focus on Early Control
Weeds that emerge simultaneously with the wheat crop, or shortly thereafter, cause the greatest yield losses. Weed species typically include annual ryegrass, wild oats, Italian ryegrass, Canarygrass species, London rocket, various mustard species, and Fiddleneck. Fields should be free of weeds at the time of planting and must be kept clean until the onset of the tillering stage.
Cultural Practices Remain Essential
Recommended Herbicide Programs (Examples of Options)
Note: Always follow the instructions on the product label specifically for your state, wheat variety, soil type, and growth stage. The rates listed below reflect ranges commonly used in the Southwestern United States.
OR
OR
OR
Resistance and Integrated Pest Management (IPM) Considerations
Rotate the use of multiple effective herbicides.
Recommended Reading
Growers and PCAs are strongly encouraged to review the following foundational reference guide for additional details regarding weed biology, application timing, and appropriate herbicide options for Arizona’s low deserts:
Bottom Line
Successful weed management in wheat and barley crops across the Southwestern United States relies on early-season management, high crop competitiveness, and the timely and appropriate application of herbicides. Integrating extension-based cultural practices with weed management programs that are mindful of herbicide resistance remains the most effective strategy for safeguarding both crop yield and grain quality.
Extension Support and Collaboration
I am more than happy to conduct field visits, assist with weed identification and management decisions, or collaborate on herbicide efficacy trials targeting weed problems in wheat and barley. Please feel free to reach out to discuss on-farm visits or research opportunities.
Diamondback moth (DBM), Plutella xylostella, is one of the most important insect pests of Brassica crops, including cabbage, broccoli, cauliflower, and some leafy greens. Heavy infestations can cause significant feeding damage, reduce crop quality, and increase production costs. Because DBM has a long history of developing resistance to insecticides, regular monitoring of insecticide susceptibility is essential to ensure effective management in the field. In the western United States, resistance to several key insecticide classes has been documented in field populations. For example, populations in California have shown high levels of resistance to several insecticide classes, including diamides, spinosyns, avermectins, pyrethroids, and Bacillus thuringiensis (Bt) products. More recently, outbreaks in Arizona have been associated with confirmed resistance to diamide insecticides and reports of reduced field efficacy. This highlights the importance of regular insecticide susceptibility monitoring and the implementation of resistance management strategies to maintain effective control of the pest. While field resistance and reduced efficacy reflect outcomes under production conditions, laboratory susceptibility monitoring provides a standardized approach to detect shifts in population response before or alongside observable field control issues.
To evaluate current insecticide susceptibility levels in DBM populations across Arizona's major Brassica-growing regions and some California populations, DBM larvae were collected from multiple locations in Arizona and California. DBM populations were reared in the laboratory, and their progenies were tested under controlled laboratory conditions.
In the laboratory, larvae were exposed to insecticides at the highest labeled field rate using a leafdip bioassay. The insecticides evaluated included Exirel®, Coragen®, Harvanta®, Radiant®, Baythroid® XL, Proclaim®, Torac®, DiPel®, and XenTari®, representing commonly used conventional and organic management options, as well as IncipioTM, which was recently registered. These laboratory bioassays measure relative susceptibility under controlled conditions using field-collected populations reared in the laboratory. Results represent mortality at the maximum labeled rate under leaf-dip assay conditions and may not directly reflect field performance, where environmental conditions, application timing, coverage, larval stage, and population dynamics influence control. These data are intended to monitor susceptibility trends and help inform resistance management decisions. Accordingly, extrapolation to field performance should be made cautiously, recognizing both the constraints of laboratory bioassays and the mosaic of susceptibility that may exist within and among field populations.
Several Insecticides Showed High Laboratory Mortality Across Field-Collected Populations
Several of the insecticides evaluated caused high mortality of DBM larvae in laboratory assays across field populations. Proclaim, Incipio, Radiant, DiPel, and XenTari caused high mortality, indicating laboratory susceptibility across the tested populations. These products remain promising options for managing DBM in Brassica crops based on current laboratory susceptibility data. In contrast, Coragen, Exirel, and Baythroid XL resulted in lower mortality in laboratory assays than the other insecticides (Figure 1).
Figure 1. Mean mortality (%) of DBM larvae exposed to the maximum label rate of
insecticides, combined across 10 field populations from California and Arizona.
DBM Mortality by Collection Location, with Some Population-Level Variability
The levels of larval mortality caused by each evaluated insecticide were generally consistent across populations. For DBM populations collected during fall 2025, Proclaim, Incipio, Radiant, DiPel, and XenTari resulted in high larval mortality under laboratory assay conditions for most DBM populations tested (Figure 2). However, slight reductions in larval mortality were observed in specific populations. For example, DiPel resulted in approximately 70% larval mortality in the Coachella Valley, CA population # 4, and Radiant caused approximately 55% larval mortality in the Coachella Valley, CA population #2. Despite this variability, these insecticides generally resulted in high mortality of DBM larvae across geographic locations under laboratory assay conditions (see Figure 1). In contrast, Coragen, Exirel, and Baythroid XL consistently resulted in low DBM larvae mortality in the field-collected populations from Salinas Valley, CA; Coachella Valley, CA; and Gila Valley, AZ (Figure 2).
We observed similar mortality trends among populations collected during winter/spring 2026 for the evaluated insecticides except for DBM mortality caused by Dipel, which declined in many populations (Figures 3 and 4). Harvanta was evaluated for two populations (Imperial Valley, CA and Somerton, AZ), which exhibited similar reduced larval mortality to that of the other diamides. Torac was also evaluated for the Imperial Valley, CA, population, resulting in a high level of DBM mortality (Figure 4). We also tested a population collected from the Yuma Ag Center, which resulted in susceptibility levels very similar to those of the susceptible laboratory colony (Figure 3). Previous DBM monitoring studies performed by Dr. Palumbo over several years also showed that while populations collected from commercial fields showed very low susceptibility to diamides, populations collected at the Yuma Ag Center remained highly susceptible. This indicates that transplant sources and/or field-specific DBM management practices are also affecting DBM susceptibility and control across Arizona’s brassica-growing regions.
The reduced and variable levels of larval mortality observed in laboratory assays across multiple field populations for some tested insecticides highlight the importance of routine susceptibility monitoring to better understand resistance dynamics and to support informed field-level decisions regarding product selection and rotation.
Figure 2. Mortality (%) of DBM larvae from field populations collected from Arizona
and California as affected by maximum label rates of 8 insecticides, Fall 2025.
Figure 3. Mortality (%) of DBM larvae from a susceptible lab colony and field
populations collected from Arizona as affected by maximum label rates of 8
insecticides, Winter/Spring 2026.
Figure 4. Mortality (%) of DBM larvae from two field populations collected from
Arizona and California as affected by maximum label rates of ten insecticides, Spring 2026.
Key Laboratory Findings
Implications for Resistance Monitoring and Field Decision-Making
Laboratory monitoring informs our understanding of resistance trends, but field scouting remains essential to evaluate how populations respond under production conditions. As always, when in doubt, scout!
Additional Reading Materials
The figure shows a wind rose based on daily wind direction data from the AZMet Yuma Valley station from January 1, 2021, through May 24, 2026. This figure (Figure 1) summarizes where the wind came from most often, not how fast the wind was blowing. Longer bars indicate more frequent wind direction. In this dataset, winds most frequently came from the north–northeast and south–southeast directions, while winds from the east and west occurred less frequently.
Figure 1. Wind rose showing the frequency distribution of daily wind direction at the AZMet Yuma Valley station from January 1, 2021, through May 25, 2026. Longer bars indicate more frequent wind directions. Dominant winds were primarily from the north–northeast and southsoutheast directions. Data source: Arizona Meteorological Network (AZMET) – University of Arizona
Wind direction is important because it indicates the likely origin and pathway of air moving into crop fields. Depending on surrounding land use and regional weather conditions, air moving from different directions may be warmer, cooler, drier, more humid, or dustier. For example, air passing over irrigated agricultural areas may carry relatively more humidity, while air moving across desert, fallow ground, or dry soil surfaces may be hotter, drier, and dustier. Therefore, wind direction can influence field microclimate, soil moisture loss, crop stress, pest movement, and spray drift pathways.
For irrigation management, prevailing wind direction can help growers interpret field variability. In flood or furrow irrigation, wind direction may influence surface drying after irrigation, especially along exposed beds, field borders, and tail-end areas. Wind moving across hot, dry ground can accelerate evaporation from wetted furrow surfaces and contribute to soil crusting. This may affect seedling emergence, salinity accumulation near the bed surface, and the timing of the next irrigation. In sprinkler-irrigated fields, wind direction during irrigation can influence water distribution patterns. In drip-irrigated vegetables, wind direction may help explain recurring dry zones, edge effects, or uneven crop vigor when combined with soil moisture, ET, and field observations.
Wind direction can also affect crop growth and development by modifying canopy microclimate. Air movement from different directions may influence canopy temperature, humidity, dust movement, and drying conditions. Young vegetable seedlings are especially sensitive to windblown soil particles, which can cause abrasion, stand injury, and delayed early growth. Leafy vegetables may also show reduced market quality when exposed to repeated dust or sand movement. This figure does not directly measure crop stress, but it helps identify the directions from which wind-related exposure is most likely to occur.
The wind rose should not be interpreted as direct evidence of yield loss. It only shows the dominant wind direction. However, yield and quality can be affected when prevailing wind direction interacts with high wind speed, dry air, poor irrigation uniformity, soil moisture stress, or physical crop injury. Long-term wind direction information can help growers evaluate whether certain field margins, bed orientations, or crop rows are repeatedly exposed to incoming air movement and associated stress factors.
Wind direction is also important for IPM because it influences the movement of insects, pathogens, dust, and pesticide droplets. Aphids, whiteflies, thrips, leafhoppers, and some airborne pathogens may move with prevailing air currents. Knowing the common upwind directions can help growers identify potential pest-source areas and improve scouting strategies. Wind direction is also important for pesticide applications because it determines the likely pathway of spray drift and off-target movement.
Overall, wind rose information provides a practical decision-support tool for understanding dominant airflow pathways in Yuma vegetable production. This figure should be used together with real-time wind speed, wind direction, pesticide label restrictions, droplet size, boom height, irrigation records, soil moisture monitoring, ET data, and field scouting. When combined with these additional data, wind direction can support better irrigation planning, crop stress interpretation, yield protection, and IPM decision-making.
References: Arizona Meteorological Network (AZMet) https://azmet.arizona.edu/
