With local temperatures expected to increase in the next few weeks, insect activity should also start to pick up. Even though worm pressure has been light in the field for the past month or so, it would be wise to keep an eye out for corn earworm (CEW).  In the past, unusually high CEW abundance has occurred on late head lettuce, particularly in the Dome/Wellton areas.  Our areawide trap catches show that moth counts have been nearly bottomed out for some time.  However, we did see an increase in CEW trap captures in Roll and Dome Valley last week (>1 moth/trap/night).  This is above normal for early February (see Areawide Trap Network) and could be the calm before the storm. In the spring seasons where lettuce was heavily infested with larvae (i.e., 2014-2015), trap catches started to increase in early February and peaked by mid-March. These increased trap numbers should be used as an early indicator that moths are actively flying at night at higher abundance and PCAs should start to scout more aggressively for eggs and larvae.  CEW can be very damaging in spring crops once head formation begins; larvae will usually bore into the head 1-2 days upon hatching.  Corn earworm is much more likely to bore into lettuce heads than other Lepidoptera larvae, rendering the heads unmarketable.  If fields are not watched closely, infestations may not be noticed until the head is harvested. Once inside the head, control of larvae with currently available insecticides is not possible. Thus, pay careful attention for newly oviposited eggs (laid singly) on lettuce plants. If you are beginning to find eggs and suspect that CEW are active in the field when plants are beginning to head, it would be a good idea to treat at regular spray intervals (~ 7days). The UA nominal threshold for CEW in head lettuce from early heading to harvest is 1-2 larvae / 100 plants. Essentially, you find one, you spray.  Repeated insecticide treatments may be required to maintain low population levels near harvest.  Most contact insecticides recommended for Lep larvae are active against CEW. Furthermore, the addition of a pyrethroid with thrips, aphid and/or fungicide sprays may be cheap insurance against larvae infesting heads as you approach harvest. For more information on CEW management and control recommendations see Corn Earworm Management on Desert Produce and the 2024 Lep Control Chart.

Arizona consistently produces the highest yields and quality of crops found anywhere.  This is a testimony to not only the skill of Arizona farmers and the capacity to irrigate and produce crops in the desert but also to the productive soils that we are fortunate to have in this region.  Soils are commonly considered to serve as the foundation of agriculture (Parikh and James, 2012) and that is certainly true in Arizona.
 
Many people think Arizona is composed of nothing but rocks and sand.  While we do have an abundance of rocks and sand in Arizona, this region also has many areas with soils that are geologically young, with high natural fertility, and extremely productive if provided with sufficient water and good management.
 
Most Arizona crops are produced on alluvial soils that are formed from the deposits of sediments from periodic flooding of rivers and streams over thousands and millions of years.  The alluvial soils used in Arizona crop production vary tremendously across the state and the entire desert Southwest.  They vary in texture, depth, and structure.  Our crop production capacities in this region are supported by the irrigation water and these incredible soil resources.
 
A great example of the rich alluvial soils of this region can be found in the lower Colorado River valleys including Mohave Valley, AZ; Parker Valley, AZ; Palo Verde Valley, CA; Imperial Valley, CA; Bard Valley, CA; Yuma Valley, Arizona; and the lower Gila Valleys of Arizona.  The alluvial sediments that have formed the soils we encounter in the lower Colorado River system are estimated to have been deposited to several hundred meters (several thousand feet) in depth in the current river channel approximately 4.63 to 4.80 million years ago (Crow et al., 2021 and Kimbrough et al., 2015).  These sediments are geologically “young” in contrast to the ancient crystalline rocks at the bottom of the Grand Canyon, often referred to as the “Vishnu Basement Rocks” that are 1,680 – 1,750 Ma (1.68 – 1.75 billion (b) years ago), (Crow, et al., 2021 and Mathis and Bowman, 2018).
 
In 1941, Dr. Hans Jenny from the University of California – Berkeley, published Factors of Soil Formation: A System of Quantitative Pedology.  [Pedology is the scientific study of soils and their weathering profiles, including the formation, nature, ecology, and classification of soil.] This book is a classic and fundamental piece of work in the study of soil science due to Jenny’s capacity to translate the basic concepts into a quantitative scientific framework through a rigorous definition of the soil system and its genesis, including the identification and separation of dependent and independent variables.  
 
Jenny described soil formation as a function of five factors: climate, biota (plants and animals), relief (topography), and parent material, all operating together over time (Figure 1).
 
Alluvial soils tend to be highly variable both horizontally across the landscape and vertically down through the soil profile.  This can sometimes present challenges in field-level management for crop production, particularly when soil types change within a given field or relatively small area, which is rather common.

Figure 1. Hans Jenny and the factors of soil formation, 1941.

We commonly find that soil formation will vary tremendously across a landscape with natural soil variation across the landscape in both the horizontal and vertical dimensions with the distinct organization of soil horizons (Figures 2 and 3).
 
It is important to recognize soil variability in a crop production system because different soil types will have unique physical and chemical properties that can affect things like soil-water holding capacity, drainage, leaching capacities, soil fertility, and plant root development.
 
The United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) has conducted extensive soil surveys across the United States for each state by county.  Soil surveys can be accessed on-line at the following site: https://websoilsurvey.nrcs.usda.gov/app/
 
A broad map selected from NRCS Soil Survey of the Yuma area graphically depicts both the alluvial nature and variability of the soil resources commonly encountered in this area.  https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx
 
It is important to recognize the importance and high value of these natural soil resources that our agricultural systems are developed and dependent upon and be alert to the natural variability associated with the alluvial soils in this region.

Figure 2.  Natural variation of soils across a landscape



Figure 3.  Layers of soil, basic organization of soil horizons.



Figure 4. Soil volume, soil texture, and water holding capacity
relationships.  Source: Kansas State University Agronomy Department, Soil
Laboratory Manual.

References:
Crow, R.S., J. Schwing, K.E. Karlstrom, M. Heizler, P.A. Pearthree, P.K. House, S. Dulin, S.U. Jänecke, M. Stelten, and L.J. Crossey.  2021.  Redefining the age of the lower Colorado River, southwestern United States: Geology, v. 49, p. 635–640, https://doi.org/10.1130/G48080.1
 
Jenny, H.  1941.  Factors of Soil Formation: A System of Quantitative Pedology.  McGraw-Hill Book Company.  New York and London.
https://netedu.xauat.edu.cn/sykc/hjx/content/ckzl/6/2.pdf
 
Kimbrough, D.L., M. Grove, M., G.E. Gehrels,, R.J. Dorsey, K.A. Howard, O. Lovera, A. Aslan, P.K. House, and P.A. Pearthree.  2015.  Detrital zircon U-Pb provenance of the Colorado River: A 5 m.y. record of incision into cover strata overlying the Colorado Plateau and adjacent regions:
Geosphere, v. 11, p. 1719–1748, https://doi.
org/10.1130/GES00982.1.
 
Kansas State University Agronomy Department, Soil Laboratory Manual: Soil and water relationships. https://kstatelibraries.pressbooks.pub/soilslabmanual/chapter/soil-and-water-relationships/
 
Mathis, A. and C. Bowman. 2018.  Telling Time at Grand Canyon National Park.  United States Department of the Interior, National Park Service.
 
Parikh, S. J. and B.R. James. 2012. Soil: The Foundation of Agriculture. Nature Education Knowledge 3(10):2
https://www.nature.com/scitable/knowledge/library/soil-the-foundation-of-agriculture-84224268/#

This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Variety: Deluxe (HMX2595) was seeded, then sprinkler-irrigated to germinate seed on March 20, 2024on 84 inches between bed centers. All other water was supplied by furrow irrigation or rainfall. Treatments were replicated five times in a randomized complete block design. Each replicate plot consisted of 25 ft of bed. Treatment beds were separated by single nontreated beds. Treatments were applied with a tractor-mounted boom sprayer that delivered 50 gal/acre at 100 psi to flat-fan nozzles spaced 12 in apart. 

Spray treatments were done on 05-21-2024, 05-31-2024, 06-07-2024 and 06-14-24. Powdery mildew was first seen on 06-05-24. Please see excel file for additional details.  

Disease severity of powdery mildew (caused by Sphaerotheca fuliginea and S. fusca) severity was determined 6-17-2024 by rating 10 plants within each of the four replicate plots per treatment using the following rating system: 0 = no powdery mildew present; 1 = one to two mildew colonies on leaves  ;2 = powdery mildew present on one quarter of leaves; 3 = powdery mildew present on half of the leaves; 4 = powdery mildew present on more than half of leaf surface area ; 5 = powdery mildew present on entire leaf. These ratings were transformed to percentage of leaves infected values before being statistically analyzed.

The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Most treatments significantly reduced the final severity of powdery mildew compared to nontreated plants. Quintec, Merivon, Tesaris, Luna Sensation, and V6M-5-14 V gave the best disease control. Phytotoxicity symptoms were not noted for any treatments in this trial.

ANR Webinar 10/17 click link for details.

Based on the recent weather, it’s definitely summer in my book. Things seem to be slowing down, so I thought I’d change pace this week and take the opportunity to ask you, the readers of this newsletter, if there are any ag automation/mechanization topics you would be interested in hearing more about. I am happy to do some research and share the findings in future articles. Drop me a line at siemens@cals.arizona.edu or feel free to call me at 928.782.5869. I’d love to know your interests.

Fig. 1. Futuristic weeding. (Image credits: S.L. Young)

Devrinol (napropamide), herbicide has been used in cole crop production at preemergence for control of many annual grasses and some broadleaf weeds. With the revision of Dacthal (DCPA) some of our PCAs are turning their attention to this tool for weed control. Like many other preemergence herbicides, it works by inhibiting root growth after weed seeds have germinated. It has sometimes injured lettuce when too much was concentrated near the lettuce seed.

Napropamide is a selective systemic herbicide that according to the label limits the growth of grasses in our area such as Barnyard grass, Junglerice, Wild barley, Annual bluegrass, Canarygrass, Foxtail, Goosegrass, Johnsongrass, Wildoats, Sandbur and Sprangletop.
Broadleaves that we have included in the label are: Lambaquarter, Prickly lettuce, Little Mallow, Redroot pigweed, Common and Horse purslane and Sowthistle.
 The PCA study guide for Arizona states that minimum re-cropping intervals of 12 months for cotton, melons and alfalfa, 6 months wheat, 2 for lettuce and 0 for cole crops. We have seen in our trials that head lettuce showed sensitivity to Devrinol when planting too soon.
The following chart from a broccoli trial conducted in 2015 (Tickes/Peña) show the efficacy of napropamide for control of London Rocket and Lambsquarter.

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:           
CEW moth counts peaked about 2 weeks ago, but declined areawide this week. About average for October.
 
Beet armyworm:         
Trap counts increased in most locations, but remain well below average compared to previous years.
 
Cabbage looper:          
Cabbage looper up in most traps, particularly active in Yuma Valley and Wellton. About average for this time of season.
 
Diamondback moth:      
DBM moths peaked about 2 weeks ago (Dome and Wellton); below average for this time of the year.
 
Whitefly:                      
Adult movement increased in the past 2 weeks, particularly in areas near melons but remains below average for this time of year. 
 
Thrips:                           
Thrips adults increased markedly in many locations (Tacna, Dome, Yuma Valley), and currently about average for October.
 
Aphids:                        
Aphid movement beginning; light numbers in North Yuma Valley and Tacna. Below average for October.
 
Leafminers:                  
Adult activity up in a few locations (Yuma Valley), below average for September.