It’s never too early to start planning for next produce season. And this includes maintaining a clean culture with effective weed control.  I think most PCAs and growers recognize that weed management is important in leafy vegetable and melon production for all the obvious reasons.  However, weed management is also essential for another important, but often overlooked, reason.  Weeds can serve as host plants for many important insect pests, and when not controlled in the field they can be an impediment to insect control. Weeds can serve a beneficial role by harboring insect natural enemies and pollinators, but the consequences resulting from weeds harboring insect pests often outweigh benefits they potentially provide.   Although flowering weeds can provide a reservoir for natural enemies, and a source of nectar and pollen for a pollinators, these same weedy refuges can serve as host sources for many key insect pests that cause economic damage to vegetable crops.  Weeds found on field margins and ditch banks can provide insect pests with suitable resources needed for rapid population growth which subsequently can lead to insect infestations occurring in adjacent vegetable crops.   In addition, many weed species can provide insects with host plants that serve as a bridge between cropping seasons when vegetables crops are not in production (May-August).  Volunteer melons and cotton are also considered weeds (“a plant out of place”). If not removed in a timely manner, these weedy volunteer plants can sustain large numbers of insect pests that can disperse onto newly planted fields.  Weeds can also serve as alternate host plants for three important groups of viruses that affect vegetable and melons; tospoviruses, potyviruses and criniviruses.  For example, one of the primary insect vectors of the crinivirus Cicurbit Yellows Stunt Disorder Virus (CYSDV) is the sweetpotato whitefly.  Not surprising, common desert weed species such as alkali mallow, groundcherry, pigweed, lambsquarter, London rocket, and silverleaf night can all harbor large whitefly numbers  and serve as reproductive hosts. More importantly, these weeds are known reservoirs of CYSDV.   Finally, weeds can serve as impediments to insecticide applications.  Dense weed foliage in vegetable and melon fields can negatively influence foliar spray applications by intercepting spray droplets before reaching the crop, which can result in less insecticide deposition and unacceptable crop damage.  Soil applied systemic insecticides (e.g., imidacloprid, Coragen, Verimark) can also be impacted by unmanaged weed growth. Weeds growing unchecked during stand establishment can compete with the seedling plants for water and fertilizer, but they can also compete with crop plants for soil insecticides. Excessive weed densities can significantly intercept insecticides in the soil profile and reduce the amount available for uptake by the target crop.   For more information on this topic, please visit Weed Interference with Insect Management in Desert Crops.

Following more than two decades of drought in the western U.S., many watersheds are experiencing critical water shortages.  In the Colorado River basin, the two largest reservoirs, Lakes Powell and Mead, are now at less than 20% capacity.  In contrast, these reservoirs were nearly full in 2000.  With agriculture responsible for 70-80% of the Colorado River water diversions, a great deal of scrutiny is being applied to crop production systems in this region.
 
Crop production systems in the desert Southwest are experiencing increasing pressure to document and improve irrigation efficiency.  There are several ways to define irrigation efficiency.  The primary definitions for irrigation efficiency include orientations with water conveyance and delivery systems (engineering efficiency), and also agronomic, economic, and environmental efficiencies.
 
It comes as no surprise that as an agronomist and soil scientist I tend to focus on the agronomic efficiency of irrigation for the benefit of the crop being grown.  The crop plants are the centerpiece of the production process and the soil-plant system itself represents the primary objective of irrigation management.
 
Agronomic Irrigation Efficiency
 
Agronomic (crop and soil) considerations are centered on our ability to provide irrigation water for the sustainable production of a crop in the field.  The three primary demands for crop water management include: 1) providing water for seed germination and stand establishment, 2) providing irrigation water to match crop-consumptive water use and avoid crop water stress, and 3) provide sufficient irrigation water to leach soluble salts from the root zone so that the soils can support crop production in a sustainable manner (Figure 1).  
 
Agronomic efficiency at the field level focuses on the crop water demand (CWD) through crop consumptive use (crop evapotranspiration, ETc, which is the combination of evaporation and transpiration from the crop) and leaching requirements (LR) which are dependent upon the crop and salinity of the irrigation water.  
 
Agronomic efficiency can be estimated by considering the difference between ETc + LR = CWD and the volume of irrigation water applied (IWA).
 

 
The leaching requirement (LR) can be estimated by use of the following calculation:
 

 
Where: 
 
EC_w = salinity of the irrigation water, electrical conductivity (dS/m)
EC_e = critical plant salinity tolerance, electrical conductivity (dS/m)
 
This is a good method of LR calculation that has been utilized extensively and successfully in Arizona and the desert Southwest for many years.  We can easily determine the salinity of our irrigation waters (EC_w) and we can find the critical plant salinity tolerance level from readily available tabulations of salinity tolerance for many crops (Ayers and Westcot, 1989).  Additional direct references are from Dr. E.V. Maas’ lab at the University of California (Maas, 1984: Maas, 1986; Maas and Grattan, 1999; Maas and Grieve, 1994; and Maas and Hoffman, 1997).  
 
 
 
 
 
To deal with crop water management at the field level agronomically, the crop and soil factors, there are some fundamentals that we can refer to for assistance.  
 
Dr. Jeremy Weiss, program manager for the University of Arizona AZMET system, has recently developed two valuable tools that provide actual crop evapotranspiration (ETc) estimates from several AZMET sites in the lower Colorado River Valley and for several key vegetable crops, including lettuce (iceberg and romaine), broccoli, cauliflower, cabbage, and spinach.  
 
The first tool provides accumulations of ETc values over the previous week and a range of dates and the planting date selected by the user.  In both models, the Kc values presented in FAO-56 are used for appropriate stages with each crop.  The reference evapotranspiration (ETo) measurements are taken directly from each AZMET site listed.  Thus, by this method ETc estimates are made by use of equation 3.
 
 

 
 
To access this first crop-water estimate tool please refer to the following link:
 
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates/
 
The second tool provides accumulations of ETc values over the crop production cycle from planting (or the wet date) and the date of harvest.  
 
This second tool gives us the opportunity to estimate agronomic efficiency of irrigation management.  With this tool we can review total crop water use estimates (ETc) and include the leaching requirements (LR) to compare with the irrigation water applied (IWA) as described in Equation 1.
 
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates-post-harvest/
 
For example, using lettuce (either iceberg or romaine) with a 10 October 2022 planting date through 9 January 2023, we can see from this model the reference evaporation, listed as “water use” (ETo) and the cumulative crop evapotranspiration (ETc, or total crop-water use) of lettuce since the selected planting date. 
 
Since planting on 10 October 2022 through 9 January 2023, a total crop water use (evapotranspiration), or the ETc, has been 11.27 ~ 11.3 inches in the Yuma Valley.  We can also determine crop water use (ETc) last week (3-9 January), has been 0.52 inches in the Yuma Valley.
 
Similar estimates are provided by this model for several AZMET weather stations in the Yuma production area: Roll, Yuma North Gila, Yuma South, and the Yuma Valley.  The Yuma Valley station is at the University of Arizona Yuma Agricultural Center and the Yuma South site is near Somerton, Arizona.  A map of AZMET sites can be found in the following link:
 
https://ag.arizona.edu/azmet/mapsites.htm
 
When we include the leaching requirement for the crop, an overall estimate of field level irrigation efficiency can be made.  We can estimate the leaching requirement as follows assuming an electrical conductivity for Colorado River water of 1.1 dS/m and the lettuce crop salinity tolerance of 1.3 dS/m.
 
 Leaching Requirement (LR) = ECw/((5 X ECe)-ECw)
 
LR = 1.1 dS/m / (5 X 1.3) – 1.1 = 1.1/5.4 = 0.20 = 20% leaching requirement
 
LR = 11.3 X 0.2 = 2.26 ~ 2.3 inches
 
Thus, total crop water demand in this case = 11.3 + 2.3 = 13.6 or ~ 14 inches total
 
Contrasting this estimate of total crop-water use with the amount of irrigation water that was applied to the field can provide an estimate of the agronomic efficiency for a given field of lettuce.  We can do the same thing using this tool for other primary leafy green vegetable crops.
 
We are working in times of decreasing water availability and increasing scrutiny in agriculture.  It serves us well to measure and understand the relationship between crop demand (consumptive use plus leaching requirements) versus the irrigation water that is applied to a given field.  
 

 
References
 
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9), D05109.
 
Ayers, R.S. and D.W. Westcot. 1989 (reprinted 1994). Water quality for agriculture.  FAO Irrigation and Drainage Paper 29 Rev. 1. ISBN 92-5-102263-1. Food and Agriculture Organization of the United Nations Rome, 1985 © FAO.
https://www.fao.org/3/t0234e/t0234e00.htm
 
Erie, L.J., O.A French, D.A. Bucks, and K. Harris. 1981. Consumptive Use of Water by Major Crops in the Southwestern United States.  United States Department of Agriculture, Conservation Research Report No. 29.
 
Khokhar, T. 2017.  World Bank. 
https://blogs.worldbank.org/opendata/chart-globally-70-freshwater-used-agriculture
 
Maas, E.V. 1984. Crop tolerance to salinity.  California Agriculture, October 1984. https://calag.ucanr.edu/archive/?type=pdf&article=ca.v038n10p20
 
Maas, E.V. 1986. Salt tolerance of plants. Appl. Agric. Res., 1, 12-36.
 
Maas, E.V. and S.R. Grattan.  1999. Crop yields as affected by salinity, Agricultural Drainage, Agronomy Monograph No. 38.
 
Maas, E. V., and Grattan, S. R. (1999).  Crop yields as affected by salinity, agricultural
drainage, Agronomy Monograph No. 38, R. W. 
 
Maas, E. V., and Grieve, C. M. 1994. “Salt tolerance of plants at different growth stages,” in Proc., Int. Conf. Current Developments in Salinity and Drought Tolerance of Plants. January 7–11, 1990, Tando Jam, Pakistan, 181–197.
 
Maas, E. V., and Hoffman, G. J. (1977).  “Crop salt tolerance: Current assessment.”

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.

For those of you interested in the latest thoughts on ag technology, you might want to check out the keynote address given by John Deere¹ at CES 2023 earlier this week. The presentation can be found on YouTube at: https://youtu.be/1kjZMHZl538. As you might expect, there is a fair amount of company self-promotion, but it is interesting to hear their view on the current state and future of agricultural technologies. A couple of key takeaways for me were that telematics and computer vision systems will play key roles in the future and that increased precision will continue to be a focus.

 

[1] Reference to a product or company is for specific information only and does not endorse or recommend that product or company to the exclusion of others that may be suitable.

Growers and PCAs asked about the potential injury that can be caused by a mix of RoNeet (Cycloate) with Dual Magnum (S-Metholachlor) to spinach. We did a small evaluation including the treatments suggested by our friends to collect data. A 1.33pt rate for Dual M and 4.0 pt of RoNeet was sprayed for this trial. The lower Dual M recommended rate by the SLN for AZ in 2017 of 0.33 - 0.67 pt per acre was not included in the trial¹. Here’s the treatment list:

1. RoNeet 2.0pt + Dual M 1.33pt
2. RoNeet 4.0pt
3. Dual M 1.33pt
4. UTC

Plots consisted of two rows 30ft long replicated four times with 10 seed lines, and the test was established in a randomized complete block design with four replications. All treatments were applied preemergence. A CO₂ backpack with a 4 flat fan nozzle boom spaced at 20” was used delivering 20 gallons/acre. 
Planting was done on Nov 17, 2022, then the next day the treatments were applied and incorporated immediately with sprinkler irrigation. Crop injury was evaluated on December 13, 21 and 27. The combination of RoNeet plus S-Metolachlor at the rates above mentioned presented the highest phytotoxicity symptoms in the form of chlorosis, stunting, and reduced stand. Also, Dual Magnum alone at the rate of 1.33 pt/a presented injury in lower proportion compared to the combination with RoNeet. In this evaluation the 4pt/a rate of RoNeet did not present injury to the spinach as you can see comparing with the untreated plots below. Weed control data was not collected due to inconsistent and low populations. Only phytotoxicity was evaluated.

The most representative images of the plots are below:

Figure 1. Preemergence herbicide injury evaluation on spinach

1. https://searchagriculture.az.gov/esd/slns/AZ120006.pdf

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth counts increased in most locations over the past 2 weeks; but remain average for this time of the season. 
 
Beet armyworm:         
Trap counts remain low in all locations, increased slightly, but remain below average for late season.
 
Cabbage looper:         
Cabbage looper trap counts increased in all locations, but still below average for late season.
 
Diamondback moth:    
Adult activity increased in some locations in the past two weeks. Overall, activity is below average for this time of year. 
 
Whitefly:                       
Adult movement remained low in all locations consistent with previous seasons.
 
Thrips:                         
Thrips adult movement increased in all locations recently particularly in the Tacna/Roll are. Activity is well below average for late season.
 
Aphids:                        
Aphid movement low in most locations but increased slightly in Gila Valley. Trap captures well below average for this time of the season.   
 
Leafminers:                   
Adult activity increased in many areas, and below average for this time of season.