Now that the weather has finally returned to normal for mid-October, the worm pressure we’ve been experiencing should begin to decline.  However, with many early-planted head lettuce crops beginning to rosette and folding-in to form heads, it would be wise to keep a keen eye out for corn earworm (CEW).  This fall could be important as pheromone trap catches spiked last week in several locations (see graph below) moths continue to be active.  Although pheromone trap counts don’t always correlate to field infestations, we have associated peak CEW moth trap counts with larval CEW activity in surrounding lettuce fields in the past.  Even with the cooler temps expected this next week, moth activity should be expected in fields and that means eggs being deposited in or on developing heads.

CEW can be very damaging in early fall head lettuce crops. On older plants beginning to form heads, larvae will migrate to the succulent terminal growth. If not controlled before the plant leaves fold in, they are protected from foliar sprays. By this time in plant growth, at-planting soil applications of Coragen or Verimark may not be effective enough to protect the heads. Once head formation begins, newly hatched larvae will usually bore into the head almost at once upon hatching.  Corn earworm is much more likely to bore into lettuce heads than other Lepidoptera larvae, rendering the heads unmarketable.  Larvae may enter the head from any point on the plants but can often be found burrowing in from the lower half of the head. 

If fields are not watched closely, infestations may not be noticed until the head is harvested. Once inside the head, it is virtually impossible to control the larvae with insecticides. 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 or cup over, you should treat as soon as possible.  Moreover, during mid-late October and early November it is probably a good idea to prophylactically apply a pyrethroid, methomyl or acephate in combination with larvicides (e.g., Radiant, Coragen, Proclaim) when heads begin to form.  The UA nominal threshold for CEW in head lettuce from the beginning of heading to harvest is1-2 larvae / 100 plants (1-2%).  Repeated insecticide treatments may be required to maintain low population levels if heavy pressure is sustained near harvest. Lab bioassays have shown that CEW larval mortality is most rapid when exposed to Lannate at 0.5 lbs (>90% mortality in 1 hour after exposure) and pyrethroids at high rates (>90% mortality in 3 hours), followed by Radiant,5 oz (>90% mortality in 12 hours).  By 24 hours, mortality was 100% for all the treatments including Coragen and Proclaim.  For more information on CEW management see Corn Earworm Management on Desert Produce.

As Colorado River flows have decreased over the past 23 years due to the prolonged drought, many proposals have been presented for management of the river.  One group from the research community presented some intriguing scenarios last year with a publication in Science on 22 July 2022 (Wheeler et al., 2022).  
 
I found some of their points and suggestions interesting in relation to the Colorado River management discussions.  These recommendations are not reflected in the recent U.S. Bureau of Reclamation (USBR) operational plans for 2024 that were presented with their 24-Month study reports and projections (USBR, 15 August 2023).  It will be interesting to see if these points become part of the negotiations in the next two years for the interstate and international shortage agreements that expire in 2026.
 
Basic parameters for any proposed management plans of the Colorado River must include the “Law of the River” which is an amalgam of interstate compacts, court decrees, federal laws, secretarial guidelines, and an international treaty with Mexico.  Key elements associated with the Law of the River include the 1922 Colorado River Compact among the seven basin states in the United States and the 1944 Treaty between the United States and Mexico.
 
The 1922 Colorado River Compact divided the watershed into upper and lower basins with the dividing point at Lees Ferry in Arizona.  Each basin was allocated 7.5 million acre-feet (MAF) per year.  The 1944 Treaty established a delivery requirement of 1.5 MAF/year to Mexico.  Thus, a total of 16.5 MAF/year is allocated from the Colorado River.
 
The 1922 Compact negotiators based their work on an assumption of 17.5 MAF/year of river flow.  However, the 20^thcentury average natural flow rates on the Colorado River have been 15.2 MAF/year, resulting in an approximate 2.3 MAF/year structural deficit.  This deficit in the water budget for the Colorado River and evaporation losses were never corrected in a formal manner, primarily due to some states not using their full allocations.
 
Changes in the climate since 2000 have resulted in much drier conditions in the Colorado River watershed with reduced annual natural flows to an average of 12.3 MAF/year.  Thus, the challenge in addressing the shortages on the Colorado River is that of basic mass balance and developing a consumptive use water budget that matches water supply in the river.
 
Like many people working in agriculture, I have been watching and studying the situation with great interest.  Based on my understanding, one of the key issues in the negotiations has been the differences in perspectives between the upper and lower basin states.  
 
The lower basin states and Mexico have been fully utilizing their water allocations from the Colorado River in contrast to the upper basin which has not been using its full 7.5 MAF/year allocation.  For example, between 2000 and 2020 the upper basin consumptive use averaged 3.7 MAF/year plus at least 0.7 MAF/year of reservoir evaporation.  The upper basin states have plans for further development and utilization of their full allocation and they want to protect their Colorado River water allocation to support these development plans.  
 
Agriculture is responsible for approximately 70% of the Colorado River water use.  As noted by the USBR (2015), the lower basin has been irrigating less than one-half of the area irrigated by the upper basin, but the agricultural revenues of the lower basin are more than three times that of the upper basin.
 
Additional upper basin development and water use from the river exposes a much higher level of uncertainty on Colorado River water management for the future and there are concerns that it could violate the non-depletion obligation in the 1922 Compact which states the upper basin will not deplete the river’s flow to less than 75 MAF during any 10-year consecutive period.  This implies an assumption on the part of the 1922 Compact negotiators that a short-term drought could be managed within a 10-year period, but we are now dealing with drought extending over 23 years.  In addition, the 1944 Treaty requires all seven states to share in the obligation to Mexico.
 
The upper basin states have been adamant that the responsibility of balancing the shortages on the river rests fully with the lower basin states and Mexico.  The upper basin states have also consistently emphasized the importance of equality between basins as outlined in the Compact but that has not actually happened in 100 years.  
 
The upper basin representatives have proposed reductions in lower basin water use of 1.2 – 1.5 MAF, which is a reduction of about 17-22%.  In addition, upper basin states suggest that evaporation losses from the river system should be subtracted out of the lower basin allocations.
 
On the other hand, important economic and equity considerations must also be recognized and a good argument exists that the loss of an established agricultural industry in the lower basin is much more harmful than reducing plans for future development in the upper basin.  Thus, the proposed development in the upper basin is being questioned and appropriately so in my view.
 
Wheeler et al. (2022) ran a series of hydrological models, including the USBR Colorado River Simulation System (CRSS, 2021) and developed a set of 100 scenarios.  More details are provided in their article (Wheeler et al., 2022).  
 
In my review of the work presented by Wheeler et al., there are two scenarios that are particularly intriguing, outlined in Tables 1 and 2.

Basin	Proposed Consumptive Use (MAF)	Percent of Full Allocation and Reduction.
Upper	4.5	60% of full allocation and0.8 MAF > recent use
Lower	6.0	66.7% of full allocation (33.3% reduction)

Table 1.  The first scenario presented in the Wheeler et al. (2022) article.
 
 

Basin	Proposed Consumptive Use (MAF)	Percent of Full Allocation and notes.
Upper	4.0	53% of full allocation and0.3 MAF > recent use
Lower	7.0	77.8% of full allocation (22.2% reduction)

Table 2.  The second scenario presented in the Wheeler et al. (2022) article.
 
Both scenarios represent significant reductions in Colorado River water allocations to balance consumptive use with recent natural flow rates.  But the fact remains that the mass balance of natural Colorado River flow and consumptive must be dealt with to realize a sustainable system of management.  All these scenarios present major challenges in implementation.
 
For 2024, the operational plans for the Colorado River are basically the Tier 1 plans from the 2019 Drought Contingency Plan (USBR, 2023; Table 3).  Tier 1 reductions include an 18% reduction in Colorado River water use by Arizona and approximately 3 MAF of additional water is being held in Lake Mead by voluntary reductions from several Arizona municipalities.  
 
In 2023, Tier 2a guidelines were in effect in 2023 and Arizona has been dealing with a 592,000 AF allocation, which is a 21% reduction of the 2.8 MAF full allocation (Table 4).  Thus, Arizona, Mexico, and other lower basin states have already been meeting the reduction requirements outlined in the second scenario described above (Table 2).

Table 3.  Drought Contingency Plans (DCP) for Arizona, Tiers 1-3.  Tier 1
reductions (red) will be in place in 2024.



Table 4.  Drought Contingency Plan for lower basin states and Mexico, rows are
Tier 0, Tier 1, and Tier 2a.  Arizona is highlighted with the red arrow and Tier 2a
for all lower basin states and Mexico in red letters.

The consideration of scenarios such as the ones described by Wheeler et al. offer some good points for consideration over the next two years as all interstate and international shortage agreements expire in 2026.  The negotiations in the next two years will be challenging and any proposal offered will meet resistance, particularly under the conditions of the Colorado River after 23 years of serious drought and the high level of demand that exists in the basin.
 
Reference
 
USBR.  2015. “Moving Forward Effort: Phase 1 Report” (DOI, May 2015); https://www.usbr.gov/lc/region/programs/crbstudy/MovingForward
 
USBR. 2021. “DRAFT CRSS: Key modeling assumptions, June 2021 model” (DOI, 2021); http://bor.colorado.edu/Public_web/CRSTMWG/CRSS.
 
USBR, 15 August 2023. 24-Month Study reports.   

• 24-Month Study Projections (Overview)
• August 2023 Most Probable  
• August 2023 Probable Minimum 
• August 2023 Probable Maximum   

August 2023 Lake Powell and Lake Mead: End of Month Elevation Charts
 
Wheeler, K.G., B. Udall, J. Wang, E. Kuhn, H. Salehabadi, and J. C. Schmidt.  2022.  What will it take to stabilize the Colorado River?  Science, 377 (6604), DOI: 10.1126/science.abo4452
https://www.science.org/doi/10.1126/science.abo4452c

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.

Last week, we initiated our first trial of the season examining the use of band steam for controlling Fusarium wilt of lettuce. The premise behind this research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae (FOL) the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing FOL disease incidence by 45-98% (Matheron and Porchas, 2010).

In our trials, we are using steam heat to raise soil temperatures. Steam is delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). As the device travels through the field, steam is injected into the soil in narrow bands centered on the seedline. Soil temperatures are raised to about 190°F. After cooling (< ½ a day), the crop is planted into the disinfested soil.

Trials conducted in prior years show the technique holds promise; however further work is needed. In these studies, a 4” wide by 2” deep band of soil was treated. Although soil assay results showed that steam treatment significantly reduced the number of Fusarium colony forming units by over 95% as compared to the untreated control, this didn’t always translate into reductions in disease incidence. At one field site with a known history of FOL, band-steam provided no benefit with virtually all lettuce plants succumbing to the disease (Fig 2a). However, at another site, disease incidence was reduced by more than 40%, plants appeared to be healthier and more vigorous, and yield was increased by more than 90% (Fig. 2b). One explanation for this difference is that at the first site, Fusarium inoculum loads were very high and the width and/or depth of soil treated by steam wasn’t large enough to protect seedling roots, which are highly susceptible to FOL, from infection. The aim of this year’s trial is to determine the width and depth of soil that needs to be disinfested to prevent disease. In the current study, we are examining bands of soil receiving steam treatment that range in size from 4” wide by 2” deep to 6” wide by 4” deep. 

Stay tuned for final trial results and reports of our other studies examining the efficacy of using steam heat to control sclerotinia lettuce drop, pythium and weeds.
As always, if you are interested in trying band-steam on your farm or would like more information, please feel free to contact me.

References
Matheron, M. E., & Porchas, M. 2010. Evaluation of soil solarization and flooding as management tools for Fusarium wilt of lettuce. Plant Dis. 94:1323-1328.

Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council.  We greatly appreciate their support.
A special thank you is extended to Fatima Corona, Roberto Delgado, Chad VanMatre, Matt McGuire and JV Farms for their generous assistance and allowing us to conduct this research on their farm.

Fig. 1. Band-steam applicator principally comprising a 35 BHP steam generator
mounted on a bed-shaper applicator sled. The device injects steam into the soil
as beds are formed.



Fig. 2. Lettuce seedlings at field sites with (a) very high and (b)
moderate levels of Fusarium wilt of lettuce inoculum. Band-steam
(left) and untreated (right) plots are shown.

Last May we conducted our “Lettuce Insect and Weed Losses Survey”.  Thank you for participating and providing real world data that shows us what are the trends in herbicide usage. Arizona does not require complete reporting for private applicators, so we appreciate you providing this difficult to acquire information. In addition to herbicide use we included in the most recent survey questions related to other methods of weed control such as automated thinning, automated weeding, cultivation hand weed control. The results reflect data obtained from PCAs in more than 54,000 acres of lettuce scouted and we believe they accurately represent what goes on in the field. Results of the weed section for the 2022-2023 season appear in the chart below. Some highlights of last season (Figure 1) are that 79% of the acres were treated with Pronamide (Kerb), 53% with Bensulide (Prefar) and 19% with Benefin (Balan). This is consistent with a previous survey conducted in 2018-2019. Regarding grass herbicides, last season shows that 24% of the acres surveyed received an application of Clethodim products and 1% of Sethoxidim. With respect to automated thinning, it is interesting that it was used in 60% of the acres representing an increase from 2018-19, which was reported on 40% of the reported acres (Tickes 2019)¹.
Additionally, we can see that automated weeding was used in 6% of the acres, which could increase in the future as the industry prepares for possible scarce labor resources. The hand weeding and cultivation continues to be a control strategy that is used in most of the acres in our area with 91 and 71% respectively.

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth counts decreased considerably y in the past 2 weeks and about average for late summer
 
Beet armyworm:         
Trap counts stay active in some locations (Yuma Dome Valley and Wellton) particularly near alfalfa
 
Cabbage looper:         
Cabbage looper numbers remain low consistent previous years.
 
Diamondback moth:      
DBM moths were not caught in any trap since June; average for this time of the year.
 
Whitefly:                      
Adult movement decreased in past 2 weeks consistent with previous years
 
Thrips:                         
Thrips adults decreasing in most locations, and trending comparable to previous years.
 
Aphids:                       
Aphid movement is absent and consistent with what we typically see during the summer.
 
Leafminers:                  
Adult activity has highest in Wellton and Dome Valley, and slightly above average for early August