So far this season, WFT numbers have been below average based on sticky trap counts, field reports and population densities here at YAC. This is likely due to the cooler temperatures and timely rainfall we’ve experienced this winter.  Although INSV incidence in Yuma has been mostly negligible this season, the potential for infection in late plantings is possible.  However, now that the season is winding down and weather is warming up, you can expect WFT numbers to begin to increase. We typically observe “bioconcentration" of WFT during March and April on late lettuce as surrounding produce acreage declines. Each time a produce field is harvested, and disked, adult thrips disperse from these areas into the next available lettuce field.  As the number of lettuce acres becomes reduced near the end of the season, this creates a bottleneck effect that concentrates high numbers of thrips adults on the remaining fields under production. This can often make chemical control of WFT very difficult, particularly in March, as adults can continually re-infest fields following spray applications.   So, what management approaches can you take to manage thrips and hopefully reduce the potential incidence of INSV.  The first line of defense should be sanitation. Growers should disc under produce crops immediately following harvest. The longer a harvested crop remains above ground, the more insects that can build up and move to adjacent lettuce fields, especially with the dry, windy, and warm weather we typically have in March.   If only light INSV incidence is present in pre-harvest fields, PCAs should consider rogueing and removing suspect plants from the field.  Ultimately, this may curb secondary infection within the field. 
Effective WFT control with insecticides is the best approach to minimizing cosmetic feeding damage and may reduce spread of INSV within fields. For adults, which can easily be found on the leaf surface, your best choices of insecticide are methomyl (Lannate) or acephate at a high label rate.  They are the most efficacious products against adult WFT. Radiant is not as consistently efficacious against adults, but a 7 oz or higher rate will provide the best knockdown and residual control of WFT larvae.  Remember, the key to preventing cosmetic damage by WFT is to maintain larval populations at low levels.  The cryptic or thigmotactic behavior of thrips often makes them difficult to find on lettuce plants. Research has shown that if you can see a few adults and larvae on the plant, it means that there are likely 10-fold more thrips actually on the plant (hiding near the base of the plant between midribs).  This behavior also means that spray coverage is important, particularly with contact insecticides like Lannate. There are other insecticides such as Torac, Minecto Pro, Exirel, Movento and Assail that can provide suppression (~50% control) against WFT larvae, but don’t expect much activity against adults. For more information on the identification, biology, ecology, and management of thrips on desert produce please visit Thrips and INSV Management on Desert Letttuce-2024

When we go to the field to evaluate a crop, there are typically three fundamental points we need to take into consideration that include: 1) stage of growth, 2) general crop vigor and yield potential, and 3) anticipate the next stage of development and what we need to do in terms of field crop management.  These considerations are commonly focused on aboveground crop evaluations.  However, the root system is also an extremely important part of the crop condition to evaluate.  
 
Root systems were described in a basic manner in a recent article on 2 May 2023 (UA Vegetable IPM Newsletter Volume 14, No. 9).  
 
The effective root zone depth is the depth of soil used by the bulk of the plant root system to explore a soil volume and obtain plant-available moisture and plant nutrients.  Effective root depth is not the same as the maximum root zone depth. As a rule of thumb, we commonly consider about 70% of the moisture and nutrient uptake by plant roots takes place in the top 24 inches of the root zone; about 20% from the third quarter; and about 10% from the soil in the deepest quarter of the root zone (Figure 1).  
 
The small and very fine root hairs are the most physiologically active portion of a developing root system.  It is important that the plants continue to develop and generate fresh young roots and an abundance of fine root hairs to maintain water and nutrient uptake.

Figure 1.  General pattern for plant-water and nutrient uptake from the soil profile. 

It is important to point out that these general root development patterns are dependent on the nature of the soil profile in the fields.  Soil profiles with compaction layers, as well as rock or caliche layers will limit root development and full exploration of the soil volume that the plants are capable of (Figure 2).

Figure 2.  Generalized soil profile with major horizons.

Therefore, in scouting fields and making crop evaluations, examining the root systems is an important part of the process.  Leafy green vegetable crops need to develop a marketable plant in a relatively short amount of time and a strong root system is essential.  
 
It does take more time and effort to check root systems and it is a plant destructive process since we need to literally excavate the roots.  So, it is also important to be careful of where and how we sample plants and the root systems in a field.
 
Crop species can vary significantly in their patterns of root development and it is important to know what is “normal” when evaluating crops in the field.  An excellent reference for vegetable crop root system development is a 1927 publication by Dr. John E. Weaver and William E. Bruner from the University of Nebraska (Root Development of Vegetable Crops).  This publication can be found at the following link:
 
https://soilandhealth.org/wp-content/uploads/01aglibrary/010137veg.roots/010137toc.html
 
A few basic examples from the Weaver and Bruner publication are provided in the following figures (Figures 3-10).

Figure 3.  Cauliflower, 3 weeks after transplanting.



Figure 4.  Cabbage roots, 55 days after transplanting.



Figure 5.  Cabbage roots, 75 days after transplanting.



Figure 6.  Pepper roots, 24 days.



Figure 7.  Pepper roots, 45 days (6 weeks).



Figure 8.  Pepper roots, mature.



Figure 9.  Lettuce roots, 3 weeks.  The roots on the right were grown in compact soil, the roots on the
left were grown in loose/open soil.



Figure 10.  Lettuce roots, 60 days.

Bindu Poudel-Ward, Martin Porchas Sr., Martin Porchas Jr., Neeraja Singh, Johan Murcia, and Rebecca Ramirez, and Jason Furr 
Yuma Agricultural Center, University of Arizona, Yuma, AZ
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%). Lettuce was seeded, then sprinkler-irrigated to germinate seed on Nov 15, 2022 on double rows 12 in. apart on beds with 42 in. 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, which contained two 25 ft rows of lettuce. Plants were thinned Jan 5, 2023  at the 3-4 leaf stage to a 12-inch spacing. 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.

Month	Max	Min	Avg	Rain
November (2022)	74°F	47°F	60°F	0.00
December (2022)	69°F	42°F	54°F	0.11
January	67°F	42°F	55°F	0.16
February	70°F	41°F	55°F	0.37
March	75°F	46°F	62°F	0.28

 
Powdery mildew (caused by Golovinomyces cichoracearum) efficacy trial treatments were made on January30 2023, February 9, 2023 and February 22, 2023.Disease was first seen on February 13,2022. Disease rating was done on March 15, 2022.   Disease severity was determined  by rating 10 plants within each of the four replicate plots per treatment using the following rating system: 0 = no powdery mildew present; 0.5 = one to a few very small powdery mildew colonies on bottom leaves; 1 = powdery mildew present on bottom leaves of plant; 2 = powdery mildew present on bottom leaves and lower wrapper leaves; 3 = powdery mildew present on bottom leaves and all wrapper leaves; 4 = powdery mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = powdery mildew present on entire plant. These ratings were transformed to percentage of leaves infected values before being statistically analyzed. Yield loss due to rejected lettuce heads would likely begin to occur on plants with a powdery mildew rating above 2.0 (percentage of leaves infected value of 40).         
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. All treatments significantly reduced the final severity of powdery mildew compared to nontreated plants. The most effective fungicides were Rhyme, Merivon and Quintec.

It’s October and planting season is well underway. When planting lettuce, uniform seed spacing is critical for efficient, economical crop thinning. Due to their lack of precision, automated lettuce thinning machines cannot eliminate lettuce plants that are spaced closer than about 1 1/8” apart. These closely spaced plants, commonly referred to as “doubles”, must be carefully removed by hand which is time consuming and expensive.

Several years ago, we conducted trials examining the influence of planter travel speed on seed spacing uniformity (Siemens and Gayler, 2016). In the study, two types of planters - a vacuum planter (Stanhay 785 Singulaire) and a belt planter (Stanhay 870) were tested with pelleted lettuce seed at travel speeds of 1.0, 1.5, 2.0 and 2.5 mph at the Yuma Agricultural Center. Vacuum planter test results showed that the percentage of “difficult to thin” spacings, defined seeds spaced less than 1.1” apart, increased from about 5% to 10% as speed increased from 1.0 mph to 2.5 mph (Fig. 1). Concomitantly, the percentage of seeds “precisely placed” within 0.5” of the target location (i.e., 2.0 ± 0.5”) decreased from 70% to less than 45%, and the percentage of skips increased from 7% to 30%. Variability of seed spacing uniformity as measured by the coefficient of variation (COV) of seed spacings also increased.

Similar declines in planter performance were found with the belt planter (Fig. 2). As travel speed increased from 1.0 to 2.5 mph, the percentage of difficult to thin spacings increased from 2% to 10%, precise spacings decreased from 93% to 65% and skips increased from 2% to 8%.

For both planter types, there was a significant decline in performance as speed increased from 1.5 mph to 2.0 mph, a difference in speed of only 0.5 mph. These results suggest that it is prudent to check planter performance at the chosen operating speed prior to establishing an entire block to ensure that seed spacing uniformity, not just the number of seeds per foot, is acceptable.

In short, the study showed that planter travel speed had a significant effect on seed spacing uniformity and difficult to thin, close spacings - the higher the speed, the poorer the performance.

You may be asking what is the reason for the phenomenon observed? A logical explanation is that seeds are traveling at the speed of the planter when they are released and tend to “bounce and roll” in the direction of travel when they hit the soil surface. Thus, the higher the travel speed, the further seeds bounce and roll resulting in increased seed spacing variability.

References

Siemens, M.C., & Gayler, R.R. (2016). Improving seed spacing uniformity of precision vegetable planters. Appl. Eng. Agric., 32(5), 579-587

Fig. 1. Seeding performance of a vacuum vegetable planter sowing lettuce when
operated at four travel speeds.


Fig. 2. Seeding performance of a belt vegetable planter sowing lettuce when operated at
four travel speeds

Sprinkler irrigation systems are delivering water throughout the Yuma CO. Either for germination or chemigations in lettuce, broccoli, onion and spinach.
How are you handling the absence of Dacthal (DCPA) in your operations?
One strategy that worked good in onions at the Gila Valley for one of our PCAs this week was the application of Prefar (bensulide) at 6qt. The weed complex he has included Purslane (Portulaca oleracea), Pigweed (Amaranthus palmeri), and is expecting later germination of Lambsquarter (Chenopodium album), Gooseofot (Chenopodium murale), and London Rocket (Sisymbrium irio). Control looks great and scouting the field in untreated areas we noticed there was a seedbank that could have caused a problematic infestation of purslane. The Pest Control Advisor is planning to include Goal (oxyfluorfen) in his weed control program later. 
We all know bensulide is an old treatment and many of you are using it, but our friend PCA wanted to share his experience just in case some of us have the same weeds.
If you would like to share with the Ag. community some of your experiences or new strategies used successfully, email the IPM Team. Also, please let us know if you like include a treatment or see a product tested in the onion and broccoli herbicide trials we are starting at the Experiment Station.
 
Following are links to some articles on onion weed control:
 
https://ag.arizona.edu/crop/vegetables/weeds/Dacthal.html
 
https://escholarship.org/content/qt91w4d06x/qt91w4d06x.pdf?t=lnryxp&v=lg
 
https://repository.arizona.edu/bitstream/handle/10150/221477/370100-165-167.pdf
 
https://ipm.ucanr.edu/agriculture/onion-and-garlic/herbicide-treatment-table/

Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 21, Oct 16, 2024

Results of pheromone and sticky trap catches can be viewed here.

Corn earworm: CEW moth counts increased in most traps; about average for early October.

Beet armyworm: Trap counts increased in most areas last week, and about average for early October.

Cabbage looper: Cabbage looper trap counts still most locations, below average for this time of the season.

Diamondback moth: Adult activity increasing, particularly in the Yuma and Dome Valleys.  Above average for this time of season.

Whitefly: Adult movement increasing Tacna, Dome Valley and Yuma Valley; overall about average for early October.

Thrips: Thrips adult movement picking up in all areas, overall activity above average.

Aphids: Winged aphids beginning to show up the north Yuma and Gila Valleys; about average for early October.

Leaf miners: Adult activity light in all areas, about average for this time of season