Lettuce Aphid (LA)      The first report of Lettuce aphid (LA)  arrived last week. We identified a small colony infesting the terminals of romaine at two separate locations in the Wellton area. This is about normal for LA; they typically begin to show up in late January.  In contrast, last season we began finding heavy colonies of LA in early December and they became worse as the spring season progressed.  We’ve also seen an increase in trap catches of winged aphids, particularly in the Yuma and Gila Valleys. A few things to remember about LA relative to the other aphids we commonly see. First, the immature nymphs are small and have a red appearance, whereas the apterous (wing-less) adults are typically a large brown  colored aphid with dark bands running across the abdomen. . For a quick guide to identifying aphids see the UA Aphid ID Guide  Second, among the crops we grow locally, lettuce aphids are only found on the lettuce types (Lactuca spp.). Lettuce aphids prefer to colonize the terminal growth, and can often be found in heads or hearts, whereas green peach aphids are often found on the frame leaves in high numbers before moving into the heads. Third, they can reproduce prolifically, producing many more alates (winged) adults than other species, which can quickly lead to widespread abundance in a field and dispersal throughout a growing area.  Finally, as the saying goes "he who hesitates is lost". If lettuce aphids are found on your lettuce, it is recommended that you respond quickly with an insecticide treatment.  For more information on lettuce aphid please refer to Lettuce Aphid on Spring Produce-2023.   
Diamondback moth   Although armyworm, looper and earworm activity has been seasonably light over the past month, peak DBM adult activity occurred in early January across all traps and remain high (see Areawide DBM Moth Trapping Network). More troubling though are recent PCA reports of DBM infestations requiring treatment in cauliflower, cabbage and broccoli. You might not expect this given the cooler temperatures. DBM is a different beast though. It will remain active in much lower temperatures than the other Leps.  On many days during the past month, DBM continued to develop unchecked, whereas the other Lep species were essentially quiescent. Remember:  prompt destruction of crop residue following harvest is helpful.  So, be very aware of DBM on young brassica crops (including seed crops) and expect to see DBM more abundant this spring than normal. For more guidance on DBM management see (2023 Guidelines for Diamondback Moth Management in Desert Cole Crops).
 
Thrips              Thus far, sticky traps and plant samples have shown relatively little activity, but trap counts are usually low this time of the year – see Areawide Insect Trapping Network PCAs have generally reportedlight thrips activity throughout the area, and populations on lettuce at YAC are light. However, do not be complacent when it comes to managing thrips, especially with the threat of INSV out there. As temperatures increase , thrips populations can build up quickly.  I recommend you be vigilant in keeping thrips suppressed, particularly in lettuce field near transplants and weedy areas where viruliferous adults might be awaiting.

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.

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 28, 2023 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 4, 2024  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	MaxTemp(°F)	Min Temp (°F)	Average Temp (°F)	Rainfall
November	80	51	65	0.08 in
December	71	44	57	0.82 in
January	68	42	54	1.14 in
February	73	47	59	0.50 in

 
Downy mildew (caused by Bremia lactucae) rating was done on variety Eblin, Bobcat, and 180 (partially funded by AILRC grant). Disease was first seen on 1-30-24. Please see attached excel file for chemicals application date. Disease rating was done on February 29^, 2024. 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; 0.5 = one to a few very small downy mildew colonies on bottom leaves; 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.
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Most of the treatments exhibited activity against the disease to some extent. Latitude, Amara, Eject, Previcure flex exhibited good control in the variety Eblin (highly susceptible variety). Whereas Cevya, Stargus, Latitude, Amara, Revus, Thrive 4 M, Actigard, Instigo+Carbose+intereact showed activity against the pathogen in variety Bobcat. Please see excel file for the full list of chemicals and their efficacy. The lettuce variety 180 was resistant to the disease and no downy mildew was observed on the particular variety. No phytotoxicity was observed in this field.

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