Aphid Control: We mentioned in our previous update that foxglove aphid was beginning to colonize lettuce in the Yuma Valley. In the past two weeks aphid activity, and particularly foxglove aphid, has increased dramatically in our untreated control lettuce plots here at the Yuma Ag Center. Keep in mind, foxglove aphids can be difficult to locate in fields due to their patchy distribution. It is not unusual to sample dozens of plants and find nothing and then find 2-3 plants heavily colonized with foxglove aphids. Green peach aphid is generally more uniformly distributed among plants. They are also still prevalent in our lettuce, albeit not at the same densities as foxglove aphids. We’re also finding green peach aphid and cabbage aphid beginning to rapidly colonize young cabbage plants. Because all these aphid species are inherently different in their susceptibility to insecticides, the proper choice of product is important for achieving adequate control. For more information on insecticide alternatives for the various aphid species please view our Aphid Control Chart-2014.
 
Cabbage looper: Our pheromone trap captures of cabbage looper moths the past 2 weeks have been unusually high for this time of the year (see Areawide Trapping Network). This is largely due to the above average temperatures we’ve been experiencing in the desert. In addition, corn earworm trap catches remain consistent from our last update, particularly in the Wellton/Dome Valley area. These increases in moth activity may indicate potential increases in egg laying and larval infestation in your later produce fields. It would be well advised to pay careful attention for fresh feeding signs and small larvae when scouting for aphids/thrips. Addition of a pyrethroid to aphid and thrips sprays can help with looper and earworm control.

Mystery Lepidopterous Larvae Identified: In out last update we mentioned that an unidentified lepidopterous larvae has been found burrowing into the roots of young romaine plants in the Yuma Valley. Because of the unusual coloration and patterns on the larvae, as well as the root boring behavior, we were uncertain of its identity. After rearing the larva to a moth, we were able to identify the Lep species. With the assistance of Ian Watkinson, we have identified the species as a beet armyworm, Spodoptera exigua (see Mystery Larvae Identified). This was surprising given the odd markings and coloration. In fact many (including myself) speculated that it might be a cutworm. But beet armyworm larvae are known to vary in color based on temperature and food source among other things. Why these larvae were boring into the lettuce roots is more confusing. In 24 years of working with beet armyworms in lettuce, I’ve never seen this occur. However, Peter Ellsworth noted that he has occasionally seen them bore into cotton terminals. You learn something new every day.

Root systems are responsible for all water and nutrient uptake by the plant and they provide the physical anchoring and support of the plant structure.  Each plant and crop species has its own “personality” and growth habits, and root systems have unique characteristics among plants species.  
 
In general plant root systems constitute 30-50% of the total plant dry matter.  When post-harvest plant residues are incorporated into the soil, the root systems provide a significant contribution to that plant material and final carbon (C) contributions to the soil.
 
The first thing a seed develops in the germination process is a primary root that grows downward into the soil.  We often refer to this as the “stinger” root that extends from a germinating seed.  New cells are formed at the tip of the primary root as it extends downward into the soil forming a “thimble-shaped” cluster of cells called a root cap (Figure 1).  The root cap serves as a type of shield that helps the root penetrate the soil matrix and protect the developing root tissue.  As the root grows downward into the soil the root cap cells are sloughed off creating a slimy surface that helps lubricate the root as it extends deeper.
 
The growing point (apical meristem) for the developing root is just behind the root cap and this is the zone of new cell formation that facilitates root growth and replaces the cells that are sloughed off as the root grows through the soil.  The new cells elongate and serve to extend the roots into the soil (Figure 1).
 
The most active parts of the plant root system for mineral nutrient and water uptake are in the tiny root hairs that are formed in zone behind the apical meristem.  Root hairs are only formed in the relatively new and freshly developed root tissue.  The root hairs are extremely small, tender, and physiologically active.  Root hairs are often referred to as “feeder roots” due to their high-level of activity in securing water and nutrients from the soil for the growing plant.  In the process of transplanting, it is important to protect the feeder roots and promote their health to ensure rapid adaptation to the new soil environment.
 
Young plants have the capacity to develop basic aboveground tissue to perform sufficient photosynthesis for establishment and growth due to the plant’s ability to take up mineral nutrients and water from the soil from the root system.  Sometimes it can appear that plants are not growing rapidly while the young crop is investing energy and resources into root system development, which is the foundation for the subsequent plant growth and development.
 
The depth of the roots will vary according to the soil physical conditions and effective soil depth, soil fertility and salinity management, plant-available water, and of course the natural rooting characteristics of the plant.
 
In general, there are two basic types of plant root systems.  Broadleaf plants (dicotyledonous) and coniferous plants (gymnosperms) commonly have a taproot system the extends downward through the soil developing root branches from the primary root stem (Figure 2).
 
Grass plants and their relatives (monocotyledonous plants) produce fibrous root systems that branch extensively and radiate out into the soil from the plant base (Figure 2).
 
In general, taproots tend to be deeper with extensive branching from the primary root, develop woody tissue on older roots, and generally long-lived.  In contrast, fibrous roots tend to be smaller, short-lived, with less branching.  As roots age, they become more important in conducting nutrients and water to the growing points of the plant, both above and belowground.  In all cases, the young and freshly developed root hairs (feeder roots) are the primary zone of water and mineral nutrient uptake.
 
As root systems age, the older roots will die, and new root tissue is formed.  As dead roots are sloughed off, the discarded tissue is attacked by naturally occurring, beneficial soil organisms (bacteria, fungi, protozoa, and worms) the release mineral nutrients and produce soil organic matter.  Turnover of root tissue is an extremely important aspect of plant contributions to soil carbon (C) and organic matter.
We do not see the plant root system on a regular basis and we cannot watch root hair development.  But it is good to be conscious of root system development since all mineral nutrient, water uptake, and structural support is provided through the roots. So, it is good to review and understand root structure and function as we work to manage crop plants for optimum growth and development.

Figure 1.  Basic root tip anatomy.


Figure 2.  Examples of taproot and fibrous root systems.

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.

 

There are many innovative automated weeding technologies coming out of Europe. One of these is ARA, the high precision, smart spot sprayer being developed by Ecorobotix[1] (Yverdon-les-Bains, Switzerland). Early prototypes were solar powered, autonomous robots that used a spider-like, three-axis delta robot to precisely deliver herbicides to target weeds (Fig. 1). The company has since moved on to a much simpler solution – a fixed boom, tractor pulled implement (Fig. 2). The 19.7’ wide smart sprayer is equipped with 156 individually controlled spray nozzles spaced 1.6” resulting in a spot spray resolution of 5.6 inch² (2.4 x 2.4 inch). The claimed travel speed is 4.5 mph equating to a work rate of 10 ac/hr. The machine uses computer imaging and artificial intelligence for crop/weed differentiation to identify and target weeds. AI weeding models have been developed for a variety of crops including several vegetable crops – onion, lettuce and spinach. In addition to weed control, the smart sprayer can be used to spot apply beneficial products to only the crop plant. A company video showing the device operating in onion crops can be found by clicking here or on the image below.

As I have mentioned before, robotic and automated weeding technologies are advancing at a very rapid pace. If you know of a new technology that would be of interest and appropriate for this newsletter, please feel free to contact me.

Fig. 1. Solar powered autonomous weeding robot developed by Ecorobotix,
Switzerland. The unit is equipped with three-axis delta robot arms to precisely
deliver herbicides to target weeds (Photo credit – Ecorobotix).

Fig 2. Precision weeding machine developed by Ecorobotix, Switzerland. The unit
is equipped with individually controllable spray nozzles mounted on a 19.7’ wide
spray boom to spot spray weeds (Photo credit – Ecorobotix).



Fig. 3. Precision spray boom comprising 156 spray nozzles for spot spraying
weeds. Spot spray resolution is 2.4 x 2.4 inches. (Photo credit – Ecorobotix,
Switzerland).

[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.

 

Image by: Jim Daily

Hare Barley, Mediterranean, or Wild Barley (Hordeum murinum) has been increasing at the Yuma AZ Valley and the Mesa. This week we have received some samples for ID from our friend PCAs.
It’s a cool season annual grass native to Europe with dense stiff awned spikes. Early in the season before the flower spikes develop, they can be a very good forage but when the spikes emerge can injure the mouths, eyes, nasal passages, ears, and skin of animals.
It is found throughout California up to 3300 feet (1000 m) and inhabits agricultural land, disturbed sites, and unmanaged natural areas³. The seedheads are unbranched, awned, stems and leaves are hairless, and the ligules are membranous. The collar region and auricles can be seen in the image below from the University of CA³.
The Hordeum murinum complex is the most widespread of all the other Hordeum species, and the origin of its distribution of was Central Europe, the Mediterranean area, North Africa, and Western Asia¹.

Collar region of (left to right) hare (wild) barley, wheat, and wild oat.
Photo by Jack Kelly Clark³.

References:

1.   Retrieved from: https://en.wikipedia.org/wiki/Hordeum_murinum
2. https://weeds.nmsu.edu/factsheet.php?weed_id=101
3. https://ipm.ucanr.edu/PMG/WEEDS/hare_barley.html

Area wide Insect Trapping Network   VegIPM Update, Vol. 11, No. 22, October 28, 2020

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

Corn earworm: Moth activity is about average for mid-October, particularly in Dome Valley and south Yuma Valley. 

Beet armyworm: Moths remain active throughout the desert, especially active in Tacna and Dome Valley.

Cabbage looper: Cabbage looper activity remains unusually low for early October, likely a result of unusually hot weather. Larvae are yet to show up in many fields.   

Whitefly: Adult movement has been about average for this time of year. Activity highest in Tacna near fall melons.

Thrips: To date, thrips activity has been seasonably low at all trap locations; activity increased significantly in Roll. 

Aphids: Aphids beginning to show in most traps along the Colorado River (Bard, Gila, Yuma) which is normal for this time of year.  Recent high winds may begin to disperse them throughout the area.

Leafminers: Adult activity below normal for September, but high numbers caught in Wellton in areas where melon harvest has commenced.