Lettuce aphids, Nasonovia ribisnigri, (aka., red aphid) have been found colonizing lettuce fields since mid-December in several locations throughout Yuma. In the desert, we typically associate lettuce aphid with the warm growing conditions in February and March.  However, in the past few weeks, I've identified lettuce aphids infesting head and romaine lettuce in both organic and conventional production in Dome Valley, Gila Valley, Yuma Valley and across the river in Mexico. This is by far the heaviest I’ve seen lettuce aphids colonize lettuce in the desert this early in the season. Foxglove aphid is also being reported in the Yuma Valley, albeit at much lower abundance. Given their widespread incidence, Lettuce aphids likely came into the desert on one of the multiple flights of winged aphids we’ve reported from sticky traps this winter (see Areawide Trapping Network). Experience has taught us high temperatures in the 70-80's are ideal for population growth of these aphid species. Our local weather forecast suggests that high temperatures will be in the 70’s with moderate nighttime lows in the 50’s for the next two weeks. Assuming the weather service is correct, these conditions may be conducive for lettuce and foxglove aphid population growth.  So be prepared to battle them.
 
A few things to remember about lettuce aphids 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 (see images below).  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. Sampling should be focused on the terminal growth of young plants, and in the heads and hearts of older plants. 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.  The product of choice is Movento at 5 oz/ac. Because of its systemic activity, Movento will reach aphids in the protected terminal growth.  Be sure to include a penetrating adjuvant for best results at a rate of at least 0.25% v/v.  It normally requires about 7 days of activity before significant reduction in the infestation is observed but can take longer (10-14 days) in cool, cloudy weather. Sequoia and Sivanto are good rotational products if coverage of terminal areas can be attained. For more information on lettuce aphid please refer to Lettuce Aphid on Spring Produce-2022.   If you’re finding lettuce aphid on organic lettuce, good luck; we’ve not had much success in controlling them with available biopesticides.

When going into 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.  The root system is also an important part of the crop condition to evaluate but we commonly do not evaluate roots because of the difficulty in doing so.
 
Root systems and early development were described in a basic manner in a recent article on 4 September (UA Vegetable IPM Newsletter Volume 15, No. 18).  
 
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 that 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 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.

Root development patterns are dependent on the nature of the soil profile in the field.  Soil profiles with compaction layers, as well as rock, clay, or caliche layers can limit and alter root development and the full exploration of the soil volume that the plants are capable of (Figure 2).  So, it is good to know what the soil profile looks like in the field and understand how that will impact plant root development.

Figure 2.  Generalized soil profile with major horizons.

Leafy green vegetable crops need to develop a marketable plant in a relatively short amount of time and a strong root system is essential.  Transplants are commonly being used in vegetable crop production systems and the transition of transplants to field conditions is a major step in the production process.  The transition is primarily experienced by the plant below ground.  
 
Transplanted crops will have altered root systems due to the constraints within the rooting cone.  Further root development beyond the original cone rooting mass is important for crop success.  Thus, it is important to monitor the root system transition and the relationship to overall plant development.
 
Checking root systems is a plant destructive process since we need to literally excavate the roots from the soil and it does take time and effort.  Accordingly, it is also important to be careful of where and how we sample plants and evaluate 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.

Fungal transmission of viruses: 
Several viruses have been shown to be transmitted by soil-inhabiting fungi and protists. Two of the common fungal species that transmit viruses in vegetable cropping system are Olpidium spp. and Polymyxa spp. There is specificity between the zoospores of the fungi and virus particles for successful transmission to occur. Not all soilborne fungi can transmit viruses.
 
Olipdium brassicae  is an obligate parasite, meaning the fungus always needs a host plant to survive (brassicae, cucumber, carrot, lettuce). The fungius that act as vector of s viruses survive the time in between crops by producing resting spores.  A virus that we  often see in the desert, Lettuce big vein virus, is transmitted fungus  Olpidium brassicae . Lettuce big vein virus is acquired by the zoospore (motile spores)  as well as resting spores of the fungus. 
  
The other soilborne virus  we see in the desert it Tomato bushy stunt virus is soilborne but has no known vectors. Thought it was predicted to be transmitted by a fungus Olpidium brassicae, new research findings have rules out the particular fungal vector. But there is a possibility that the virus has some soilborne vector. 
 
Common viruses with fungal vector: 
Cucumber necrosis virus: Olpidium bornovanus
Beet necrotic yellow vein virus: Polymyxa betae
Beet soilborne mosaic virus: Polymyxa betae
Beet soilborne virus:Polymyxa betae
Beet virus Q: Polymyxa betae

Nematode transmission of viruses
Two genera of viruses; Nepoviruses (Nematode transmitted polyhedral viruses) and Tobraviuses (Tobacco rattle virus) are transmitted by nematodes.  Xiphinema (Dagger nematodes), Longidorus (needle nematodes), Trichodorus and Paratrichodorus (stubby root nematode) vector transmit different species of viruses. 
 
Virus transmission by nematodes happens in 7 different but interrelated steps. 

1. Ingestion: The process where the nematode ingests the virus particle along with the plant material/sap. 
2. Acquisiton: The unique receptor in the nematode identifies the virus, so he virus particles stay intact within the nematode. 
3. Adsorption: The phenomena of the virus being intact instead of disintegrated in the nematode digestive system. 
4. Retention: Once adsorption, the virus particle can be retained in nematodes anywhere from months to even years. 
5. Release: The virus particles are released onto new plants when nematodes commences feeding on new plant. 
6. Transfer: The virus particles get transferred to new plant. 
7. Establishment: The virus particles establish themselves in the new host plant. 

 
It is a sophisticated and thankfully a very specific relationship. Not all nematodes transmit virus, neither do nematodes vector all plant viruses. Nematodes and nematode transmitted viruses are of a bigger problem in perennial agriculture system such as citrus orchards and grapevines. 
Viruses transmitted by nematodes: 
tobacco rattle virus: beans, beets, peppers, potatoes, and spinach and 100 others
tomato ringspot virus-tomato, cucumbers, 
peach rosette mosaic virus: peach, blueberries 
cherry rasp leaf virus: cherry, apple
grapevine fanleaf virus: grapes and grape hybrids

Seed transmission:  About 1/7 th of the known plant viruses are transmitted through seeds. Different viruses have different host ranges (the plants that they can infect). Tobacco mosaic virus, Cucumber mosaic virus are some viruses with a very wide host range and they may not be seed transmissible in all plants they infect. Seed transmission plays a huge role in virus epidemiology. Not only they can be a primary source of infection, leading to an epidemic in the field upon conducible environment, seed transmission is an effective way for long distance travel of the virus, thus introducing the virus to new places. You have heard of USDA regulations/restrictions on different crops, from certain foreign countries to avoid introduction of infected seeds/plant materials. 
 
Seed transmission can occur simply by contamination of seeds, as in tomato seeds by Tobacco mosaic virus. This can be readily inactivated by seed treatments. 
The second type of seed transmission occurs when the virus is present in the embryo tissue that can happen prior to fertilization or takes place at pollination. Pea seed-borne mosaic virus is a well studied plant virus in this category. 
 
 
Pollen Transmisison:  Some viruses are transmitted from plant to plant via pollen. As in seed transmission, pollen transmission has two mechanisms, gametic infection of embryo and direct infection of mother plant. 
 
Vegetative propagation: An important horticultural practice, and unfortunately a very effective method for perpetuating and spreading viruses. In clonally propagated plants, an infected mother plant which could be asymptomatic could be used to make hundreds and thousands of daughter plants, which will all have the virus. Any vegetative parts  such as bulbs, corms, runners, and cutting will be infected. 
 
Grafting: Essentially a form of vegetative propagation, once the organic union has been established and plants (Scion and Stock) function as a single plant. In experimental front, grafting is used as a virus transmission methods, when all other methods fail.

Last week, we initiated our first on-farm demonstration of soil steaming of the season with our self-propelled steam applicator. The machine is designed to inject steam into the soil and raise soil temperatures to levels sufficient to kill weed seed and soilborne pathogens (140°F for > 20 minutes). After the soil cools (< ½ day), the crop is planted into the disinfested soil.
In this trial, we are examining the viability of soil steaming for controlling weeds in organic carrot at the field scale level (plot size > ½ ac). The machine performed well in that it was able to reach target soil temperatures at reasonable travel speeds (> 0.4 mph), provide uniform temperature distribution across the bed and form nicely shaped beds suitable for subsequent planting. Stay tuned for reports of weed control efficacy, crop yield and overall profitability as compared to the grower standard. 
We are seeking collaborators to conduct similar field-scale trials/demos in Yuma, AZ. The primary objectives are to assess the viability of soil steaming at the field-scale level and obtain grower feedback on the device’s commercial potential. The machine can be adjusted to work with most bed configurations including narrow (40”, 42”) and wide (80”, 84”) beds, and is suitable for use in conventional or organic crops (soil steaming is organically compliant). To date, the device has been successfully trialed in iceberg lettuce, romaine lettuce, baby leaf spinach and carrot crops.
If you are interested in an on-farm demo of soil steaming, please let me know. I’d be happy to work with you.

Fig. 1. On-farm demonstration of a self-propelled steam applicator for weed and
disease control

Acknowledgements

This project is sponsored and funded in part by the Arizona Specialty Crop Block Grant Program and the Propane Education and Research Council (PERC). We greatly appreciate their support.

According to some authors "the human brain can process images up to 60,000 times faster than words"¹. So, I decided to share images of some of the onion herbicide tests we did at the Yuma Ag Center last season hoping you find them interesting as the season progresses. As you know the products tested could respond differently under different environments. Also some of these trials are done searching for new alternatives, therefore products are not registered.

1. https://www.linkedin.com/pulse/why-pictures-speak-louder-than-words-laura-nicole-brown#:~:text=According%20to%20Krista%20Neher%2C%20the,that%20really%20captures%20your%20attention.

Included below is a list of organic insecticides that you can consider for your organic IPM programs. Although there are few alternative organic insecticide options, it is important to rotate when possible. Like conventional insecticides, continuous exposure to the same biopesticides may pose some risk of further reducing their efficacy and leading to the development of resistance. 

A good organic insecticide rotation practice is to alternate selective organic insecticides with broad-spectrum organic insecticides. The use of selective organic insecticides favors the increase of beneficial arthropod populations which may help to keep the pest population in check and delay repeated application of insecticides. When the population of lepidopteran pests is low, spraying Bt-based insecticides first and pyrethrin or Spinosad-based products thereafter is a good strategy. Proper insecticide rotation is important because it can help reduce insecticide application frequency, resulting in reduced crop production costs. 

 Tank-mixing can help improve the efficacy of some organic insecticides against some target pests. Tank-mixing a Bt insecticide with pyrethrin, such as Xentari + Pyganic or Dipel + Pyganic, can be an effective combination for controlling lepidopteran larvae. Additionally, combining Pyganic and a neem-based insecticide like Aza-Direct can be a favorable combination for small lepidopteran larvae. Tank-mixing Entrust (spinosad) and M-Pede can help suppress flea beetles and bagrada bugs.

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.