A significant decline in insect pest activity has occurred over the past 2 weeks. These changes are consistent with cooler, seasonal temperatures. We all know that weather influences produce crop growth and ultimately can create wide fluctuations in markets. Similarly, our local weather can significantly impact pest activity on desert produce crops.    For insects, their activity and abundance are closely aligned with weather. The reason for this is that temperature is the driving force behind insect development, growth and behavior. Unlike many animals, insects are poikilothermic (“cold-blooded”); that is, they are unable to regulate their body temperature. Their internal temperature varies along with that of the ambient environmental temperature. Consequently, insect pests such as whiteflies, beet armyworms, cabbage loopers, diamondback moths, and leafminers are more active and develop rapidly when temperatures average ~85° F; in contrast, they are less active and develop much slower under cool, winter conditions.   That is one of the primary reasons these insect pests can be so abundant on produce crops in Sep-Nov.  Behavioral activities such as flight, movement, reproduction, feeding and oviposition are similarly influenced by seasonal temperatures. But temperature requirements for insects vary with species (see table below).  For example, the optimal temperature for growth and development of beet armyworms and diamondback moths is the same (86 °F), but diamondback moth can complete a generation quicker and can develop over a much broader range of temperature. Thus, they can be more problematic season long if not adequately controlled.  Then you have Bagrada bug which tend to prefer warmer weather for development.  Its lower developmental threshold of 62°F is the main reason the pest is generally hard to find during the winter months.  In contrast, green peach aphids, which we annually find on winter produce crops, thrive in cooler weather (optimal temperature for growth is 55°F.  That’s the main reason we don’t find them on summer crops; it is just too hot for them.  In general, extreme hot (> 120°F) or cold temperatures (<32 °F), can be lethal or greatly restrict insect growth and behavior.  Consequently, temperature and other weather factor such as rainfall, humidity, sunlight and wind play a major role in determining insect activity and abundance on local desert crops; but they can also suppress insect abundance under extreme conditions. For a more detailed explanation on the impact of weather on insects visit Weather Influences Desert Insect Pests.

Soil Nitrogen
Nitrogen (N) is the essential nutrient that is required in largest amounts by plants.  Following water, N is the most limiting factor in the growth and development of non-leguminous crops. 

In most places around the world, sunlight is the first most limiting factor in terrestrial ecosystems, including crop production systems.  This is followed closely by water as a limiting factor to plant growth and biologically available N (NO3- -N) is commonly the third most limiting factor.

Nitrogen goes through many natural transformations in the soil and cycling of N can take many routes and forms. Thus, the management of N is also one of the most challenging plant nutrients to work with efficiently.

Even though N is often a limiting factor in terrestrial ecosystems and crop production systems, N is ubiquitous in the atmosphere and biosphere.  For example, 78% of the Earth’s atmosphere is made up of N gas or N2, a molecule made of two nitrogen atoms bonded together by a strong, stable, triple bond.  As a result, N gas is biologically inert.

Nitrogen is the mineral element required by plants in the greatest amount and it serves many functions in plant physiology.  Nitrogen is an integral component of amino acids, which are the building blocks for proteins.  

Proteins are present in the plant as enzymes that are responsible for metabolic reactions in the plant.  Because N is so important, plants often respond dramatically to plant-available N, which is nitrate-nitrogen (NO3- -N), (Havlin, et al. 2014; Thompson and Troeh, 2005; Warren, et al., 2017; and Weiland Brady, 2017).

Nitrogen is central to global crop production.  Many parts of the world do not have enough to achieve food and nutrition security, in other cases excess N from fertilizer leaks into the environment with damaging consequences.

Though it makes up a large portion of the air we breathe, most living organisms cannot access N in this form.  Atmospheric N must go through a natural process called “nitrogen fixation” to transform before it can be used for plant nutrition.

In both plants and humans, N is used to make amino acids, which make the proteins that construct cells, including the building blocks for DNA.  It is also essential for plant growth because it is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide(photosynthesis).

The Nitrogen Cycle
The N cycle is the multi-faceted process through which N moves from the atmosphere to earth, through soils and organisms, and is released back into the atmosphere, with conversions in and out of organic and inorganic forms (Figure1).

Figure 1. The nitrogen cycle.

A good point to begin the review of the N cycle is with biological N fixation, the process of converting biologically inert N2 gas into an organic compound or an inorganic form such as NO3- -N.  Nitrogen fixation can take place through basic routes: 1) biological fixation or 2) conversion of N2 gas to NO3--N by lightning.  The basic routes of N fixation are shown in the upper left-hand side of Figure 1.

Biological fixation occurs when naturally occurring N-fixing symbiotic and some non-symbiotic bacteria convert N2 gas from air into forms like ammonium-nitrogen (NH4+ -N) and then into nitrate-nitrogen (NO3- -N).  A very important form of biological N fixation is carried out by symbiotic bacteria that live in the root nodules of legumes converting N2 gas into ammonium (NH4+) and then nitrate (NO3-), which are commonly incorporated very quickly into organic forms.

Plants preferentially absorb nitrate -N (NO3- -N) from the soil through the root hairs and use it in their physiological systems to create the N forms they need (amino acids, proteins, enzymes, complex compounds, etc.).  Some ammonium-N (NH4+-N) can be taken up by some plants.  The preferential form of N for plant uptake and utilization is nitrate-N (NO3--N).

Organic forms of N are not taken up by the plant and incorporated into the plant physiology.

Denitrifying bacteria convert excess nitrate back into inorganic N which can be released back into the atmosphere in gaseous forms (N2O and N2).

Nitrogen fixation can also begin with lightning, the heat from which ruptures the triple bonds of atmospheric nitrogen (N2 gas), freeing its atoms to combine with oxygen and creating nitrous oxide gas (N2O), which dissolves in rain forming nitric acid (HNO3) which then can be absorbed by the soil.

Excess nitrate in the soil can be lost through leaching, the process where nutrients mobile in the soil, including nitrate-N, can pass through the soil profile and into groundwater and potentially polluting streams.

Because N is so important and plant-available forms are often limiting, plants often respond dramatically to available N.  There are no substitutes for sufficient plant-available N and management is a critical part of a crop production system.

References
Havlin, J.L., Beaton, J.D., Tisdale, S.L. and Nelson, W.L. 2014.  Soil Fertility and Fertilizers; An Introduction to Nutrient Management.  6th Edition, Prentice Hall, Upper Saddle River, NJ.

Troeh, F.R. and Thompson, L.M. (2005) Soils and Soil Fertility.  Sixth Edition, Blackwell, Ames, Iowa, 489.

Warren, J., H. Zhang, B. Arnall, J. Bushong, B. Raun, C. Penn, and J. Abit.  Oklahoma Soil Fertility Handbook. Published Apr. 2017; Id: E-1039

Weil, R.R. and Brady, N.C. (2017) The Nature and Properties of Soils.  15th Edition, Pearson, New York.

Gearing up for the produce season, we already have been seeing plenty of fields transplanted with cole crops (cauliflower, cabbage, broccoli, etc). Below are the few common diseases to watch out for.  

Damping-Off
This disease commonly affects seeds and young transplants and is caused by the soil-borne fungi such as Pythium, Fusarium, Rhizoctonia etc. Infected seeds decay in the soil. Seedlings and young transplants will “damp-off” or rot at the soil line, before they eventually collapse and die.

The fungus, Rhizoctonia solani, causes wirestem. Stems of plants become constricted and brittle at the soil line. The plant becomes stunted and may rot at the soil line. This disease is more severe on fall cole crops when the soil is warm. We have seen lot of this problem in the fields last year. Make sure you get certified disease-free seedlings.

Prevention & Treatment: Cultural controls include planting on raised beds and providing good drainage. In greenhouse where transplants are grown, use new potting soil and new or thoroughly cleaned and disinfested containers and trays. Wash used containers with soapy water to remove all traces of old soil mix, and then briefly submerse containers in a 10% bleach solution. Allow to dry before planting in containers. Both in greenhouse and fields: avoid overwatering and wet feet in plants/seedlings.

Black Rot
Black rot is another common disease we observed in the fields last growing season. Black rot is caused by a bacterium, Xanthomonas campestris pathovar campestris, and can affect all vegetables in the crucifer family. Above-ground parts of the plant are primarily affected, and symptoms may vary depending on the type of plant, age of the plant and the environmental conditions. In general, yellow, V-shaped lesions appear along the tips of the leaves with the point of the V directed toward a vein. When lesions enlarge, wilted tissue expands toward the base of the leaves. Veins turn black or brown. Infection may spread into the stems. Cutting into the stems often reveals a black-brown discoloration with a yellowish slime present. Symptoms on cauliflower may appear as numerous black or brown specks, black veins and discolored curds.

Prevention & Treatment: With no effective curative measures available, preventative measures are very important. The bacteria survive the winter on plant debris and on weeds, such as wild mustard and Shepherd’s purse. It also can survive in and on seeds from infected plants. It can remain alive on plant residue buried in the soil for up to two years. The disease is easily spread by splashing water, wind, insects and garden tools. High temperatures and humidity favor development of the disease.

Use certified disease-free seed and transplants. If source of the seeds is unknown, or infested seed lots must be used, treat seed with hot water to eradicate pathogenic bacteria. Cabbage, broccoli, and Brussels sprouts can be treated at 122 °F for 25 minutes, while seeds of cauliflower, kale, turnip, and rutabaga are treated for 15 minutes. However, this treatment may reduce the viability of seed.

Choose varieties tolerant to black rot.  Do not plant cole crops where black rot has occurred in the past two to three years. Select well-drained sites with good air circulation.

Downy Mildew
This disease is caused by the fungus Peronospora parasitica and can attack both seedlings and mature vegetable plants. Infected plants develop a gray mold on the lower leaf surface. The upper leaf surface of infected plants first turns yellow and then may turn brown or necrotic. Leaves wither and die. Symptoms differ from powdery mildew in that the downy mildew fungus grows only on the lower surface of the leaf. Development of the disease is favored by moist conditions.

Downy mildew symptoms on a collard leaf. Justin Ballew, ©2020, Clemson Extension

Prevention & Treatment: Use varieties with resistant varieties. Rotate with crops other than cole crops or greens. Remove plant debris immediately after harvest. Use wide plant spacing to promote drying of leaves. For chemical control, make sure to rotate the fungicides to avoid development of resistance.

Alternaria Leaf Spot
Alternaria leaf sport is a common problem and sometimes may not be of economic importance.  However, if the plants are already weak or physically damaged providing the site of infection, the disease can cause economic losses.

Article source: https://hgic.clemson.edu/

2024-2025 growing season field trials

With the start of 2024-2025 growing season, we are gearing up for field trials on vegetables. Listed below are our research trials for this growing season addressing the need of the agriculture community. For more information and/or collaboration contact: bpoudel@arizona.edu

Integrated management of downy mildew in lettuce:

Three different most commonly grown iceberg lettuce cultivar will be evaluated against downy mildew using different chemical treatments. Partially funded by Arizona Iceberg Lettuce Research Council.

Sclerotinia drop of Lettuce

In this game of all or nothing, we will continue to do field trials on sclerotinia drop of lettuce. We will be doing trials on both species S. sclerotirum and S. minor.

Powdery mildew of lettuce

We will continue with field trials on powdery mildew of lettuce.

Downy mildew of Spinach

Downy mildew of spinach field trials won’t start until January of next year. But we are requesting industry reps to contact us ahead of time.

For more information: bpoudel@arizona.edu,928-920-1110

 

Plans are firming up for The Desert Difference: A Showcase of AgTech Opportunities for Growing in the Desert. The two-day event will be held November 13-14th in Yuma, AZ. The first day will be a Field Day, the second will be a standard conference with keynote speakers, breakout sessions and trade booths. Details of the event and Conference Day (Day 2) activities can be found here.

The focus of this article is the Field Day which will be held Wednesday, November 13th at the Yuma Agricultural Center. Registration begins at 7:00 am and the program starts at 7:30 am (agenda below). As with our previous AgTech Field Days, the educational workshop will feature 12 of the latest automated and robotic technologies for pest control and improved vegetable production being demonstrated in the field.

This year, we’ve added a twist we think you’ll like. We’ve asked people demonstrating equipment to treat one of their plots two weeks prior to the event. This way, attendees can “see” the end result of the treatment – how well an automated weeding machine technology actually controlled weeds, for example. A second set of plots, immediately adjacent to the pre-treated plots, will be used for a live demonstration of the equipment used.

As mentioned in previous articles, we would love to showcase as many innovations as possible. All live demo slots have been filled, however there are still openings for static displays. If you are interested in being added to the program, please let me know and we will do our best to try and accommodate you. It’s an open invitation - private companies, university and government researchers are all welcome.

Fig.1. Technologies for controlling weeds “pre-treating” plots two weeks prior to the AgTech Field Day. Weed control efficacy of the pre-treated plots along with the technology being operated in the field will be shown at the event.


Fig. 2. Field Day agenda (Day 1) for The Desert Difference: A Show case of AgTech Opportunities for Growing in the Desert event. More information about the event and Conference Day activities (Day 2) can be found here.

 

In the August 21, 2024 update we mentioned our plans to do a demonstrative trial at the Yuma agricultural center. This experiment was established on 10/2 and planted on 10/14. Sprinklers started on10/15. A stand evaluation was done on 10/28 and Phytotoxicity and Goosefoot and Lambs quarter weed control evaluated yesterday. The treatments suggested by some of the people in the industry, where:

Treflan (trifluralin) Applied at 1.5pt/A pre-emergent on the flat ground incorporated mechanically by disking approximately at 4” depth, then make the beds and plant the crop to germinate with sprinkler irrigation. The same method was utilized for Prowl herbicide at the rate of 1 pint product per acre. Also Prowl non-incorporated was included, Devrinol2XL at 1 pt/A, Prefar at 6qt/A as well as a high rate of Goal (oxifuorfen)preemergence to evaluate and document phytotoxicity levels.

As mentioned, we have collected weed control data, which we plan to include in future updates.

Would you like to see how this treatments look? Some of the observations, even though expected are very interesting, such as the fact that 16 floz of Goal Tender preemergence didn’t allow goosefoot germination but similarly broccoli didn’t emerge.

Come to the NW corner of the Ag Center (you can’t miss it) and look at the plots and see the difference between Prowl mechanically incorporated and non-incorporated, compare it with Prefar and Devrinol. We put some very clear signs as you can see in the picture above. The experimental plots are 14x60ft so they are large enough to illustrate what would happen in a farm.

We thank you for your suggestions ...please come and let us know what you think!

Preliminary results from a field trial that is being conducted at the Yuma Agricultural Center experimental farm to evaluate seven bioinsecticides against whiteflies in broccoli demonstrate that M-Pede (Potassium salts of fatty acids aka insecticidal soap), BotaniGard 22WP (Beauveria bassiana Strain GHA), Pyganic (Pyrethrins), Surround (Kaolin clay), and Venerate (Burkholderiaspp. Strain A396) may favor measurable control of the pest. These insecticides exhibited a reduction in whitefly adults and nymphs density relative to the nontreated control (Figure 1). The trial is ongoing, and more data will be collected to assess the efficacy of these insecticides further. Potassium salts of fatty acids are produced by adding potassium hydroxide to fatty acids found in animal fats and in plant oils. Pyrethrins is a mixture of six active compounds extracted from Chrysanthemum cinerariifolium plants. Beauveria bassiana is a naturally occurring entomopathogenic fungus. Burkholderia spp. strain A396 is a naturally occurring entomopathogenic bacteria. Kaolin clay is a soft, white, naturally occurring clay mineral.

Figure 1. Means whitefly adults (A), small whitefly nymphs (B), large whitefly nymphs (C),
and total whitefly nymphs (D) as affected by bioinsecticide applications. Bars with the
same letters are not statistically significant.

Area wide Insect Trapping Network   VegIPM Update, Vol. 13, No. 13, Jun 29, 2022
Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth counts decreased in the past 2 weeks, but still higher than the previous 2 summers.
 
Beet armyworm:        
Trap counts remain active in some locations (Yuma Dome Valley and Tacna) but lower than last summer
 
Cabbage looper:         
Cabbage looper numbers remain low consistent with the lack of activity this fall compared to last season.
 
Diamondback moth:      
DBM counts very low and average for this time of the year.
 
Whitefly:                      
Adult movement continuing to increase, particularly in Gila and Yuma Valleys consistent with melon harvests.
 
Thrips:                           
Thrips adult decreasing in most locations, and trending comparable to this previous years.
 
Aphids:                        
Aphid movement is absent consistent with what we typically observe in the summer. 
 
Leafminers:                   
Adult activity has highest in Yuma Valley, and above average for late-June.