Pest pressure in desert produce has begun to subside, but don’t become complacent. Although the local weather forecast calls for slightly below average temperatures for the next 10 days, conditions are still within the biological range of most of our fall pests. And it will soon become ideal for aphids. Below are some thoughts on the status of local pest pressure based on field sampling, trap counts and PCA reports.

Diamondback Moth (DBM): Reports of DBM on coastal transplants has quieted down a bit, but some PCAs have reported spraying more than usual for DBM on coastal transplants. So far, no reports of “uncontrollable” populations. Trials at the Yuma Ag Center are showing that standard insecticide products (Proclaim, Radiant, Exirel, Harvanta) are showing significant efficacy against DBM larvae as expected (see graph below). Of course, this is on direct-seeded cabbage and not coastal transplants. The weather should help slow down their growth and development but don’t become complacent. Populations still have the potential to get out of hand if not controlled properly, even in cooler weather. Beet Armyworm (BAW): BAW numbers are still above average in trials at YAC (1.0-1.2 larvae /plant) in untreated checks but have slowed a little. Furthermore, pheromone trap catches peaked last week and remain high areawide (see graph below). The cooler weather will slow them down, but expect light to moderate numbers for a while. Standard insecticides were effective in controlling BAW in efficacy trials at YAC this fall.

Cabbage looper (CL): CL numbers have been unusually light this fall and areawide trap catches have been historically low (see Areawide insect Trapping Network). Numbers have been significantly lower than normal in untreated plots at YAC this fall (<0.1-0.2 larvae / plant). But CL trap counts did increase in all areas last week, so keep an eye out for increased oviposition and hatches on your produce crops.

Western Flower Thrips (WFT): WFT started out strong in some locations in mid-October following winds and melon/alfalfa harvests, but have subsided considerably with cooler temps. Adult and larvae numbers are light in our insecticide efficacy trials at YAC. We did see a large population of Bean Thrips (BT) show up following the heavy winds out of the west in mid-October. Lannate and Radiant have been effectively controlling both WFT and BT in our trials. We have had our first report of INSV infected lettuce this fall, and as you can expect the infected plants were found on romaine transplants originating from the coast.

Aphids: Surprisingly, I’ve had several reports from PCAs throughout the area (Bard, Yuma Valley, Wellton and Dome) of small colonies showing up on lettuce and brassica crops. We’re also picking up small colonies at YAC. More than we typically see in Oct/Nov. Above average sticky trap catches have

shown a sharp increase in winged aphid movement in the past 3 weeks (see Areawide insect Trapping Network). This has coincided with windy conditions from the west-northwest. Most of the aphids I have seen colonizing have been green peach aphids. Weather has not been conducive for aphid population growth so far, but with the cooler temps coming you might expect to see more given the winged aphid movement we’ve been trapping. Historically, green peach aphid infestations are worse on spring

produce following above average monsoon and fall/winter rainfall. So far, it’s been pretty dry, and could be an indicator of things to come. Nevertheless, keep a close watch on aphids as we transition into our “normal” winter weather.

The cycling of nitrogen (N) in the soil is one of the most important aspects of soil health. Nitrogen cycling in the soil includes a multitude of interactions with microbes and plants. 

The basics of soil N were described in a recent article posted in this UA Vegetable IPM newsletter (Silvertooth, 2024).  The overall N reactions in the soil system are described in the N Cycle, Figure 1.

Figure 1. The nitrogen cycle.

Most of the N in soil is present as organic N in the soil organic matter.  As a general estimate, there are approximately 1,000 pounds N per acre of this form for every one percent organic matter in the soil.  However, soil organic matter (SOM) is relatively stable and resistant to further decay.  So, most of the organic N in the soil is not biologically available during any given growing season (Havlin, et al. 2014;Thompson and Troeh, 2005; Warren, et al., 2017; and Weil and Brady, 2017).

Normally, about two percent of the N from SOM will be released each year to mineral forms when soil is cultivated.  Arizona soils typically have approximately 1%organic matter.  Thus, 10 to 20 pounds of N per acre is a typical amount of N present in unfertilized soils after cultivation and seed bed preparation.

We commonly assume that 1 acre-furrow slice of soil (soil that is 6 inches deep covering an area of 1 acre (43,560 ft.2))contains approximately 2 million pounds of soil.  Thus, a soil in Arizona with 1% organic matter will have approximately 20,000 lbs. of SOM per acre and that will contain about1,000 lbs. of total N (5% N in the SOM), most of that being in an organic form that is not directly plant-available. 

If 2% of the N in the SOM does mineralize in one year, that would release approximately 20 lbs. of N per acre per year. Accordingly, it is common to find 10 to 20 pounds of mineral plant-available N (NO₃^--N) per acre.  That is a typical amount of mineral plant-available N present in unfertilized soils after cultivation and seed bed preparation.  Higher levels of residual NO₃^--N is commonly a remnant of prior fertilizations.

 

Nitrogen Mineralization and Immobilization

Because N release from SOM is dependent upon decay by microorganisms, which themselves require N, the amount of N available for a crop is in constant flux.  Unlike plants, which get their carbon (C) as carbon dioxide (CO₂) from the air, many microorganisms get C from decaying organic matter.

Nitrogen availability depends upon the relative amount of C and N in the SOM, its resistance to decay, and the environmental conditions supporting microbial activity. Figure 2 illustrates how mineral N becomes more concentrated in SOM as soil organic matter decays.

Figure 2.  The carbon to nitrogen ratio (C:N) declines and narrows as residues decay
until mineral nitrogen finally becomes available.
(Source: B. Raun In: Warren, et al., 2017)

Note that mineral N is not released during the first stages of decay.  This is because N that is released is immediately consumed by active microorganisms.  With time, remaining SOM becomes more resistant to decay, microorganisms die off, and there is more mineral N present than can be consumed by the few active microorganisms.

The first step of measurable mineral N release from SOM is usually in the form of ammonia NH₃.  The ammonia readily reacts with soil moisture to form ammonium NH₄⁺. These two reactions can be stated simply as:

1) Organic nitrogen → NH₃ (gas)                                     

2) NH₃ + H₂O → NH₄⁺ + OH^- (ammonia + water → ammonium + hydroxide)

The process of converting or transforming N from organic compounds to inorganic compounds is called mineralization.  This results in increasing N available for crops.  The N form that is directly available to plants is nitrate-N (NO₃^--N).

When the reverse happens, plant-available N, primarily nitrate-N (NO₃^--N) is absorbed by crops or microorganisms.  This process is called immobilization and results in a decrease in the amount of N in the soil solution that is immediately available for crops.

These processes of mineralization and immobilization and their interacting nature for a typical field soil situation are illustrated in Figure 3 starting with 2,060 lbs. of total soil N in this case partitioned among the SOM, crop residues, and mineral N pools.

Figure 3.  Interacting pools of soil nitrogen.
(Source: B. Raun In: Warren et. al., 2017)

Approximately 98 percent of the soil N is unavailable for plant uptake.  This large reservoir of organic N provides an important buffer against rapid changes in available N and plant stress.  The small reservoir of mineral N can often be slowly replenished by mineralization (Figure 2.2).

Supplemental N as fertilizer usually must be added to support high, economic crop production levels when native levels of soil N mineralization are not sufficient.  This is common in Arizona agricultural soils.

 

Example Case:

Immediately following fertilization with 120 pounds N, the system may be illustrated by Figure 4a. Note the increase of 120 pounds of N in the mineral N pool in addition to the original 60 lbs. of mineral N in this case (Figure 3), for a total of180 lbs. of mineral N.  While the organic N pool is static at 2000 lb./acre in this example.  In this case there is a total of 2,180 lbs. Nin the soil.

Addition of fertilizer N will stimulate microorganism activity, resulting in consumption of mineral N by soil microbes as they attack and break down some of the crop residues (immobilization) as illustrated in Figure 4b.  Note that the total amount of N among the three pools (mineral, organic, and crop residues) is the same at 2,180lbs. N/acre.

The immobilized N will be present as microbial tissue and other new material in the organic pool.  Note that the N present in the crop residue has dropped from 30 to 20 lbs. N/acre in this example due to microbial decomposition.

An important point to follow in this example is the transformations taking place with the consistent base of 2,180 lbs. of N partitioned into various forms.  This is a rather simplified example, but it illustrates the natural dynamics taking place in the soil-plant system with the cycling of N and the challenges of managing N fertilizers effectively.

Figure 4.  Relative amounts of organic and mineral nitrogen in soil immediately after
fertilizing (a) and several days after active immobilization (b).

As indicated by the two arrows pointing in opposite pathways, mineralization and immobilization are both taking place simultaneously.  Immobilized fertilizer N will again become available in a few weeks if conditions favor crop uptake.

This exchange in N forms is often referred to as the mineralization-immobilization transformation (MIT) process and it is a core element of N cycling and management for crop production.

There is a constant flux taking place in soils among these pools of soil N through the MIT process and it is important to be aware of and understand this process when managing fertilizer N inputs for crop production.

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

Silvertooth, J.C.  2024.  Soil Health – Nitrogen.  University of Arizona Vegetable IPM Newsletter, Volume     15, No. 22,

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. 2017. Oklahoma Soil Fertility Handbook. Published Apr. 2017; Id: E-1039

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

Alfalfa is a major crop grown in Yuma area as a cover crop before vegetable season. Generally expected to be a hardy and easy crop to grow, alfalfa production still can be affected by many pests and diseases. Root and crown rot in alfalfa is a common problem. The pathogen Phytopthora megasperma thrives on standing water, overwatered, and/or in soil that lacks good drainage. 
 
The above ground symptoms typically consists of plant wilting, usually occurring quite suddenly, often followed by plant death. The roots turn tan-to-brown, sometimes turning black. Sometimes orange-to-reddish streaks can be observed in rotted roots. Occasionally, the symptoms can be seen in crowns too. Infected plants succumb to death, or grow at reduced rate thus being susceptible to other pests and diseases. 
 
The causal organism survives in soil as mycelia in infected plant tissue or as thick-walled oospores. It also produces thin-walled sporangia that release motile zoospores in the presence of free water. So it is important to grow alfalfa in well drained soils. Adjust irrigation water in a way that there is no standing water for too long. If possible, install tailwater ditch to remove excess water. Plant resistant cultivars like Agate, Apollo if the problem persists in the field. Till the soil deeply to reduce wet soil pockets, level the land before planting. 
 
If diagnosis is confusing, bring the samples to the Yuma Plant Health Clinic for proper diagnosis. Make sure to bring healthy looking plants, sick plant, plants in between etc. Remember, there is no such thing as too much plant tissue when it comes to submitting samples to the clinic!

The Desert Difference: A Showcase of Ag Tech Opportunities for Growing in the Desert begins TODAY Wednesday, November 13th with a Field Demo Day at the Yuma Agricultural Center. The educational workshop will feature 13 of the latest automated and robotic technologies for pest control and improved vegetable production being demonstrated in the field. Registration begins at 7:00 am and the program starts at 7:30 am (agenda below).

The Field Demo Day is part of at wo-day event. The second day will be a standard conference with keynote speakers, breakout sessions and trade booths. The event will be held Thursday, November 14th at the Yuma Civic Center. Details of the event and Conference Day (Day 2) activities can be found here.

Looking forward to seeing everyone at both events!

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

In our last update we invited you to come to the YAC to look at a broccoli trial we have in which we sprayed Trifluralin at 1.5 pt/ acre preemergence (PREE) incorporated mechanically. The application was made to the flat ground then incorporated with a disk at 4”depth. Then rows were built, and broccoli was direct seeded and germinated with sprinkler irrigation. Last year we saw injury in a non-disked Treflan treatment, so I didn’t include it in the trial. The same procedure was done with Prowl at 1pt with disked and non-disked plots.

Additional PREE treatments were Devrinol at 1pt/a, Prefar6qt/a, Goal tender at high rate of 16floz/a.

Prowl mechanically incorporated looks better than Non-incorporated in weed control as well as not showing phytotoxicity symptoms. Treflan, also mechanically incorporated looks similar to Prowl with a very small number of large weeds. The predominant weeds in this field are goosefoot and lambs quarter and are showing restricted growth not being as competitive with the broccoli.  

Our observations in the trial as well as your field experience shows us with Treflan and Prowl available PREE and the post emergence herbicides such as Clethodim and Goal the absence of Cacthal can be mitigated to some degree. Again, you are welcome to come look at it at the Yuma Ag Center.

Treatments didn’t affect broccoli stand except Goal high rate PREE as expected.

Prowl and Treflan applied on the flat and incorporated mechanically did not produced phytotoxicity symptoms at 13 and 28 days after planting.

All weeds were about 0.5” at the time of this evaluation.

Some of the small weeds observed in the 13-day evaluation in the disked Prowl and Treflan died or didn’t grow by day 28.

We have conducted a field efficacy trial evaluating the efficacy of 14 biological insecticides alone or as a tank mix against lepidopteran pests, including diamondback moth (DBM), beet armyworm(BAW), and cabbage looper (CL). The insect pressures were relatively low when we initiated the insecticide applications; the CL number was never high enough to be considered for statistical analysis and treatment comparisons.

We applied all insecticides at the highest label rate when sprayed alone or at mid-rate when sprayed as a mixture of two insecticides using an application volume of 40 gal/ac. The adjuvant, Oroboost, was added to each of the insecticide treatments at a rate of 0.4% v/v. Most of the insecticides evaluated in our trial are registered for lepidopteran control except for M-Pede, BotaniGard, and PFR-97.

The results of our trial showed that Xentari, Xentari + Pyganic, and Entrust provided the highest level of BAW suppression. We also found that other insecticides/mixes, including Aza-Direct, Dipel, Dipel +Pyganic, Gargoil, Grandevo, Venerate, M-Pede, and PFR-97, may also cause some levels of BAW suppression (Figure 1A). Xentari and Dipel + Pyganic provided the best DBM suppression, followed by Xentari + Pyganic, Dipel, and Entrust, which provided 50-60% of DBM suppression (Figure 1B). Pyganic alone did not control either BAW or DBM (Figure 1A&B).

Table 1. List of bioinsecticides evaluated


Figure 1. Means Beet armyworm larvae (A) and Diamondback moth (B) per cabbage plant as affected by bioinsecticide sprays.

Area wide Insect Trapping Network   VegIPM Update, Vol. 14, No. 22, Nov 1, 2023            

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth captures have steadily decreased over the past 2 weeks, and areawide about average for late-October.
 
Beet armyworm:         
Trap counts reached their highest levels so far this season, particularly in Tacna, Wellton and Yuma Valley, and about average for late October.
 
Cabbage looper:           
Cabbage looper numbers decreased areawide, and are still below average for this time of the year.
 
Diamondback moth:   
Sporadic DBM activity in low numbers throughout the area, trending well below average for late October.
 
Whitefly:                      
Adult movement increased in the past 2 weeks and above average for late October.
 
Thrips:                           
Thrips adult activity peaked in the last two weeks, and trending below average in October.
 
Aphids:                        
Winged adults continue to be captured for the season, consistent with heavy winds from W-NW. Aphid captures thus far have been well below average. 
 
Leafminers:                  
Adult activity decreased in most areas, and trending about average for late October.