Historically, our Areawide Pheromone and Sticky Trap monitoring for insects was terminated around the first of April as the produce season ended. Beginning 4 years ago however, we continued our Areawide Trapping Network throughout the summer to collect trapping data from all 15 areawide trap locations year-round.  So why is this additional trapping data useful? For several reasons: 

1) Understanding the activity of some of our key pests when produce is not grown during the summer may give us an indication of what to expect as the fall produce season begins. This may be particularly helpful for predicting moth flights and whitefly flights in August-September coinciding with early transplanting and direct seeded crops.  Another example is keeping track of corn earworm which can unexpectedly show up near the beginning of fall harvests. 
 
2) Trapping for pests during the summer has shown us that 2 of our more important produce pests are not caught in traps during the summer. We presume this is due to the absence of brassica crops and weeds for diamondback moth, and high daytime/nighttime temperatures lethal to aphids. The fact that trap catches resume in the fall supports our conclusion that these pests are absent in the summer, only to reenter the desert via winds and/or transplants in the fall. And finally, 

3) It gives me something to do in the summer other than write reports and papers. 
So, visit the Areawide Summer Trap Network if you’re curious what our key pests are up to.

The Mobility Concept
Plant nutrient management is strongly dependent on nutrient mobility in the soil.  Nutrient mobility in the soil is different among the essential plant nutrients and nutrient management in the field needs to take this into account.

In 1954, Dr. Roger H. Bray at the University of Illinois proposed a nutrient mobility concept that has proven to be very important in the management of nutrients for optimum efficiency (agronomically, economically, and environmentally).  Bray essentially simplified all soil nutrient chemistry to the fact that some plant nutrients are mobile in the soil and some are not. 

A great deal of research has been conducted in the past 70years supporting Bray’s nutrient mobility concept and it is now considered a fundamental feature of soil fertility and soil health management (Raun, 2017; Warren et al., 2017, Havlin et al. 2014; Troeh and Thompson, 2005).

 

Mobile Nutrients
Mobile plant nutrients in the soil move with the soil water.  Thus, plants can extract mobile nutrients from a large volume of soil beyond the direct root system.  Accordingly, plants take up mobile nutrients from a “root system sorption zone”, Figure 1.  This gives plants the capacity to utilize most of the mobile nutrients in the root system sorption zone as those nutrients will move to the plant roots with the soil water as it is taken up by the plant.

We consider the mobile plant nutrients to be nitrogen (N), sulfur (S), boron (B), and chlorine (Cl).  These mobile plant nutrients are taken up by the plant in the following forms: nitrate-nitrogen (NO₃^--N), sulfate-sulfur (SO₄^2- - S), boric acid (H₃BO₃) and borate ions (BO₃^3- - B), and chlorine is taken up as the chloride ion (Cl^-).

Figure 1.  The root system sorption zone and an illustration of the large volume of soil
from which plants extract mobile nutrients.

In a crop field where many plants are growing together, there are root system sorption zones commonly overlap.  Therefore, the root system sorption zones for adjoining plants are competing for water and mobile nutrients, Figure 2.  This is one of the main reasons that appropriate plant populations are important for optimum yield.

Due to this overlapping and potential competition among plants in the root system sorption zone, we need to fertilize and manage nutrients in direct proportion to the number plants or the potential yield of the crop.  We can make an estimate of the mobile nutrient requirements by first calculating the amount of mobile nutrient that will be taken up by the crop.

Knowing the average concentration of the nutrient in the crop and the yield goal for the crop can provide a good estimate of total mobile nutrient demand by the crop.  This is important for estimating the crop requirements for nutrients like nitrogen(N) for a crop.

Taking a soil test for plant available N, which is nitrate-nitrogen (NO₃^--N), will only provide a snapshot of the plant-available N over the season due to myriad N transformations that are constantly taking place in the soil.  Thus, it is better to work with a yield goal approach for mobile nutrients such as N.

Figure 2. Competition among plants brought about by increasing yield goal.

Yield goal example: Consider a lettuce yield goal of 30 Tons (fresh weight)/acre.  Nitrogen uptake studies on lettuce have shown that for iceberg lettuce a general average of 2.6 lbs. N/Ton is taken up by the crop and 3.62 lbs. N/T for romaine. Thus, using 3.0 lbs. N/Ton of fresh lettuce with a projected yield of 30Tons/acre indicates a total N demand by the crop of 90 lbs. N/acre (Bottomset al., 2012; USDA-ERS, 2013; Doerge et al., 1991). 

We can subtract residual nitrate-nitrogen (NO₃^--N) found in a pre-season soil test from the total N crop demand estimate and identify a target N fertilization rate. For example, 10 parts per million (ppm) residual nitrate-nitrogen (NO₃^--N) as a preseason level is very common in agricultural soils. 

Using an estimate of 2 million lbs. of soil per acre-furrow slice (6-inch-deep layer of soil in an acre area), 10 ppm X 2 = 20 lbs. nitrate-nitrogen (NO₃^--N) residual in the soil.

90 lbs. N projected requirements – 20 lbs. residual N = 70lbs. N/acre fertilizer requirement.

This would be fine if everything in the field was 100%efficient.  Due to inefficiencies that are inherent in a field production system, higher rates of fertilizer N can be required in some cases.

Our management goal is to achieve the highest levels of efficiency (agronomically, economically, and environmentally) in the field as possible (Bottoms et al., 2012 and Doerge et al. 1991).  Using the nutrient mobility concept is good to incorporate into our crop management strategy.

References:
Bottoms, T.G., Smith, R.F., Cahn, M.D., Hartz, T.K.  2012.Nitrogen requirements and N status determination of lettuce.  Hort Science 47, 1768-1774.

Doerge, T. A., R. L. Roth, and B. R. Gardner.  1991. Nitrogen Fertilizer Management in Arizona. College of Agriculture Doc. 19102.  University of Arizona.

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.

Raun, W.R. 2017.  In: Warren et al.  2017. Oklahoma Soil Fertility Handbook, Id:E-1039

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

USDA-ERS. 2013. U.S. lettuce statistics 2011. http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID= 1576 (accessed 21 Feb. 2013).

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

Lettuce fields right now are covered with downy mildew after the unusual amount of rain we got this year in the desert southwest. 
Another disease to look out for is downy mildew in onions. 
Downy mildew of onions is caused by the fungus-like Stramenopile Peronospora destructor. The pathogen produces abundant sporangia that germinate directly acting as spores. Reported hosts other than onions include shallot, leek, garlic and chives. The fungus overwinters in volunteer hosts, cull piles, and debris. Spores are wind borne. Spores of the fungus present in soil or bulbs germinate, infect roots or bulbs, and develop systemically in the plant. Spores that land on the leaf surface produce local lesions that can spread systemically to younger tissue. Infection can spread rapidly under cool (34– 82 °F), damp conditions generating new inoculum within the field. The pathogen requires cool temperatures and the presence of free moisture (rainfall, dew or overhead irrigation) to infect onion plants. Under those conditions repeated disease cycles may occur with devastating results.
 
Disease management:  Regular scouting of field is important to assess disease severity and apply preventative fungicides. Disease management includes use of disease free bulbs, transplants and seed. Crop rotation with non host (small grain, corn) for at least 3 years. Destroy volunteer plants, debris and cull pile. Better soil drainage, better circulation, preventative spray can reduce disease occurrence. Always rotate fungicide from different groups to avoid resistance build up.

Pics : https://www.omafra.gov.on.ca/IPM/english/onions/diseases/downy_mildew.html
Sources: 
https://amarillo.tamu.edu/
https://barron.extension.wisc.edu/

New automated/robotic ag technologies are coming out all the time. Ever wonder how they function in the “real world” and whether they are cost effective? Western Growers recently released a case study report on the economic impact of Stout Industrial Technology, Inc.’s Smart Cultivator on overall weeding costs. The study tracked expenses, productivity, and labor savings of the machine operating over one year on five types of lettuce crops and 2,700 acres at Triangle Farms in Salinas, CA. It is a well done, detailed study with machine costs and labor savings broken down by crop type and acreage. It’s an easy read and worth the time for those interested in the economic and overall feasibility of automated mechanical weeding. Check it out here or by clicking the image below. I don’t want to be a spoiler, but I was surprised to learn that costs for hand weeding lettuce in Salinas, CA were so high - $525/acre (conventional) and $750/acre (organic) and that in these conditions, the return on the $330K investment for the machine was less than one year when used on 2,700 acres.

Stay tuned. Western Growers plans to release four more automation technology case study reports within the next year. Upcoming reports include grower case studies experiences with the automated weeding machine from Ecorobotix; and with autonomous ag platforms from Burro, GUSS Automation, and Bluewhite. Their first report, which examined the economics of Carbon Robotics’ Laser Weeder at the commercial scale, can be found here.

Fig. 1. Western Growers case study report on the economic impact of Stout Industrial
Technology, Inc.’s weeding machine on weeding costs in lettuce on 2,700 acres at
Triangle Farms, Salinas, CA. Click here or on the figure to view. (Photo credit: The
Western Growers Centerfor Innovation & Technology)

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.

As the weather gets cooler, aphids are migrating back to the desert to harm our crops. It is important to revisit some factors that may favor aphid infestations in your crops, specifically lettuce, and take appropriate action against them.  

Nitrogen availability is one of the most important factors in the development of insect pest populations. Excessive application of nitrogen fertilizer to crops will likely increase the feeding preference and consumption rate of insect pests such as aphids, resulting in greater survival, growth, and reproduction. In some situations, high nitrogen levels in plant tissue can decrease resistance and increase susceptibility to aphids’ attacks.

Water management is also very important to consider. Excess water around plant roots can increase nitrogen uptake, which may favor an increase in the aphid population. Thus, supplying proper amounts of nitrogen and water to your crops can tremendously help with aphid management. See this link for more information on this topic.

Results of pheromone and sticky trap catches can be viewed here.
Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 15, July 24, 2024

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:             
CEW moth counts increased in Roll and Yuma Valley; about average to previous years.
 
Beet armyworm:          
BAW moth counts increase in Tacna and Wellton; comparable to previous summers.
 
Cabbage looper::          
Cabbage looper numbers are low consistent with previous years and higher temperatures.
 
Diamondback moth:      
DBM moths were not caught in any trap; average for this time of the year.
 
Whitefly:                      
Adult movement continues to increase consistent with disking of late melon fields and volunteers.
 
Thrips:                          
Thrips adult activity continues to decline, comparable to mid-summer.
 
Aphids:                        
Aphid movement is absent and consistent with what we typically see during the summer.
 
Leafminers:                  
Adult activity about average for early July.