Insects are notorious for hitching a ride on the winds associated with the remnants of hurricanes and tropical storms originating from Baja and Sonora, Mexico. As the storms dissipate upon arriving in the desert, they tend to “release” the flying pests onto our fall crops.  The Diamondback Moth (DBM) is a good example. Following the DBM outbreaks during the 2016-2017 season, we established over 50 pheromone traps from Texas Hill to San Luis to monitor year-round DBM activity over a three-year period.  The most important thing we learned from that study was that trap catches during the summer months in all three years showed that DBM were non-existent in the desert during July and August due to the unavailability of suitable brassica host plants. In essence, the populations become “extinct” during this two-month window when Cole crops and brassica weeds are absent. Thus, DBM disappear in the summer, only to reappear in the fall on transplanted and direct-seeded cauliflower, cabbage and broccoli.  We observed that sharp increases in areawide moth trap captures in late October occurred following the remnants of Hurricane Rosa (Sep 30), and in again 2019 following remnants of Hurricane Lorena (Sept 24). Shortly thereafter, PCAs began reporting DBM larvae appearing on seedling stands and newly transplanted crops throughout Yuma. This strongly supports our contention that these DBM populations migrated into the area on these storms.  
Of course, we know from firsthand experience that DBM will come into the desert from transplants produced in CA coastal growing areas where DBM are active year-round. In the fall of 2020, monsoon activity was negligible in Yuma and no tropical storms/hurricanes were reported in the region.  Yet DBM showed up again. Accordingly, we only observed significant DBM activity adult in areawide traps adjacent to transplanted cauliflower and cabbage fields in mid-September further suggesting that transplants certainly play an important role in annual reinfestation of fall Cole crops. 
Bottom line:   DBM are like a box of chocolates: “You never know what you’re going to get”.  Do not become complacent.  Because DBM populations disappear from the desert each summer and reappear on fall crops from outside sources, we never really know where they originate from, when they will show up, how abundant fall populations will be, or how susceptible to insecticides they are. This has become important lately with consistent reports of difficulty controlling DBM with diamides in coastal CA regions. Locally based on last year’s efficacy trials against DBM populations at YAC, all standard products ( Radiant, Proclaim, Coragen, Harvanta, Exirel, Verimark, and Xentari) were effective against the pest. But you never know what kind of DBM population will migrate into the desert this fall.  So be vigilant in scouting and follow IPM guidelines for DBM management.  For more information see:

• Impact of Diamondback Moth on Desert Cole Crops, 2016-2021

• 2024 Guidelines for Diamondback Moth Management in Desert Cole Crops

 

In a recent edition of this University of Arizona Vegetable Integrated Pest Management (UA VIPM) Newsletter, Vol. 14, No. 18, I provided a brief review of some basics related to soil physical properties and soil health.  Physical properties have strong impacts on some basic functions of healthy soil.  In irrigated agriculture one important physical feature is that of water holding capacity and the plant-available water (PAW) a soil can hold (Baver et al., 1972).
 
Soil serves as the storehouse or reservoir of water available for plant uptake.  As noted in the article regarding soil physical properties, soils vary significantly in terms of texture (relative proportions of sand, silt, and clay) and water holding capacity.  The relationship between total water in a soil and the portion of that water that is “available” to plants is shown in Figure 1 and Table 1.
 
In review of Figure 1, it is important to note several points: 1) The total amount of water that can held by a soil increases with finer textured soils.  2) The PAW held in a soil generally increases with finer textured soils but only to a certain extent with a salient bulge of maximum proportions of PAW in medium textured soils, such as loams and silt loams.  3) Soils with higher clay content, e.g. clay loams and clays, have high total water holding capacity but lesser amounts of PAW (Kirkham, 2023).  
 
Water that is held very tightly by the soil particles is referred to as “hydroscopic” water and the finer soil particles, such as clay, exert the strongest hydroscopic forces, and holding that soil-water unavailable to most plants.
 
The lower boundary of PAW shown in Figure 1 is a general line of demarcation at -1500 kilopascals (kPa) of potential energy and is referred to as the permanent wilting point (PWP).  The PWP was originally considered as a more universal limit among plant species (Briggs and Shantz, 1912).  However, the PWP as defined in Figure 1 is very general since all plant species vary considerably in their capacities to extract water from the soil.  Many native desert plant species for example can extract some hydroscopic and most crop species will suffer water stress long before soil-water content approaches the PWP line shown in Figure 1 (van Lier et al. 2023).  

Figure 1.  Soil-water holding capacity as a function of soil texture.  Source: The
COMET program.  University Corporation of atmospheric research.



Figure 2.  Soil-water relations and energy levels with soil-water content,
kilopascals (kPa) of potential energy.

Figure 2 describes a close-up view of two adjacent soil particles and the pore space between them.  Soil-water is held as a film around the soil particles leaving some air space in the pores after the gravitational water has drained away, leaving the soil at field capacity (FC).  As the soil dries the water film on the soil particles become thinner and held more strongly by the physical forces associated with the soil particles.
 
Following irrigation with the gravitational water drained away (dependent upon good internal soil drainage), which commonly takes about 24 hours, a soil will be at FC.  A crop will extract water most easily at FC and can continue to do so until the soil dries to a point where the crop plant can no longer pull water from the soil against the forces of the soil exerting holding that water.  
 
That critical point below which plants begin to encounter water stress is unique among crop plants but for most leafy green vegetable crops this limit is commonly about the 65-70% PAW level.  Other common desert crops like alfalfa, cotton, wheat, and melons can maintain adequate water levels before stressing and starting to show signs of wilting at approximately 55% PAW.
 
This means the actual range of soil-water that is available to plants is in the range from FC to the specific point of crop stress level, that is unique to each crop species, and the corresponding wilting point (WP).  Thus, PAW = FC – WP.
 
One of our challenges in managing crop production systems is identifying the WP for the crop in question and FC for the soil in that field.  In addition, with irrigation management we strive to maintain crops in a non-stressed condition and manage the fields in that range of PAW between the FC of the soil and the WP for the crop.  
 
This is a good example of a healthy soil that does not limit water availability due to physical or chemical restrictions and benefits the growth of a healthy crop.  
 
The Web Soil Survey in the following link can be used to assess soil textures dominant in a given field.  https://websoilsurvey.nrcs.usda.gov/app/

 

Texture	AWC range (in/in)	AWC range (in/ft)	Est. typical AWC (in/ft)
Coarse sand	0.01-0.03	0.1-0.4	0.25
Sand	0.01-0.03	0.1-0.4	0.25
Fine sand	0.05-0.07	0.6-0.8	0.75
Very fine sand	0.05-0.07	0.6-0.8	0.75
Loamy coarse sand	0.06-0.08	0.7-1.0	0.85
Loamy sand	0.06-0.08	0.7-1.0	0.85
Loamy fine sand	0.09-0.11	1.1-1.3	1.25
Loamy very fine sand	0.10-0.12	1.0-1.4	1.25
Coarse sandy loam	0.10-0.12	1.2-1.4	1.3
Sandy loam	0.11-0.13	1.3-1.6	1.45
Fine sandy loam	0.13-0.15	1.6-1.8	1.7
Very fine sandy loam	0.15-0.17	1.8-2.0	1.9
Loam	0.16-0.18	1.9-2.2	2
Silt loam	0.19-0.21	2.3-2.5	2.4
Silt	0.16-0.18	1.9-2.2	2
Sand clay loam	0.14-0.16	1.7-1.9	1.8
Clay loam	0.19-0.21	2.3-2.5	2.4
Silty clay loam	0.19-0.21	2.3-2.5	2.4
Sandy clay	0.15-0.17	1.8-2.0	1.9
Silty clay	0.15-0.17	1.8-2.0	1.9
Clay	0.14-0.16	1.7-1.9	1.8

 
Table 1.  Available Water Capacity (AWC), range, and typical levels for various soil textures. Source: Chapter 2 “Soils”, Irrigation Guide, Natural Resources Conservation Service- National Engineering Handbook.
 

 
References 
Baver, L.D., Gardner, W.H., and Gardner, W.R. 1972. Soil Physics. 4th ed. Wiley: New York.
 
Briggs, L.J., and Shantz, H.L. 1912. The wilting coefficient for different plants and its indi-rect determination. USDA Bur Plant Industry Bull No. 230. U.S. Dept. Agr: Washington, DC.
 
Kirkham, M.B. 2023.  Field capacity, wilting point, available water, and the non-limiting water Range. Ch. 8. In: Principles of Soil-Plant-Water Relations. 2023 Elsevier Ltd.
 
“Soils”, Irrigation Guide, Natural Resources Conservation Service- National Engineering Handbook.https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17837.wba
 
van Lier, Q. J., S.D. Logsdon, E. A. Rodrigues, and P.I. Gubianid,  Plant-Available Water.  Ch. 12.  In: Principles of Soil-Plant-Water Relations. 2023 Elsevier Ltd.

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%). Variety: Deluxe (HMX2595) was seeded, then sprinkler-irrigated to germinate seed on March 20, 2024on 84 inches 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. 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. 

Spray treatments were done on 05-21-2024, 05-31-2024, 06-07-2024 and 06-14-24. Powdery mildew was first seen on 06-05-24. Please see excel file for additional details.  

Disease severity of powdery mildew (caused by Sphaerotheca fuliginea and S. fusca) severity was determined 6-17-2024 by rating 10 plants within each of the four replicate plots per treatment using the following rating system: 0 = no powdery mildew present; 1 = one to two mildew colonies on leaves  ;2 = powdery mildew present on one quarter of leaves; 3 = powdery mildew present on half of the leaves; 4 = powdery mildew present on more than half of leaf surface area ; 5 = powdery mildew present on entire leaf. These ratings were transformed to percentage of leaves infected values before being statistically analyzed.

The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Most treatments significantly reduced the final severity of powdery mildew compared to nontreated plants. Quintec, Merivon, Tesaris, Luna Sensation, and V6M-5-14 V gave the best disease control. Phytotoxicity symptoms were not noted for any treatments in this trial.

ANR Webinar 10/17 click link for details.

At the 2023 FIRA USA Robotics and Autonomous Farming Solutions Forum last month, some of the latest automated and robotic technologies were demonstrated in the field. Several of the weeding technologies demonstrated are brand new to the U.S. and/or Yuma, AZ area. These included a 5.6 inch² (2.4 x 2.4 inch) resolution fixed boomed spot sprayer (Fig. 1), a high precision, “turret gun” spot sprayer  (Fig. 2) and a mechanical weeder that utilizes a unique rotating blade design and lidar for depth control (Fig. 3). Although the test runs were short, I was impressed with the performance and possibilities of these machines. Company representatives said that they would be traveling to Arizona for the season and would like to meet with growers interested in their technology. Company contact information can be found at their respective websites, or feel free to contact me if you would like additional information.

Fig. 1. Ecorobotix¹ (Yverdon-les-Bains, Switzerland) precision weeding machine
demonstration at 2023 FIRA USA. The unit has modular, 6.6’ wide spray booms
equipped with 156 individually controllable spray nozzles to spot spray weeds
(top right). A vision system is used to detect weeds and spot spray resolution is
2.4 x 2.4 inches. Darker soil indicates where spray (water) was delivered to
targeted weeds (bottom).



Fig. 3. GreenTech Robotics’ Weed Spider¹ (Kelvin Grove, New Zealand)
demonstration at 2023 FIRA USA. The unit is equipped with a vision system for
detecting crop plants and weeds, and blades that move in and out of the crop row
to remove in-row weeds. Lidar sensors are used to create a 3-D image of the soil
surface and control cultivating blade depth.

---

[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.

This is lettuce..what do you think?:

send your comments here: marcop@ag.arizona.edu

---

EPA’s Herbicide Strategy & Proposed Mitigations on Agricultural Herbicide Use 

We Need Your Input! 

The Environmental Protection Agency (EPA) is seeking public comment on its Draft Herbicide Strategy Framework to protect threatened and endangered species. The Strategy outlines proposed mitigations on agricultural herbicide use that could have significant impacts on production of field crops and specialty crops in the lower 48 states.

The Arizona Pest Management Center will submit comments to EPA to communicate the challenges and potential impacts of the proposed Strategy on our agricultural industries, but we need your help.  

We need input from growers, applicators, pest control advisors, registrants and others on the feasibility of EPA’s proposed mitigations for Arizona production systems. If you are concerned about EPA’s proposed changes and willing to share your knowledge of what works in Southwest agriculture, your input could help make a difference. 

Please consider responding to our online survey by clicking the link below. The survey takes around 12 minutes and is completely anonymous. The data you provide will only be used in aggregate to communicate local farming practices and concerns related to herbicide mitigations to the EPA. Taking this survey is also a good way to learn more about EPA’s specific proposed changes. Please respond by October 19.

Survey Link: https://uarizona.co1.qualtrics.com/jfe/form/SV_1QUBcXiIVgjFDHE

Thank you!

Area wide Insect Trapping Network   VegIPM Update, Vol. 12, No. 21, Oct 20, 2021.
 
Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth counts declined considerably over the past 2 weeks across all locations and below average for this time of the season. 
 
Beet armyworm:         
Trap counts increased slightly in many locations, but well below average for mid-October. Most activity in Dome and Yuma Valleys.
 
Cabbage looper:         
Cabbage looper trap counts remain steady in most areas. Activity average to below normal compared to previous years. 
 
Whitefly:                      
Adult movement increased sharply in the Dome Valley, Wellton Gila Valleys and Bard two weeks ago, but down in all locations last week. 
 
Thrips:                         
Thrips adult movement variable across locations; up in Wellton and Tacna, but remains below average for mid-October.
 
Aphids:                        
Aphid movement Increased sharply in Bard and Yuma and Gila Valleys consistent with N and E winds over the past 2 weeks.
 
Leafminers:                  
Adult activity increased sharply in the Gila Valley 2 weeks ago, but remains low in most location; below average for this time of season.
 
Diamondback moth:    
Significant increase in moth activity in the last 2 weeks, particularly in Dome Valley, Gila Valley and S. Yuma Valley. Traps located adjacent to cauliflower and broccoli transplants had significantly higher counts.  Areawide counts are trending upward.