As reductions of agricultural water allocations in central Arizona from the Colorado River are going into place in 2022 from the Tier 1 of the Drought Contingency Plan provisions, it is important for every segment of the Arizona agricultural industry to continue to review and refine our practices associated with water management and irrigation.  
 
Seasonal conditions are a benefit to vegetable crop production and water management in the desert Southwest, both in terms of cool-season crop adaptability and the general climatic conditions that create lower environmental crop water-use demands.  For example, based on more than 30 years of Arizona Meteorological Network (AZMET) data from the Yuma, Arizona area, approximately 20% of the total annual reference evapotranspiration demand typically occurs between August and May.  
 
Reference evapotranspiration (ETo) is the collective loss of water from a field due to evaporation from the soil surface and transpiration of water vapor from the plants.  Collectively, water movement from the soil and plant systems in a field is referred to as “crop consumptive use” and it is important to understand this process in our efforts to steward our water resources most effectively.
 
The AZMET system provides both historical and real-time weather information that can be used to assist in crop water and irrigation management.  Reference evapotranspiration values can be obtained daily from AZMET for the Yuma area and applicable to most of the region in the lower Colorado River Valley.  The ETo values multiplied by an appropriate crop coefficient (K_c) can provide very good estimates on actual crop evapotranspiration (ET_c) rates, ETo * K_c = ET_c.
 
The appropriate K_c values are specific for each crop species and stage of growth.  We commonly use crop coefficient K_cvalues that are provided in the publication “Consumptive Use by Major Crops in the Desert Southwest” by Dr. Leonard Erie and his colleagues, USDA-ARS Conservation Research Report No. 29 (see attached).  Reference information for K_cvalues can be obtained in this publication for common vegetable crops grown in this region.
 
Estimating and tracking actual crop-water use can be a valuable tool in understanding crop-water demand and irrigation management.  Comparing actual crop-water demand and crop irrigation practices can be important in our efforts to understand crop management needs and field-level efficiencies.
 
Reference:
Erie, L.J., O.A French, D.A. Bucks, and K. Harris. 1981. Consumptive Use of Water by Major Crops in the Southwestern United States.  United States Department of Agriculture, Conservation Research Report No. 29.

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%). Lettuce was seeded, then sprinkler-irrigated to germinate seed on Nov 28, 2023 on double rows 12 in. apart on beds with 42 in. 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, which contained two 25 ft rows of lettuce. Plants were thinned Jan 17, 2024  at the 3-4 leaf stage to a 12-inch spacing. 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.

Month	Max Temp (°F)	Min Temp (°F)	Average Temp (°F)	Rainfall
November	80	51	65	0.08 in
December	71	44	57	0.82 in
January	68	42	54	1.14 in
February	73	47	59	0.50 in

 
Powdery mildew (caused by Golovinomyces cichoracearum) efficacy trial treatments were made on February 15,2024, February 23, 2024, March 4, 2024, and March 12, 2024and .Disease was first seen on February  26,2024. Disease rating was done on March 15, 2024.   Disease severity was determined  by rating 10 plants within each of the four replicate plots per treatment using the following rating system: 0 = no powdery mildew present; 0.5 = one to a few very small powdery mildew colonies on bottom leaves; 1 = powdery mildew present on bottom leaves of plant; 2 = powdery mildew present on bottom leaves and lower wrapper leaves; 3 = powdery mildew present on bottom leaves and all wrapper leaves; 4 = powdery mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = powdery mildew present on entire plant. These ratings were transformed to percentage of leaves infected values before being statistically analyzed. Yield loss due to rejected lettuce heads would likely begin to occur on plants with a powdery mildew rating above 2.0 (percentage of leaves infected value of 40).         
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. The most effective fungicides were Rhyme, Merivon, Quintec, Cevya, Luna Sensation, Luna Experience, and Elisys. 

Automated thinning machines were first commercialized in 2012, almost a decade ago. Since then, the machines have improved tremendously, and automated thinning has become the standard practice in the Yuma area. As such, I was a bit surprised to see numerous fields this year with sub-optimal thinning performance. Many fields had numerous amounts of excess seedlings that should have been thinned and needed to be removed during the follow-up hand weeding/double removal operation (Fig. 1). There were also some fields where the machine was apparently not readily detecting seedlings, resulting in long sections of plants being killed and a poor final stand (Fig. 2).

Automated thinning machines can and do perform very well when set-up and calibrated correctly. However, field conditions such as lighting, levels of crop residue, weed pressure or soil type (color) change throughout the day and affect the way the machine chooses which plants to selectively thin.

As such, it may be beneficial to establish a protocol where automated thinning machine settings are checked at regular intervals throughout the day to ensure the unit is performing optimally. Doing so may help lower follow-up hand weeding/double removal costs and minimize the number of unnecessary skips in the field.

Fig. 1. Labor crew removing excess lettuce seedlings and weeds after machine thinning.


Fig. 2. Poor final stand establishment following machine thinning, possibly due to incorrect machine calibration.

Preemergence herbicides can be very effective in the low deserts where weeds germinate with every irrigation or rainfall. It is essential, however, that they be at the right place in the soil at the right time. The right place is in the soil around the weed seeds and the right time is when they are germinating. Only fumigants kill seeds. For most other herbicides, the seed must germinate and are then killed. When weed seeds germinate is dependent on a great number of interdependent variables.  These include weed species, soil type, moisture, soil temperature, condition of the seed, depth, organic matter and many other variables. Many seeds have biochemical mechanisms that allow them to germinate at different times. Some may germinate in a few hours and some may not germinate for several years. If they all germinated at the same time, they would be easier to control. The methods used to predict when weed seeds will germinate vary from very sophisticated numeric equations to Farmers Almanac type lore involving things like the thickness of gopher fur, the cuticle thickness of plants or the condition of bird feathers. Some methods will give you a precise day while others will give you just a general idea. In general, as the precision goes up, the accuracy goes down. Regardless of what technique you use, now is the best time to apply preemergence herbicides for the control of annual weeds. It is better to be a month early than a day late.We have conducted trials to determine how long it takes various weed species to germinate after receiving moisture. Results will vary depending on several factors and from field to field and year to year. Results are summarized below for individual species and by month. An average of all species is also included.
   Hours to Germination

NG = No germination

Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 17, Aug 21, 2024
 
Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:           
CEW moth counts decreased areawide; about average to previous years.
 
Beet armyworm:         
BAW moth counts are up in south Yuma valley and Dome valley; about average for August

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 decrease, average for this time of the year.
 
Thrips:                          
Thrips adult activity continues to decline, average for August.
 
Aphids:                       
Aphid movement is absent and consistent with what we typically see during the summer.
 
Leafminers:                  
Adult activity declining and average for August.