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.

It’s unfortunately a very great season to be a plant pathologist…

We have confirmed the first sample of Fusarium wilt on lettuce submitted to the Yuma Plant Health Clinic from Yuma County. The stunted seedlings looked like any other typical case of damping-off at the seedling stage. When plated on culture media, subsequently confirmed Fusarium colonies grew abundantly from the declining plant tissues. If you’re not already on guard and scouting, this is a warning that Fusarium is active in Yuma County.

Adding on to this early alert, we’ve received a surge of submissions of young brassicas to the clinic. Several severely wilted and declining plants from around Yuma County have cultured positive for Pythium, likely as an opportunistic invader coming in on the back of all the early-season rain that brought stress to seeds and young transplants. Growers may want to consider oomycides, but only if the seedling disease is first confirmed to be Pythium. Remember, many seedling diseases caused by true fungi are indistinguishable from those caused by Pythium.

If you have any concerns regarding the health of your plants/crops please consider submitting samples to the Yuma Plant Health Clinic for diagnostic service or booking a field visit with me:

 

Chris Detranaltes
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109

For those of you interested in the latest developments in Vegetable IPM and/or in need of CEU’s, Yuma County Cooperative Extension has created online workshops that offer CEU and CCA credits for both AZ and CA. Current offerings are the recorded workshops from 2021 Southwest Ag Summit. There’s an ag mechanization session entitled “Advances in Weed Control Technologies”, and several others that readers of this newsletter may be particularly interested in including: “Integrated Pest Management Regulatory Update” “Pesticide-Related Updates: The Latest Presented by the Greatest” “Influence of Irrigation on Insect, Weed, and Disease Management” “Advances in Weed Control Technologies” “Integrated Pest Management in Vegetables”. Please recognize Yuma County Cooperative Extension Agricultural Agent Robert Masson who spearheaded this effort. Click on the image, or here to access Veg IPM workshops online.

March is traditionally the time when winter annual weeds die and summer annual weeds start growing. Weed management would be much easier if this transition was clear cut. Like everything else, many factors can complicate this. Especially here in the low desert with mild winters and summer irrigation. Some summer annual weeds have survived all winter and some winter annual weeds will grow all summer. Sprangletop and sandbur are classified as summer annuals but they have survived all winter in a dormant state and will regrow from established crowns as it continues to warm up. Not all survive the winter and some new plants will come from seed this summer. Those coming from seed can be controlled with preemergence herbicides but those coming from crowns can not be. Other weeds that survive year round that are not normally classified as perennials are clovers, mallows and thistles. Many of these have become very widespread and difficult to control. White sweet clover and both green and Mexican sprangletop have become more widespread every year.

This time of year, John would often highlight Lepidopteran pests in the field and remind us of the importance of rotating insecticide modes of action. With worm pressure present in local crops, it’s a good time to revisit resistance management practices and ensure we’re protecting the effectiveness of these tools for seasons to come. For detailed guidelines, see Insecticide Resistance Management for Beet Armyworm, Cabbage Looper, and Diamondback Moth in Desert Produce Crops .

VegIPM Update Vol. 16, Num. 20

Oct. 1, 2025

Results of pheromone and sticky trap catches below!!

Corn earworm:  CEW moth counts declined across all traps from last collection; average for this time of year.

Beet armyworm: BAW moth increased over the last two weeks; below average for this early produce season.

Cabbage looper: Cabbage looper counts increased in the last two collections; below average for mid-late September.

Diamondback moth: a few DBM moths were caught in the traps; consistent with previous years.

Whitefly:  Adult movement decreased in most locations over the last two weeks, about average for this time of year.

Thrips: Thrips adult activity increased over the last two collections, typical for late September.

Aphids: Aphid movement absent so far; anticipate activity to pick up when winds begin blowing from N-NW.

Leafminers: Adult activity increased over the last two weeks, about average for this time of year.