Hope you are all enjoying the summer so far and keeping busy. Although, we’re about 2 months away from the Fall produce season, it’s never too early to begin thinking of IPM strategies you will be using to battle key insects next season. This includes keeping track of their typical activity during June, July, and August. Below is a short discussion detailing individual pests. 
 
Aphids:    The major aphid species that are important in produce crops each spring include green peach aphid, foxglove aphid and lettuce aphid.  These species are considered cool season pests and are biologically most active during the winter and spring when temperature average around (55-65 F). We do not find them on summer crops as the high temperatures prohibit their activity. They essentially become extinct each summer only to reappear in the fall to infest crops in Oct-Dec migrating into the desert via N-NW winds that occur after the monsoons.
 
Diamondback Moth (DBM):  Another key pest that disappears each summer but for a different reason than for aphids. The host range of DBM is limited to only brassica crop and plants. Although brassica weeds are common during the winter, they occur during the summer due to high temperatures and dry conditions. Thus, like aphids, DBM populations from the previous spring season become “extinct” during the summer when brassica crops and weeds are absent. They reappear each fall migrating in on high winds associated with tropical storms from Mexico, or arrive into the region on infested brassica transplants from California.
 
Whiteflies:       A resident pest that occurs year-round in the desert. As a warm season pest, whiteflies are most abundant on fall produce crops, and seldom a problem during the winter and spring. Peak abundance occurs during the summer months on cotton, and again in September and October on fall brassica, leafy vegetables and fall melons.  Thus, it is important to control whiteflies in cotton prior to defoliation to prevent large migrations onto fall produce crops. In addition, whiteflies can be found on several weeds such as purslane, goosefoot, ground cherry, and sunflowers. Sanitation in and around fields being prepped for fall plantings is important. Preventatively, growers should consider applying soil, system insecticides such as imidacloprid or Verimark to Fall produce crops.

Beet Armyworm (BAW):  A warm-season pest that occurs year-round in the desert. They tend to reach peak abundance twice a year. Generally, the most important peak is in September and October when weather is ideal for their development, flight activity and oviposition. A second peak occurs in late spring. Activity is generally light during the summer, occurring primarily in alfalfa crops. Although BAW is known to migrate in from the south via monsoon and tropical storms, resident populations annually occur in alfalfa during the summer. Thus, to minimize BAW in fall produce crops, thorough management of BAW can potentially minimize their movement onto Fall produce crops.  Weeds are also a source of BAW (purslane, goosefoot, sunflower and Russian thistle) so sanitation in and around fields is important.
 
Western Flower Thrips (WFT):  Also a resident pest that can be found in the desert year round. WFT is most abundant on lettuce in the late spring, but peaks in abundance on alfalfa, melons and cotton is May and June. During the late summer numbers drop dramatically where thrips can be found on melons and alfalfa and weeds like goosefoot and purslane. WFT slowly move into lettuce again in early fall and are least abundant during the winter.  Sanitation of weeds during the summer can help reduce thrips abundance moving into the fall crops and weeds.
 
For more information see:   Keeping Track of Major Produce Pests During the Summer

The Colorado River Compact of 1922 was formally negotiated among the basin states (Figure 1) to define how much water from the Colorado River can be used by each state on an annual basis. In 2007, basin states negotiated interim guidelines to deal with Colorado River water shortages and determine the reductions for each state depending on the elevation of water in Lake Mead. Another series of agreements have included Colorado River water allocations to Mexico.
 
After more than 20 years of drought, reduced flows in the Colorado River are now resulting in some serious steps to arrest the decline of the major storage reservoirs, Lakes Powell and Mead. The Drought Contingency Plan (DCP) Authorization Act was agreed to by the lower basin states (Nevada, California, and Arizona) and signed into law on 16 April 2019.  Participation from Mexico was determined through a U.S. – Mexico agreement.  The DCP outlines a process, dependent upon triggers related to the water level in Lake Mead that is intended to protect the Colorado River system through voluntary reductions and increased conservation. Similarly, there is an Upper Basin DCP involving Colorado, New Mexico, Utah, and Wyoming.  The lower basin DCP will extend through the end of 2026 and consists of increasing reductions in Colorado River use in Tiers 1, 2a, 2b, and 3 at Lake Mead elevation levels of 1,075; 1,050; 1,045; and 1,025 ft. above sea level, respectively.
 
With the water level in Lake Mead below 1,075 ft. above sea level, the Tier 1 water allocation reductions have been called into action by the Bureau of Reclamation beginning in January 2022.  
Arizona will suffer the biggest cuts because it agreed in the 1960s to take the lowest priority among the Lower Basin states.  According to the DCP, Tier 1 requires a 512,000 acre-foot (AF) reduction in Arizona’s 2.8M AF annual allocation.  This reduction is primarily being taken from the agricultural irrigation districts associated with the Central Arizona Project (CAP), primarily impacting agricultural areas in Pinal County.  It is important to note that urban/metropolitan and tribal water allocations are not reduced in Tier 1 conditions.
 
Even with the DCP reductions, there are still valid concerns regarding the rate of decline of Lakes Mead and Powell with a continuation of existing weather patterns.  As a result, representatives from Nevada, California, and Arizona have been working on an additional plan to provide further reductions of water use from the Colorado River with a goal of achieving an additional 500,000 AF to leave in Lake Mead.  This would serve to nearly double the conservation of Colorado River water and further reduce the rate of decline in Lake Mead.  For California, this would mean they will take water reductions prior to reaching DCP levels that would trigger their mandatory cuts.
 
This new proposal is being referred to as the “500+ Plan” and it would be entirely voluntary among entities in the lower basin states to participate.  It is estimated that $100M will be needed to implement this plan for further reductions in Colorado River water allocations from the three lower basin states.  For example, this plan would include payments to farmers for fallowing land to save water.  The water agencies from these three states are working to find the money to support this effort and Arizona has pledged $40M to support the plan.  California and Nevada have the plan under review for funding and additional funding could possibly come from the federal government to make this work.
 
In the past two decades, the flow of water in the Colorado River has declined nearly 20% below the 20th century average.  As a result, extractions from the Colorado River have exceeded inputs and in time this catches up with us.  These plans are important for everyone living in the desert Southwest with a dependence on the Colorado River.  Therefore, all steps to conserve this valuable source of water are critical.  In Arizona, approximately 70% of the water is diverted to agriculture and our efforts to provide good stewardship and management of our water resources are always important and increasingly so under these circumstances.
 
Reference:
Colorado River Drought Contingency Plan: Arizona Implementation. Central Arizona Project
https://library.cap-az.com/documents/departments/planning/colorado-river-programs/CAP-FactSheet-DCP.pdf

Figure1. The Colorado River watershed of seven western U.S. states and 2 Mexican states.  Source: USGS.

Downy mildew has always been one of the major problem for PCAs and growers in the desert southwest. The symptoms observed are green to yellow angular spots on the upper surface of the leaves and fluffy growth on the lower side (See Picture). Symptoms usually start from older leaves. As disease progresses the lesion turn brown and dry up and in some occasions, the disease can become systemic causing dark discoloration of vascular tissue.

Favorable condition for disease development:
The pathogen Bremia lactucae thrives in damp, cool condition, with moisture present on leaves. Spores are short-lived but dispersed efficiently by wind during moist period. Cultivated lettuce is the main host of the pathogen but it has also been reported to infectartichoke, cornflower and strawflower.

Why is downy mildew difficult to manage?
One of the main reason that hinders the disease management is the complexity of the pathogen. Bremia lactucae consists of multiple races (pathotypes), and new races continue to occur as pathogen evolves. The pathogen is one of the fastest evolving plant pathogen. And each pathotypes have developed insensitivity to fungicides to different extent.

One of the best practice is to grow resistant cultivar, but there are limitations. As the pathogen is highly variable and dynamic, resistant cultivars are not a permanent solution as the pathogen overcomes the resistance by evolving into virulent strains and isolates.

Preventative application of fungicides are effective to some extent. Reducing leaf wetness and humidity by using drip or furrow irrigation can be helpful. However, weather condition like rain during cool weather as we had in past couple of weeks is conducive to development of epidemics and we have very little control on that matter.

It Takes a Village:
And better yet the whole nation or world! Downy mildew is a bigger problem than we think. It is just not a problem in Arizona and California, it is a nationwide, worldwide problem. Thousands of plant pathologist/scientists/labs are working hard everyday to combat the disease.

Dr. Michelmore’s lab in University of California-Davis has been working on downy mildew for years and they need our help to build the database for next several years. Please let me (Bindu Poudel) or anyone in University of Arizona know if you see symptoms in the field. We can come collect the samples and send it to UC-Davis. The goal is to racetype as many isolates as possible to understand the genetic variability, the more information we have about the pathogen, the more it helps the scientific community come up with better management practice, better resistant cultivars etc.

Learn more about the Bremia Project:
https://bremia.ucdavis.edu/index.php

Bremia Project Database:
https://bremia.ucdavis.edu/bremia_database.php

Watch this video

Herbicide Chemigation Injecting products through irrigation systems is an old technique. It was first used with fertilizers but is now used with pesticides as well. The first herbicide that was chemigated was EPTC (Eptam) in the early 1960’s. Chemigation is generally simple, cost effective and can be done when the field is too wet for equipment operation. Off target drift is minimized and those herbicides that require incorporation into the soil are incorporated at the same time as application. Disadvantages are that sprinklers are designed to apply large volumes of water over several hours and days. It works well for water and fertilizer because any ununiformed is masked by the volume and time it takes to apply them. With herbicides we might be applying only a few ounces per acre and if they are applied in less than an hour or if it is windy ununiform applications can result. Probably the major disadvantage is that sprinkler pipe must be in the field. The pipe commonly has been removed when postemergence applications are made. Most of the common soil applied preemergence herbicides including Prowl,Treflan, Dacthal, Eptam and others can be chemigated. Some of the postemergence, foliar herbicides including Buctril/Maestro, Goaltender, Clethodim, Sencor, Diuron, Dual, Prometyrn and others are also registered for chemigation. This is surprising because you would expect them to be washed off with the water, but they work well and some work better when chemigated. Chemigations typically require lower rates so check the label for rates.

Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 21, Oct 16, 2024

Results of pheromone and sticky trap catches can be viewed here.

Corn earworm: CEW moth counts increased in most traps; about average for early October.

Beet armyworm: Trap counts increased in most areas last week, and about average for early October.

Cabbage looper: Cabbage looper trap counts still most locations, below average for this time of the season.

Diamondback moth: Adult activity increasing, particularly in the Yuma and Dome Valleys.  Above average for this time of season.

Whitefly: Adult movement increasing Tacna, Dome Valley and Yuma Valley; overall about average for early October.

Thrips: Thrips adult movement picking up in all areas, overall activity above average.

Aphids: Winged aphids beginning to show up the north Yuma and Gila Valleys; about average for early October.

Leaf miners: Adult activity light in all areas, about average for this time of season