Overall insect pressure has been below average this fall in my estimation.  Of course, there have been a few surprises like bagrada bug incidence, and heavy worm and/or whitefly pressure in some isolated areas. However, these pests should start to decline quickly as winter approaches.  With the seasonal transition of cooler temperatures, along with shorter days and consistent N-NW winds, you will likely begin to see an increase in winged aphids showing up on desert produce crops. Winged aphid numbers on yellow sticky traps have suddenly increased in the North Gila and North Yuma Valleys over the past 2 weeks or so (see Areawide Trapping Network).  This is not a surprise; experience tells us that this is an annual occurrence.  The key aphid pests found on winter produce (i.e., green peach aphid, foxglove aphids) do not over-summer here, rather they migrate into our cropping system from mountainous regions of southern California via wind currents during the fall.  Once the aphids reach our desert valleys, they typically land on native vegetation and then into produce fields, moving among weeds and crops until they find a suitable host to feed and colonize on.  No need to panic if you suddenly find a few winged aphids on a lettuce plant. But it is important that you correctly identify the aphid species present.  It is not uncommon to find winged aphids on lettuce that do not colonize on the crop. For example, winged cabbage aphids can typically be found on lettuce this time of the year but will not colonize on the plants. Other examples of winged aphids that can be found on produce crops would include aphids that colonize small grains (i.e., oat bird-cherry aphid) or alfalfa (i.e., blue alfalfa aphid).  Because these aphid species will not colonize produce crops, it is important to be able to distinguish them from the aphids that do colonize and require management to prevent problems at harvest (i.e., green peach aphid, foxglove aphid, lettuce aphid, cabbage aphid, turnip aphid).  Proper aphid ID can also influence your choice of insecticide, but more on that in a later update.  Don’t be surprised if you start finding small colonies of cowpea aphids or melon aphids showing up on frame leaves in lettuce. That is a common occurrence every fall.  Not to worry, experience has shown us that although small cowpea aphid colonies may be found on lettuce, the populations generally stay low on the plant on the frame leaves and rarely increase to levels causing contamination issues.   But you never know.  So, keep a close watch out for these aphids found in your crops, as our weird weather this year may be more conducive to their development than normal. Bottom Line:  proper aphid identification is important; it can save a PCA time and money and prevent unnecessary insecticide applications. If you find an unusual aphid in your produce, don’t hesitate to drop it by the Ag Center and we’ll get it identified for you.    If you want to make fast and accurate IDs, you might use the attached publication Aphid Identification in Desert Produce Crops that can assist you in identifying winged and wingless (apterous) aphids important in leafy vegetables and cole crops.

Identify the insect pest below 

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 4, 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	MaxTemp(°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

 
Downy mildew (caused by Bremia lactucae) rating was done on variety Eblin, Bobcat, and 180 (partially funded by AILRC grant). Disease was first seen on 1-30-24. Please see attached excel file for chemicals application date. Disease rating was done on February 29^, 2024. Disease severity was determined  by rating 10 plants within each of the five replicate plots per treatment using the following rating system: 0 = no downy mildew present; 0.5 = one to a few very small downy mildew colonies on bottom leaves; 1 = downy mildew present on bottom leaves of plant; 2 = downy mildew present on bottom leaves and lower wrapper leaves; 3 = downy mildew present on bottom leaves and all wrapper leaves; 4 = downy mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = downy mildew present on entire plant.
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Most of the treatments exhibited activity against the disease to some extent. Latitude, Amara, Eject, Previcure flex exhibited good control in the variety Eblin (highly susceptible variety). Whereas Cevya, Stargus, Latitude, Amara, Revus, Thrive 4 M, Actigard, Instigo+Carbose+intereact showed activity against the pathogen in variety Bobcat. Please see excel file for the full list of chemicals and their efficacy. The lettuce variety 180 was resistant to the disease and no downy mildew was observed on the particular variety. No phytotoxicity was observed in this field.

Steam sterilization of soils is commonly used in plant nurseries and greenhouses for effective control of soilborne pathogens and weed seeds. The technique, however, is highly energy intensive as the entire soil profile is heated. This is too costly and slow to be practical for field scale vegetable production.  To reduce energy consumption and cost, use of band-steaming, where steam is applied only in the area where it is needed – in the plant root zone, is proposed. In this method, narrow strips of soil centered on the seed line are treated with steam rather than the whole bed.

Over the course of the last year, we developed a prototype band-steam and co-product applicator that is designed to raise soil temperatures in a band 2” deep by 4” wide to levels sufficient to control soilborne pathogens (140 °F for > 20 minutes) and weed seed (150 °F for > 20 minutes). The device is principally comprised of a 35 BHP steam generator and a co-product applicator mounted on top of a bed shaper (Fig.1). The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper. An exothermic compound can be co-applied via shank injection and/or a banding spray nozzle. The rationale behind co-applying an exothermic compound with steam is that exothermic compounds react and release heat when combined with water, thereby reducing energy requirements and increasing travel speed.

Preliminary testing of the device this spring in Yuma, AZ were very promising. Trial results showed that application of steam alone effectively raised soil temperature in the center of the seed line to levels required for effective pest control (140 °F for more than 20 minutes). Use of the exothermic compound increased soil temperature by about 10 °F. A video of the device in action can be found at the link provided below.

We are currently evaluating the device in field trials with lettuce in Salinas, CA. Target pests in these experiments conducted in collaboration with Steve Fennimore, UC Davis, are soil pathogens which cause Sclerotinia lettuce drop and in-row weeds. Future articles will report the findings of this research.

This fall, we will be replicating these tests in Yuma, AZ and also investigating the effectiveness of band-steam for controlling Fusarium oxysporum f. sp. lactucae which causes Fusarium wilt of lettuce. Heat has been shown to effectively kill Fusarium oxysporum spores and control Fusarium wilt disease. As an example, soil solarization, where clear plastic is placed over crop beds during the summer, raises soil temperatures to 150-155˚F at the soil surface, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010). 

These projects are sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council.  We greatly appreciate their support.

If you are interested in seeing the machine operate or would like more information, please feel free to contact me.

See the band-steam and co-product applicator in action!

References: Matheron, M. E., & Porchas, M. 2010. Evaluation of soil solarization and flooding as management tools for Fusarium wilt of lettuce. Plant Dis. 94:1323-1328.

Sprangletop has become increasingly widespread in Arizona mostly because of its growth habits and tolerance to many commonly used herbicides. It is in the Leptochloa genus which is derived from the Greek words leptos (thin) and chloa (grass). There are more than 150 species of sprangletop worldwide but only three in Arizona and two in Yuma County. The two that are the most common in the low desert are Mexican Sprangletop, which is Leptochloa uninervia and Red Sprangletop, Leptochloa filiformis. A third species, Bearded Sprangletop, Leptochloa fascicularis, is more common at higher elevations of 1500 feet or higher. It is not uncommon to find both Red and Mexican Sprangletop in the same field and it is not hard to distinguish them when they are side by side. Red Sprangletop has a light green leaf blade which is similar in width to watergrass and barnyardgrass. It has very fine hairs and very small and fine branches and spiklets. It also has a long membranous ligule. The name Red refers to the leaf sheath, which is characteristically red, rather than the seed head. Mexican Sprangletop has a thinner leaf blade which is darker green or grayish in color and similar in appearance to common bermudagrass. The seed head is distinctly coarser than that of Red Sprangletop. Side by side, leaf color and size of the seed make it easy to distinguish these two. Both of these grasses are classified as summer annuals, but they grow more like perennials in the low desert. Sprangletop does very well in the hottest part of the summer and typically germinates from seed during the hottest period between July and September. Once established, however, it often survives through the cold winter months. It grows into clumps that often appear to be dead during the winter. New shoots commonly grow from these established crowns the next season. When this occurs, preemergent herbicides such as Trifluralin or Prowl are ineffective. Some Sprangletop plants stay green and grow through the winter. Many of the postemergence, grass specific herbicides that control many grasses are ineffective on Sprangletop. This also has contributed to the spread of these weeds. Sethoxydim (Poast) and Fluazifop (Fusilade) do not control either Red or Mexican sprangletop. Only Clethodim (Select Max, Select, Arrow and others) is the only one of these grass herbicides that is effective and only at the highest labeled rates. Two applications are often necessary to achieve season long control.

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:           
CEW moth counts decrease to very low levels  in areas, about average for this time of year.
 
Beet armyworm:          
Trap counts low; increased slightly in the Yuma Valley; lower than average compared to previous years.
 
Cabbage looper:          
Cabbage looper counts deceased in most traps and below average for this time of season.
 
Diamondback moth:       
DBM moths counts increased Gila and Yuma Valley. About average for this time of the year.
 
Whitefly:                    
Adult movement negligible, average for late winter. 
 
Thrips:                         
Thrips adult counts remain low, likely in response to cumulative rainfall this winter. Currently, numbers are below average compared with previous years.
 
Aphids:                       
Aphid movement decreased significantly in the past two weeks; about average for mid-February
 
Leafminers:                   
Adults remain low in most locations, increased in Yuma Valley; average for mid February.