Every season is different, and this fall was quite unusual in terms of insect pressure. A quick look at both recent and historic data on pest abundance recorded from our University of Arizona research plots and areawide and pheromone/sticky traps suggests that insect pressure this fall low for most key pests. For the supporting data see Pest Abundance on Desert Produce and Melon Cops in Fall 2019.

Whitefly/CYSDV: Areawide, whiteflies in fall melons were relatively light like last season. CYSDV incidence at harvest in cantaloupe fields remains low, but incidence in 2019 was higher than we’ve seen in the past 3 years. The virus incidence at harvest in 25 commercial plantings was very low in most fields (1-5%), with a couple of hot spots observed in Texas Hill and Wellton. For the most part, whitefly adult abundance and immature densities were similarly light in produce crops with an occasional flare-up in some isolated areas. Data from our untreated broccoli/cauliflower plots at the Yuma Ag Center (YAC) showed that colonization by whitefly nymphs was moderate; higher than the previous 2 seasons, but lower than what we normally expect.

Beet armyworm, cabbage looper, and corn earworm: For the second year in a row, we had a very light worm year in Yuma. Population abundance on untreated lettuce at YAC this fall was similar to last season, and one of the lightest falls we’ve seen since 1996. Similarly, numerous reports from PCAs suggest that fewer sprays were applied this fall, and some field broccoli fields reportedly received only a single spray. Not sure why we had light activity, but cooler nighttime temps in Oct, coupled with the storms we had in late September appeared to slow down activity dramatically in most areas. Trap catches for both BAW and CL were some of the lowest we’ve seen in the past 7 years.

Diamondback moth (DBM): DBM was not a problem again for local desert growers in 2019; about what we normally expect for the fall. We received no calls from PCAs reporting difficulty in controlling populations. Areawide pheromone trapping showed that DBM moth activity began later this fall but trended very similarly to 2017 and 2018. At YAC, the larval populations were the lightest we’ve seen since 2001, and lab bioassays of DBM collected from cabbage plots at YAC showed that this population was very susceptible to Coragen. Caution: DBM can increase quickly in the spring in the desert, so keep your eyes open for larvae and damage on those Feb-Apr cole crops and late seed crops.

Thrips: Similarly, the western flower thrips pressure was light this fall; lighter than the previous three falls. These lower population levels were likely a result heavy rainfall associated storms in September and November. Bean thrips abundance was also lighter in 2019 relative to the previous three seasons. Scarring damage was negligible in untreated lettuce plots at YAC.

Bagrada bug: In contrast, bagrada bugs made a comeback in 2019. Adult populations at YAC were the highest we’ve seen in the past 5 years, but well below the outbreak numbers we saw from 2010-2013. Populations picked up in mid-September and then began to increase rapidly with warm October weather. There were numerous reports of bagrada throughout the desert southwest, with minor damage showing up in late November. Likely the wet spring contributed to enhanced numbers in the spring and survival during the summer. Much like Lygus. Have a Merry Christmas, and we’ll visit again next year!

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%). Spinach ‘Revere’ was seeded, then sprinkler-irrigated to germinate seed Jan 18, 2024 on beds with 84 in. between bed centers and containing 30 lines of seed per bed.  All irrigation water was supplied by sprinkler irrigation.  Treatments were replicated four times in a randomized complete block design.  Replicate plots consisted of 15 ft lengths of bed separated by 3 ft lengths of nontreated bed.  Treatments were applied with a CO₂backpack sprayer that delivered 50 gal/acre at 40 psi to flat-fan nozzles. 
 

Month	Max	Min	Average	Rainfall
January	68	42	54	1.14 in
February	73	47	59	0.50 in
March	77	50	63	0.31 in

 
Downy mildew (caused by Peronospora farinosa f. sp. spinaciae) was first observed in plots on Feb 19  and final reading was taken on February 26, 2024. Spray date for each treatments are listed in excel file with the results.   Disease severity was recorded by determining the percentage of infected leaves present within three 1-ft² areas within each of the four replicate plots per treatment.  The number of spinach leaves in a 1-ft² area of bed was approximately 144. 

The data (found in the accompanying Excel file) illustrate the degree of disease reduction obtained by applications of the various tested fungicides. Products that  provided effective control against the disease include Orondis ultra, Thrive 4 M, Fungout, Cevya, Eject and Zampro. No phytotoxicity was observed in any of the treatments in this trial.

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.

Area wide Insect Trapping Network 
 
Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:             
CEW moth counts are down areawide and average for this time of year. 
 
Beet armyworm:         
Trap counts decreased in most locations; about average compared to previous years.
 
Cabbage looper:         
Cabbage looper counts increased in most traps and about average for this time of season.
 
Diamondback moth:      
DBM moths continue to increase in all areas. Below average for this time of the year.
 
Whitefly:                      
Adult movement negligible, typical for early January. 
 
Thrips:                           
Thrips adult counts beginning to decreased in the past 2 weeks, likely in response to rainfall in late December. Currently, numbers are about average compared with previous years.
 
Aphids:                       
Aphid movement increased significantly in the past two weeks, particularly in North Yuma and Gila Valleys. Well above average for early January.
 
Leafminers:                  
Adults decreased in most locations, but above average for January.