Green Peach Aphids / Lettuce Aphids: Aphid populations remain relatively heavy so far this season. We are currently finding green peach aphid and lettuce aphid on lettuce/brassicas at YAC. Green peach aphid populations have remained steady on lettuce (10-15 /plant) and increasing rapidly on the brassica crops (>40/plant). This is above average for February, but I’ve seen heavier numbers in previous years. In contrast, this is the earliest, most abundant, and most widespread I’ve observed lettuce aphid in desert lettuce since they first appeared over 20 years ago. Reports of heavy lettuce aphid pressure continue throughout the area, particularly Yuma Valley and Dome. This season lettuce aphids have been colonizing plants since December and are currently exceeding 100 aphids/plant in some untreated plantings at YAC. Unfortunately, they are likely to increase more rapidly with warmer weather forecasted for next week. Trials at YAC show that common aphid products (Sequoia, Beleaf, Versys, PQZ, Movento) are providing ~10 to 14-day activity on Green peach aphid; Movento is providing ~21 d control on Lettuce aphid. For more information on control visit Lettuce aphids on Desert Lettuce and 2022 Aphid Control.
INSV: It was about this time last year that INSV was reported in the desert for the first time. Not surprisingly, INSV symptomatic plants have begun to show up again in desert lettuce following a similar pattern to what we observed last spring. Currently we have confirmed INSV infected plants in direct-seeded lettuce grown in Tacna, Roll, Wellton, Dome Valley, Gila Valley, Yuma Valley and Bard. However, the rate of INSV incidence in most fields is very light so far, averaging less < 0.5%. Our surveys show that lettuce fields currently infected with INSV are in proximity to ranches where transplanted lettuce was grown last fall. Reports of symptomatic plants in late planted lettuce are starting to trickle in from growers and PCAS as well. I encourage anyone spotting symptomatic plants in the next few weeks to let us know. The more we understand this problem, the better chance we can solve it.
Thrips: Western flower thrips abundance is beginning to build throughout the area and will likely increase rapidly on late lettuce where “bioconcentration" of thrips occurs each year as produce acreage declines near end of the season. This bottleneck effect concentrates high numbers of thrips adults on the remaining lettuce fields under production. This can often make chemical control of thrips difficult, particularly in March, as thrips adults may continually re-infest fields following spray applications. It can also increase this risk of primary INSV spread by adults dispersing from infected fields. Accordingly, PCAs should be vigilant in their thrips control to minimize larval development and the potential for secondary, within-field INSV infection. Efficacy trials at YAC currently show that Radiant and Lannate are providing a solid 10-day control of nymphs. For more information on thrips management and insecticide options in desert lettuce go to: Rethinking Thrips Management in Desert Lettuce.
Leps: Activity has been unusually low for the past several months and pheromone trap counts have been the lowest we’ve seen in several years for cabbage looper, beet armyworm and corn earworm. We’re picking up an occasional report of looper activity from PCAs. No reports of corn earworm to date. DBM moth trap counts are down compared to this time last season (see DBM Trap Network), and only a few reports of DBM in commercial fields. No issues in control have been reported. Nonetheless, keep a close eye out for all these Leps as they tend to build up rapidly with warmer weather late in the season
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.
Band-Steam Applicator for Controlling Soilborne Pathogens and Weeds in Lettuce
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.
