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
2023-2024 Sclerotinia Drop of Lettuce Fungicide Trial
Bindu Poudel-Ward, Martin Porchas Sr., Martin Porchas Jr., and Neeraja Singh
Yuma County Cooperative Extension, University of Arizona, Yuma, AZ
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 17, 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
Max Temp (°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
Sclerotia of Sclerotinia minor were produced in 0.25 pt glass flasks containing 15 to 20 sterilized 0.5 in. cubes of potato by seeding the potato tissue with mycelia of the fungus. After incubation for 4 to 6 wk at 68°F, mature sclerotia were separated from residual potato tissue by washing the contents of each flask in running tap water within a soil sieve. Sclerotia were air-dried at room temperature, then stored at 40°F until needed. Inoculum of Sclerotinia sclerotiorum was produced in 2 qt glass containers by seeding moist sterilized barley seeds with mycelia of the pathogen. After 2 months incubation at 68°F, abundant sclerotia were formed. The contents of each container were then removed, spread onto a clean surface and air-dried. The resultant mixture of sclerotia and infested barley seed was used as inoculum. Lettuce ‘Magosa’ was seeded and then sprinkler-irrigation was initiated to germinate seed in double rows 12 inches apart on beds with 42 inches between bed centers. Plants were thinned Jan 17, 2024 at the 3-4 leaf stage to a 12-inch spacing. For plots infested with Sclerotinia minor, 0.13 oz (3.6 grams) of sclerotia were distributed evenly on the surface of each 25-ft-long plot between the rows of lettuce and incorporated into the top 1 inch of soil. For plots infested with Sclerotinia sclerotiorum, 0.5 pint of a dried mixture of sclerotia and infested barley grain was broadcast evenly over the surface of each 25-ft-long lettuce plot, again between the rows of lettuce on each bed, and incorporated into the top 1-inch of soil. Treatment beds were separated by single nontreated beds. Treatments were replicated five times in a randomized complete block design. Each replicate plot consisted of a 25 ft length of bed, which contained two 25 ft rows of lettuce. Control plots received sclerotia but were not treated with any fungicide.
For treatments first applied at seeding, sclerotia were introduced into plots before the first application of treatments. The first application for at seeding treatments was made on Nov 28, with an additional application on January 17, 2024. Some treatments had second application on Jan 30, 2024 (See table). For treatments first applied after thinning, sclerotia were introduced into plots after thinning before the first application of these treatments, with additional applications as noted in the data sheets. An initial sprinkler irrigation supplied water for seed germination, with subsequent furrow irrigations for crop growth. First sign of disease was observed on January 29, 2024. The final severity of disease was determined at plant maturity by recording the number of dead and dying plants in each plot due to Sclerotinia minor or Sclerotinia sclerotiorum (March 5, 2024). As a point of reference, the original stand of lettuce was thinned to about 65 plants per plot.
In nontreated plots, about 32% of lettuce plants were dead or dying due to infection with Sclerotinia sclerotiorum and about 24 % due to S. minor, at the end of the trial. Please refer to the data tables to compare treatments of interest, using the Least Significant Difference Value listed at the bottom of each table to determine statistically significant differences among treatments. Miravis Prime, Luna Sensation and Elisys gave the best results against Sclerotinia minor. Luna Sensation, Miravis Prime and Fontellis gave the best control against S. sclerotiorum (see table). From the list of treatments applied at seeding, Endura fb Merivon gave the best control against both species of Sclerotinia (see table).
Phytotoxicity was not 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.
Corn earworm:
CEW moth counts declined considerably over the past 2 weeks across all locations and about average for this time of the season.
Beet armyworm:
Trap counts decreased in most locations, but well below average for late-October. Most activity in Yuma and Gila Valleys.
Cabbage looper:
Cabbage looper trap counts declined in most areas but increased in the N. Yuma Valley. Activity below normal compared to previous years.
Whitefly:
Adults remains active in Dome Valley, Wellton, and Gila Valley, but below average movement for this time of season.
Thrips:
Thrips adult movement increased in most locations in the last 2 weeks, particularly in Dome, Wellton, and Tacna. Activity remains below average for late-October.
Aphids:
Aphid movement increased again sharply in Bard and Yuma and Gila Valleys consistent with N and E winds over the past 2 weeks. Capture about average for this time of year.
Leafminers:
Adult activity increased sharply in the Gila Valley and Wellton but remains below average for this time of season.
Diamondback moth:
Adults remains active in Dome Valley, Wellton, and Gila Valley, but below average movement for this time of season. Traps located adjacent to cauliflower and broccoli transplants had significantly higher counts, particularly in Dome Valley and Gila Valley.