Our annual Lettuce Crop Losses Workshop was recently held in April and the results of the surveys continue to show consistent trends in insecticide usage on desert head lettuce. In general, the most commonly used insecticides in fall and spring lettuce correspond directly to the key pests that typically occur during these growing periods. By far, the pyrethroids, applied both as foliar sprays and chemigations, were the most commonly used insecticide class. No surprise there. Over the past 11 years, pyrethroid usage has remained steady. The reason for this is quite clear to me: pyrethroids are one of the most inexpensive and safe broad spectrum insecticides still available for use in tank-mixtures for effective control of flea beetles, crickets, plant bugs and some Lep larvae (looper and earworm). The overall use of OPs and carbamates continues to decline, but Lannate (methomyl) and acephate are still relied upon for thrips management. Their usage is being replaced primarily by several reduced-risk chemistries, of which the spinosyns remain the second most commonly used class of insecticides. In 2014-2015, nearly 95% of the lettuce acreage in our area was on average treated with > 2 applications of Radiant or Success. Their use against both lepidopterous larvae and thrips has remained steady since they were first registered. Foliar uses of Diamides (Coragen, Voliam Xpress, Vetica, Belt) were the third most commonly used chemistry in lettuce in 2014-2015. Since they were first registered in 2008, PCAs have steadily incorporated this new chemical class into their management programs. The use of Belt increased significantly this season, whereas soil uses of Coragen continue to decline. Ketoenol usage (Movento) on fall and spring lettuce increased this season likely due to heavier whitefly and aphid pressure. Another important class of chemistry used in fall and spring lettuce is the neonicotinoids driven primarily by soil-applied imidacloprid for whiteflies and aphids. The usage of imidacloprid on both fall and spring lettuce has increased markedly since 2009 and is used on almost 90% of the acreage, albeit at top of the label rates. Foliar neonicotinoid usage also increased last season, presumably due to heavier whitefly/aphid infestations in 2014-15. Finally, for the fifth season in a row, PCAs treated a greater percentage of their acreage with selective, reduced-risk products than with the broadly toxic, older chemistries. To view a summary of the estimated insecticide usage by chemical class, as well as the 12 most commonly used insecticides on head lettuce this past growing season, go to Insecticide Usage Summary in Arizona Lettuce 2015.
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 remain low across all locations; average for this time of the season.
Beet armyworm:
Trap counts decreased in all locations, and a little below average for late-January.
Cabbage looper:
Cabbage looper trap counts remained low in all locations; below average for January.
Diamondback moth:
Adult activity decreased across all locations, except the North Gila Valley where trap is adjacent to with nearby brassica seed crops. Overall, activity is a little below for this time of year.
Whitefly:
Adult movement remained low in all locations consistent with previous seasons.
Thrips:
Thrips adult movement beginning to increase slightly in most locations last week but increased sharply in Roll. Activity about average for January.
Aphids:
Aphid movement low in most locations, increased slightly in N. Yuma and E. Gila Valleys. Trap captures slightly below average for this time of season.
Leafminers:
Adult activity increased in many areas, particularly in the Yuma Valley; above average for this time of season.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
