Aphids: Aphid populations have remained relatively high even with the cold weather we experienced last week. However, with the weather in the 70-80’s for the next 10 days, PCAs should be on the lookout for foxglove and lettuce aphids on late season lettuce. We’ve had reports of both showing up throughout the area. Green peach aphids remain abundant on cole crops; we can find high densities on small cabbage and broccoli at the Yuma Ag Center (YAC). Because aphid species are inherently different in their susceptibility to insecticides, the proper choice of product is important for achieving adequate control. In my lettuce trials, to date Sequoia, Sivanto, and Beleaf have provided the most consistent knockdown and residual control of green peach and foxglove aphids. Movento has been inconsistent, but this is not surprising given our cole, rainy and cloudy weather which reduces plant transpiration. For more information on control visit Foxglove aphids on Desert Produce and 2019 Aphid Control.
Thrips: In contrast to aphid, western flower thrips have been quite low this winter due to the rainfall and cooler weather. However, PCAs should expect thrips numbers to rapidly increase with the warmer weather. This is particularly important on late-lettuce where “bioconcentration" of thrips occurs each year as lettuce acreage declines. Each time a lettuce field is harvested and disked, adult thrips disperse from these areas into the next available lettuce field. As the number of lettuce acres becomes reduced near the end of the season, this creates a bottleneck effect that concentrates high numbers of thrips adults on the remaining fields under production. They should also expect thrips adults to begin migrating into late season lettuce fields from recently harvested fields. This can often make chemical control of thrips populations very difficult, particularly in March, as thrips adults may continually re-infest fields following spray applications. See theThrips Control Chart 2019 for more information.
Corn Earworm: We have had no reports of corn earworm larvae showing up in head lettuce from local PCAs, although there ahave been some reports of CEW found in celery. However, we did have a pheromone trap spike last week in the Yuma Valley. Since, March and April are historically the time when spring lettuce tends to be infested heaviest by earworms, PCAs should consider intensifying their scouting efforts and begin tightening their spray intervals on the last remaining head lettuce fields. For more information see Corn Earworm Management on Desert Produce
Diamondback Moth (DBM): Compared to this time last year, DBM moth counts are higher in trap counts (see DBM Trap Network), but significantly lower than 2017. We can find larvae on seedling cabbage and broccoli at YAC, but they are very low levels at this time. We have had a few reports from PCAs of larval activity in cabbage crops. If larvae do become abundant in late cabbage, based on trials conducted this fall, all the key insecticide products used to control DBM larvae should provide control. However, bioassays of DBM populations collected from cauliflower fields in November from Roll, Yuma Valley and the Yuma Ag Center showed reduced susceptibility to Coragen. Thus, PCAs should avoid using products containing chlorantraniliprole at this time. For more information see Diamondback Moth Management Spring 2019
This study was conducted at the JV farms at Gila Valley. Lettuce variety ‘Guapo’ was seeded, then sprinkler-irrigated to germinate seed on September 19, 2023, on double rows 12 in. apart on beds with 42 in. between bed centers. Rest of the irrigation 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 on October 9, 2023 at the 3-4 leaf stage to a 12-inch spacing. Treatment beds were separated by single nontreated beds. Treatments were applied by incorporating in soil before seeding or with a tractor-mounted boom sprayer that delivered 50 gal/acre at 100 psi to flat-fan nozzles spaced 12 in apart.
Month
Max
Min
Avg
Rain
September
100
71
86
0.71 in
October
93
61
77
0.00 in
November
80
51
65
0.08 in
December
71
44
57
0.82 in
Fusarium wilt (caused by Fusarium oxysporum f. sp. lactucae ) rating was done in the field by observing the typical symptom of lettuce wilt. Confirmation was done by cutting the cross section of roots. Disease scoring/rating was done on December 6, 2023.
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. The disease pressure was extremely high in 2023, and most treatments showed little or no control against the disease. The treatments that showed some activity were Bexfond, Cevya, Rhyme, and Serifel. Plant vigor was normal and phytotoxicity symptoms were not observed in any treatments in this trial.
Controlling Fusarium Wilt of Lettuce Using Steam Heat – Trial Initiated
Earlier this week, we initiated a trial examining the use of band steam for controlling Fusarium wilt of lettuce. The premise behind this research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae, the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010).
In our trials, we are using steam heat to raise soil temperatures. Steam is delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). Preliminary results were encouraging. The device was able to increase the temperature of the top 3” of soil to over 180°F at a travel speed of 0.5 mph as shown in this video of the machine in action (shown below). These temperatures exceed that of those known to control pathogens responsible for causing Fusarium wilt of lettuce (> 140°F for 20 minutes).
Stay tuned for final trial results and reports on the efficacy of using steam heat to control Fusarium wilt of lettuce.
If you are interested in evaluating the technique on your farm, please contact me. We are seeking additional sites with a known history of Fusarium wilt of lettuce disease incidence to test the efficacy and performance of the device.
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.
Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support.
A special thank you is extended to Cory Mellon and Mellon Farms for allowing us to conduct this research on their farm.
Weeds are one of the most visible of all agricultural pests. They can’t move or hide and once established often stick up over the crop. Just one weed in a 10 acre field is annoying to look at. With insects and diseases, the damage is often more visible than the pest. That is not the case with weeds. A moderate weed infestation is approximately 10 weeds per square foot. If a herbicide produces 90% control, that leaves 1 weed per square foot or 43 weeds per acre. Without an untreated check, this can look like the herbicide failed! It is easy to leave an untreated spot in a field and it is well worth doing. Many applicators do so unintentionally because of skips, powerlines and other causes. They help determine crop injury and weed control. Here are some examples of what various levels of control looked like from one of our cole crop trials:
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
