Aphid Control: We mentioned in our previous update that foxglove aphid was beginning to colonize lettuce in the Yuma Valley. In the past two weeks aphid activity, and particularly foxglove aphid, has increased dramatically in our untreated control lettuce plots here at the Yuma Ag Center. Keep in mind, foxglove aphids can be difficult to locate in fields due to their patchy distribution. It is not unusual to sample dozens of plants and find nothing and then find 2-3 plants heavily colonized with foxglove aphids. Green peach aphid is generally more uniformly distributed among plants. They are also still prevalent in our lettuce, albeit not at the same densities as foxglove aphids. We’re also finding green peach aphid and cabbage aphid beginning to rapidly colonize young cabbage plants. Because all these aphid species are inherently different in their susceptibility to insecticides, the proper choice of product is important for achieving adequate control. For more information on insecticide alternatives for the various aphid species please view our Aphid Control Chart-2014.
Cabbage looper: Our pheromone trap captures of cabbage looper moths the past 2 weeks have been unusually high for this time of the year (see Areawide Trapping Network). This is largely due to the above average temperatures we’ve been experiencing in the desert. In addition, corn earworm trap catches remain consistent from our last update, particularly in the Wellton/Dome Valley area. These increases in moth activity may indicate potential increases in egg laying and larval infestation in your later produce fields. It would be well advised to pay careful attention for fresh feeding signs and small larvae when scouting for aphids/thrips. Addition of a pyrethroid to aphid and thrips sprays can help with looper and earworm control.
Mystery Lepidopterous Larvae Identified: In out last update we mentioned that an unidentified lepidopterous larvae has been found burrowing into the roots of young romaine plants in the Yuma Valley. Because of the unusual coloration and patterns on the larvae, as well as the root boring behavior, we were uncertain of its identity. After rearing the larva to a moth, we were able to identify the Lep species. With the assistance of Ian Watkinson, we have identified the species as a beet armyworm, Spodoptera exigua (see Mystery Larvae Identified). This was surprising given the odd markings and coloration. In fact many (including myself) speculated that it might be a cutworm. But beet armyworm larvae are known to vary in color based on temperature and food source among other things. Why these larvae were boring into the lettuce roots is more confusing. In 24 years of working with beet armyworms in lettuce, I’ve never seen this occur. However, Peter Ellsworth noted that he has occasionally seen them bore into cotton terminals. You learn something new every day.
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:
Corn earworm:
CEW moth counts declined considerably over the past 2 weeks across all locations and below average for this time of the season.
Beet armyworm:
Trap counts increased slightly in many locations, but well below average for mid-October. Most activity in Dome and Yuma Valleys.
Cabbage looper:
Cabbage looper trap counts remain steady in most areas. Activity average to below normal compared to previous years.
Whitefly:
Adult movement increased sharply in the Dome Valley, Wellton Gila Valleys and Bard two weeks ago, but down in all locations last week.
Thrips:
Thrips adult movement variable across locations; up in Wellton and Tacna, but remains below average for mid-October.
Aphids:
Aphid movement Increased sharply in Bard and Yuma and Gila Valleys consistent with N and E winds over the past 2 weeks.
Leafminers:
Adult activity increased sharply in the Gila Valley 2 weeks ago, but remains low in most location; below average for this time of season.
Diamondback moth:
Significant increase in moth activity in the last 2 weeks, particularly in Dome Valley, Gila Valley and S. Yuma Valley. Traps located adjacent to cauliflower and broccoli transplants had significantly higher counts. Areawide counts are trending upward.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
