Prior to the cold snap that occurred in the low desert last week, green peach aphid populations were steadily colonizing untreated lettuce plots here at the Ag Center. Very few winged forms were found, but small colonies of 5-6 nymphs could be found on about 10% of the plants. However, temperatures in the Yuma Valley dropped below 32° F several nights during that period. Past experience has shown that aphid populations tend to significantly slow down under these “cold” desert temperatures. That is essentially what we observed this weekend; population growth had stalled out and aphid numbers were the same as they were 2 weeks prior. However, past experience has also shown us that aphid population growth begins to increase again once the weather warms up. Our local forecast suggests that temperatures will be in the mid-70’s for the next 10 days. Assuming the weather service is correct, these conditions may be ideal for aphid population growth. Thus far, in the Yuma Valley we have been finding mainly green peach aphid nymph colonies, and we have been picking up winged aphids on our yellow sticky traps. However, we have found winged Foxglove aphids in our celery. As temperatures increase foxglove may become more abundant (they tend to be more biologically active under warmer temperatures; that is when avg. temps > 60 F). If the field has been treated with imidacloprid at planting, then chances are you are in pretty good shape. But, continue to monitor as imidacloprid doesn’t generally last all season, and the neonicotinoids are marginally effective against foxglove and lettuce aphids. Regardless of whether you find green peach aphid or foxglove aphid the key to effective aphid management with foliar insecticide is to initiate sprays at the time apterous (wingless) aphids begin to colonize. Of course, this requires diligent scouting and sampling. On older lettuce, make sure you thoroughly examine developing heads/hearts. Fortunately, PCAs have alternatives for foliar aphid control. For more information on insecticide alternatives and Aphid Identification please visit Aphid Management in Desert Produce Crops – 2015 andAphid Identification in Desert Produce Crops So keep your eyes open for new aphid colonies and plan accordingly. As Thomas Edison once said “Good fortune is what happens when opportunity meets with planning”.
Prior to the cold snap that occurred in the low desert last week, green peach aphid populations were steadily colonizing untreated lettuce plots here at the Ag Center. Very few winged forms were found, but small colonies of 5-6 nymphs could be found on about 10% of the plants. However, temperatures in the Yuma Valley dropped below 32° F several nights during that period. Past experience has shown that aphid populations tend to significantly slow down under these “cold” desert temperatures. That is essentially what we observed this weekend; population growth had stalled out and aphid numbers were the same as they were 2 weeks prior. However, past experience has also shown us that aphid population growth begins to increase again once the weather warms up. Our local
Prior to the cold snap that occurred in the low desert last week, green peach aphid populations were steadily colonizing untreated lettuce plots here at the Ag Center. Very few winged forms were found, but small colonies of 5-6 nymphs could be found on about 10% of the plants. However, temperatures in the Yuma Valley dropped below 32° F several nights during that period. Past experience has shown that aphid populations tend to significantly slow down under these “cold” desert temperatures. That is essentially what we observed this weekend; population growth had stalled out and aphid numbers were the same as they were 2 weeks prior. However, past experience has also shown us that aphid population growth begins to increase again once the weather warms up. Our local
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 4, 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
MaxTemp(°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
Downy mildew (caused by Bremia lactucae) rating was done on variety Eblin, Bobcat, and 180 (partially funded by AILRC grant). Disease was first seen on 1-30-24. Please see attached excel file for chemicals application date. Disease rating was done on February 29, 2024. Disease severity was determined by rating 10 plants within each of the five replicate plots per treatment using the following rating system: 0 = no downy mildew present; 0.5 = one to a few very small downy mildew colonies on bottom leaves; 1 = downy mildew present on bottom leaves of plant; 2 = downy mildew present on bottom leaves and lower wrapper leaves; 3 = downy mildew present on bottom leaves and all wrapper leaves; 4 = downy mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = downy mildew present on entire plant.
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Most of the treatments exhibited activity against the disease to some extent. Latitude, Amara, Eject, Previcure flex exhibited good control in the variety Eblin (highly susceptible variety). Whereas Cevya, Stargus, Latitude, Amara, Revus, Thrive 4 M, Actigard, Instigo+Carbose+intereact showed activity against the pathogen in variety Bobcat. Please see excel file for the full list of chemicals and their efficacy. The lettuce variety 180 was resistant to the disease and no downy mildew was observed on the particular variety. No phytotoxicity was observed in this field.
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
