Just because we’re experiencing cool weather, don’t become complacent when it comes to aphids. Before the frigid weather occurred in the low desert over the holidays, aphids were beginning to colonize celery plants here at the Ag Center. However, recent temperatures in the Yuma area, and particularly in Wellton and Roll, dropped below 32° F on several nights. Experience has shown that aphid populations tend to slow down significantly under these “cold” desert temperatures. That is essentially what we observed the past week; population growth had stalled out and aphid numbers were very low. However, past experience has also shown us that aphid population growth begins to increase once the weather warms up. Our local weather forecast suggests that temperatures will be in the mid-to-high 60’s with moderate nighttime lows for the next two weeks. Assuming the weather service is correct, these conditions may be ideal for aphid population growth. Green peach aphid population growth tends to increase when temperatures average around 55-57 °F. Because we’ve seen an increase in winged aphids on our yellow sticky traps over the last month, PCAs should be extra vigilant in scouting for aphid colonies. As average temperatures increase later, foxglove aphid 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, particularly in cool, cloudy weather when the plants are not transpiring at a rapid rate. Regardless of whether you find green peach 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 please visit Aphid Management in Desert Produce Crops – 2016. So, keep your eyes open for new aphid colonies and plan according. As Sun Tzu, the ancient Chinese military strategist once said, “Rely not on the likelihood of the enemy’s not coming, but on our own readiness to receive him”
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 increased in Tacna/Roll/Dome last week, and areawide about average for mid-October.
Beet armyworm:
Trap counts highest in Tacna, Wellton and Dome, but below average for October.
Cabbage looper:
Cabbage looper numbers increased in Tacna/Roll/Wellton but still below average.
Diamondback moth:
DBM moths are beginning to appear in traps in Yuma Valley, trending below average for early October.
Whitefly:
Adult movement increased in the past 2 weeks and above average for mid-October.
Thrips:
Thrips adult activity down over the past week, and trending below average in October.
Aphids:
Winged adults continue to be captured for the season, consistent with heavy winds from W-NW. Can expect aphid flights to increase in the coming weeks.
Leafminers:
Adult activity increased significantly in Tacna associated with maturing melons. Trending above average for mid-October.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
