Seems like each year about this time we begin to think how unusual this produce season is compared to previous years. Of course, every season is different. That’s the complicated nature of farming and pest management; so many factors (some explained and many unexplained) influence plant growth and insect pest activity and abundance. I thought I’d share some of my recent observations of “unusual” insect activity from research plots at the Yuma Ag Center (YAC), local commercial fields and conversations with PCAs. Whitefly populations were the lightest we’ve seen in years, but have been heavy in a few small areas. Many, like myself, believe this is due to the lower cotton acreage this summer, but there could be other factors as well. Beet armyworm pressure remains steady, but seems much lower than what I usually see this time of the year. Cabbage looper numbers have been considerably lower from what we typically see in lettuce and cole crops in October. There numbers seem to be increasing now. Corn earworm larvae populations appear to be about normal relative to the past 3 or 4 years. So far, I have not had any complaints in regards to controlling these worm pests, and all the standard Lep materials are performing up to par in my efficacy trials. In contrast, diamondback moth larvae are very abundant in my broccoli trials. They showed up earlier and in higher numbers than I typically see this time of the year. Not sure why? Good news is, they remain easy to kill with standard materials. Another interesting pest showing up is the Hawaiian beet webworm. They were very abundant last year and PCAs have been sending me images from this season (see image below). The larvae prefer spinach and beets and can cause damage if left untreated. Again, easy to control. Western flower thrips are on the increase, consistent with the warm weather. Adult and immatures are increasing on my lettuce plots, and several PCAs have mentioned that they are quite abundant. Have also had some reports of cowpea aphids showing up in lettuce. Experience has shown us that cowpea aphid will begin to colonize lettuce about this time of the year, but never seems to amount to much and colonies disappear when it gets colder. If the weather breaks in the next week or so as anticipated, worm and thrips pressure should slow down considerably. However, given our current and predicted El Nino weather pattern, all bets are off. So, have your weather reports handy, and keep your eyes open for the unexpected. Remember: When in Doubt-Scout.
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
