Bob Dylan wrote in one of his songs that “you don’t need a weatherman to know which way the wind blows”. I don’t claim to be a meteorologist, but I think most would agree with me that this was one of the warmest winters we’ve experienced in a long while. In fact, I’m not sure when winter ended and spring started; sometime in January I think. Given the temperatures we’re currently experiencing one might wonder if summer has arrived early this year. But in fact, spring begins tomorrow and the produce season is quickly winding down. I made the comment last year that Yuma’s weather (2012-2013) was certainly unusual. Same this year, but don’t we say this every year? Same for insect pressure. The warm winter weather certainly had an influence on insect pressure at the Yuma Agricultural Center. It was not the worst I’ve seen but certainly heavier pressure than the previous 2 seasons; particularly for aphids, thrips and Leps. Based on weather data collected from AZMET weather stations throughout Yuma County, average temperatures were warmer in January and February than in the past several years (and compared to “normal” temp). This may explain to a large degree why aphid and thrips populations have been higher this year compared with what we historically see during this period. Although winter rainfall (Oct-Feb) was higher in some locations, almost no precipitation was recorded in Jan and Dec, which is about 0.75” lower than normal. This certainly can have an influence on insect infestations, especially thrips. Not surprisingly, thrips numbers are presently building exponentially on our late lettuce trials at YAC, but this is expected given our warm, dry growing conditions and the migration of adults from recently disked lettuce fields. Cabbage lopper, corn earworm, and diamondback moth numbers were high this spring, and are likely a reflection of the warmer nighttime temperatures experienced in the last 6-8 weeks. Not surprisingly, Bagrada bugs are quite abundant on our cole crops at YAC; keep an eye out for them on your Brassica seed crops. Whitefly adults have been found on early melons since early February. Although we think of CYSDV as a fall problem, high whitefly numbers in April and May could lead to more virus than expected on spring melons. Finally, seed corn maggots have not been a problem to date on either melons or cotton, but this recent warm, dry weather is not considered especially conducive to maggot outbreaks. For a detailed summary of the winter weather data described above, please view the update on Winter Weather Conditions for Yuma County, 2013-2014.
2023-2024 Powdery Mildew of Lettuce Fungicide Trial
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 17, 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
Max Temp (°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
Powdery mildew (caused by Golovinomyces cichoracearum) efficacy trial treatments were made on February 15,2024, February 23, 2024, March 4, 2024, and March 12, 2024and .Disease was first seen on February 26,2024. Disease rating was done on March 15, 2024. Disease severity was determined by rating 10 plants within each of the four replicate plots per treatment using the following rating system: 0 = no powdery mildew present; 0.5 = one to a few very small powdery mildew colonies on bottom leaves; 1 = powdery mildew present on bottom leaves of plant; 2 = powdery mildew present on bottom leaves and lower wrapper leaves; 3 = powdery mildew present on bottom leaves and all wrapper leaves; 4 = powdery mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = powdery mildew present on entire plant. These ratings were transformed to percentage of leaves infected values before being statistically analyzed. Yield loss due to rejected lettuce heads would likely begin to occur on plants with a powdery mildew rating above 2.0 (percentage of leaves infected value of 40).
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Most treatments significantly reduced the final severity of powdery mildew compared to nontreated plants. The most effective fungicides were Rhyme, Merivon, Quintec, Cevya, Luna Sensation, Luna Experience, and Elisys.
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 decrease to very low levels in areas, about average for this time of year.
Beet armyworm:
Trap counts low; increased slightly in the Yuma Valley; lower than average compared to previous years.
Cabbage looper:
Cabbage looper counts deceased in most traps and below average for this time of season.
Diamondback moth:
DBM moths counts increased Gila and Yuma Valley. About average for this time of the year.
Whitefly:
Adult movement negligible, average for late winter.
Thrips:
Thrips adult counts remain low, likely in response to cumulative rainfall this winter. Currently, numbers are below average compared with previous years.
Aphids:
Aphid movement decreased significantly in the past two weeks; about average for mid-February
Leafminers:
Adults remain low in most locations, increased in Yuma Valley; average for mid February.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
