Every fall about this time we begin to think how unusual the produce season has been compared to previous years. And of course, every season is different. That’s the complicated nature of desert agriculture and insect pest management; so many abiotic and biotic factors influence plant growth and insect activity. This year is no different and here are a few observations I have so far based on field observations, PCA reports and areawide insect trapping over the past 6 years (see Insect Trapping Trends– 6 Year Avg).
Whitefly populations have been very light this year. Areawide trapping shows that whitefly counts are the lowest we’ve seen in 6 years. Whitefly flights we used to see during September just didn’t occur this year. The relatively light virus seen on the fall melons out east is another indicator. Populations have been so light on the experiment station that we are having difficulty starting whitefly trials in broccoli. Of course, there are some high numbers in localized areas based on PCA reports. Beet armyworm pressure started strong in early September, but decreased significantly following the remnants of Hurricane Rosa and Sergio, and the break in weather. This is reflected in trap counts compared with previous seasons. However, I have recently seen an increase in egg masses consistent with the warmer weather. Cabbage looper numbers have similarly been considerably lower from what we typically see in lettuce and cole crops in October and again, trap counts support this observation. Corn earworm adult trap counts however have been as high as we’ve seen in 6 years (see Graph below). This is important since the last time we saw trap counts this high (fall 2014), earworm larvae were common in the head lettuce fields harvested in November. Furthermore, I’ve had a few reports of occasional earworm larvae showing up in lettuce. Be careful with this one. The good news is that when the weather finally breaks, we should a marked decreased in worm pressure. Diamondback moth trap counts are lower this year overall, but did tend to increase following the recent storms (see DBM Trap Network). Larval numbers are light in our broccoli trials, and have had very few reports of larvae infesting commercial fields. To date, most reports of DBM larvae in fields are occurring in transplants. Keep in mind though, DBM can complete a life cycle (egg-adult) in a bout 2 weeks with temperatures we’re presently experiencing. Western flower thrips have also been unusually light this fall. The cooler weather and earlier rain in early October likely had a negative impact on population growth, but now thrips appear are on the increase, consistent with the warm weather and drier conditions. Adult and immatures are increasing on my lettuce plots, and several PCAs have mentioned that they are becoming abundant. Bean thrips are also showing up, albiet in lighter numbers than the previous 2 fall season. Aphid colonies are beginning to show up in brassica crops in some areas, and we are finding winged aphids showing up on small seedlings at YAC. Aphid trap counts have been low, but with the lush desert and winds from the N-NW, would anticipate aphid flights to begin picking up in the next few weeks as they typically do.
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%). Variety: Deluxe (HMX2595) was seeded, then sprinkler-irrigated to germinate seed on March 20, 2024on 84 inches 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. 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.
Spray treatments were done on 05-21-2024, 05-31-2024, 06-07-2024 and 06-14-24. Powdery mildew was first seen on 06-05-24. Please see excel file for additional details.
Disease severity of powdery mildew (caused by Sphaerotheca fuliginea and S. fusca) severity was determined 6-17-2024 by rating 10 plants within each of the four replicate plots per treatment using the following rating system: 0 = no powdery mildew present; 1 = one to two mildew colonies on leaves ;2 = powdery mildew present on one quarter of leaves; 3 = powdery mildew present on half of the leaves; 4 = powdery mildew present on more than half of leaf surface area ; 5 = powdery mildew present on entire leaf. These ratings were transformed to percentage of leaves infected values before being statistically analyzed.
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. Quintec, Merivon, Tesaris, Luna Sensation, and V6M-5-14 V gave the best disease control. Phytotoxicity symptoms were not noted for 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
