With the spring melon deal in Yuma finishing up in the next couple of weeks, now is the time to begin thinking about whitefly management in fall produce and melons crops. The first line of defense in avoiding whitefly issues in the fall produce/melon plantings is for PCAs and growers to do a good job of controlling whiteflies on cotton this summer. But before whitefly management begins in cotton, it is important that whitefly populations be prevented from building up to large numbers in the spring melons that have recently finished, or are nearing completion of harvest in the next few weeks. Although whitefly numbers have been relatively lighter this spring than normal, adult numbers have increased significantly over the past week in melons coinciding with higher temperatures and area-wide melon harvests. Growers and PCAs should not hesitate to knock these populations down on pre-harvest melons. Under current weather conditions (high temperatures) immature populations can rapidly build-up and cause honeydew/sooty mold contamination on fruit. Once harvest is complete, proper sanitation in spring melons is critical for preventing unnecessary whitefly migration into cotton and alfalfa. It is highly recommended that melon growers quickly destroy plant residue as soon as possible following harvest. A delay in disking under melon fields following harvest can provide a large source of adult whiteflies that will readily disperse into cotton and alfalfa, especially if they don't need to fly very far. These fields also potentially extend the host-acquisition period for CYSDV. Once melons fields have been disked and flat-watered, it is common for volunteer melons to germinate and emerge during July. The destruction of volunteer melons between spring and fall crops can reduce the source of whiteflies and virus inoculum available. This can be done by disking or applying a contact, burn-down herbicide to fields. Volunteer plants should be destroyed as soon as practical. Recent research has indicated that fall melons grown near spring melons with volunteers (<1.8 miles) are at a high risk of CYSDV infection. Ideally, it is in the Ag community’s best interest to use sound cultural management practices, such as sanitation, to eliminate and avoid unnecessary sources of whiteflies before the fall produce and melon seasons begin. For more information on cultural management practices for whiteflies see: Insect Management on Desert Melons: Whiteflies.
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:
Corn earworm:
CEW moth counts increased significantly ober the past 2 weeks reaching their summer peak so far.
Beet armyworm:
Trap counts remain high in some locations (Yuma and dome Valleys, Wellton) comparable to numbers we saw in October.
Cabbage looper:
Cabbage looper numbers remain low consistent with the lack of activity this fall and summer compared to last season.
Whitefly:
Adult movement continuing to increase slowly, particularly in the Yuma Valley consistent with melon harvest and destruction of plant residue.
Thrips:
Thrips adult movement highest in Dome Valley, but areawide started to decline to numbers comparable to this time last year.
Aphids:
Aphid movement has been has been absent for the past month similar to what we typically observe in the summer.
Leafminers:
Adult activity has highest in Dome Valley and Tacna, and about average for mid-summer.
Diamondback moth:
DBM counts continue to slowly decline and adults captured only in areas where seed crops were recently harvested.