Diamondback Moth Control and Resistance Management 2016
What started off as a moderate insect season this fall has turned into one of the heaviest worm seasons on produce crops we’ve experienced in many years. Since mid-September, cabbage looper (CL), beet armyworm (BAW) and diamondback moth (DBM) populations have been consistently abundant in various areas throughout Yuma county. For the most part, CL and BAW have been relatively easy to manage with our standard insecticide products. In some cases, however, spray intervals have been shorter than normal due to overlapping egg lays and warm nighttime temperatures driving development of multiple overlapping generations in fields. Given the warm weather this fall and the abundance of alternate hosts for them to develop on all summer, the presence of these heavy infestations is not unusual. However, what is unusual is the emergence of DBM as a fall pest of cole crops. It’s been my experience, that DBM has become a fall pest within the past 7-8 years. Prior to then, I considered DBM a spring pest, occurring in fields once the weather warmed up. Because DBM attacks only cole crops (Cruciferae), they do not survive in any great abundance during the summer due to the lack of host crops. However, it is not unusual for fall transplanted cole crops to arrive in fields with DBM, regardless of the origin of the transplants. This certainly would explain how DBM populations can become so quickly established in September and October. Under ideal temperatures (~85 °F), DBM can complete a generation in a little over two weeks, as opposed to about 3 weeks necessary for CL and BAW. Thus once established, DBM populations can rapidly build up multiple generations in the field. Management of DBM in cole crops often requires intensive insecticidal management, more so than what is generally required to control BAW and CL. In some growing locations (e.g., Florida or Hawaii), protection of cole crops often requires multiple spray applications, sometimes as often as twice per week to break the DBM cycle. Unfortunately, intensive management can lead to insecticide resistance which DBM has a long history of. I’ve become concerned this fall because I’ve had numerous reports from PCAs who have had difficulty in controlling DBM in transplanted cole crops; transplanted cabbage and cauliflower in particular. In some cases, there have been reports of our standard insecticide products not providing adequate efficacy against DBM. It impossible to explain for sure why this poor field performance has occurred as many operational and biological factors (i.e., application timing and frequency, rates, choice of product, weather and resistance) ultimately determine the level of efficacy. Although resistance is unlikely, we will be collecting and bioassaying a few of these populations just to make sure this is not one of these factors. For the short term though, PCAs should remain diligent in their worm control using the standard products (Radiant, Proclaim, Coragen, Exirel, Intepid) at labeled rates and by ground whenever possible. Also it is important that PCAs practice sound insecticide resistance management (IRM) by rotating modes of action following each application. This is particularly important with the Diamide group of insecticides (IRAC group 28) because these products can be applied as both foliar sprays and soil injections and multiple generations of Lep larvae can potentially be exposed to variable doses of this chemistry. The most effective way to delay the onset of resistance by worms in leafy vegetables is to consider the recommendations provided in the guidelines recently updated entitled Insecticide Resistance Management Guidelines for Lepidopterous Larvae in Lettuce.
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: